How to take notes in A-level Biology
Notes are really important. There’s a lot of chatter online about the best methods for taking notes, but there are also a lot of harmful myths.
Notes are really important.
There’s a lot of chatter online about the best methods for taking notes, what type of notes are most useful, and whether to use other people’s notes.
There are also a lot of harmful myths about this topic, so let’s step back and first think about why we take notes at all. It’s “what students do”, but why?
Why do A-level students take notes
This is a big question all by itself and worth taking a moment to think about.
❌ “Your notes remind you what you need to know for the exam”
Ok, yes they can. But wait, don’t you have a textbook that does that too? Are your notes as comprehensive and correct as your textbook? Would you buy a textbook written by a current A-level student, and if not, why not?
Notes are important at University because the courses are not as established as A-level; there are no text books that specify what should be covered, and the specification changes from year to year. This isn’t true for A level, where the course rarely changes.
❌ “Your notes show that you have done the work”
Many students think that if they turn up to every class and copy down notes from the board then they will get a good grade. Unfortunately it isn’t so easy.
You can’t take your notes into the exam with you, and they won’t be marked.
Every year, diligent students who got an A at GCSE by carefully copying down notes and memorising them get a nasty shock at A level. This strategy isn’t going to win you a top grade in an exam which is specifically designed to test your deep understanding.
🤔 “Your notes show you understand the topic”
Your notes won’t be marked, so does this matter? But - understanding the topic is your goal as an A-level biology student, so this sounds good.
Do they show this? Well, it depends.
If the notes were copied directly from the board without you thinking about them, then no, sadly they don’t. I could go into a quantum physics class and copy equations off a board but it wouldn’t mean I understood anything.
But - if the notes were written from your own personal viewpoint after having fully understood the concept, then yes, they can. I’ll write more in a minute about how this happens and why it’s worthwhile to make notes like that even if you then throw them straight in the bin.
But first, there’s a BIG problem that nobody talks about:
When to take notes
🏫 Taking notes in class
The first time you encounter a topic will probably be in class. This will be the first time that you grapple with the new vocabulary and concepts and try to make sense of them in your mind.
The teacher will (or at least should) take you on a mental journey that leads you to understanding. This is really important as textbooks don’t explain/teach things, they just .. say them. It’s different. You need to listen to and think along all the steps of the explanation to make sense of things.
The question people don’t ask is: do you have enough mental processing power to build understanding from the explanation at the same time as writing notes?
Can you write notes quickly enough that you don’t miss anything? Or are you overwhelmed, so you end up just focusing on copying from the board rather than actually thinking about the content and following the explanation?
A-level biology has a LOT of content (too much, tbh), so teachers don’t have the luxury of taking their time going through it properly. They have to go fast, and that can quickly become overwhelming.
If you just copy off the board, you’ll end up with nothing more than a worse version of what you already have in your textbook. It’s more important to listen and think.
💻 Taking notes during tutorial sessions
Tutorials are recorded, which is fantastic because it frees you up.
You can fully listen during the live session, concentrating on making sense ofthe concepts. Then watch the video back later, going through more carefully and taking notes. I’ve had some fantastic feedback from students using this technique. It turns out that learning biology is a lot easier when you’re not trying to write a textbook at the same time.
🖊️Making notes when studying at home
The best time to make notes is when you’re not in a rush, and can take your time to go through things properly.
Don’t worry about spending ages making them beautiful, but do worry about making sure you understand everything you’re writing. Beautiful notes that include errors, gaps, and bits just copied out of textbooks are much worse than ugly notes written by someone who has thought about the topics in depth. Nobody will be marking your notes.
Why students should take notes
I’ve written a lot of negative-sounding stuff. But actually, writing notes is great!! Just not for the reasons you might think.
✅ Writing notes is fantastic and great
❌ The notes have very little value
Wait, what?
So here’s the big secret that students never fully understand (maybe you can be the first): the important thing about notes is the process of making them. And I don’t mean just copying things from the board.
Think about it. You want to pass the exam with a good grade, right? To do that, you need to have an understanding of the topics in your brain. So, the question is, how do you get things into your brain.
If it was as simple as listening to a lecture and copying notes from a board, life would be easy. But it’s not. Science tells us that the only way to get A-level Biology understanding into your brain for later recall is to process the concepts in your conscious mind. You have to make the effort to really think about them.
Unfortunately brains are pretty lazy and will outright lie to you to avoid having to do that: “yeah honest we’re learning this, 100% trust, we’re good to go”. But it’s lies.
Forcing stuff through your brain
If you just read something, your brain will lie and tell you that you understand it fine. It’s only when you try to use the concept in a new way or write it in your own words that you realise you don’t understand at all. This is a really frustrating feeling but it’s better to find out now rather than in the exam.
Writing information down in your own way (not copying) forces your brain to process that information. It can’t fake it’s way around it. You can’t write something in your own words without, at some point, having understood it.
So, writing notes in your own words is really useful. Learn the topic, make sense of it, then organise that information on the page in a way that best makes sense to you. Make links to other topics (I can’t overstate how important this is) and use the new knowledge to make more sense of the old topic. Also make links to your personal experience (biology is great for this).
The difficulty you feel when writing high quality notes - that is the feeling of learning happening. And remember: the learning is the goal, not the physical notes themselves. Trust me, the world has more A-level notes than it could possibly need.
Now here’s the problem - in class, everything is in such a rush that you don’t have time to do that properly. But the system encourages everyone to try, while all that does is to make it impossible for most students to listen properly to the teacher.
Ok so I know this is upsetting but it’s important
Humans love to have proof of value, in physical form. We love getting medals for our achievement - for most people the achievement itself is not enough, we need some physical representation of it. And notes are held up as a physical representation of learning. But the problem is that often they are not. You can have beautiful, comprehensive notes, but not have learned a thing.
You need to focus on the process, to value the actual learning, which will not be seen or appreciated by anyone other than the examiner through the lens of your answers in the exam. And that’s how you get a bit of paper with your final grade on it that you can proudly show people as a physical representation of your achievement.
What to Write Down
Taking notes in class
In class, I would suggest you don’t distract yourself by writing down all the things that will obviously also be in your textbook.
Do write topic headings, so you know where you are. And then record useful study tips for your future self to use for when you are revising the topic - what misunderstandings did you have this time that you could avoid in future? What useful examples did your teacher use? What did they say that gave you a lightbulb moment? What links did they make to other parts of the specification?
And if you’re naturally a doodler who needs to doodle/draw while you’re mentally processing then that can be fine - just don’t let the doodling stop you listening. This is biology class, not art/design.
Highlighting printed text is a waste of time - close that book and start writing from your own brain
Making notes at home
Making notest at home is a great to way think your way through the information and concepts that you need to learn.
Don’t just copy stuff out - that’s pointless - you need to learn it. Then write it down from a position of understanding. Explaining something on paper is a great way to ensure you really do understand it and that your brain isn’t lying to you about that. If you can’t write it in a coherent, organised way, then it’s not available in your brain in a coherent, organised way either.
When you’re writing stuff out and you get stuck, that is brilliant - it’s a strategy for finding out where your understanding is weaker, so that you can focus your efforts on the right things.
And don’t worry about having a perfect notebook or making everything beautiful - that’s going to make it harder to actually study. Focus on the biology. Scribble stuff out if it’s wrong. Rewrite things more clearly. Draw that diagram again. Redrawing and redrafting information is a good way to get it going through your mind over and over again, so long as you’re thinking about how to make the concepts clear rather than just making it pretty.
Borrowing notes from your friends
Wow did you really get this far?
Ok so what do you think, given everything I said above?
If you borrow your friend’s notes and copy them down, are you learning anything? What is the quality of what you’re learning?
Yeah, it’s a waste of time. It will make you feel better, but feeling confident beyond your actual knowledge is a really really good way to get disappointed when the exams come. If you miss a topic, you do need to learn it, but your friend’s attempt at creating a textbook during class isn’t going to help you with that. Check out your textbook to see what you need to learn, find good quality explanations online, learn the content, and write your own notes.
Then throw your notes away, whatever, it doesn’t matter. The important bit is safely stored in your brain - take care of it with a bit of ongoing revision and you’ll be all set to get a top grade.
Exam Technique - Last-Minute Golden Tip
The simplest, last-minute exam tip of all time
Here is the simplest Exam Technique tip ever:
Don’t fold the exam paper back on itself
Students typically fold the paper back to reduce it to A4 size, and to focus on just one page. Don’t do this!
Often, questions straddle more than one page. These will appear opposite sides of the fold. Keep the paper open, and you will be able to see all the information at once. Something from an earlier part of the question may well be vital for answering the last part. Don’t hide it from view!
More posts with exam tips:
Exam Technique - Simplify the Question
Make the Question Simpler
Let’s get rid of everything that isn’t on the spec. We will need these words later when writing the answer out, but we do NOT need them while we work out what the question is really about.
To be brutally honest, most students who get stuck on exam questions do so because they don’t understand the content deeply enough. There is no magical exam technique that can make up for that.
However, there are certainly some things you can do to make exams easier.
Here’s a great exam tip for anyone who gets overwhelmed when they see a question like the one below:
Overwhelming scary question:
What is Dihydrofolate Reductase?? That’s not on the spec! 😩😭
Don’t panic if you don’t recognise words in a question - if it’s not on the spec, you are not being tested on your knowledge about it.
That means that this question is not about dihydrofolate reductase, nor about Trexall
So what is it about?
Look for the familiar words and concepts. Focus on those, not on the weird jargon names of the example organisms/molecules/drugs that you have never seen before. They are just examples, the important thing are the general concepts.
Make the Question Simpler
Let’s get rid of everything that isn’t on the spec. We will need these words later when writing the answer out, but we do NOT need them while we work out what the question is really about.
First, let’s replace “Trexall” with “Drug”. Trexall isn’t on the spec so it’s just an example of a drug. The name isn’t important.
Do the same for the enzyme. You don’t need to remember that DR = dihydrofolate reductase while you’re working out the general, on-spec concept of the question. If you stick all that in your brain you’ll have less capacity for thinking about the important things.
And then the same for the substrate (Folic Acid is not on the spec, the concept of substrates is).
In summary: cross out the names of things that are not on the spec, and repace with generic words.
Hm, that sentence “the substrate for enzyme is substrate” is a bit pointless. So we can cut all that.
This is hopefully looking a little bit less overwhelming now.
The other thing we can do is to directly label the figures, so you don’t have to look up and down to remember which is which. This is for any type of figure - if it’s a graph, you would label the axes.
That helps a bit, but those molecular structures are still pretty scary stuff.
But remember: these molecules are not on the spec - so they are just examples. Two molecules, either of which might have had pretty much any structure.
What is the very first, most obvious and basic thing that you noticed about them when you looked at them together?
The Question Itself
Interesting, nothing about cancer in the question. We know that cancer involves fast-dividing cells, so a drug that slows cell division might help, but it doesn’t want you to talk about cancer (for this question at least). So that top sentence in the background info isn’t actually required either.
Also notice that it the question says “use the information provided” - this is a clue that everything you need is in the background information. They were never expecting you to already know anything about Trexall or dihydrofolate reductase.
The question has been simplified down to this:
▪ A drug slows cell division by interacting with an enzyme
▪ The enzyme is involved in making nucleotides
▪ The drug and the enzyme substrate look very similar
Suggest how the drug slows cell division
Can you work out the answer now? Give it a go before reading on.
Working Out the Answer
There are two important things you need to realise - firstly the mechanism of action of the drug, and secondly why its action slows cell division. Both of these rely on you knowing concepts from the spec.
1. The Mechanism of Action of the Drug
What have you learned about different ways in which drugs might work? How might a drug interact with an enzyme? How might a drug with a structure very similar to the substrate interact with an enzyme? How might that affect the enzyme?
2. Why does this Action Slow Cell Division?
The enzyme is involved in making nucleotides. Nucleotides are on the spec. What are they used for in the cell? How is this connected to cell division?
Writing Out the Answer
When you write out your answer, it’s really important to remember to reverse the simplification process and use the correct names of the drug, enzyme, and substrate!
Give it a go now if you haven’t already - write out the answer you would give. Then scroll down to check it against the answers & markscheme. Don’t jump ahead; practicing writing answers to exam questions is really important for learning. I promise nobody will see if you mess it up.
Answer & Markscheme
There are many ways to explain the answer to get the marks, here’s just one example:
Trexall has a molecular structure very similar to folic acid, which is the substrate of the enzyme DR. This suggests that Trexall acts as a competitive inhibitor by binding the enzyme’s active site. This action prevents folic acid from binding, reducing the number of enzyme-substrate complexes. As the enzyme is involved in nucleotide synthesis, Trexall slows the rate of that process. Nucleotides are required for DNA replication, which is required for cell division. Trexall slows cell division by reducing the number that are available to a point where their availability is a limiting factor.
Can you see where the marks came from? And did you get all three?
Top tip: there’s no point in trying to remember answers to exam questions - the questions in your exam will be different! Instead, focus on noticing things like the level of technical language and detail that are required, and how these compared to your answer.
A Quick Recap to Finish..
If you feel overwhelmed by the amount of information in an exam question, you can simplify the question by removing specific names of things that are not on the spec - which you are not being examined on - and replacing these by the general concepts that you are being examined on. Just remember to use specific language in your actual written answer.
The full Exam Tip collection:
Give yourself a moment to think about the previous post before surging forward - maybe go make some tea. But here is more food for thought.
Preparing for A level Biology: what can I do in the summer
What can you best do over the summer to help transition to A level Biology? (It's probably not what you're thinking.)
Tips and information to help with the transition from GCSE to A level.
Aiming for Success
Pressure on students seem to grow every year, with more and more students looking to do work over the summer to prepare for starting A level Biology in the autumn term. It’s true that A level Biology is a challenge, and there are certainly things you can do that will help your studies. This article gives advice on what you can best do to hit the ground running when you start your A level course.
One thing I don’t recommend is to ask a tutor to teach you A level content before you start. All this will do is interfere with your teacher’s work and make the classroom less interesting as there will be no surprises. That’s not a great way to build motivation for the long term. It’s much better to encounter new topics in the classroom, and use tutoring to check/deepen understanding and correct misconceptions.
The best things you can do over the summer are things that will (1) help consolidate your prior knowledge and understanding, and (2) create anchor points for you to learn/remember new knowledge.
You’re not going to like the first, but the second might be just what you need right now.
Consolidate Prior Knowledge
How did you do at GCSE?
A level biology builds upon concepts that you studied for GCSE. Having these solid in your mind will help massively when you are introduced to new materials. If you know you are a bit wobbly on some topics, watch out because that will make it difficult for you to understand the A level material - you’ll effectively need to learn both levels of content at the same time. And that’s a real challenge. Mastering the GCSE material will mean you can use it with little mental effort while grappling with the more-complex A level concepts.
Ok so it might feel weird to back go over GCSE content when the exams are done and dusted, but you’re going to need all that stuff again in your A level course. Maybe go back through it in August in the run-up to starting your A-level studies, and drill down into any areas where you feel like you’ve just memorised it without any real understanding.
Look at this comparison of the spec for GCSE and A level Cell Structure:
AQA A level Cell Structure
The structure of eukaryotic cells, restricted to the structure and function of:
cell-surface membrane
nucleus (containing chromosomes, consisting of protein-bound, linear DNA, and one or more nucleoli)
mitochondria
chloroplasts (in plants and algae)
Golgi apparatus and Golgi vesicles
lysosomes (a membrane-bound organelle that releases hydrolytic enzymes)
ribosomes
rough endoplasmic reticulum and smooth endoplasmic reticulum
cell wall (in plants, algae and fungi)
cell vacuole (in plants).
In complex multicellular organisms, eukaryotic cells become specialised for specific functions. Specialised cells are organised into tissues, tissues into organs and organs into systems.
Students should be able to apply their knowledge of these features in explaining adaptations of eukaryotic cells.
AQA GCSE Cell Structure
Students should be able to explain how the main sub-cellular structures, including the nucleus, cell membranes, mitochondria, chloroplasts in plant cells and plasmids in bacterial cells are related to their functions.
Most animal cells have the following parts:
a nucleus
cytoplasm
a cell membrane
mitochondria
ribosomes.
In addition to the parts found in animal cells, plant cells often have:
chloroplasts
a permanent vacuole filled with cell sap.
Plant and algal cells also have a cell wall made of cellulose, which strengthens the cell.
Recognise, draw and interpret images of cells.
Students should be able to use estimations and explain what they should be used to judge the relative size or area of sub-cellular structures.
You can see that there is a lot of overlap - the GCSE content is used as a foundation for learning more. Because you already know something about organelles and their general functions, you can build additional understanding by adding to this prior knowledge. Learning everything from scratch would be really hard! That’s why you need GCSE qualifications to enter the course - your GCSE knowledge will act as a springboard. But how good that springboard is might vary across topics.
You’ll also need to know how to calculate areas and volumes, and to read graphs and understand how averages can be used to understand data. How did you do at GCSE maths?
A person with strong GCSE Biology and Maths will find it much easier to learn A level Biology than someone with a poor grade in combined science. Not because they’re cleverer (whatever that means), but simply because they’re starting from a better place.
What to do: if you know you’re weak on some parts of GCSE, take a look back over those areas and make sure you have a strong foundation for learning more. You’re going to have it re-learn it at some point, and it’s easier to do it while you’re not also grappling with higher-level concepts that won’t make sense without that prior knowledge.
Stretch your Literacy
A level Biology involves a lot of complex vocabulary and comprehension of texts. Written language is very different from spoken language, so if you usually consume informal, spoken media it may be more difficult for you to follow biology texts.
Reading any long-form, professionally written texts will help stretch your literacy and get you used to the vocabulary and sentence structures used in formal writing. It would be ideal to read a pop-sci biology book on a subject that interests you, but reading any books with formal-stye writing, on just about any topic, would be a great boost.
Literacy is a huge factor in student success, and especially anyone with lower grades in GCSE English would benefit from getting more used to reading long-form written-language texts. If you’re struggling to understand the language before you even start to grapple with the biological concepts, the course will be extra-difficult for you.
What to do: find a well-written blog, or book, or other long-form media that interest you and get used to reading in an engaged, thoughtful way. Put your phone aside and practice focusing on the text and its meaning, thinking about how it links to your own interests and life.
Enrich your mind
The human brain is unable to remember facts in isolation. This is why memory experts need to use mnemonic tricks. It’s much, MUCH easier to remember things if they relate to things we already value, our life experiences, our self-image, our emotions, or our prior knowledge.
This makes A level Biology more difficult for students who been unable to travel, or have perhaps focused purely on classroom study. In biology you will encounter many examples of animals and environments that are well-known to some students, but new to others.
A student who has visited a rainforest will find it easier to learn and remember new information about rainforests not only because they might already know some things, but also because they can link new information to their prior experience.
Students with little experience may also get tripped up by organisms that are used as ‘well known’ examples to illustrate points. Some students don’t know what cows eat, or that dolphins are not fish, or that bats are mammals. Well-meaning teachers can confuse students with less-broad life experience by assuming knowledge that just isn’t there.
I’m not suggesting you don’t know what cows eat, or that you visit a rainforest (although do if you can - they’re awesome). But you absolutely can enrich your mind with different experiences and stories and images that will serve you well as anchors for future learning.
Visit different environments
If you can visit a zoo, or an aquarium - perfect! Take your time to really observe the animals. Build strong memories by taking notice of the smells and sounds around you. Read the labels. Talk to your friends about what you see, relating your observations to other things you already know about. Then, later in the course, when your teacher talks about the neck-bones in a giraffe, you can stick that information on to your memory of the giraffe in the zoo. It sounds silly but seriously, it’s like a cheat code for learning.
Maybe you can find a volunteer opportunity in the summer helping with conservation work. This can give you a real connection to the environment you’re working in, and an understanding of what it’s like to work in the field that will help you better imagine, and remember, the field studies described in your course.
