Avoid tragedy
There are some problems that come up again and again in students’ answers to A level Biology exam questions, and which lose them marks. Sometimes it’s a misconception, sometimes misuse of technical language, and sometimes just a lack of required information.
Here are some examples of problem answers that come up a lot in cell topics. Get a step ahead of other students by not including any of these in your A level Biology exam answers!
See if you can spot the problem first, then scroll down to find out if you were right.
What’s the problem with these exam answers?
❌ 1. The ion can’t pass through the hydrophobic inside of the membrane because the ion is polar ❌
❌ 2. The nucleolus contains rRNA ❌
❌ 3. Fatty acids interact with hydrophobic bonds ❌
❌ 4. Ribosomes have a single membrane ❌
❌ 5. Carrier proteins require ATP hydrolysis, and so only do active transport ❌
❌ 6. Mitochondria carry out respiration ❌
Did you spot the problems? Here’s what you should say instead:
✅ 1. The ion can’t pass through the membrane because it is charged. Ions are not polar. This trips up a lot of students but there is an important difference - see this article for a full explanation.
✅ 2. The nucleolus is where rRNA (which forms part of ribosomes) is synthesised. But the nucleolus itself is made of DNA wrapped around histones.
✅ 3. Fatty acids (and other non-polar molecules) have hydrophobic interactions. There is no such thing as a ‘hydrophobic bond’. A bond formed is when an electron is shared between two atoms/molecules. In contrast, hydrophobic interactions happen because the fatty acid molecules do not want to be exposed to water - they cluster together to minimise their exposure. They are ‘water avoiding’ because, being unpolar, they are unable to satisfy water’s hydrogen-bonding potential. (When you think about it, it’s more like the water is avoiding them - but overall it works out as the same thing.)
✅ 4. Ribosomes do NOT have membranes! They are made of protein subunits and rRNA. Ribosomes are sometimes attached to membranes (they are “membrane-bound”), and are sometimes free-floating. In eukaryotes, they are often attached to the membrane of the Rough Endoplasmic Reticulum - but this membrane does not belong to the ribosome, it’s just where it’s hanging out.
✅ 5. Carrier proteins can also be used for faciliated diffusion. This is possible because some carrier proteins can change confirmation without ATP hydrolysis. Facilitated diffusion using carrier proteins moves the carried substance in both directions, but more often away from the higher-concentration side of the membrane - this happens simply because it’s more likely that it will encounter the substance on that side.