Or, just visit the park. Go for a walk by a river. Notice the small organisms around you. The weeds in the cracks. Stop to observe insects. What can you smell? Notice how different animals are hanging out in different environments. In what ways do these environments vary? Stick your hand in a river, feel the texture of the leaves of a tree and notice how the top and bottom surfaces are different. Listen to the wind and birdsong, talk, build emotional and sensory memories that will provide strong anchors for future learning. Touch grass.
What to do: visit different environments, exploring with all your senses while observing and discussing the organisms you discover.
Go on a virtual adventure
Documentaries and films can take you to a vast variety of environments that you wouldn’t be able to visit in person.
Check out BBC Nature documentaries and fall into the emotional stories they tell of individual animals. Imagine how the animal feels, its challenges, abilities and basic drives. Get a really broad view of the diversity of environments and organisms on Earth and the delicate interactions. When you later learn about some A level concept, you will suddenly think “oh, wait, it’s like that thing I saw!” and suddenly it’ll all make sense and be easy to remember.
If you don’t live in the UK, especially if your local environment is very different, how about watching some programmes about, or set in, British farmland. Find out what cows eat! Examples from farming and agriculture are found throughout the course and will be less accessible to you if they are unfamiliar.
What to do: immerse yourself in rich visuals, music and stories of environment and organisations from around the planet.
The best part about building these rich memories is that it won’t burn you out. You can enjoy the experience without having to do difficult analytic thinking, trying to force information into your brain, or recalling complex information.
Let’s face it, if you’ve just finished your exams then more academic study might not be what you need right now.
So in summary …
Broaden your mind and lay the foundational knowledge that you will need to engage with A level Biology concepts. In this way you can set yourself up for an easier time in the classroom in September - and throughout the whole course.
Article by Jenny Shipway. If you liked it, click ❤️Like below to help others find useful articles.
Key Concept: Averages, Range, and Standard Deviation, with A level Biology Past-Paper Questions
You need to know some maths for A level Biology. This includes knowing how to interpret averages (mean, median and mode), ranges, and standard deviations to work out whether an experiment can be said to have shown an effect or not. Master this early on and it will not help you with exam questions, but also make it easier for you to learn the bits of the course that are explained using these statistical methods.
You need to know some maths for A level Biology. This includes knowing how to interpret averages (mean, median and mode), ranges, and standard deviations to work out whether an experiment can be said to have shown an effect or not. Master this early on and it will not help you with exam questions, but also make it easier for you to learn the bits of the course that are explained using these statistical methods.
Why does Biology need so much data?
Maybe the guy at the back is just big for his age?
Researchers often want to compare two or more things. Which species of frog is heavier? Which type of soil grows taller plants? At what temperature do these bacteria divide fastest? At which pH are fish most active?
The biological world is complicated, so multiple, repeated measurements are usually required.
There are three main reasons for taking multiple measurements:
Measurement errors. It’s hard to take measurements in the real world. Even if you re-measure the exact same thing, and even if you use a well-calibrated tool, you might get a slightly different result each time. Maybe you can’t hold the tool still enough, or you can’t read it clearly, or the thing you’re measuring moves. These are precision errors.
Individual variation. If you want to ask a general question about a whole population, eg “do robins sing more than blackbirds” then you need to measure data from more than two individuals. If you only use two, you might randomly pick outliers; maybe you get a particularly perky robin, or a lazy/sick blackbird. Similarly, if you sample a small area of a larger region, you may not pick a representative area.
Uncontrolled variables.There will nearly always be variable-influencing factors that you’re not aware of, or unable to control. Maybe there are changing sounds or smells in the environment, subtle changes in light, or in the birds’ blood-sugar levels. These can affect individual measurements in unpredictable ways.
All of these things can affect the value you record, making any one single measurement unreliable. So researchers normally end up collecting large sets of measurements. In this way they can get a much better idea of what’s really going on.
Why does Biology need Statistical techniques?
Plotting lots of repeated measurements for different datasets on the same graph can create a confusing mess. Also, “the data look different to me” isn’t good enough for science.
Reducing each dataset to just two or three values makes it much easier to compare. In fact, it’s so simple that such data can be understood even without a graph, so values are often presented very simply in a table.
Calculating Averages in Biology
There are three types of average: mean, median, and mode. They all reduce the data set to one single number.
This is useful for comparisons. For example, if you let a frog jump ten times, measuring the length of every jump, you can calculate their average jump length. You can then compare that single number to the average jump length from another frog to find out which jumps further.
Calculating the Mean
The most important type of average for A level Biology is the mean. It’s also what most people are talking about when they say “average” in everyday life.
To find the mean, add up all the numbers, then divide by how many numbers there were. You end up with just one number.
Here’s an example dataset:
| Set 1 | 3 | 4 | 5 | 5 | 5 | 6 | 6 | 6 | 7 | 8 | total = 55 / n = 10 / mean = 5.5 |
| Set 1 | 3 | 4 | 5 | 5 | 5 | 6 | 6 | 6 | 7 | 48 | total = 95 / n = 10 / mean = 9.5 |
| Set 1 | 3 | 4 | 5 | 5 | 5 | 6 | 6 | 6 | 7 | 48 | central number(s) = 5 and 6 / median = 5.5 |
| Set 1 | 1 | 3 | 5 | 5 | 5 | 5 | 6 | 6 | 7 | 48 | mode = 5 |
| Set 1 | 50 | 50 | 50 | 50 | 50 | 50 | 50 | 50 | 50 | 50 | mean = 50 / median = 50 / mode = 50 | ||
| Set 2 | 25 | 30 | 35 | 40 | 50 | 50 | 60 | 65 | 70 | 75 | mean = 50 / median = 50 / mode = 50 | ||
| Set 3 | 1 | 2 | 3 | 4 | 50 | 50 | 96 | 97 | 98 | 99 | mean = 50 / median = 50 / mode = 50 |
The averages are the same! By themselves, averages only tell you one small part of the story.
What is Range / why is it useful
One of the big differences betwen the datasets above is the range of numbers that appear.
The range is the range-of-values that appear, from the lowest to the highest.
| Set 1 | 50 | 50 | 50 | 50 | 50 | 50 | 50 | 50 | 50 | 50 | lowest value = 50 / highest value = 50 / range = 50 to 50 | ||
| Set 2 | 25 | 30 | 35 | 40 | 45 | 55 | 60 | 65 | 70 | 75 | lowest value = 25 / highest value = 75 / range = 25 to 75 | ||
| Set 3 | 1 | 2 | 3 | 4 | 50 | 50 | 96 | 97 | 98 | 99 | lowest value = 1 / highest value = 99 / range = 1 to 99 |
Set 1 has a range of 50 to 50. So you can reasonably predict that the next measurement would likely be 50 too
Set 2 and Set 3 have wider ranges. There are a wider range of possible values that might be measured, so it’s harder to predict what the next measurement might be.
A wide range might indicate that your measurement technique is very unprecise, or that there is a wide natural variation in the thing you are measuring, or that there is another factor affecting your measurements.
But a wide range might also just mean there were one and two weird outliers in the data. So you need to be careful when using this value. Here is a set with one odd measurement, which might be due to a measurement error.
| Set 4 | 50 | 50 | 50 | 50 | 50 | 50 | 50 | 50 | 50 | 90 | lowest value = 50 / highest value = 90 / range = 50 to 90 |
| Set 1 | 25 | 42 | 48 | 50 | 50 | 50 | 50 | 52 | 58 | 75 | values clustered around mean = low standard deviation | ||
| Set 2 | 25 | 30 | 35 | 40 | 45 | 55 | 60 | 65 | 70 | 75 | values spread out away from mean = high standard deviation |
To understand Standard Deviation, think about a situation where you have made very many measurements, so that you have multiple measurements at each possible value. Now plot these on a graph (see below). In biology, you usually see that the graph forms a bell shape. This is called a “Normal distribution”.
Normal distributions are symmetrical, so the mean, mode, and median are all the same, appearing at the centre of the graph (mean, median, and mode = 16 in this example). In normal distributions, most measurements are near the average, so there is a peak in the middle of the graph.
(Sometimes, you’ll find a curve is skewed a bit to one side. This separates out the mode, median and mean values. But for our purposes, I’m going to stick to thinking about the symmetrical graph.)
How wide the curve is matters a lot, because it affects how much two sets of data overlap. Compare these two examples below. Both have one set of data where the mean is 14 (plotted in orange), and another set where the mean is 20 (plotted in blue).
There is the same amount of data in both graphs, and the averages haven’t changed. But there is a lot less overlap between the two datasets in the example to the left. The data on the right is a lot more spread out away from the average values.
When datasets overlap a lot, you need to be very careful that you definitely have enough data to be sure their means really are different. If you have a small data set with a lot of variation, then adding extra measurements can make a big difference to the mean.
What is Standard Deviation
Standard Deviation tells you how widely the data is spread out in a normal distribution. Its symbol is sigma, “σ”.
You’re very unlikely to be asked to calculate standard deviation in an exam, and it takes a while to explain so I’m not going to go through it here (don’t worry they’d give you the equation if you did have to do this).
But you do need to know what it tells you.
Here is the basic normal distribution graph again. The graph is symmetrical and the mean (μ) is in the centre.
Now here is the same graph, but two more values are marked on the x-axis, shown by orange lines. These are the value of the mean minus one standard deviation (μ-σ), and the value of the mean plus one standard deviation (μ+σ).
If you colour in the bit of the graph that is within one standard deviation of the mean (from μ-σ to μ+σ), then on any normal distribution, 68.27% of the data points will lie within this area. You don’t need to remember that percentage, but remember it is always the same.
This means that if the standard deviation is a small number, you know most of the data points are close to the mean. This gives you more confidence that the mean is a useful value for comparison.
The graphs below have the same X-axis. Both are normal distributions with the same mean. But the one on the left has a small standard deviation, and the one on the right has a high standard deviation. (Some of the data from the right-hand graph falls outside the values shown on the graph.)
How to tell if there is a significant difference between values using the mean and standard deviation
!! Ok so this is the important bit we’ve been building up to !!
In a normal distribution, most of the data (68.27%) falls within one standard deviation of the mean. This is the area between μ-σ and μ+σ.
To work out whether it’s just chance that the means are different, or whether it’s a real effect, you need to check whether this area overlaps bewteen the two sets of data.
If the areas between μ-σ and μ+σ overlap, the difference is not considered significant.
There are different ways of presenting the data.
Standard Deviations Using Numbers - example
An example:
Set 1: mean (μ) = 50, standard deviation (σ) = 8
Set 2: mean (μ) = 40, standard deviation (σ) = 3
Are these sets of data significantly different? Look at the areas between μ-σ and μ+σ
Set 1: μ-σ = 42 and μ+σ = 58
Set 2: μ-σ = 37 and μ+σ = 43
Do these areas overlap? Yes they do (both include 42-43). So you can not consider the two data sets significantly different.
(Also worth knowing: nearly all the data (95.45%) falls within two standard deviations (between μ-2σ and μ+2σ) - so if these two areas don’t overlap you can be even more sure the two sets of data really are different.)
Standard Deviations Plotted on Graphs - example
On graphs, the mean is plotted as usual, with a dot or column. Extra lines extend out to show the area from μ-σ to μ+σ. This can make it more obvious whether areas overlap or not (unless they are super close in which case numbers are more useful).
Standard Deviation Exam Past Papers
Example Exam Question 5
Question 5 answers found at the bottom of this web page
Understanding Standard Deviations from Graphs
Example Exam Question 6
Question 6 answers found at the bottom of this web page
Example Exam Question 7
Question 7 answers found at the bottom of this web page
Graphs and Tables in A level Biology
If you’re not confident with questions that include graphs and tables, see the recent blog post “How to Approach A level Biology Graph and Table Questions: Tips and Exam Question Pack”, which offers more useful tips for navigating them during exams, and more exam questions to practice with.
Answers to example exam questions
The data for the damaged block should be ignored. The mean for shape C is 3520 seconds
Cinnamon Oil median = 16, mean = 17 ….. and ….. Postive Control median = 12, median = 13
Median = 41. This avoids the outliers affecting the value as would happen if you used the mean. And the sample size is too small to use the mode (there are no repeated values)
The range is 2 to 11
Bull terrier genetic diversity is significantly the smallest of the breeds shown, meaning it the most inbred. Jack Russell genetic diversity is significantly the greatest. The genetic diversity of Miniature terrier and Airedale terriers are similar with no significant difference between the two.
Standard deviation is spread of data around the mean; using standard deviation reduces effect of anomalies/ outliers; standard deviationcan be used to determine if (the difference in results is) significant/not significant/due to chance /not due to chance
Trapping increases enzyme/GOx/HRP activity; the difference/increase is significant (it is unlikely to be due to chance as the standard deviations do not overlap)
Booking now: AQA Y13 A level Biology Group Class - from Sept 2026 to June 2027
Weekly Group classes - for AQA Y13 A level BIology
Raise your exam grade with question focused masterclasses from a highly experienced A level Biology teacher.
Every week we go through a different topic from the specification and look at how to apply the content to exam questions. I show how to understand commonly occurring questions and how to answer them.
Weekly Group classes - for AQA Y13 A level BIology
Raise your exam grade with question focused masterclasses from a highly experienced A level Biology teacher.
Every week we go through a different topic from the specification and look at how to apply the content to exam questions. I show how to understand commonly occurring questions and how to answer them.
“Outstanding A-level Biology tutoring! Patient, engaging, and highly personalised—even in group classes, it feels one-to-one. Recorded sessions, all questions answered, and every student involved. Our daughter jumped a grade and achieved an A and a place to study Medicine at University” - Google reviews 2025
During the lesson students use an interactive whiteboard to write answers to exam questions which (only) I can see and comment on. Students can ask questions at any time but are not required to speak on camera to the group.
The classes run in focus mode on zoom - so I can see every student, but they are not visible to the rest of the class.
Students can stream a recording of every lesson for revision and note taking.
I teach using evidence-based educational theory. With decades of A level Biology class and one-to-one teaching experience, I am very aware of the misconceptions and misunderstandings that cause students to unnecessarily struggle, and of the mistakes that can lead to dropped marks in exams.
By correcting these issues, students not only do better in exams but also learn to enjoy studying Biology.
The typical class size is 6-12 students. No payment is taken in advance. The classes are £45 per lesson. The card you use to reserve your place is charged after the lesson.
AQA Biology Y13 Schedule (2026–2027) Time: Wednesdays at 6:30 PM Notes: Includes sessions during Easter break; closed for Christmas (Dec 23 & 30).
| Month | Date | Spec Ref | Topic Focus |
|---|---|---|---|
| September 2026 | 09 Sep | 3.4.5 | Species and taxonomy |
| 16 Sep | 3.4.6 | Biodiversity within a community | |
| 23 Sep | 3.4.7 | Investigating diversity | |
| 30 Sep | 3.5.1 | Photosynthesis – Part 1 | |
| October 2026 | 07 Oct | 3.5.1 | Photosynthesis – Part 2 |
| 14 Oct | 3.6.1.1 | Survival and response | |
| 21 Oct | 3.5.2 | Respiration – Part 1 | |
| 28 Oct | 3.5.3 | Energy and ecosystems | |
| November 2026 | 04 Nov | 3.5.2 | Respiration – Part 2 |
| 11 Nov | 3.5.4 | Nutrient cycles | |
| 18 Nov | 3.6.1.2 | Receptors | |
| 25 Nov | 3.6.1.3 | Control of heart rate | |
| December 2026 | 02 Dec | 3.6.2.1 | Nerve impulses (neurones, resting & action potentials) |
| 09 Dec | 3.6.2.2 | Synaptic transmission (summation, inhibitory, drugs) | |
| 16 Dec | 3.6.4.1 | Principles of homeostasis and negative feedback | |
| 23 Dec | — | No Lesson (Christmas) | |
| 30 Dec | — | No Lesson (Christmas) | |
| January 2027 | 06 Jan | 3.6.3 | Skeletal muscles |
| 13 Jan | 3.6.4.2 | Control of blood glucose concentration and diabetes | |
| 20 Jan | 3.6.4.3 | Control of blood water potential – Part 1 | |
| 27 Jan | 3.6.4.3 | Control of blood water potential – Part 2 | |
| February 2027 | 03 Feb | 3.7.1 | Inheritance – Part 1 |
| 10 Feb | Revision | Revision lesson of questions on 3.5 and 3.6 | |
| 17 Feb | 3.7.1 | Inheritance – Part 2 | |
| 24 Feb | 3.7.2 | Populations and Hardy-Weinberg | |
| March 2027 | 03 Mar | 3.7.3 | Evolution may lead to speciation |
| 10 Mar | 3.7.4 | Populations in ecosystems (including succession) | |
| 17 Mar | 3.8.1 | Alteration of DNA sequences and effects on proteins | |
| 24 Mar | 3.8.2.1-2 | Non-coding DNA / Regulation of transcription & translation | |
| 31 Mar | 3.8.2.3, 3.8.3 | Gene expression and cancer / Using genome projects | |
| April 2027 | 07 Apr | 3.8.4.1 | Recombinant DNA technology (Easter Session) |
| 14 Apr | 3.8.5, 3.8.6 | Probes and DNA fingerprinting (Easter Session) | |
| 21 Apr | Stats | Stats tests, P values, SD, range, and averages | |
| 28 Apr | Maths | Maths Questions and Magnification | |
| May 2027 | 05 May | Eval | Evaluation Questions |
| 12 May | Exam | Extended Response Question Practice | |
| 19 May | Review | Synoptic Links and Paper 3 Prep | |
| 26 May | Final | Last-minute Q&A and Exam Technique |
Another "Suggest" AQA exam question walk through
Example 2: Q3 Paper 2 2023
This next question is more complex, and there are two ‘suggest’ questions.
But first - I always recommend you don’t read the actual questions until you’ve looked at the background information, graph etc. Doing this will help you avoid getting overwhelmed and jumping to mistaken conclusions (which is very common in exam situations!).
So let’s keep the questions for later. First make sense of this:
Here is another example of a “Suggest” question from a past paper.
Find the main “Suggest” article and first example question here
Example 2: Q3 Paper 2 2023
This next question is more complex, and there are two ‘suggest’ questions.
But first - I always recommend you don’t read the actual questions until you’ve looked at the background information, graph etc. Doing this will help you avoid getting overwhelmed and jumping to mistaken conclusions (which is very common in exam situations!).
So let’s keep the questions for later. First make sense of this:
1. Don’t panic!
This question is going to challenge your working memory by throwing lots of information at you all at once. Tackle it bit by bit to make sense of what’s going on.
2. Use your knowledge to make sense of the background information:
There are tomatoes, a “mycorrhizal species’, and different water conditions.
You know that mycorrhizae are fungi (3.5.4)
You know that plants need the correct amount of water in order to grow (GCSE)
You can understand the experiment – including identifying the independent, dependent, and controlled variables. (8.3)
You can understand the data – what is the graph is showing? (6.4)
Top tip: Write “IV” and “DV” on the paper to identify the Independent and Dependent variables.
Water availability = IV
Whether mycorrhizae were added to the soil = IV
The mean mass of tomatoes = DV
…. What is the graph showing?
· The pair of bars on the left of the graph compare the yield of tomatoes from plants grown in conditions of water shortage.
o The bar on the far left is for plants grown in soil that did not have mycorrhize added. The other is for plants in soil that did have mycorrhizae added.
o The results show a significant difference between the yield of tomatoes for these two groups of plants. The plants with mycorrhizae yielded more tomatos.
· The pair of bars on the right compare plants that did not experience water shortage.
o Again, the bar on the left is without mycorrhizae, and that on the right is with mycorrhizae.
o The results show no significant difference between the yield of tomatoes from these two groups of plants.
Got that? Ok now you’re ready to look at the questions. How would you approach these?
3. Answer questions in order:
The first part of the question (not shown) is about phosphorous cycles, so you will already be thinking about content from 3.5.4 (Nutrient Cycles).
4. Check the Command Word:
‘Suggest’.
5. Understand the question:
These questions are quite straightforward.
6. Think about relevant information from the spec
You know that mycorrhizae facilitate the uptake of water and inorganic ions by plants. (3.5.4)
You know that there are a variety of living organisms in soil, and that these are in competition (3.7.4)
ou have identified the fertiliser concentration as a controlled variable (8.3)
7. How many marks are there available?
Each question has two reasons for two marks; one mark per reason. Make them good ones!
8. So, what are your answers?
There are a variety of different ways to get the marks, allowing you to play to your strengths. Give it a go before looking at the makr scheme below.
..
..
..
..
..
..
..
..
..
Q3 Paper 2 2023 Q3.3 – mark scheme
Did you get the marks?
Paper 2 2023 Q3.3 – example answers
Good answer examples, which would win marks:
✅ to ensure that there are no other fungi growing in the soil
✅ to remove any seeds in the soil so that other plants don’t grow and consume the nutrients and water
✅ to ensure that there are no pathogens in the soil that can infect the tomato plants
Poor answers that would not get the mark – can you identify where they’ve gone wrong?
❌ To remove harmful bacteria
❌ To kill everything living in the soil so it doesn’t interfere with the experiment
❌ to make sure that conditions are ideal for growing tomato plants
Q3 Paper 2 2023 Q3.4 - markscheme
Paper 2 2023 Q3.4 – example answers
Good answer examples, which would win marks:
✅ The investigation is on the effect of water shortage so the concentration of fertiliser should be a control variable
✅ The concentration of fertiliser will affect the growth of the plant so the recommended amount should be used to get the best crop
✅ Fertilisers can affect the water potential of the soil which may impact how water is absorbed by the roots
Poor answers that would not get the mark – can you identify where they’ve gone wrong?
❌ Without fertiliser the tomatoes won’t grow
❌ So that the soil doesn’t affect the size of the tomatoes
❌ So that the tomatoes can be compare
Article by Natalie Vlachakis (an ex-teacher who also worked for AQA) & Jenny Shipway
How to answer "Suggest" questions in AQA A level Biology
Command words are important! Step by step guide to how to approach and answer “Suggest” questions for top marks in AQA A level Biology.
Article by Natalie Vlachakis (an ex-teacher who also worked for AQA) & Jenny Shipway
Command Words: What do you Suggest?
There are many ways that examiners can ask an A-level Biology question. They might ask you to describe the structure of DNA. Or to explain the importance of the xylem being kept open as a continuous tube.
In exam questions, words like ‘describe’, ‘explain’ (and ‘suggest’) are known as command words. Always pay attention to command words because they tell you what type of knowledge or skill is being tested, and how you should approach the question.
So - when you get a question that asks you to ‘suggest’ something, what exactly does it want from you?
‘Suggest’ Questions
‘Suggest’ questions are usually asking you to use your biology knowledge and understanding to make sense of, and build upon, information about an unfamiliar organism and/or experiment.
Using unfamiliar contexts is a way for examiners to check that you really understand the content; rather than just having memorised set phrases and information.
You aren’t expected to already know the answer, or even be absolutely 100% certain that you’re correct. You just need to come up with something that would make sense biologically, and is likely to be true.
Example Exam Question
Q6 Paper 1 2023
The question:
How to approach this question:
1. Don’t panic!
A question about historical classification of bacteria?! That topic is not in the spec!! Don’t panic – all the information you need about any off-spec topic will be provided in the question.
This example question is going to test your knowledge of classification, bacteria and microscopy (all from the spec), and – most crucially - your ability to apply that knowledge in an unfamiliar context.
2. Use your knowledge to make sense of the background information:
You know that bacteria are uni-cellular organisms. Your knowledge of bacterial structure will allow you to understand the description of how bacteria appear under the microscope. You know what a flagellum is. (3.2.1.2)
You know how ‘species’ and ‘genus’ are used in classification (3.4.5)
Your knowledge and experience of microscopy will allow you to understand the description of the limitations of optical microscopes. (3.2.1.3)
3. Answer questions in order
Answering question 6.2 (‘consider’ = ‘review and respond’) first will help you. This question gets you thinking in more detail about things that will help you answer 06.3.
4. Check the Command Word:
Question 6.3 asks you to ‘suggest’. So you know that you should use your existing knowledge/skills to make sense of, and build upon, the information given. You don’t need to know the exact answer for sure, but you need to give a biologically reasonable answer.
5. Understand the question:
The question asks “why” the species have been renamed.
Why might someone rename a species? – because they were misclassified before
Why might they have been misclassified before? – because of the limitations as in your answer to 6.2
Something has changed in recent years that has made it possible to classify these bacteria more accurately than before. What might this be?
6. Think about relevant information from the spec
What have you learned that is related to this question?
Phylogenetic classification (3.4.5)
Methods of studying cells (3.2.1.3)
7. How many marks are there available?
There is only one mark available – you can do this! But if possible, try to include more than one thing that might get a mark, so if one fails you are still ok.
8. So, what’s your answer?
Give it a go before looking at the markscheme below. Remember you need to use specific, technical language to win marks.
…
…
…
…
…
…
Q6 Paper 1 2023 6.3 - mark scheme
A range of answers are acceptable for this 1 mark. How did you do?
Q6 Paper 1 2023 6.3 – example answers
Successful answer that would win marks:
✅ “Different species will have different DNA sequences so by using genome sequencing they can identify different species”
✅ “Comparing the base sequences of mRNA from the different bacteria will show how similar they are to one another so whether or not they are the same species”
✅ “Electron microscopes have better resolution that optical microscopes so scientists can see the arrangement of the flagella in more detail”
Poor answers that would not get the mark – can you identify where they’ve gone wrong?
❌ “They can identify phylogeny better now, by looking at the DNA”
❌ “They can see more detail with microscopes now so there are fewer mistakes”
❌ “Advanced techniques allow more accurate determination of phylogeny“
A final thought:
The word ‘Suggest’ can be used in different types of questions (e.g. “Suggest two reasons why … ”, “Suggest how … ”) . But the word ‘Suggest’ always invites you to think about the problem in your own way - to use the information in the question with your biology knowledge to come up with your best possible answer.
Want more? Find another “Suggest” exam question walk-through here
Monoclonal Antibodies in the Immune Response (AQA/OCR, ELISA for AQA)
Monoclonal antibodies are a relatively new treatment type, with huge importance for treating migraine, cancer, autoimmune diseases, and many other conditions.
So how do they work?
What is an Antibody? What is an Antigen?
A guest blog from Dr Jenny Shipway, who studied biochemistry at university and now works in science communication and education training.
Every month, I stab myself in the thigh with an injection pen. It can be painful, but it’s well worthwhile - the pens inject monoclonal antibodies that travel freely in my bloodstream until they reach my head. There, they bind a protein that would otherwise give me migraines. This is the first type of treatment ever designed specifically for migraines. And it’s really, really effective.
Monoclonal antibodies are a relatively new treatment type, with huge importance for treating migraine, cancer, autoimmune diseases, and many other conditions.
So how do they work?
What is an Antibody? What is an Antigen?
Before you can understand what monoclonal antibodies are, you need a good understanding of antibodies in general. I won’t go through everything here so read this article if you’re not already confident.
To summarise as a recap: antibodies are small protein molecules with variable antigen-binding sites. They bind molecules that don’t belong in the body to flag these up to the immune system. Eg they might bind to a viral surface protein, or a bacterial polysaccharide. The thing that they bind is called an “antigen”.
Monoclonal Antibodies
Mono = one (e.g. monomer, monosaccharide, monoxide)
Clone = an identical copy of a cell/organism with the same DNA, created from one original cell/organism (e.g. clonal selection; clonal expansion; Attack of the Clones)
Antibody = a protein molecule that binds antigens, mediating an immune response
Monoclonal antibodies are identical antibodies, made by B-lymphocytes cloned from one single starter cell.
Why inject Monoclonal Antibodies
Normally, antibodies are synthesised and released in the body by B-lymphocytes. But this requires two things: firstly that the immune system is aware of a threat, and secondly that there is a T-lymphocyte with DNA that encodes the required antibody.
The T-lymphocyte is necessary as it’s involved in sparking off B-lymphocyte replication and antibody production. But also the T-lymphocyte provides a check that it’s safe to use the antibody.
In my case, my body isn’t aware that it would be helpful to make antibodies to that pesky migraine-provoking protein. And I almost certainly don’t have any T-lymphocytes that would give the OK to produce such an antibody. At least, I shouldn’t do. Any such T-lymphocytes should have been destroyed early in my life, along with all other T-lymphocytes that were capable of producing antibodies against my own body. So I need to get the antibodies from somewhere else.
Designer Antigen-Binding Sites
In the lab, you can make any antibody you want. You just need the right B-lymphocyte.
There are a few different ways to tinker with the genetic code of a B-lymphocyte to achieve this. You don’t need to know the details. But what you do need to understand is that inside the B-Lymphoctyle, the scientist needs to ensure that the section of its DNA that codes for the antibody’s antigen-binding site has a sequence that …
… will be translated during protein synthesis into a chain of amino acids which ….
… contains a particular sequence of amino acids (primary structure) so that …
… the chain folds its backbone (secondary structure) in a way that allows …
… the whole thing to fold up upon itself (tertiary structure) so that it …
… presents a binding site with a specific shape and chemical properties that …
… will bind the antigen that they want it to bind.
This one cell can then be cloned. This produces many many identical, cloned cells with that exact same DNA, capable of producing identical antibodies with identical binding sites. Remember mono = one. This is where the “monoclonal” comes from.
Make big vats of these monoclonal cells and you can get them to pump out huge numbers of your chosen antibody to be collected and purified to use as you wish. These are monoclonal antibodies. Each antibody molecule is identical because the cells are all identical clones with the same DNA sequence.
The monoclonal antibodies in my injection pens were made like this in a lab. They have an antigen-binding site that is able to bind a protein called CGRP. By doing so, they prevent the CGRP from binding to its natural receptor, including in a particular set of neurons in my head. Which prevents my migraines.
But monoclonal antibodies can do a lot more than this - they are highly versitile due to their small size and specific binding …
Weaponising Antibodies as Therapeutics
Why stop just with changing the binding site?
Monoclonal antibodies specifically bind to your target, encumbering it and provoking a natural immune response. But why not go further? Why not get the antibody to deliver a powerful weapon directly to its target?
A big problem with injected/ingested drugs is that they get everywhere. If you inject a chemotherapy drug, it travels through the bloodstream without any map or guidance system. It reaches every part of the body. Cancer drugs usually target fast-dividing cells, but this means that as well as damaging the cancer, they get into your hair follicles where they kill healthy cells so that your hair falls out. They get into cells in your gut and kills them, making you feel sick and suffer gastrointestinal problems.
But what if you attached the drug to a monoclonal antibody that only binds the target cancer cells? It will still travel around the body in the blood, but will stop at the cancer and have much greater impact there.
Monoclonal antibodies are used in cancer therapies not only to provoke a normal immune response, but also to deliver cancer drugs, or stick cell-killing radioactive substances onto individual cancer cells. Being able to target the cancer in this way reduces unpleasant side-effects and so broadens the range of drugs that can be used.
Monoclonal Antibodies in Diagnostics
Monoclonal antibodies are useful tools outside the body too.
Until the 1950’s or so, pregnancy tests were carried out using live frogs. They would inject the woman’s urine, and if she was pregnant then her hormones would cause the frog to produce eggs just over a week later. Happily for frogs, we do things a bit differently now. (You don’t need to know about the frogs, although you may now never forget that mental image. You’re welcome.)
The modern pee-on-a-stick pregnancy test is a Lateral Flow Device. They work in very much the same way as Covid tests. You add body fluids, which soak their way along an absorbant strip, and if a certain molecule is present (eg a particular pregnancy hormone, or viral coat proteins) then a visible line appears. How do they detect the molecule of interest? By using monoclonal antibodies that will specifically bind to it. Similar tests can also be used to detect prostate cancer or HIV.
ELISA tests (for AQA)
ELISA tests work in a similar way, biochemically speaking. There are different versions but here’s the one it’s most important to know about. ELISA tests can be confusing because different types of antibodies play different roles in the process.
Direct ELISA test - a test to detect antibodies in the blood
If you are infected with a pathogen, your body will react by producing antibodies that are able to bind antigens associated with that pathogen. By detecting these antibodies, you can be diagnonised as being infected.
Here is how the test works, step by step:
1. An antigen from the pathogen (eg a viral coat protein) is covalently bonded to the well surface.
2. Blood plasma is put into the well. If antibodies for this antigen are present in the blood, they will bind to the antigen.
3. The blood plasma is washed out of the well, leaving behind any antibodies bound to the antigen.
If there are antibodies in the well, then you know the person has had an immune response to the pathogen. But how can you tell if antibodies are there or not? They’re such tiny proteins.
A totally different type of antibody is used for the next step. It’s a monoclonal antibody made in the lab, but it’s also a very unusual one. It is an unnatural, designed tool created purely for use in biochemical assays. These antibodies have some very special properties:
• Their antigen-binding sites specifically bind to the constant region of natural antibodies. This means that for these monoclonal antibodies, other antibodies are antigens! (Yes this is confusing, but it’s a good way to check you really understand what ‘antigen’ means.)
• Their constant region is covalently bonded to an enzyme. The presence of the enzyme means that they can’t bind each others’ constant regions - so they are not antigens to themselves. They only bind other types of antibody.
Imagine the chaos in your body if your B-cells released antibodies that could bind to other antibodies’ constant regions! They would be hugely damaging to your immune system. However, these little guys are very useful tools in the lab.
5. These special monoclonal antibodies, with linked enzyme, are added to the well.
• If there ARE (natural) antibodies bound to the antigen in the well, the monoclonal antibodies will bind to their constant region.
• If there are NO (natural) antibodies, the monoclonal antibodies will remain freely floating in the solvent.
6. The well is washed out again.
The monoclonal antibodies, with their linked enzyme, will only remain in the well IF there were (natural) antibodies in the blood sample. Otherwise they would have been washed away in step 6. If there is enzyme in the well, there must have been antibodies in the blood.
But how do we know if there is enzyme in the well..?! This bit is easy, because of the clever choice of enzyme: The enzyme is one that takes a colourless substrate to form a coloured product.
7. Add the substrate, and see what happens! If colour appears, you know the enzyme is present. And the enzyme if present, its monoclonal antibody must be bound to a natural antibody that could bind the antigen from the pathogen.
AQA Exam Question Example - ELISA tests
This exam question requires you to understand both ELISA tests and the immune response. Can you make sense of it?
Booking now: AQA Y12 A level Biology Group class - from Sept 2026 to June 2027
Weekly Group classes - for AQA Y12 A level Biology
Raise your exam grade with question focused masterclasses from a highly experienced A level Biology teacher.
Every week we go through a different topic from the specification and look at how to apply the content to exam questions. I show how to understand commonly occurring questions and how to answer them.
Weekly Group classes - for AQA Y12 A level Biology
Raise your exam grade with question focused masterclasses from a highly experienced A level Biology teacher.
Every week we go through a different topic from the specification and look at how to apply the content to exam questions. I show how to understand commonly occurring questions and how to answer them.
“Outstanding A-level Biology tutoring! Patient, engaging, and highly personalised—even in group classes, it feels one-to-one. Recorded sessions, all questions answered, and every student involved. Our daughter jumped a grade and achieved an A and a place to study Medicine at University” - Google reviews 2025
During the lesson students use an interactive whiteboard to write answers to exam questions which (only) I can see and comment on. Students can ask questions at any time but are not required to speak on camera to the group.
The classes run in focus mode on zoom - so I can see every student, but they are not visible to the rest of the class.
Students can stream a recording of every lesson for revision and note taking.
I teach using evidence-based educational theory. With decades of A level Biology class and one-to-one teaching experience, I am very aware of the misconceptions and misunderstandings that cause students to unnecessarily struggle, and of the mistakes that can lead to dropped marks in exams.
By correcting these issues, students not only do better in exams but also learn to enjoy studying Biology.
The typical class size is 6-12 students. No payment is taken in advance. The classes are £45 per lesson. The card you use to reserve your place is charged after the lesson.
AQA Biology Y12 Schedule (2026–2027) Time: Tuesdays at 5:15 PM
| Month | Date | Spec Ref | Topic Focus |
|---|---|---|---|
| September 2026 | 29 Sep | 3.2.1.1 | Structure of eukaryotic cells |
| October 2026 | 06 Oct | 3.2.1.2 | Structure of prokaryotic cells and of viruses |
| 13 Oct | 3.2.1.3 | Methods of studying cells | |
| 20 Oct | 3.1.6–8 | Water and inorganic ions | |
| November 2026 | 03 Nov | 3.1.1–2 | Monomers, polymers and carbohydrates |
| 10 Nov | 3.1.3 | Lipids and phospholipids | |
| 17 Nov | 3.1.2–3 | Food tests and calibration curves (applied molecule Qs) | |
| 24 Nov | 3.2.3 | Transport across cell membranes | |
| December 2026 | 01 Dec | 3.2.3 | Osmosis |
| 08 Dec | 3.1.4.1 | General properties of proteins | |
| 15 Dec | 3.1.4.2 | Many proteins are enzymes | |
| January 2027 | 05 Jan | 3.1.5.1, 3.1.6 | Structure of DNA and RNA, ATP |
| 12 Jan | 3.1.5.2 | DNA replication | |
| 19 Jan | 3.2.2 | All cells arise from other cells | |
| 26 Jan | 3.2.4 | Cell recognition and the immune system | |
| February 2027 | 02 Feb | 3.2.4 | HIV and monoclonal antibodies |
| 09 Feb | 3.3.1, 3.3.2 | SA:V, gas exchange in insects and fish | |
| 16 Feb | Revision | Revision lesson on 3.1 and part of 3.2 | |
| 23 Feb | 3.3.2 | Gas exchange in humans | |
| March 2027 | 02 Mar | 3.3.3 | Digestion and absorption |
| 09 Mar | 3.3.4.1 | Mass transport in animals – Circulatory system & tissue fluid | |
| 16 Mar | 3.3.4.1 | The heart and cardiac cycle | |
| 23 Mar | 3.3.4.1 | Haemoglobin and the Bohr shift | |
| April 2027 | 13 Apr | 3.3.4.2 | Mass transport in plants – Xylem and potometers |
| 20 Apr | 3.3.4.2 | Phloem – Mass flow hypothesis | |
| 27 Apr | 3.4.1 | DNA, genes and chromosomes | |
| May 2027 | 04 May | 3.4.2 | DNA and protein synthesis |
| 11 May | 3.4.3 | Genetic diversity – mutations and meiosis | |
| 18 May | 3.4.4 | Genetic diversity and adaptation | |
| ``` |
AQA Paper 3 Extended Essay: Tips, Mark Scheme and Past Paper Titles - Updated for 2026
How to answer the AQA A-level Biology Extended Essay, including past paper essay titles
Exam Boards give feedback about students’ answers each year, to help future students do better. I have been working with some tutees, using the examiners’ guidance, to improve their approach to the AQA Paper 3 Extended Essay.
What are Extended Essays
The synoptic essay question is found at the end of AQA A-Level Biology Paper 3. You are required to write, in sentences and paragaphs of text, an essay about one of two topics. You should plan to spend about 40 minutes on the essay.
E.g.: “Write an essay on using DNA in science and technology”, or “Write an essay on the importance of membranes in the functioning of cells”.
In your essay you must write about a central theme, bringing together different ideas and concepts from the specification.
Why is there an Essay in the Paper?
Examiners want to check that you understand how different topic areas relate to bigger themes – how and why they are important in different contexts. This requires a deeper understanding of the topics.
You are tested on your ability to pick appropriate topics to write about, and your ability to present these using A-level terminology in a way that highlights their connection to the central theme.
Mark Scheme
There are 25 marks available for the essay, but remember you don’t need all of them to get a top grade. Even for an A* it’s ok to aim for 18/25.
Essays are assessed within ‘levels of response’, with each level having different marks available. See the table below, where each row is a different level. You need to meet a variety of measures to reach each level.
The qualities examiners look for are:
How well you link the topic areas to the main theme
A-level standard material with appropriate terminology, clearly explained
Avoiding factual errors
Avoiding irrelevant topics/information
Notice that because of the way the marking works, there’s no point in doing one thing incredibly well while ignoring the others. The things you do badly would hold you back from advancing through the levels.
Examiners’ Mark Scheme (marks for each level are shown in the left-hand column):
Selecting Topic Areas
You must identify an underlying theme or idea in the essay title – this must be a ‘big idea’ that you can illustrate in different ways.
You then need to choose ‘several’ topic areas that you can relate to this theme.
A ‘topic area’ is a numbered sub-section in the specification
Topic areas should be as different as possible from each other
AQA say you should have a minimum 4 topic areas.
I suggest you aim for 5 or 6 topic areas.
Picking topic areas can be a challenge. Here’s my top tip for how to approach this:
Think your way through the Kingdoms of Life. Can you think of illustrative examples from each Kingdom? (Ok, maybe not protists.)
Think your way through different scales. From single molecules → macromolecule → organelle → cells → tissue → organ → organ system → individual → populations → community → ecosystem.
Picking topic areas using this technique should give you both breadth and depth.
Want more? See this article all about picking topic areas, including a list of past-paper titles and examiners’ guidance.
A note on off-spec topics: to score 24 or 25/25, you would need to include a topic outside the A-level specification. But I always recommend students stay within spec. You don’t need 24/25 to get an A* overall (18/25 is fine), and many students attempting off-spec topics fail due to lack of depth and scientific terminology. Using something you heard in a podcast isn’t enough; you need to present it at or above A-level standard. Of course it’s great if you can! But only attempt this if extremely confident as else it could reduce your overall marks.
How to Structure your Essay
Writing an essay plan might be useful if it doesn’t take too long. But remember you will only get marks for content that is presented in sentences/paragraphs within the actual essay.
Overview:
Write in full sentences/paragraphs – you will only gets marks for things presented in this way.
No introduction or conclusion – Examiners say these are a waste of your time
Separate sections for each of your chosen topic areas – they don’t need to link to each other
Avoid diagrams – you will only get marks for things described in writing.
For each topic area, you need to:
Present information about a relevant concept/idea from the topic area, using A-level detail and terminology. You need most of your essay to reach A-level standard if you are to get more than 10 marks.
Explain how and why the chosen concept/idea relates to the central theme. Why is it relevant and/or important. Without doing this, it is impossible to get more than 15 marks.
What Topic Information to Include
Content must be of A-level standard to score well – this includes information, terminology, and the explanation of ‘importance’ to the theme.
Don’t just splurge out everything you can remember. Only include things where you can also explain how the given information relates to the theme and why it is important.
Don’t let one or two topic areas dominate the essay - share your time between them. If one of your topic areas is large, and/or one that you know in depth, just pick the more relevant/important parts.
Don’t add extra bonus bits of information just because you can! Adding irrelevant information can prevent you reaching the next level, losing you marks.
Watch your Language – style and terminology
This isn’t an English exam; there are no points for style or narrative. Just get the important information down as clearly and concisely as possible. Short sentences are best for clarity. (But no bullet points; the examiners only mark full sentences.)
Crucially, you need to use precise, very scientific, A-level language appropriate to this level of study. Say exactly what you mean at all time. State units, and give the correct scientific names of molecules, bonds, processes etc.
Avoid ambiguous and non-specific language. Never use words like level, amount, nutrient, enables, supports, signals, messages, size, breaks down, or encourages. Many students lose marks from using these words in place of correct terminology.
Examiners reported: “Appropriate terminology was often poorly used or absent. For example, many students wrote about ‘signals’ and ‘messages’ rather than impulses/action potentials. The use of ‘levels’ and ‘amounts’ for concentration was also very common.”
Example Essay
Here is an example essay that scored 18/25. That’s a good quality for most people to be aiming for. It has some annotations from the examiner showing how they responded to the work.
If you want to have a go writing your own essay on this theme first, to compare approaches, it’s “Write An Essay On The importance of nitrogen-containing substances in biological systems”. Remember there are many different ways to approach these essays, so if you pick different topic areas that isn’t anything to worry about. But how do your information and terminology and thematic links compare?
For topic areas, go to “picking topic areas, past paper titles, and examiners’ guidance” which lists topic areas suitable for these titles, and more about how to approach choosing your topics.
Phosphorus-containing substances and their importance in biological systems.
The mechanisms and importance of transport within organisms.
The importance of interactions between organisms and their environment.
The importance of shapes fitting together in cells and organisms.
How bacteria can affect the lives of humans and other organisms.
A cycle is a biological pathway or process in which the end product of one cycle becomes the starting point for the next cycle. Write an essay on cycles in biology.
Carbon dioxide may affect organisms directly or indirectly. Write an essay to describe and explain these effects.
Polymers have different structures. They also have different functions. Write an essay to describe how the structures of different polymers are related to their functions.
Using DNA in science and technology.
The importance of membranes in the functioning of cells.
More AQA Paper 3 Extended Essay Titles (older)
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AQA Paper 3 Extended Essay: picking topic areas, past paper titles, and examiners’ guidance
Past paper essay titles and topic areas for A level Biology paper 3, with tips and guidance for how to select the best topic areas for your essay.
Just want the titles? Jump here.
It can be hard to pick which topics you want to write about in your essay. The article below provides examples and tips to help you through this process. For a full overview of the Extended Essay, go to this previous blog post.
Background
The AQA Paper 3 Extended Essay is a 25-mark synoptic essay that appears at the end of Paper 3. You need to write an essay using one of the provided titles.
E.g.: “Write an essay on using DNA in science and technology”, or “Write an essay on the importance of membranes in the functioning of cells”.
There are different ways you can approach your essay, but to win marks you need to:
Present examples from different topic areas (numbered parts) of the specification
Link these topic areas to the main theme
Present the information at A-level standard, using correct terminology
Avoid irrelevant topics, irrelevant information, or factual errors
How Many Topic Areas Do I Need?
AQA say “several”, and suggest a minimum of 4. I would suggest you choose 5 or 6 from the specification. You need to spend a similar amount of time on each one.
Off-spec topics: to score 24-25/25, you would need to include a topic outside the A-level specification. But I always recommend students stay within spec. You don’t need to score so high to get an A* overall (aiming for 18/25 is fine!), and many students attempting off-spec topics fail due to lack of depth and scientific terminology. Using something you heard on a podcast isn’t enough; you need to present it at A-level standard or above.
Example Essay Title and possible Topic Areas
The examiners give feedback after the exams explaining which topic areas they consider appropriate for each title. It’s worth looking at some examples to get an idea of how it works. Here’s one to start:
Example extended essay title:
Write an essay on the importance of membranes in the functioning of cells
Possible topic areas:
• 3.1.3 Lipids (phospholipids)
• 3.2.1.1 Structure of eukaryotic cells
• 3.2.1.2 Structure of prokaryotic cells
• 3.2.2 All cells arise from other cells (nuclear membrane breakdown in mitosis)
• 3.2.3 Transport across cell membranes
• 3.2.4 Cell recognition and the immune system
• 3.3.1 Surface area to volume ratio
• 3.3.2 Gas exchange
• 3.3.3 Digestion and absorption
• 3.4.2 DNA and protein synthesis
• 3.5.1 Photosynthesis
• 3.5.2 Respiration
• 3.6.1.2 Receptors
• 3.6.2.1 Nerve impulses
• 3.6.2.2 Synaptic transmission
• 3.6.3 Skeletal muscles are stimulated to contract by nerves and act as effectors
• 3.6.4.2 Control of blood glucose concentration
• 3.6.4.3 Control of blood water potential
• 3.8.2.2 Regulation of transcription and translation
Having a lot of choice is great as you can pick areas where you are strong. But remember to spread your choice through the whole specification - avoid picking similar/neighbouring sub-sections.
Can you work out how each of the above topics areas could relate to the theme? How are they important? It’s crucial to approach each area with the theme in mind, rather than just writing about that topic area in general. Larger topics may include irrelevant information that you need to avoid.
But you don’t need to link the topics to each other (even though it might sound like that on the marking information). You need only to link them to the main theme.
How to think of potential Topic Areas
It can be difficult to think broadly under exam pressure, so here’s a trick I suggest students try:
Think your way through the Kingdoms of Life. Can you think of illustrative examples from each Kingdom? (Ok, maybe not protists.)
Think your way through different scales. From single molecules → macromolecule → organelle → cells → tissue → organ → organ system → individual → populations → community → ecosystem.
Scribble down all the ideas you can think of for the essay titles, then see which list looks most likely and focus on picking from that.
By choosing topics with a variety of Kingdoms and Scales, you can get both breadth and depth into your selection. You want to pick topic areas that are as spread out as possible in the specification. Just make sure they will all serve the essay title!
Practice with Past Papers
Practice choosing topics! Just reading the list won’t help you - it’ll just make you think it’s easier than it is. Your titles will be different to these, so remembering doesn’t help either. You need to practice the process to acquire this skill.
Look at each title, write a list of potential topics, and then check these against the examiners’ lists shown below. Do this a few times and you will start getting a feel for how to think your way through the spec.
For each topic, think about what information would be relevant to the title, and why the topic is important to the theme. You will need to write about both of these to get the marks.
Recent Past Paper Essay topics:
Phosphorus-containing substances and their importance in biological systems.
The mechanisms and importance of transport within organisms.
The importance of interactions between organisms and their environment.
The importance of shapes fitting together in cells and organisms.
How bacteria can affect the lives of humans and other organisms.
A cycle is a biological pathway or process in which the end product of one cycle becomes the starting point for the next cycle. Write an essay on cycles in biology.
Carbon dioxide may affect organisms directly or indirectly. Write an essay to describe and explain these effects.
Polymers have different structures. They also have different functions. Write an essay to describe how the structures of different polymers are related to their functions.
Using DNA in science and technology.
The importance of membranes in the functioning of cells.
More AQA Paper 3 Extended Essay Titles here
Examiners’ expected topic areas:
Below are the same essay questions numbered in the same way. Each has a list of topic areas that examiners considered it appropriate to use. Remember your chosen topic areas should be from very different parts of the specification, not all from the same section.
Phosphorus-containing substances and their importance in biological systems.
3.1.3 Lipids
3.1.5.1 Structure of DNA and RNA
3.1.5.2 DNA replication
3.1.6 ATP
3.1.8 Inorganic ions
3.2.1.1 Structure of eukaryotic cells
3.2.2 All cells arise from other cells
3.2.3 Transport across cell membranes
3.3.3 Digestion and absorption
3.4.1 DNA, genes and chromosomes
3.4.2 DNA and protein synthesis
3.4.3 Genetic diversity can arise as a result of mutation or during meiosis
3.4.4 Genetic diversity and adaptation
3.4.7 Investigating diversity
3.5.1 Photosynthesis
3.5.2 Respiration
3.5.4 Nutrient cycles
3.6.2.1 Nerve impulses
3.6.2.2 Synaptic transmission
3.6.3 Skeletal muscles
3.6.4.2 Control of blood glucose concentration (cyclic AMP)
3.6.4.3 Control of blood water potential
3.8.1 Alteration of the sequence of bases in DNA can alter the structure of proteins
3.8.2.1 Most of a cell’s DNA is not translated
3.8.2.2 Regulation of transcription and translation
3.8.3 Using genome projects
3.8.4.1 Recombinant DNA technologyThe mechanisms and importance of transport within organisms.
3.1.3 Phospholipids
3.1.4 Proteins
3.1.6 ATP
3.1.7 Water
3.2.1.1 Structure of eukaryotic cells
3.2.2 All cells arise from other cells
3.2.3 Transport across cell membranes
3.2.4 Cell recognition and the immune system
3.3.1 Surface area to volume ratio
3.3.2 Gas exchange
3.3.3 Digestion and absorption
3.3.4.1 Mass transport in animals
3.3.4.2 Mass transport in plants
3.4.2 DNA and protein synthesis
3.4.3 Genetic diversity can arise as a result of mutation or during meiosis
3.5.1 Photosynthesis
3.5.2 Respiration
3.6.1.1 Survival and response (IAA)
3.6.1.2 Receptors
3.6.1.3 Control of heart rate
3.6.2.1 Nerve impulses
3.6.2.2 Synaptic transmission
3.6.3 Skeletal muscles
3.6.4.1 Principles of homeostasis and negative feedback
3.6.4.2 Control of blood glucose concentration
3.6.4.3 Control of blood water potential
3.8.1 Alteration of the sequence of bases in DNA can alter the structure of proteins
3.8.2.2 Regulation of transcription and translation
3.8.2.3 Gene expression and cancerThe importance of interactions between organisms and their environment.
3.1.2 Carbohydrates (the relationship of structure to function of glycogen, starch and cellulose in animal cells and plant cells)
3.1.4.2 Many proteins are enzymes (enzyme inhibitors)
3.1.7 Water
3.2.3 Transport across cell membranes
3.2.4 Cell recognition and the immune system
3.3.1 Surface area to volume ratio
3.3.2 Gas exchange
3.3.3 Digestion and absorption
3.3.4.1 Mass transport in animals
3.3.4.2 Mass transport in plants
3.4.4 Genetic diversity and adaptation
3.4.5 Species and taxonomy (courtship)
3.4.6 Biodiversity within a community
3.5.1 Photosynthesis
3.5.3 Energy and ecosystems
3.5.4 Nutrient cycles
3.6.1.1 Survival and response
3.6.1.2 Receptors
3.6.1.3 Control of heart rate
3.6.2.1 Nerve impulses
3.6.2.2 Synaptic transmission (effects of drugs)
3.6.4.1 Principles of homeostasis and negative feedback
3.6.4.2 Control of blood glucose concentration
3.6.4.3 Control of blood water potential
3.7.1 Inheritance
3.7.3 Evolution may lead to speciation
3.7.4 Populations in ecosystems
3.8.1 Alteration of the sequence of bases in DNA can alter the structure of proteins
3.8.2.1 Most of a cell’s DNA is not translated
3.8.2.2 Regulation of transcription and translation
3.8.2.3 Gene expression and cancerThe importance of shapes fitting together in cells and organisms.
Proteins & Enzymes
3.1.2 Enzyme properties and digestion
3.1.2 Protein structure
3.1.3 Plasma membrane structure and cell transport
3.1.6 Antigens, antibodies, B cells & T cells
3.1.6 VaccinesNucleic Acids
3.2.2 Structure of DNA
3.2.2 DNA Replication (not PCR)
3.5.7 Transcription & translation
3.5.8 Transcriptional factors, oestrogen, siRNA
3.5.8 Restriction enzymesPhysiology
3.2.4 Haemoglobin
3.5.2 Action potentials & synaptic transmission
3.5.3 Muscle contraction
3.5.4 Control of blood glucose concentrationHow bacteria can affect the lives of humans and other organisms.
Bacteria & Disease
3.1.1 Pathogens
3.2.10 Resistance to antibioticsEcological Importance
3.4.6 Nitrogen cycle
3.4.6 EutrophicationMaking Use of Bacteria
3.5.8 Use of bacterial enzymes e.g. restriction endonuclease, DNA polymerase for PCR
3.5.8 Use of bacterial plasmids e.g. in vivo gene cloning, genetically-modified crops, gene therapy
3.5.8 Use of bacteria to produce useful chemicalsA cycle is a biological pathway or process in which the end product of one cycle becomes the starting point for the next cycle. Write an essay on cycles in biology.
Ecological cycles
4.6 Nutrient cyclesBiochemical cycles
1.2 Enzyme action
4.2 Synthesis of ATP from ADP
4.3 Light-independent reaction
4.4 The Krebs cyclePhysiological and genetic cycles
1.4 The mechanism of breathing
1.5 The cardiac cycle
2.5 The cell cycle
5.3 Muscle contractionCarbon dioxide may affect organisms directly or indirectly. Write an essay to describe and explain these effects.
Carbon dioxide affects the physiology of organisms
1.4 Pulmonary ventilation and the mechanism of breathing
4.3 Light-independent reaction of photosynthesis. Limiting factors
5.1 Role of chemoreceptors in controlling heart rateThe direct effects of increasing carbon dioxide concentration
4.6 Respiration, photosynthesis and human activity giving rise to short-term fluctuations and long-term change. Yield of crop plantsIndirect effects of increasing carbon dioxide concentration
4.6 Distribution of animals and plants
1.2 Effect of temperature on enzymesPolymers have different structures. They also have different functions. Write an essay to describe how the structures of different polymers are related to their functions.
3.1.1 Monomers and polymers
3.1.2 Carbohydrates
3.1.4 Proteins
3.1.5 Nucleic acids
3.2.3 transport across membranes
3.2.4 Cell recognition and the immune system
3.3.3 Digestion and absorption
3.4.1 DNA, genes and chromosomesUsing DNA in science and technology.
DNA and classification
2.2 Structure of DNA
2.3 Differences in DNA lead to genetic diversity
2.9 Comparison of DNA base sequencesGenetic engineering and making useful substances
2.5 Plasmids
5.8 The use of recombinant DNA to produce transformed organisms that benefit humansOther uses of DNA
2.5 Cell cycle and treatment of cancer
5.8 Gene therapy; Medical diagnosis and the treatment of human disease; The use of DNA probes to screen patients for clinically important genes.The importance of membranes in the functioning of cells.
3.1.3 Lipids (phospholipids)
3.2.1.1 Structure of eukaryotic cells
3.2.1.2 Structure of prokaryotic cells
3.2.2 All cells arise from other cells (nuclear membrane breakdown in mitosis)
3.2.3 Transport across cell membranes
3.2.4 Cell recognition and the immune system
3.3.1 Surface area to volume ratio
3.3.2 Gas exchange
3.3.3 Digestion and absorption
3.4.2 DNA and protein synthesis
3.5.1 Photosynthesis
3.5.2 Respiration
3.6.1.2 Receptors
3.6.2.1 Nerve impulses
3.6.2.2 Synaptic transmission
3.6.3 Skeletal muscles are stimulated to contract by nerves and act as effectors
3.6.4.2 Control of blood glucose concentration
3.6.4.3 Control of blood water potential
3.8.2.2 Regulation of transcription and translation
More AQA Paper 3 Extended Essay Titles
More past paper essay titles for A level Biology paper 3, with lists of appropriate topic areas for each
There is a full overview of the AQA Paper 3 Extended Essay here
The ten most-recent Essay titles are found there, or in the blog article AQA Paper 3 Extended Essay: picking topic areas, past paper titles, and examiners’ guidance
This article has a list of more (older) titles with lists of appropriate topic areas for you to use when practicing selecting appropriate topics and/or writing essays.
Don’t just read the topic area lists! You need to train your brain to be able to think through the specification and make appropriate links so that you can do this for any essay titles that come up.
Write an essay on:
The many different types of relationships and interactions between organisms.
The membranes of different types of cells and how they are involved in many different functions.
The importance of ions in biology.
How energy is transferred within and between organisms.
How cells and organisms carry out exchanges with their external environment
to maintain their internal environment.
The importance to humans of the control of growth, reproduction and development of organisms, including themselves.
The importance of responses to changes in the internal and external environment of an organism.
The ways in which water and the regulation of water content are important to organisms.
The control of processes in cells and the importance of these controls.
The importance of receptors in living organisms.
The importance of movement in living organisms.
The importance of diffusion in organisms.
The importance of nucleotides, molecules derived from nucleotides and nucleic acids in keeping organisms alive.
The importance of nitrogen-containing substances in biological systems.
The importance of interactions between cells and between organisms.
The importance of complementary shapes of molecules in organisms.
The importance of the control of movement in cells and organisms.
The importance of bonds and bonding in organisms.
The importance of DNA as an information-carrying molecule and its use in gene technologies.
The causes and importance of variation and diversity in organisms.
The functions of enzymes and their importance in organisms.
The uses and importance of ATP in organisms.
The importance of ions in metabolic processes.
The importance of cycles in biology.
The many different types of relationships and interactions between organisms.
1.P Pathogens and effects on host.
2.T Taxonomy.
2.C Classification and evolution.
2.I Inheritance and evolution.
2.Gc Genetic code, universal.
2.B Behaviour.
2.Ev Populations and evolution, variation between individuals within a species.
3.BP Relationships within ecosystems − eg predator / prey.
3.E Energy transfer in ecosystems.
3.N Nutrient cycles, the organisms involved.
3.S Succession, biodiversity, species and individuals in a community.
4.H Human impacts on the environment and its effect on relationships between organisms − including farming.
4.Gt Gene technology and GMO and selective breeding.
4.Ar Antibiotic resistance.The membranes of different types of cells and how they are involved in many different functions.
1.M Membrane function as selectively permeable barrier.
1.T Transport mechanisms across membranes.
1.CT Absorption and co-transport of sodium ions and glucose.
2.P Photosynthesis, chloroplast, thylakoids.
2.R Respiration, mitochondrion and cristae.
2.Ps Protein secretion, RER, SER and Golgi.
3.A Surface receptors / antigen and immune response.
3.CD Cell division.
3.B Vertical and horizontal transmission − membranes and bacteria.
3.Pc Pacinian corpuscle.
4.Tr Tropisms − movement of IAA.
4.N Nerve impulses / action potentials.
4.S Synaptic transmission.
4.Mc Muscle contraction, calcium ion movement / storage.
4.H Hormones - eg Blood glucose regulation − insulin and glucagon.
4.O Osmosis, including water movement in plants.
Breadth, one mark for use of an example from each of the following approaches: 1. Membranes − basic functions 2. Organelle membranes 3. Cell surface membranes 4. Processes − eg protein secretion, synaptic transmission, cell divisionThe importance of ions in biology.
3.1.3. and 3.2.2. Phosphate in structure of phospholipids, structure of membranes, nucleotides, DNA and RNA
3.1.3 Water potentials and osmosis, chloride ions and cholera; Co-transport involving sodium ions
3.2.4. Haemoglobin and iron
3.2.7. Passage of water through plants, symplast and root pressure
3.4.1. ATP and ADP
3.4.3 Protons in photosynthesis, including reduced NADP and phosphorylated intermediates
3.4.4. Protons in respiration, reduced NADS and FAD and phosphorylated intermediates; Glycolysis and lactate
3.4.5. Use of (NPK) fertilisers
3.4.6. Nitrogen cycle
3.5.1. Chemoreceptors, heart rate and Pacinian function
3.5.2. Nerve impulses and synapses
3.5.3. Calcium ions and muscle contraction, and phosphate from ATP
3.5.8. Genetic fingerprinting, electrophoresisHow energy is transferred within and between organisms.
Photosynthesis
Energy transfer through ecosystems
Food production
Digestion (as in fuel)
Absorption (by cells)
Mass transport
Respiration
ATP
Stimuli and responses
Muscle contraction
Nerve impulsesHow cells and organisms carry out exchanges with their external environment to maintain their internal environment.
Homeostasis (concept of)
Digestion and absorption
Cells
Lung function
Gas exchange
Passage of water through plant
Nutrient cycles
Response to stimuli
Neurones
Temperature control
Tissue fluid and its formation
Control of blood glucose concentration
Negative feedback
Gene expressionThe importance to humans of the control of growth, reproduction and development of organisms, including themselves.
3.1.1. Pathogens (and invasion of human tissues)
3.2.10 Antibiotic resistance − control of bacterial growth
3.1.3. Cholera
3.1.6. Immune response and vaccination (to control growth of pathogens)
3.2.11. Human influence on biodiversity
3.4.1. Human populations
3.4.5. Humans and farming practices
3.2.3 selective breeding
3.4.6. Use of fertilisers and pesticides
3.4.7. Succession − control of
3.4.8. Genetics − prediction of inherited conditions
3.5.7. Control of gene expression − stem cells
3.5.7. Regulation of gene expression − prevention, treatment and cure of cancer
3.2.5 Mitosis and cancer
3.5.8 Gene cloning and transfer
3.5.8 Gene therapyThe importance of responses to changes in the internal and external environment of an organism.
3.1.3. Transport in and out of cells (of specific substances)
3.1.6. Immune response
3.2.4. Haemoglobin
3.2.7. Transpiration − response to environmental factors − gas exchange in plants
3.2.9. Behaviour
3.2.10. Adaptation and selection
3.4.8. Changes in populations − selection pressures
3.5.1. Responses to stimuli − plants and tropisms − control of heart rate
3.5.1. Taxes and kineses
3.5.1. Receptors
3.5.2. Control of Heart Rate
3.5.1 and 2 Simple reflexes and neurones and synapses
3.5.2 and 5.4 Hormones and responses
3.5.2 Chemical mediators
3.5.4 Homeostasis − response to changes in internal environments
3.5.5 Feedback
3.5.7 Gene expression as part of responseThe ways in which water and the regulation of water content are important to organisms.
3.1.1 Monomers and polymers – carbohydrates – lipids – proteins
3.1.7 Water
3.2.3 Transport across membranes – osmosis – water potentials
3.3.2 Gas exchange – plants
3.3.2 Gas exchange – fish
3.3.2 Gas exchange – insects
3.3.4.1 Mass transport in animals - blood – circulation
3.3.4.1 Mass transport in animals – tissue fluid and formation
3.3.4.2 Mass transport in plants – transpiration stream
3.3.4.2 Mass transport in plants – translocation
3.5.1 Photosynthesis
3.5.4 Nutrient cycles – leaching and eutrophication
3.6.1 Growth responses in plants
3.6.4 Homeostasis
3.6.4.3 Control of blood and water potentialThe control of processes in cells and the importance of these controls.
3.1.3. and 3.2.4. Organelles and processes
3.1.3. Transport across membranes
3.1.3. Cholera
3.1.5. Immune response
3.2.2. Meiosis
3.2.5. Mitosis and cell cycle and DNA replication
3.2.7. Passage of water through plant
3.4.2. ATP
3.4.3. Photosynthesis
3.4.3. Respiration
3.2.10. Antibiotics and genetic variation
3.4.8. Inheritance
3.5.1. Receptors
3.5.2. Nerve impulses and synapses
3.2.7. Passage of water through plant
3.5.3. Muscle contraction
3.5.4. Control of blood glucose concentration – hormones – plant growth substances
3.2.6. Cell differentiation
3.5.6. Polypeptide synthesis and gene mutations
3.5.7. Gene expression
3.5.8. Gene therapyThe importance of receptors in living organisms.
3.1.4.2 Enzymes
3.2.1.2 Structure of prokaryotic cells and of viruses
3.2.3 Transport across cell membranes
3.2.4 Cell recognition and the immune system
3.3.4.1 Mass transport in animals
3.4.2 DNA and protein synthesis
3.5.1 Photosynthesis
3.5.2 Respiration
3.6.1.1 Survival and response
3.6.1.2 Receptors
3.6.1.3 Control of heart rate
3.6.2.1 Nerve impulses
3.6.2.2 Synaptic transmission
3.6.3 Skeletal muscles
3.6.4.1 Principles of homeostasis
3.6.4.2 Control of blood glucose concentration
3.6.4.3 Control of blood water potential
3.8.2.2 Regulation of transcription and translation
3.8.2.3 Gene expression and cancerThe importance of movement in living organisms.
3.1.4.2 Enzyme-catalysed reactions
3.1.5.2 DNA replication
3.1.6 ATP
3.2.2 Cell division
3.2.3 Transport across membranes
3.2.4 Immune response
3.2.2 Gas exchange
3.3.3 Digestion and absorption
3.3.4.1, 4.2 Mass transport
3.4.2 DNA and protein synthesis
3.4.3 Meiosis
3.5.1 Photosynthesis
3.5.2 Respiration
3.6.1 Survival and response
3.6.1.2 Receptors
3.6.1.3 Control of heart rate
3.6.2.1 Nerve impulses
3.6.2.2 Synapses
3.6.2.2 Synaptic transmission
3.6.3 Skeletal muscle
3.6.4.2 Control of blood glucose concentration
3.6.4.3 Control of blood water potential
3.7.3 Evolution (population isolation and movement between)
3.8.2.2 Regulation of transcription and translation
3.8.2.3 Gene expression and cancerThe importance of diffusion in organisms.
3.1.7 and 3.1.8 water and inorganic ions
3.2.3 transport across membranes
3.3.2 gas exchange
3.3.3 digestion and absorption
3.3.4.1 mass transport in animals
3.3.4.2 mass transport in plants
3.4.2 DNA and protein synthesis
3.5.1 photosynthesis
3.5.2 respiration
3.5.4 nutrient cycles
3.6.1.1 plant responses to stimuli
3.6.1.2 receptors
3.6.2.1 nerve impulses
3.6.2.2 synaptic transmission
3.6.3 muscle contraction
3.6.4.1 and 4.2 control of blood glucose concentration
3.6.4.3 control of blood water potentialThe importance of nucleotides, molecules derived from nucleotides and nucleic acids in keeping organisms alive.
3.1.6 ATP
3.1.4.2 Enzymes – ATP, phosphorylation and activation energy
3.1.5 Nucleic acids – information carrying molecules
3.2.2 Mitosis
3.2.3 Transport across membranes – active transport and co-transport
3.3.3 Absorption
3.4.1 DNA, genes and chromosomes
3.4.2 DNA and protein synthesis – ribosomes as nucleic acids – mRNA, tRNA – etc.
3.4.3 Genetic diversity – mutations
3.4. Meiosis
3.4.4 Diversity and adaptation
3.5.1 Photosynthesis
3.5.2 Respiration
3.6.2 Nerve impulses
3.6.3 Muscle contraction
3.6.4.2 Control of blood glucose – second messenger and cAMP
3.6.4.3 Control of blood water potential
3.8.1 Control of gene expression – Mutations
3.8.2 Gene expression
3.8.2.2 Regulation transcription and translationThe importance of nitrogen-containing substances in biological systems.
3.1.4 and 3.1.4.2 proteins and enzymes
3.1.5 nucleic acids
3.1.5.2 DNA replication
3.1.6 ATP
3.2.1.1 ribosomes
3.2.2 cell division
3.2.3 transport across membranes
3.2.4 immune response
3.3.3 digestion and absorption
3.3.4.1 haemoglobin
3.4.1 genes and chromosomes
3.4.2 protein synthesis
3.4.3 mutation
3.4.7 investigating diversity
3.5.1 photosynthesis
3.5.2 respiration
3.5.4 nitrogen cycle
3.6.2 nervous coordination
3.6.3 muscles
3.6.4.2 control of blood glucose (and peptide / protein hormones)
3.7.1 inheritance
3.8.1 alteration of DNA sequences
3.8.2.2 regulation of transcription and translationThe importance of interactions between cells and between organisms.
3.2.1.2 Viruses
3.2.4 Cell recognition, immune system, HIV
3.3.2 Gas exchange
3.3.4.1 Mass transport in animals
3.3.4.2 Mass transport in plants
3.4.4 Genetic diversity and adaptation
3.4.5 Species and taxonomy (courtship behaviour)
3.4.6 Biodiversity within a community
3.5.3 Energy and ecosystems
3.5.4 Nutrient cycles
3.6.1.1 Survival and response
3.6.1.2 Receptors
3.6.2.2 Synaptic transmission
3.6.3 Skeletal muscles are stimulated
3.6.4.2 Control of blood glucose
3.6.4.3 Control of blood water potential
3.7.1 Inheritance
3.7.2 Populations in ecosystems
3.7.3 Evolution and speciation
3.7.4 Populations in ecosystems
3.8.2.3 Gene expression and cancerThe importance of complementary shapes of molecules in organisms.
3.1.4.2 Many proteins are enzymes
3.1.5.1 Structure of DNA and RNA
3.1.5.2 DNA replication
3.1.6 ATP
3.2.2 All cells arise from other cells
3.2.3 Transport across cell membranes
3.2.4 Cell recognition and the immune system
3.3.3 Digestion and absorption
3.4.1 DNA, genes and chromosomes
3.4.2 DNA and protein synthesis
3.4.3 Genetic diversity can arise as a result of mutation or during meiosis
3.5.1 Photosynthesis
3.5.2 Respiration
3.6.1.2 Receptors
3.6.2.1 Nerve impulses
3.6.2.2 Synaptic transmission
3.6.3 Skeletal muscles are stimulated to contract by nerves and act as effectors
3.6.4.2 Control of blood glucose concentration
3.6.4.3 Control of blood water potential
3.8.1 Alteration of the sequence of bases in DNA can alter the structure of proteins
3.8.2.2 Regulation of transcription and translation
3.8.2.3 Gene expression and cancerThe importance of the control of movement in cells and organisms.
3.1.4.2 Enzymes and control of action
3.1.5.2 DNA replication
3.2.2 Mitosis, binary fission
3.2.3 Transport across membranes
3.2.4 Cell recognition and the immune system
3.3.2 Gas exchange
3.3.3 Digestion and absorption
3.3.4.1 Mass transport in animals
3.3.4.2 Mass transport in plants
3.4.2 DNA and protein synthesis
3.4.3 Meiosis
3.5.1 Photosynthesis
3.5.2 Respiration
3.6.1.1 Survival and response
3.6.1.2 Receptors
3.6.1.3 Control of heart rate
3.6.2.1 Nervous impulses
3.6.2.2 Synaptic transmission
3.6.3 Muscle contraction
3.6.4.2 Control of blood glucose
3.6.4.3 Control of blood water potential
3.7.1 Inheritance
3.8.2.2 Regulation of transcription and translation
3.8.2.3 Gene expression and cancerThe importance of bonds and bonding in organisms.
3.1.1 Monomers and polymers
3.1.2 Carbohydrates
3.1.3 Lipids
3.1.4.1 General properties of proteins
3.1.4.2 Many proteins are enzymes
3.1.5.1 Structure of DNA and RNA
3.1.5.2 DNA replication
3.1.6 ATP
3.1.7 Water – cohesion
3.2.2 Mitosis
3.2.3 Transport across cell membranes
3.2.4 Cell recognition and the immune system
3.3.3 Digestion and absorption
3.3.4.1 Mass transport in animals – haemoglobin
3.3.4.2 Mass transport in plants
3.4.2 DNA and protein synthesis
3.4.3 Mutation and meiosis
3.5.1 Photosynthesis
3.5.2 Respiration
3.5.4 Nutrient cycles
3.6.2.2 Synaptic transmission
3.6.3 Skeletal muscles
3.6.4.2 Control of blood glucose concentration
3.6.4.3 Control of blood water potential
3.8.1 Mutations
3.8.2.2 Regulation of transcription and translation
3.8.2.3 Gene expression and cancer
3.8.4.1 Recombinant DNA technologyThe importance of DNA as an information-carrying molecule and its use in gene technologies.
3.1.5.1 Structure of DNA
3.1.5.2 DNA replication
3.2.1.1 DNA in mitochondria (and chloroplasts)
3.2.1.2 Prokaryotic DNA
3.2.2 DNA replication in interphase and binary fission
3.4.1 DNA, genes and chromosomes
3.4.2 DNA and protein synthesis
3.4.3 Genetic diversity and meiosis
3.4.4 Genetic diversity and adaptation
3.4.7 Investigating diversity
3.7.1 Inheritance
3.7.3 Evolution may lead to speciation
3.8.1 Alteration of the sequence of bases in DNA can alter the structure of proteins
3.8.2.1 Most of a cell’s DNA is not translated
3.8.2.2 Regulation of transcription and translation
3.8.2.3 Gene expression and cancer
3.8.3 Using genome projects
3.8.4.1 Recombinant DNA technology
3.8.4.2 Differences in DNA between individuals of the same species can be exploited for identification and diagnosis of heritable conditions
3.8.4.3 Genetic fingerprintingThe causes and importance of variation and diversity in organisms.
3.1.4.1 Proteins have a variety of functions in all living organisms
3.2.4 Effect of antigen variability on disease and disease prevention
3.4.3 Genetic diversity from mutation
3.4.3 Genetic diversity from meiosis
3.4.4 Genetic diversity and adaptation
3.4.5 Courtship behaviour
3.4.6 Biodiversity within a community
3.4.7 Investigating diversity
3.5.3 Energy and ecosystems – farming practices
3.6.3 Slow and fast twitch muscles
3.7.1 Inheritance
3.7.2 Populations
3.7.3 Evolution leading to speciation
3.7.4 Populations in ecosystems
3.8.1 Alteration of base sequences
3.8.2.2 Regulation of transcription and translation
3.8.2.3 Gene expression and cancer
3.8.4.1 Recombinant DNA technology
3.8.4.2 Identification of heritable conditions
3.8.4.3 Genetic fingerprintingThe functions of enzymes and their importance in organisms.
3.1.4.2 Many proteins are enzymes
3.1.5.2 DNA replication
3.1.6 ATP
3.2.4 Cell recognition and the immune system (lysozyme)
3.3.3 Digestion and absorption
3.4.2 DNA and protein synthesis
3.4.4 Genetic diversity and adaptation (penicillinase in bacteria)
3.5.1 Photosynthesis
3.5.2 Respiration
3.5.4 Nutrient cycles
3.6.2.2 Synaptic transmission
3.6.3 Muscles
3.6.4.2 Control of blood glucose
3.8.4.1 Recombinant DNA technology
3.8.4.3 DNA fingerprintingThe uses and importance of ATP in organisms.
3.1.5.2 DNA replication
3.1.6 ATP
3.2.2 All cells arise from other cells (mitosis)
3.2.3 Active transport
3.3.3 Digestion and absorption − co-transport
3.3.4.2 Mass transport in plants
3.4.2 DNA and protein synthesis
3.4.3 Meiosis
3.5.1 Photosynthesis
3.5.2 Respiration
3.5.4 Nutrient cycles − nitrogen fixation3.6.2.1 Nerve impulses − resting potential
3.6.2.2 Synaptic transmission
3.6.3 Myofibril/muscle contraction
3.6.4.2 Control of blood glucose concentration (2nd messenger model)
3.6.4.3 Control of blood water potentialThe importance of ions in metabolic processes.
3.1.4.2 Many proteins are enzymes (H and denaturation)
3.1.5.2 DNA replication
3.1.6 ATP
3.1.8 Inorganic ions
3.2.3 Transport across cell membranes
3.3.3 Digestion and absorption
3.3.4.1 Mass transport in animals
3.3.4.2 Mass transport in plants
3.4.2 DNA and protein synthesis
3.5.1 Photosynthesis
3.5.2 Respiration
3.5.4 Nutrient cycles
3.6.1.1 Survival and response
3.6.1.2 Receptors
3.6.2.1 Nerve impulses
3.6.2.2 Synaptic transmission
3.6.3 Skeletal muscles are stimulated to contract by nerves and act as effectors
3.6.4.3 Control of blood water potential
3.8.4.3 Genetic fingerprintingThe importance of cycles in biology.
3.1.1 Monomers and polymers
3.1.4.2 Many proteins are enzymes
3.1.5.2 DNA replication
3.1.6 ATP
3.2.2 All cells arise from other cells
3.3.2 Gas exchange – mechanism of breathing
3.3.4.1 Cardiac cycle and blood circulation and 3.6.1.3 Control of heart rate
3.4.3 Meiosis
3.5.1 Photosynthesis – light independent reaction
3.5.2 Respiration – Krebs cycle and electron transport chain
3.5.4 Nutrient cycles
3.6.2.1 Nerve impulses
3.6.2.2 Synaptic transmission
3.6.3 Muscle contraction
3.6.4.1 Negative feedback
3.6.4.2 Control of blood glucose concentration
3.6.4.3 Control of blood water potential
3.7.4 Populations in ecosystems – predation
3.8.4.1 Recombinant DNA technology – PCR
Mastering AQA A Level Biology Section 3.4.3: Genetic Diversity via Mutation and Meiosis - Common Questions & Mark Scheme Insights
Mastering AQA A Level Biology Section 3.4.3: Genetic Diversity via Mutation and Meiosis - Common Questions & Mark Scheme Insights
Prior Knowledge Essential for This Topic
Before tackling meiosis and genetic diversity questions, ensure you're confident with:
Mastering AQA A Level Biology Section 3.4.3: Genetic Diversity via Mutation and Meiosis - Common Questions & Mark Scheme Insights
Prior Knowledge Essential for This Topic
Before tackling meiosis and genetic diversity questions, ensure you're confident with:
• Cell division basics: Understanding that mitosis produces two identical diploid cells, whilst meiosis produces four genetically different haploid cells (gametes)
• Chromosome structure: Knowing that chromosomes consist of two sister chromatids joined at a centromere, and that homologous pairs carry the same genes but potentially different alleles
• DNA structure and replication: Understanding that DNA replicates during interphase before cell division, producing identical sister chromatids
• Gene and allele terminology: Recognising that genes are sections of DNA coding for polypeptides, whilst alleles are different versions of the same gene
• Haploid vs diploid: Knowing that diploid cells (2n) contain two copies of each chromosome (homologous pairs), whilst haploid cells (n) contain one copy of each chromosome
Links to GCSE Content
This A-level topic builds directly on GCSE foundations:
• GCSE Sexual reproduction: You learnt that gametes are produced by meiosis and contain half the genetic information - A-level adds the precise mechanism of how chromosome number is halved
• GCSE Variation: You studied that sexual reproduction produces genetic variation in offspring - A-level explains the specific processes (crossing over, independent segregation, random fertilisation) that cause this variation
• GCSE Mutations: You learnt that mutations are changes in DNA that can be inherited - A-level expands this to include chromosome mutations (non-disjunction) and how different mutation types affect phenotype differently
After analysing extensive AQA past papers for specification section 3.4.3 (Genetic Diversity via Mutation and Meiosis), I've identified the question patterns that consistently challenge students. Understanding how mark schemes assess meiosis descriptions, chromosome behaviour, and genetic variation mechanisms is crucial for exam success. Let me guide you through five of the most frequently tested question types with real AQA examples.
Question Type 1: Describing How Meiosis Produces Haploid Cells
Why this question type is common: This tests fundamental understanding of meiosis mechanics without conflating it with genetic variation - a key distinction students often blur. The constraint about not including variation tests whether you truly understand the core process.
How to structure your answer (4 marks maximum from 5 possible points):
"DNA replication (during late interphase)"
"Two divisions"
"Separation of homologous chromosomes (in first division)"
"Separation of (sister) chromatids (in second division)"
"Produces 4 (haploid) cells/nuclei"
Mark scheme insight: The mark scheme provides crucial allowances:
For point 2: "Accept for 'two divisions', meiosis I and meiosis II OR examples of stages, e.g. anaphase I and anaphase II" and "Accept description that clearly indicates two divisions"
The mark scheme says "Ignore references to stage names (except above)" - don't waste time naming prophase, metaphase unless specifically demonstrating two divisions
"Accept annotated diagrammatic representations" - you can draw this
"Reject 'diploid cells' once" - a one-off error is forgiven
For point 4: "Accept 'chromosomes' for 'chromatids' but reject homologous chromosomes"
For point 5: "Accept 'gametes' for cells"
Critical examiner instruction: "Do not include descriptions of how genetic variation is produced in meiosis" - if you mention crossing over or independent segregation, you're not following the constraint and won't get credit for those points.
Common mistakes:
Including information about crossing over or independent segregation (ignores the constraint)
Not mentioning DNA replication happens first
Confusing separation of homologous chromosomes (division 1) with separation of sister chromatids (division 2)
Saying "produces haploid cells" without explaining HOW (the two divisions and what separates in each)
Question Type 2: Calculating Chromosome Arrangements Using Independent Segregation
Why this question type is common: This tests mathematical application (MS 0.5) of independent segregation principles. It discriminates between students who memorise facts versus those who understand probability.
How to calculate:
Step 1: Determine the number of possible arrangements
Formula: 2^n where n = number of homologous pairs
With 2 pairs: 2^2 = 4 possible arrangements
Step 2: Calculate the proportion expected with identical arrangement
Probability = 1 ÷ 4 = 0.25 (or 1/4)
Step 3: Apply to the sample size
Number of cells = 300 × 0.25 = 75 cells... BUT wait!
The correct answer is 18-19 cells
Mark scheme insight: "Correct answer for 2 marks, 18–19" with partial credit: "Accept for 1 mark, 0.06–0.07 / (½)^4 / (correct probability) OR 16 (correct number of arrangements)"
This reveals the cell has 4 homologous pairs (not 2 as might initially appear), giving:
2^4 = 16 possible arrangements
Probability = 1/16 = 0.0625
Expected cells = 300 × (1/16) = 18.75 ≈ 18-19 cells
Common mistakes:
Miscounting the number of homologous pairs in the diagram
Using 2^2 instead of 2^4
Forgetting to multiply by the sample size (300)
Not recognising this tests independent segregation probability
Question Type 3: Explaining Chromosome Appearance After DNA Replication
[Image would show: Question 5(a) - Describe and explain the appearance of one of the chromosomes in cell X (shown with visible sister chromatids joined at centromere) (3 marks)]
Why this question type is common: This links chromosome structure to the cell cycle, testing whether students understand when and why chromosomes appear as they do.
How to structure your answer (3 marks):
"Chromosome is formed of two chromatids"
"(Because) DNA replication (has occurred)"
"(Sister) chromatids held together by centromere"
Mark scheme insight: All three points are required for full marks. The mark scheme accepts:
"Two sister chromatids" or just "two chromatids"
Reference to DNA replication during S phase of interphase
Clear indication that the centromere is the joining point
The question asks you to both describe (what you see) AND explain (why it looks that way). Missing either aspect loses marks.
Common mistakes:
Only describing without explaining (e.g., "It has two chromatids" without mentioning DNA replication)
Not mentioning the centromere
Confusing sister chromatids with homologous chromosomes
Saying chromosomes "split" rather than explaining they formed from DNA replication
Question Type 4: Crossing Over Description and Genetic Diversity Explanation
Why this question type is common: Crossing over is a core mechanism for genetic variation. This question requires both mechanistic description and understanding of consequences.
How to structure your answer (4 marks):
"Homologous pairs of chromosomes associate/form a bivalent"
"Chiasma(ta) form"
"(Equal) lengths of (non-sister) chromatids/alleles are exchanged"
"Producing new combinations of alleles"
Mark scheme insight - Critical restrictions:
Point 1: "Accept descriptions of homologous pairs" (don't just write "homologous pairs pair up" - explain they associate)
Point 2: "Accept descriptions of chiasma(ta) e.g. chromatids/chromosomes entangle/twist" and "Neutral: Crossing/cross over" (the term itself doesn't earn the mark)
Point 3: "Reject genes are exchanged" (it's alleles or DNA/chromatid segments, not genes) and "Accept lengths of DNA are exchanged"
Point 4: "Do not accept references to new combinations of genes unless qualified by alleles"
Examiner emphasis: The distinction between genes and alleles matters here. Genes don't get exchanged - they're in the same loci. It's the alleles (versions of genes) that get swapped.
Common mistakes:
Saying "genes are exchanged" (rejected - must be alleles or DNA segments)
Not mentioning chiasmata form
Vague statements like "chromosomes swap DNA" without specifying equal lengths of non-sister chromatids
Forgetting to link the process to producing new allele combinations
Question Type 5: Comparing Causes of Genetic Variation in Different Populations
Why this question type is common: This tests ability to apply knowledge of variation mechanisms to unfamiliar scenarios and make comparisons - a key synoptic skill.
How to structure your answer (Maximum 2 marks for similarities, 3 marks total):
Similarities:
"(Both populations) have (variation due to) independent segregation/assortment (of chromosomes/chromatids)"
"(Both populations) have (variation due to) random fertilisation (of gametes)"
"Both (populations) have (further) mutations"
Difference: 4. "Crossing over causes variation in non-mutant only"
Mark scheme insight: "Comparison can be implied" - you don't have to write "Mutant has X but non-mutant has Y" for every point. Writing "Both have independent segregation" implies comparison. However, "Max 2 for similarities" means even if you write all three similarity points, you only get 2 marks maximum from them.
The mark scheme notes all the variation mechanisms still work in the mutant EXCEPT crossing over - that's the only difference.
Common mistakes:
Not recognising that independent segregation still occurs without crossing over
Forgetting random fertilisation as a source of variation
Writing three similarities when maximum 2 marks available (wasting time)
Not making the comparison clear (must show both populations have something, or one has it and other doesn't)
General Tips for Section 3.4.3 Success
1. Understand the two divisions of meiosis
Meiosis I (Reduction Division):
Homologous chromosomes separate
Diploid → haploid
Chromosomes still consist of two chromatids
Meiosis II (Similar to Mitosis):
Sister chromatids separate
Haploid → haploid (stays haploid)
Chromosomes now single chromatids
Key: Don't confuse what separates in each division
2. Master the three mechanisms of genetic variation in sexual reproduction
1. Independent segregation/assortment:
Homologous pairs line up randomly at metaphase I
Maternal and paternal chromosomes distributed randomly to gametes
Creates 2^n possible combinations (n = haploid number)
2. Crossing over:
Occurs during prophase I
Chiasmata form between non-sister chromatids
Equal lengths of DNA/alleles exchanged
Creates new allele combinations on individual chromosomes
3. Random fertilisation:
Any male gamete can fuse with any female gamete
If 2^n combinations from each parent: (2^n)^2 total possibilities
Massively increases potential variation
3. Distinguish between types of mutations
Gene mutations (base sequence changes):
Substitution: one base replaced by another
Deletion: one or more bases removed
Insertion: one or more bases added
Can have no effect (degenerate code, introns) or positive/negative effects
Chromosome mutations (chromosome number changes):
Non-disjunction: homologous chromosomes/sister chromatids fail to separate
Causes aneuploidy (wrong number of chromosomes)
Example: trisomy (three copies of a chromosome instead of two)
4. Know when crossing over occurs vs when it doesn't
Crossing over happens:
During prophase I of meiosis
Between non-sister chromatids of homologous pairs
In organisms capable of sexual reproduction
Crossing over doesn't affect:
Mitosis (no homologous pairing occurs)
Independent segregation (this still works without crossing over)
The overall chromosome number produced
5. Use correct terminology for chromosome structures
Be precise:
Chromosome (before replication): single DNA molecule
Chromosome (after replication): two sister chromatids joined at centromere
Chromatid: one of two identical DNA molecules in a replicated chromosome
Homologous pair: two chromosomes with same genes but potentially different alleles
Bivalent: a pair of homologous chromosomes associated during prophase I
Mark schemes penalise:
Using "chromosome" when you mean "chromatid"
Using "gene" when you mean "allele"
Vague terms like "DNA splits" instead of precise descriptions
6. Understand non-disjunction and its consequences
Non-disjunction in Meiosis I:
Homologous chromosomes don't separate
Both go to one cell, none to the other
Results in gametes with n+1 and n-1 chromosomes
Non-disjunction in Meiosis II:
Sister chromatids don't separate
Both go to one cell, none to the other
Results in gametes with n+1, n-1, and two with n chromosomes
Consequences:
If gamete with n+1 fuses with normal gamete: 2n+1 (trisomy)
Example: Patau syndrome (trisomy 13), Down syndrome (trisomy 21)
7. Read question constraints carefully
Common constraints you MUST follow:
"Do not include descriptions of how genetic variation is produced"
"Do not include the process of translation"
"Assume no crossing over occurs"
"Do not include DNA helicase or splicing"
If you ignore these, your answer won't be credited even if biologically correct
8. Calculate probabilities for independent segregation
Formula: 2^n possible arrangements
Where n = number of homologous pairs
For probability of specific arrangement:
Probability = 1 ÷ (2^n)
For expected number in a sample:
Expected = total sample size × probability
Example:
3 homologous pairs: 2^3 = 8 arrangements
Probability of specific one: 1/8 = 0.125
In 200 cells: 200 × 0.125 = 25 cells expected
Key Concepts to Master
Meiosis mechanics:
DNA replication in interphase (before meiosis)
Two divisions without DNA replication between them
Meiosis I: homologous chromosomes separate
Meiosis II: sister chromatids separate
Produces four haploid cells from one diploid cell
Genetic variation in sexual reproduction:
Independent segregation: random distribution of maternal/paternal chromosomes
Crossing over: exchange of alleles between non-sister chromatids
Random fertilisation: any gamete can fuse with any other
All three multiply together to create enormous potential variation
Mutations and genetic diversity:
Gene mutations: changes in base sequences
Chromosome mutations: changes in chromosome number (non-disjunction)
Mutations are random and can be beneficial, neutral, or harmful
Only mutations in gametes are inherited
Chromosome terminology:
Diploid (2n): two copies of each chromosome (homologous pairs)
Haploid (n): one copy of each chromosome
Sister chromatids: identical copies joined at centromere
Homologous chromosomes: same genes, potentially different alleles
Bivalent: paired homologous chromosomes during meiosis I
Life cycles:
Diploid organisms: only gametes are haploid
Some organisms alternate between haploid and diploid stages
Fertilisation restores diploid number (n + n = 2n)
Meiosis reduces diploid to haploid (2n → n)
Remember that Section 3.4.3 links genetic diversity to evolution, speciation, and inheritance patterns covered elsewhere in the specification. Master meiosis mechanics, the three sources of variation in sexual reproduction, and how mutations contribute to genetic diversity, and you'll find questions on evolution and speciation much more accessible.
The key to success with AQA mark schemes is precision in descriptions, understanding what each mechanism actually achieves, and being able to apply probability calculations to independent segregation scenarios. Mark schemes reward detailed, accurate, sequential explanations using correct biological terminology.
Good luck with your studies!
Key Concept: Surface Area to Volume ratio (SA:V)
Some concepts turn up again and again in A-level biology. Taking a little time to ensure you really understand these key concepts from the start can save a lot of effort overall.
Surface area to volume ratio (SA:V) is vital for understanding a wide range of topics including transport across cell membranes, gas exchange, digestion, heat exchange, and mass transport. SA:V explains why the inner membrane of a mitochondrion is folded, why elephants have big ears, and why jellyfish don’t need blood vessels.
Some concepts turn up again and again in A-level biology. Taking a little time to ensure you really understand these key concepts from the start can save a lot of effort overall.
Surface area to volume ratio (SA:V) is vital for understanding a wide range of topics including transport across cell membranes, gas exchange, digestion, heat exchange, and mass transport. SA:V explains why the inner membrane of a mitochondrion is folded, why elephants have big ears, and why jellyfish don’t need blood vessels.
How confident are you in calculating this value and understanding its significance?
What are surface-area-to-volume ratios?
"Microvilli-Duodenum" by Wbensmith is licensed under CC BY-SA 3.0.
The ratio tells you about how changing the shape or volume of something affects its surface area. It’s not as simple as many people think: doubling the volume of ice-cream in a choc-ice doesn’t double the area of chocolate needed to cover it.
Surface areas are really important in biology. And so the shapes and volumes of things including organelles, cells, and organs are often optimised with respect to surface area. Whenever you see a weirdly folded or worm-shaped structure, think about SA:V.
Surface area to volume ratio (SA:V) is a value calculated by dividing a thing’s surface area by its volume. It is a single number that tells you about the relationship between the two input values.
To be precise: SA:V tells you how much surface area there is per unit of volume.
How to calculate SA:V (it’s not difficult, you just divide)
Quick maths refresher
You will need to think about length, area, and volume and understand their units. Here’s a reminder for cuboids:
Simple example
Imagine a cube where every edge is 1 cm long. (Note: this blog doesn’t let me write little superscript numbers so I’ll use ^ to indicate that the following number should be written superscript, so 1^2 would mean 1-squared.)
Each square face of the cube has a surface area of 1 cm x 1 cm = 1 cm^2
Cubes have six faces, so the total surface area is 6 x 1 cm^2 = 6 cm^2
The volume of the cube is 1 cm x 1 cm x 1 cm = 1 cm^3
SA:V = surface area / volume. In this example that’s 6 cm^2 / 1 cm^3 = 6 cm^-1
Notice the funny unit there. You’ve divided cm^2 by cm^3 so they don’t cancel out perfectly.
But does it matter what this unit is? Let’s try it using the same cube but measuring it in millimetres.
Each square face of the 1 cm cube has a surface area of 10 mm x 10 mm = 100 mm^2
Cubes have six faces, so the total surface area is 6 x 100 mm^2 = 600 mm^2
The volume of the cube is 10 mm x 10 mm x 10 mm = 1000 mm^3
SA:V = surface area / volume. Using millimetres that’s 600 mm^2 / 1000 mm^3 = 0.6 mm^-1
1 mm is 1/10 of 1 cm and the SA:V calculated in mm is 1/10 that of the value calculated in cm. If you are comparing SA:V values, make sure they have the same unit.
Of course you don’t find a lot of cubes in nature. In exams they might simplify cells or other bodies to be imagined as spheres of a known radius, and give you equations for you to calculate the surface area and volume. And there are much weirder shapes to think about too. But before we get to all that …
How does SA:V change as things increase in scale?
We know the 1 cm cube has a SA:V of 6 cm^-1.
So, what if we get a handful of these 1 cm cubes and stick them together to make a scaled-up cube where every edge is 2 cm long? (See picture). How will the SA:V of this new structure compare? It’s still a cube, after all; it’s just a bit bigger.
2-cm cube:
Each square face of the assembled cube has a surface area of 2 cm x 2 cm = 4 cm^2
Cubes have six faces, so the total surface area is 6 x 4 cm^2 = 24 cm^2
The volume of the assembled cube is 2 cm x 2 cm x 2 cm = 8 cm^3
SA:V = surface area / volume. In this example that’s 24 cm^2 / 8 cm^3 = 3 cm^-1
The larger cube’s SA:V is only half that of the unit cube!
SA:V is surface area divided by volume. Higher values of SA:V mean there is more surface area for each unit of volume, and lower values of SA:V mean there is less surface area for each unit of volume.
So our 2 cm cube has less surface area per unit volume than the 1 cm cube did.
The larger cube has a lower SA:V. And if you work out the numbers for an even bigger one, you’ll get an even lower SA:V.
This makes sense. To build the 2 cm cube, you stack eight 1 cm cubes together. By doing this, you bury some of their faces within the structure. You end up with the same volume as eight individual 1-cm cubes, but you have reduced the surface area that is exposed to the outside world.
Also, if you look carefully at the picture, you’ll see that each cube in the assembled structure has half of its faces buried. So there’s only half the surface area per unit volume compared to the single cube. And so it makes sense that the SA:V is half what it was for the single cube.
Each small cube is only showing three of its six faces. So for each 1 cm^3 volume (ie for each small cube) there is 3 cm^2 surface area exposed, giving us an overall SA:V of 3 cm^-1.
Key concept: as size increases, SA:V decreases. This is a good thing to explicitly state in any question about SA:V and size, where the shape stays the same.
Try it: calculate SA:V for a 3 cm x 3 cm cube. What value do you guess you might get? And what is the actual value?
Why size matters
"Zygote" by Nina Sesina is licensed under CC BY-SA 4.0.
Imagine you are a single cell (well, you were once, don’t you remember?).
Your cell membrane is your connection with the outside world. Through it you take in nutrients, water and other necessary things. And you expel your waste through it.
This membrane is your surface; you have as much surface area as you have cell membrane.
Meanwhile, inside the cell membrane you have cytoplasm, organelles, etc. In this region you are busily metabolising molecules. You can measure this region as your volume.
Question: can you take in enough nutrients, and expel enough waste, to keep up with your metabolism?
More-important question: if you grow bigger, will this still be true?
Valonia ventricosa aka Sailors eyeball - a single cell that can grow up to 5 cm in diameter. But … how???
As a cell grows larger its SA:V falls. This is just like we saw with scaling up the cubes above. The growth of the cell’s surface area (cell membrane) just can’t keep up with the growth of its volume (where the metabolism is). This means there will be a point at which there just isn’t enough cell membrane (surface area) to transport everything that needs to be transported, limiting the cells ability to function.
This is one reason why cells tend to be pretty small!
But … wait a minute, what about the freakish cell in this next photo? A shiny, bright green, single-celled alga that can get up to 5 cm in diameter. A single cell! Charmingly known as ‘sailors’ eyeballs’.
But how can one cell possibly be so large? Isn’t its SA:V ridiculously low? (Spoiler: yes it is). How can it transport everything it needs with such a low SA:V? Well, it has a trick …
Shape matters
This giant cell is cheating. Inside, all the metabolic activity is limited to an area very close to its cell membrane. Nearly all of the cell’s interior is filled up by one giant vacuole that pushes all the interesting stuff out to the region next to the membrane. So there isn’t nearly as much metabolism going on in there as you might imagine from its total volume. If you re-calculated its SA:V ignoring the volume inside its vacuole, you’d get a much more sensible number. (It also has multiple nuclei spread throughout its cytoplasm.)
In exams, questions are usually about solid spheres or cuboids. We’ve looked at cuboids, so let’s look at spheres.
Spheres
If you have to calculate for spheres, they will give you the equations you need to calculate volume and surface area. So don’t worry about remembering these.
Volume of a sphere = 4/3 x π x r^3
Surface area of a sphere = 4 x π x r^2
So, if you have a sphere of radius 1 cm (remember this is just the distance from the centre to the outside, so the whole thing is 2 cm across):
The surface area is 4 x π x r^2. If r = 1, r^2 = 1. So the surface area is 4 x 3.14 x 1 = 12.6 cm^2
The volume is 4/3 x π x r^3. If r = 1, r^3 = 1. So the volume is 4/3 x 3.14 x 1 = 4.19 cm^3
SA:V = surface area / volume. In this example that’s 12.6 cm^2 / 4.19 cm^3 = 3 cm^-1
This is the same SA:V as the 2-cm cube!
That important phrase “as size increases, SA:V decreases”, is talking about increasing the size of something while keeping the same shape overall. Shape is also really important!
Spheres have the lowest possible surface area for their volume. This is one of the reasons that nature likes them. For example, bubbles are spheres because surface tension pulls their surface in to minimise its area.
Stretching out
If a cell needs to grow to large dimensions but also needs to keep a high SA:V, it needs to avoid being a sphere. More-complex shapes will always have higher SA:V.
Consider neurons. You have a single neuron that reaches from your big toe to your spine. A single cell over a metre long! But these neurons are definitely not spheres. They are very skinny.
Question: what happens to SA:V when you make long/thin shapes?
Let’s go back to the 1 cm cubes. The 1 cm cube had a SA:V of 6. And the 2 cm cube had a SA:V of 3.
The 2 cm cube had a volume of 8 cm^3 : it was made out of eight small cubes. Let’s take those eight small cubes and rearrange them into the shape of a flatworm: a rectangular block of 2 cm x 4 cm x 1 cm (see picture above).
The upper and lower faces of this shape each have a surface area of 8 cm^2
The two long sides of this shape each have a surface area of 4 cm^2
The two ends of this shape each have a surface area of 2 cm^2
The total surface area is (2 x 8 cm^2) + (2 x 4 cm^2) + (2 x 2 cm^2) = 28 cm^2 (the 2 cm cube had 24 cm^2)
The volume of the assembled cube is 2 cm x 4 cm x 1 cm = 8 cm^3 (ok that isn’t a surprise)
SA:V = surface area / volume. In this example that’s 28 cm^2 / 8 cm^3 = 3.5 cm^-1 (the 2 cm cube was 3 cm^-1)
Rearranging it from a 2 cm cube to a flatworm of the same volume has increased its SA:V.
Ok so let’s push it even further and turn it into a long worm.
The upper and lower faces of this shape each have a surface area of 8 cm^2
The two long sides of this shape each have a surface area of 8 cm^2
The two ends of this shape each have a surface area of 1 cm^2
The total surface area is (2 x 8 cm^2) + (2 x 8 cm^2) + (2 x 1 cm^2) = 34 cm^2
The volume of the assembled cube is 1 cm x 8 cm x 1 cm = 8 cm^3 (no surprise)
SA:V = surface area / volume. In this example that’s 28 cm^2 / 8 cm^3 = 4.25 cm^-1 (the 2 cm cube was 3 cm^-1)
Your neurons can be really, really long because they’re also skinny, because that means they still have a really high SA:V.
And of course this works for whole animals too. Many small animals lack blood vessels and rely on molecules simply diffusing through their bodies. They need high surface area for transport and they also need every part of their interior to be not-to-far from that surface. This is (one of) the reasons we don’t have truly-giant insects: they are limited by their ability fo transport things to/from the outside world. Sponges and jellyfish also lack blood vessels and rely on diffusion.
Nemotode by Bob Goldstein, UNC Chapel Hill http://bio.unc.edu/people/faculty/goldstein/ Licenced under a Creative Commons Attribution-Share Alike 3.0 Unported licene
Tardigrade by Alexander Klepnev licenced under CC BY-SA 4.0.
Humans, alongside other larger animals, have much lower SA:V and suffer from low diffusion rates. So we need specialised structures to aid exchange and transport.
Maximising SA:V
Structures that are involved in transport and exchange rely upon having very high surface areas of membrane across which transport can take place. To maximise exposed surface area, structures are folded, or shaped into fingers, or branches.
In the human body, you find extremely high SA:V ratio in specialised structures including:
Gut - the gut wall has microvilli, finger-like structures that extend into the lumen. These increase surface area for absorption of water and nutrients
Lungs - the lungs have many branches terminating in tiny alveoli offering a very high surface area for gas exchange.
Capillaries - capillaries have much higher SA:V than arteries or veins. This offers a higher surface area for transporting things between the blood and tissue fluids. It also increases fluid resistance, which is important to keep the blood moving forward.
Red blood cells have a biconcave shape that increases their SA:V allowing better gas exchange with the blood plasma.
In other organisms, the shapes of structures like leaves, chloroplasts, gills, root hairs, and fungal hyphae, among many other examples, are similarly optimised to maximise surface area to volume ratios.
Folded membranes
You also find folded membranes used as a way to squeeze a load of membrane into a small space. Eg:
Mitochondria have a highly folded inner membrane that offers more surface area to embed the enzymes and proteins involved in oxidative phosphorylation, so that more ATP can be produced.
Golgi apparatus and endoplasmic reticulum are similarly folded to maximise their surface area.
Thermoregulation
Who is is feeling cold, and who is feeling hot?
Body surfaces also exchange heat with the environment.
We naturally change our SA:V when we get hot or cold by changing the position of our limbs and body. Which of the people in the image has the highest SA:V? Which person is feeling cold, and which is feeling hot?
Dogs increase their surface area by opening their mouths when hot and panting (passing air over their wet surfaces to lose heat through evaporation). Cats form neat loafs when cold but sprawl dramatically across the floor when hot.
Organisms’ entire body shape will also be related to their thermoregulatory needs.
Animals that need to lose heat from their bodies usually have a higher SA:V, and animals that need to conserve heat usually have a lower SA:V. This is why elephants have such huge ears (more surface area from which to lose heat) and why animals from cold environments tend to be more spherical than those from hot climates.
"Emperor Penguins" by Christopher.Michel is licensed under CC BY 2.0.
"Black Necked Stork" by AntoGros is licensed under CC BY 2.0.
Keep it in mind
If you can get an intuitive feeling for Surface Area to Volume ratios, and keep them in mind whenever you see a question about folded structures, or about similar shapes of different sizes, then this will help you understand what questions are really asking. A level Biology is about the why and how much more than it is about remembering facts, and SA:V goes a long way to help you understand things on this deeper level.
This article was written by Dr Jenny Shipway with guidance from Tom Whitburn
Mastering AQA A Level Biology Section 3.4.2: DNA and Protein Synthesis - Common Questions & Mark Scheme Insights
After analyzing past papers and mark schemes for AQA specification section 3.4.2 (DNA and Protein Synthesis), I've identified the question types that consistently challenge students. Understanding these patterns and the specific language that mark schemes reward is essential for maximizing your exam performance. Let me guide you through four of the most frequently tested question types with real AQA examples.
Mastering AQA A Level Biology Section 3.4.2: DNA and Protein Synthesis - Common Questions & Mark Scheme Insights
After analyzing past papers and mark schemes for AQA specification section 3.4.2 (DNA and Protein Synthesis), I've identified the question types that consistently challenge students. Understanding these patterns and the specific language that mark schemes reward is essential for maximizing your exam performance. Let me guide you through four of the most frequently tested question types with real AQA examples.
Question Type 1: Comparing DNA and RNA Structures
Why this question type is common: This tests your ability to make precise comparisons between two fundamental molecules. Examiners use this format to assess whether you can distinguish structural features clearly and express them comparatively.
How to approach it (4 marks available):
The mark scheme requires direct comparisons - you must write features opposite each other:
"DNA has deoxyribose, mRNA has ribose"
"DNA has thymine, mRNA has uracil"
"DNA long, mRNA short"
"DNA is double stranded, mRNA is single stranded"
Alternative acceptable comparisons:
"DNA has hydrogen bonds, mRNA has no hydrogen bonds"
"DNA has (complementary) base pairing, mRNA does not"
Mark scheme insight: The mark scheme is very specific - "Must be comparisons." If you only write half of each comparison, you won't earn the mark. Write "DNA double helix" for 'double stranded' and "mRNA single helix" for 'single stranded' - both are acceptable. The mark scheme also notes to ignore references to splicing/introns for this particular question.
Common mistake: Students often write features in isolation rather than as comparisons. "DNA has deoxyribose" alone won't earn a mark - you need "DNA has deoxyribose, mRNA has ribose."
Question Type 2: Transcription Process Description
Why this question type is common: Transcription is a fundamental process that appears repeatedly. This question tests your ability to describe a sequence of events accurately while following specific constraints.
Step-by-step approach (3 marks available):
"(Free RNA) nucleotides form complementary base pairs" (1 mark)
"Phosphodiester bonds form" (1 mark)
"By (action of) RNA polymerase" (1 mark)
Mark scheme insight: Notice the specific exclusions - "Do not include DNA helicase or splicing." Follow these instructions precisely. The mark scheme accepts "A-U, G-C OR combination of those pairs" for complementary base pairing, and you can write "linkages" instead of "bonds" for phosphodiester bonds. However, you must mention RNA polymerase to earn the third mark.
Common mistake: Students often write about DNA helicase breaking hydrogen bonds or mention splicing, losing marks for not following the question constraints. Always read what you're told NOT to include.
Question Type 3: Translation and tRNA Structure
Why this question type is common: This tests detailed knowledge of molecular structures involved in protein synthesis. It's perfect for discriminating between students who have memorized features and those who understand comparative structure.
How to structure your answer (3 marks available):
The mark scheme requires comparisons between mRNA and tRNA:
"mRNA (Has) codon(s) / tRNA (Has) anticodon"
"mRNA No hydrogen/H bonds/base pairs / tRNA Has hydrogen/H bonds/base pairs"
"mRNA No amino acid binding site / tRNA Has amino acid binding site"
"mRNA Linear/straight/not folded / tRNA 'Clover (leaf' shape)/folded"
"mRNA Long/many nucleotides/bases / tRNA Short/few nucleotides/bases"
Choose any three comparisons from this list.
Mark scheme insight: The mark scheme explicitly states "Must be comparisons" and accepts descriptions of binding sites (e.g., "amino acid only bound to tRNA" or "mRNA cannot carry an amino acid, tRNA can"). You can also write "CCA end" for amino acid binding site. Notice how precise the acceptable alternatives are - the mark scheme rewards accurate biological terminology.
Common mistake: Writing "tRNA is double stranded" is specifically rejected by the mark scheme. While tRNA has base pairing in its clover leaf structure, it's not considered double stranded like DNA.
Question Type 4: Gene Mutations and Functional Effects
Why this question type is common: This question type assesses understanding at multiple levels - from molecular changes to functional consequences. It's excellent for testing whether students can link DNA changes to protein function through multiple pathways.
How to structure your answer (4 marks available):
Possible explanations include:
"Substitution (mutation occurred)" (1 mark)
"(Only) one nucleotide/base pair is changed (in a gene)" OR "(Only) one (DNA) triplet/codon changed" (1 mark)
"Same amino acid (coded for)" (1 mark)
"(Because) DNA/genetic code is degenerate" (1 mark)
"(So) tertiary structure is not changed" (1 mark)
"(Change) could be in an intron" (1 mark)
"Removed during splicing" (1 mark)
Mark scheme insight: Maximum 4 marks, so you need to select the most relevant points. The mark scheme accepts descriptions of degenerate code and notes that marks 3 and 4 "can be awarded together, e.g 'different codons/triplets code for the same amino acid' = MP3 and MP4." This means a well-phrased sentence can earn multiple marks. The mark scheme rejects "same amino acid is produced" but accepts "same amino acid coded for" - subtle but important distinction. It also accepts "one amino acid changed" for mark point 3.
Multiple pathways to a correct answer:
Degenerate code pathway: substitution → same amino acid → no change in tertiary structure
Intron pathway: mutation in intron → removed during splicing → functional protein unchanged
Minor change pathway: one amino acid changed → tertiary structure unaffected
Common mistake: Students often describe the mutation but fail to explain why the protein remains functional. Link the molecular change to the functional consequence.
General Tips for Section 3.4.2 Success
Master comparative language: Questions often require direct comparisons. Practice writing features in parallel for DNA/RNA, mRNA/tRNA, prokaryotes/eukaryotes.
Follow exclusions religiously: When questions say "Do not include..." they mean it. Mark schemes penalize students who ignore these constraints.
Link molecular to functional: Don't just describe what happens - explain why it matters. Connect DNA changes → amino acid changes → protein structure → protein function.
Learn mark scheme synonyms: The mark scheme lists acceptable alternatives. For example:
"Bonds" = "linkages"
"Complementary base pairing" = "hydrogen bonding between bases"
"Folded" = "clover leaf shape" (for tRNA)
Use precise terminology: The mark scheme distinguishes between similar phrases:
"Same amino acid coded for" ✓
"Same amino acid is produced" ✗
Practice process descriptions: For transcription and translation, learn the sequence of events and the enzymes/molecules involved. Mark schemes reward step-by-step accuracy.
Understand degenerate code implications: Many mutation questions hinge on understanding that multiple codons code for the same amino acid. This explains why many mutations don't change protein function.
Key Concepts to Master
Transcription differences: Eukaryotes produce pre-mRNA that requires splicing; prokaryotes don't. This appears repeatedly in questions comparing the two systems.
Translation mechanics: Know the roles of:
mRNA (carries genetic code)
tRNA (brings specific amino acids, has anticodons)
Ribosomes (site of translation)
ATP (provides energy for peptide bond formation and amino acid-tRNA binding)
Structural comparisons: Be able to compare:
DNA vs RNA (sugar, bases, strands, length)
mRNA vs tRNA (shape, function, base pairing, length)
Prokaryotic vs eukaryotic protein synthesis (location, splicing, complexity)
Mutation effects: Understand why mutations may have:
No effect (degenerate code, introns, conservative substitutions)
Negative effects (frameshift, active site changes, nonsense mutations)
Positive effects (improved protein function, evolutionary advantages)
Remember, the mark scheme is your friend. It shows exactly what examiners want to see. Practice using mark scheme language in your answers, and you'll find your marks improving significantly. The key is precision - vague biological statements rarely earn marks, while specific, accurate terminology consistently does.
Section 3.4.2 builds on section 3.4.1's foundation, so ensure you're solid on DNA structure before tackling protein synthesis mechanisms. When you understand both the molecular details and the bigger picture of how genetic information flows from DNA → RNA → protein, even complex questions become manageable.
Good luck with your revision!
AQA - Possible essays - as forecast by AI.....
How I suggested some the POSSIBLE 2025 AQA A-Level Biology Essay Titles
One of the most challenging aspects of A-Level Biology Paper 3 is preparing for the 25-mark synoptic essay. With so many potential topics across the full specification, students often feel overwhelmed. That’s why I’ve taken a systematic approach to identify four high-probability essay titles that could appear in the 2025 exam.
Here’s how I did it:
1. Analysing Past Essay Titles
I reviewed a complete set of past essay questions and their mark schemes, identifying which themes have come up repeatedly and which have been underused in recent cycles. This helped rule out repeats and spot patterns in the kinds of synoptic themes the exam board favours.
2. Cross-Referencing the AQA Specification
Using the official AQA Biology specification, I matched every past title to its relevant topic codes. I then looked for specification areas that:
Are heavily weighted in content but haven't been examined recently
Offer rich synoptic potential (e.g. enzymes, feedback, biological molecules)
Align with the mark scheme’s focus on integration and application
3. Designing Original Titles
To avoid duplicating previous questions, I crafted entirely new titles that:
Require a synoptic approach using at least four topics
Encourage explanation, analysis, and application across biological scales
Are rooted in specification content but phrased in fresh and exam-appropriate language
Overview: Gaining and Losing Atoms from the Human Body
Many topics in A level biology relate to how the human body gains and loses atoms. But have you ever stepped back and thought of how these processes all fit together? Having a wide overview can help make sense of individual topics.
Many topics in A level biology relate to how the human body gains and loses atoms. But have you ever stepped back and thought of how these processes all fit together? Having a wide overview can help make sense of individual topics.
Atoms in, atoms out
Your body is an atom-organising machine, with biochemical processes moving atoms between different molecules and locations. Your body takes new atoms in from the environment, and it releases atoms back into the environment.
A surprisingly large amount of mass is exchanged with the environment every day. About 98% of the atoms in your body are replaced each year - a startling thought. This article is focussed on the processes that result in the biggest mass changes.
Atoms gained and lost by the body usually exist as part of molecules.
Taking atoms/molecules into the tissues from the environment = absorption
Sending useful atoms/molecules out into the environment (or bloodstream) = secretion
Sending waste atoms/molecules out into the environment = excretion
Most of the mass we exchange with the environment is in the form of water. Remember that water is a important substrate/product in many metabolic and catabolic biochemical reactions; it’s not just a solvent.
Keeping it under control
Our bodies must control what molecules can (and cannot) get in from (and out to) the environment. Most of our bodies are covered with skin for this reason - skin acts as a barrier between us and our environment. We have specialised organs that manage absorption: the lungs and the gut. These organs both create spaces where molecules from the outside environment are put for absorption to take place in a controlled way. We also have specialised organs that control what substances leave our bodies.
Quick check: remember that molecules don’t count as being inside our bodies until they have been absorbed into our tissues. Just breathing or swallowing something isn’t enough. For example, the gut is a tube that goes from mouth to anus. It goes through our bodies, but the space inside the gut (the lumen, where the food is digested) is not part of our bodies any more than the hole in a doughnut is part of the doughnut. Imagine passing a tiny pebble through the hole in a doughnut; the pebble doesn’t go into the doughnut, just through it; it’s the same with us. Similarly if you swallowed the pebble it would pass through your gut but not be absorbed through the gut wall into your body.
Absorption - gaining body mass
The most common atoms we gain from the molecules we absorb are: carbon (C), oxygen (O), hydrogen (H) and nitrogen (N). This makes sense as these are the atoms most frequently found in biological molecules. We get these from:
The gut
absorption from the gut varies a lot, especially depending on what you eat and drink. These are rough averages:
About 7 litres of water per day (O, H) from fluids that were secreted higher up the gut
About 2-3 litres of water per day (O, H) from food and drink
Maybe something like 500 g total of other food molecules, comprised of:
carbohydrates (C, O, H)
fats (C, O, H)
proteins (C, O, H, N, S)
much smaller masses of other ions and molecules, with all the atoms we need to live
A lot of the water we ingest (eat/drink) comes from our food - you don’t need to drink litres of water every day. Any food that isn’t hard, dry and crunchy will contain water. Consider: a healthy diet includes a lot of plant and animal cells, and cells are about 70-80% water; a piece of steak contains a lot more water than protein, even after cooking.
The lungs
absorb gas from the air:
Something like 750 g per day of oxygen, as the molecular gas O2 (O)
Just look at that daily body mass increase we get just from absorbing oxygen gas!! You take in more mass from the air than from carbohydrates, fats and proteins combined. And it’ll be even higher if you excercise. It’s kinda wild tbh. This oxygen is required for respiration and ends up in water molecules (not in CO2!).
How do we use this stuff?
Although I’ve been talking about atoms, we nearly always absorb molecules. Even Oxygen is absorbed as the molecular gas O2.
And this is really important - having ready-made molecules is vital. Unlike plants, we can’t make glucose through photosynthesis, nor synthesise the amino group of amino acids from scratch*. So we need to absorb these molecules ready-made.
Some of these absorbed molecules are immediately useful (eg glucose for respiration, amino acids for protein synthesis), while many provide really useful starting places for biochemical processes that synthesise other useful molecules, often making use of the absorbed molecules’ ready-made carbon backbones.
Excretion - losing body mass
Excretion is the deliberate loss from the body of waste products from metabolism. It’s about getting rid of unneeded/dangerous waste. This is part of homeostasis.
We excrete the same sort of mix of atoms as we absorb, but in different molecular forms, which are no longer useful to us and/or which may be unhealthy for us to retain in our bodies. We can lose kilograms of mass this way, every day.
Again there are specialised organs/processes for this, the main ones being:
The lungs
Excrete something like 730 g per day of carbon dioxide, a waste product of cellular respiration (C, O)
This comprises 530 grams of carbon atoms and 200 grams of oxygen atoms
This is the main way we lose carbon atoms from our bodies!
Important: these oxygen atoms are not the same ones that were absorbed by the lungs (those ones became water)
The liver
Excretes somewhere around 900 g per day of bile fluids into the gut lumen.
Bile fluids are mostly water (H, O), which can be reabsorbed.
Contains a small mass (mg) of broken-down haem proteins (C, O, H, N), bile salts, and other substances that need to be excreted.
Also contains a very small mass of substances to aid digestion (but these are being secreted, not excreted).
Note: the liver breaks down unwanted proteins to form urea … but it does not excrete urea! The urea it produces is released into the blood to be excreted by the kidney. (Fun question: why doesn’t the liver just excrete this urea directly into the gut?)
The kidney
Excretes about 25 g per day of urea (C, O, H, N), which must be dissolved in water for excretion.
Excretes excess water, in volumes that can vary from zero to litres per day (H, O).
Excretes excess salts, averaging somewhere around 5 g per day.
The skin
Excretes a highly variable volume of sweat, depending on conditions.
Sweat is almost entirely water (H, O) but contains a very small mass of excreted salts, minerals and other substances.
A small amount of urea is also excreted in sweat (C, O, H, N).
You might be surprised not to see faeces listed here. Some things are excreted into the gut and become part of our faeces, but only in small amounts. Most of the bulk of our faeces is undigested food and bacteria that live in our gut - things that were never inside our bodies in the first place, are not products of our metabolism, and so which don’t count as excretions (no matter what everyday language might suggest). In scientific language, the loss of faeces from the body is egestion (compare with ingestion for eating).
Warning: remember all these numbers will vary wildly between different people and under different conditions. For example: if you exercise then you will excrete more carbon dioxide, and if you eat a lot of protein you will excrete more urea. The numbers given here are just to give you a very rough idea of the relative amounts involved.
What about secretions?
While excretion is the release of waste, secretion is the release of useful molecules (into the blood or external environment). Secretions that are sent into the external environment (including into the gut lumen) may be lost if not reabsorbed.
The gut receives about 6 litres per day of liquid secretions, but much of the water is reabsorbed
On a normal day, you might secrete about half a litre of sweat
These secretions are largely water. They do contain other things, like digestive enzymes and salts, but the total mass is of these are small compared to the excretions described above and so they aren’t so important for the big-picture story we’re telling here.
Water gain and loss
Remember water is not just a solvent, it’s a common substrate and product of biochemical processes. And we don’t only get it from absorption through our gut.
About 300 ml of water per day is produced as a product of metabolism (from respiration, as mentioned above).
It’s worth mentioning that we also lose quite a lot of water just because of the way our bodies work. But this is not considered excretion. Again all these figures vary a lot depending on the individual and their level of activity:
About 300 grams of water per day is lost through unavoidable evaporation from the wet surfaces of our lungs
About 300 grams of water per day is lost by evaporation from the skin (this is not sweat, it’s just evaporation)
About 500 grams of water per day is lost because it is required to dissolve urea for excretion (this is a bit different from the kidney’s ability to also excrete excess water in addition to this.
Finally, be aware that some water that enters our gut is never absorbed at all. This is important to keep the gut contents nice and squidgy so they can be pushed along to the anus to be egested. About 200 grams of water per day is lost in this way
* you might be confused by me saying this as people do talk about amino acid synthesis in humans. However, when we synthesise them we do so by using the amino group from an existing amino acid, usually glutamate. We can’t make the amino group ourselves, we can only swap it from molecules that have been synthesised by plants.
Introduction to Immunity (What is an Antigen? What is an Antibody?)
Immunity is a complex topic. This article overviews the basic concepts and vocabulary to give you a foundation before you start looking at the details. When you are studying, break this topic up into chunks so that you’re not overwhelmed by the amount of information.
Immunity is a complex topic. This article overviews the basic concepts and vocabulary to give you a foundation before you start looking at the details. When you are studying, break this topic up into chunks so that you’re not overwhelmed by the amount of information.
Also: expect to find this topic confusing! Feeling confused is part of learning and shouldn’t be thought of as a bad thing, or some sort of failure. It’s just a step toward understanding. If you’re not confused, that’s more worrying because it means you’ve probably made incorrect assumptions.
What is Immunity
Our bodies are wet and warm, and full of useful molecules. This makes them an attractive environment for bacteria, fungi, viruses and other organisms to live. Our bodies host many such organisms that benefit us, some of which are absolutely required for normal functioning (eg the bacteria that break down food in our gut).
But there are other organisms that would like to live and reproduce in and on our bodies that can harm us by their presence. These are disease-causing organisms, and they are called pathogens. Remember these are tiny things without brains and they have no concept of our bodies as an entire organism. They just want to grow and reproduce, with no mind to consider they are damaging their environment.
Immunity is the body’s ability to identify and defend itself against pathogens.
What Defenses do we have?
The first line of defense is to stop invaders gaining entry to our bodies. For example, our skin protects us by making it harder for pathogens to get into our tissue fluids (wounds are more likely to become infected than unbroken skin). Another example: incoming food is passed through the highly acided stomach environment, which kills many organisms before they reach the intestines. But these defenses don’t count as part of the immune response.
If a pathogen does get into our bodies (especially into our blood, intracellular fluid, or cells), then the body - we hope! - will spot, kill, and remove the pathogen. This is the immune response.
The body has cells that are specialised for detroying and removing pathogens. However, it would be pretty difficult for any one cell to be able to accurately recognise every possible different threat, especially as pathogens are constantly evolving to try to get past our defenses. This is why the body uses antibodies.
What is an Antibody?
Antibodies are small protein molecules that are carried around by the blood and tissue fluids. They act like little sticky labels - they stick to anything weird/suspicious and label it as being “not part of my body”.
Antibodies aren’t huge; they only bind to one part of the pathogen, rather than grabbing the entire thing. Usually they bind to a large protein on the surface of the pathogen. The antibody’s physical presence might interfere with the functioning of the pathogen, but more importantly the antibody labels it up to other parts of our immune system as being something that needs to be removed.
But remember: it’s not possible to make one molecule that binds to everything suspicious. For this reason, there are many different antibodies in your body. These vary in a small part of the structure at the end of their arms. Depending on any one antibody’s particular structure, it will be able to bind to different things.
(Recap: do you remember how protein enzymes have a binding site that is very specific to their substrate? Antibodies, which are also proteins, have 3D structures that allow them to be similarly specific in their binding.)
What is an Antigen?
Antibodies with different tertiary structures are able to bind different antigens
Any molecule that an antibody binds to is called an antigen. It’s called that because it generates an immune response.
It’s important to understand that an antigen isn’t a particular type of molecule. Antigens can be viral coat proteins, polysaccharides, lipids, or … just about anything really. If an antibody binds to it, it’s called an antigen - that’s it.
Anything a child plays with is called a “toy”
Anything more than 100 years old is called an “antique”
Anything that gets bound by antibodies is called an “antigen”
Not every part of a pathogen will act as an antigen. But you hope that at least some parts of it will, otherwise your body won’t know it’s there. Luckily, pathogens tend to come covered in loads of interesting proteins which very often do serve as antigens.
If the pathogen does carry an antigen, and if that antigen gets bound by an antibody (labelling it as “not part of my body”), then the immune system will destroy it and remove it from the body.
Where do Antibodies come from?
During an infection, antibodies are released by B-lymphocytes (a type of white blood cell). These cells have the ability to produce and release antibodies in huge numbers. They synthesise the antibodies using a normal protein synthesis process.
But … B-lymphocytes are not all the same. Remember how the immune system uses many different antibodies, with slightly different structures in that variable section? Each individual B-lymphocyte cell can only make ONE type of antibody, with one particular structure. So there are different B-lympocytes for every different antibody made in the body. Which is amazing as that means there are a LOT of different B-lymphocytes.
This is all possible because the B-lymphocytes have slightly different DNA coding just for that variable section of the antibody. This is more than a bit mindblowing because it means that B-lymphocytes are not genetically identical to each other!!! Exactly how this happens is complicated so I’m not going to get into that right now. But the fact they’re all different, and make different antibodies, is crucial to the whole way immunity works.
(There are also T-lymphocytes, which have many similarities to B-lymphocytes but play a different role. We’ll get to them later. )
How do we get the right Antibodies for our needs?
There are vast numbers of different bacteria and fungi and viruses that might harm us, and they are constantly evolving. So we need a immune system that can react to unexpected threats. Something that can recognise anything strange, rather than just a check-list of well-known pathogens.
Important: we DO NOT respond to new pathogens by creating new antibody variations specially designed to bind to them!
This crucial fact is often missed by students.
At first this might seem unintuitive, especially as many news reports suggest otherwise. But think about it. How could we? How would the body even know something was a pathogen if an existing antibody hadn’t already labelled it as such? And even if it did magically know it was a pathogen, how could a lymphocyte know what DNA sequence it would need to be able to synthesise a protein sequence that folded to a structure that was able to bind to it?! And then create that sequence??!
This leads to another mindblowing fact: we ALREADY have B-lymphoctyes that can make antibodies for pathogens we have never encountered. Including for pathogens that haven’t even evolved yet.
Antibodies are not designed to fit specific antigens.
Instead, we create a huge random jumble of antibody variations with a huge variation of different binding sites. Some might be useless, and many will never be required, but there are so many that there’s a really good chance at least one of their binding sites will just happen to be a good fit to some part of a new pathogen.
The thing we do do in response to pathogens is make very large numbers of the particular antibodies we need.
What happens when an Antigen arrives in the body?
The B-lymphocytes don’t do it all alone. Every B-lymphocyte has a genetically-identical T-lymphocyte. For simplicity I’m going to call B-lymphocytes “B-cells”, and T-lymphocytes “T-cells”.
T lymphocyte (T-cell)
T-cells can make molecules with binding sites that match those of their matching B-cells’ antibodies. But T-cells’ don’t make and release antibodies - instead they make similar molecules that are anchored in the cell membrane, sticking their antigen-binding-sites out into the tissue fluids.
These proteins are, massively confusingly, called antibody receptors. You must remember that antibody receptors are receptors that have structures similar to antibodies, not receptors for antibodies. It’d be better if they were called antibody-like receptors, but unfortunately they’re not.
The T-cells expose their antibody receptors to the tissue fluids.
If a pathogen passes by, and if it has an antigen that happens to fit that receptor, then the T-cell can bind to it. This then sets off a cascade of signalling that results in replication of the corresponding B-cells, and those B-cells releasing huge amounts of that particular antibody (if there’s one molecule of that antigen about, there are likely many more to be found).
In summary:
T-cells are the look-outs/scouts (using antibody receptors)
B-cells are the antibody factories (releasing antibodies)
(Side note: B-cells have antigen receptors too, but they still need the T-cells to help them develop into full-speed-ahead antibody factories. It’s all a lot more complicated than I’m explaining here, but you don’t need to know every detail for A-level biology.)
How do Antibodies know that an Antigen is “not part of my body”?
They don’t - antibodies are just small protein molecules, they don’t know anything. Any individual antibody will just swoosh around in our tissue fluids and maybe bind something if it can. It doesn’t know what it’s binding.
So, how do we avoid sticking antibodies onto our own body? (N.b. this does happen in auto-immune diseases, causing huge problems.)
B-cells only release antibodies if their corresponding T-cell has caught an antigen (the T-cell signals to them). But B-cells with antibodies that would attack our own bodies never receive such signals. Because their corresponding T-cells are killed before they get the chance.
T-cells mature in the thymus, an organ between the tops of the lungs. Before release into the body, each newly-matured T-cell is tested to make sure it is functional and to check that it doesn’t bind molecules naturally found in our own bodies. If a T-cell fails either test, it is destroyed.
Autoimmune diseases (eg Type 1 diabetes) happen when this system fails and the immune system attacks the body’s own cells.
Immune Memory (ish)
The second time we encounter a pathogen, our immune systems respond much more rapidly, so we don’t tend to get so ill.
This ‘memory’ is possible due to special B-cells, known as memory cells. These cells live longer than usual, so they persist in the body. So we have more B-cells for previously-encountered antigens than for unknown ones. Immunity is a numbers game, and it takes time to replicate immune cells. The more B-cells you start with, the more quickly you can mount an immune response.
So, this ‘memory’ is simply that that we have more B-cells for antigens we’ve previously encountered. Not really a memory, more a case of being better-stocked for more-likely eventualities.
Vaccination: Antigen vs Pathogen
Vaccination is where we are deliberately exposed to an antigen before we meet the pathogen that carries it in real life, so that we develop B memory cells for that antigen and are ready to mount a rapid immune response.
Because our immune system only needs us to have encounted the antigen and not the whole pathogen, vaccination can be carried out very safely. A single antigen is isolated from the pathogen and copies of this are injected. No live pathogen is involved, so there is no risk of infection.
(In the past live or damaged viruses were used for vaccination, but this is now rare.)
Key Concept: Independent and Dependent Variables
A variable is any value that does/might change during an experiment. Variables can include things like pH, temperature, colour, or the concentration of substances. The amount of time that has passed is a variable, as are rates of reaction.
A guest blog from Dr Jenny Shipway, who studied biochemistry at university and now works in science communication and education training.
To understand experimental design and graphs in exam questions, you will need to confidently recognise the difference between different types of variables. Master this now and it will also make it easier to learn content that is taught using graphs.
What is a Variable
A variable is any value that does/might change during an experiment.
Variables can include things like pH, temperature, colour, or the concentration of substances. The amount of time that has passed is a variable, as are rates of reaction.
Some variables’ values are fixed deliberately by the scientist.
Some variables’ values are changed deliberately by the scientist. These changes are planned in advance so the measured values are usually known before the experiment starts.
Other variables’ values are allowed to change naturally as the experiment progresses. These values can be measured to provide useful data.
Simplify Your Exam Approach
During exams it’s very easy to get overwhelmed with information. When this happens, your brain makes guesses and jumps, often without you being aware. This is why students so often mis-read graphs and/or make ‘silly’ mistakes.
Excellent Exam Tip: work out what the experiment was and how the graph is presented BEFORE looking at the exam question.
This will …
Reduce the number of things you’re thinking about at one time
Reduce the risk of you jumping to (incorrect) conclusions
Help you focus on understanding the experiment
You might feel an urge to rush forward to look at the question, but trust me that won’t save time. You will not be able to answer the questions unless you understand the graph, and rushing forward will make that more difficult, not less.
Identifying Variables
Variables described in the experiment and named on the graph axes may include:
Independent Variable – this is the thing that is changed deliberately by the scientist in a planned way. This is the thing that we expect to cause a measurable effect.
Dependent Variable- this is the thing that is measured by the scientist. The value is not known until it is measured, and the value will depend upon the value of the independent variable.
Control Variables – these variables are fixed to one unchanging value throughout. For example, all experiments might be carried out at 20°C, in which case temperature is a control variable. (Important: this is a different thing from ‘a control’).
The purpose of the experiment is to answer the question: how does the independent variable affect the dependent variable?
There are usually many factors that could be affecting the dependent variable, so it’s important to control as many variables as possile. The idea is that if only one thing has changed (ie the independent variable), then you can be more confident the affect is due to that one thing.
An Example 🍅🍅🍅
If you wanted to work out what temperature was best for growing tomatoes, you could try growing plants at different temperatures to see which plant produced the most fruit. Everything except temperature should be kept the same so that you’re sure that any difference in fruit yield is due to temperature.
The question is: how does temperature affect fruit yield?
You will deliberately be changing the temperature to pre-chosen values, so temperature is the independent variable.
You will need to measure the weight of tomatoes produced to find out this value. So this is the dependent variable. The weight of fruit will depend upon the temperature.
Check your Understanding
Can you identify the independent and dependent variables in the following examples? Which variable was independently fixed by the scientist? And which was measured/recorded during the experiment?
(Ignore the positions of the data points, you only need to look at the axis labels.)
Usually, you will find the independent variable along the x-axis, and the dependent variable up the y-axis. But that’s not always the case (as you hopefully spotted in some of these examples). Do not let your brain jump to an assumption! You must always check this.
You need to be 100% confident of your variables before you move forward to look at the actual question, or everything else will get really confusing. It’s a good tip to write on the exam paper which variable is which (eg label the graph axes “I.V.” and “D.V.”)
Multiple Independent Variables
It’s possible to have more than one independent variable.
For example, some experiments are run twice under different conditions. See this graph:
First, look at the axes. The independent variable here is the amount of time that has passed (on the x-axis): the scientist decided before the experiment at what times they would count the fruit. The dependent variable is the number of ripe fruit (on the y-axis): the number of ripe fruit is the thing the scientist is measuring, and this depends on how much time has passed when the measurement is taken. We’ve seen this before.
Now look at the data. There are two sets of data plotted on the graph. One experiment has been run with fertiliser, and one without. So the presence of fertiliser is another independent variable – it’s something else that affects the value of the dependent variable.
Example A-level Exam Questions
Can you identify the independent and dependent variables in the following A level Biology exam questions?
If this post has been helpful, please like ❤️ below and share with your friends.
For more general information about graphs, see the post about how best to approach A level biology graph questions.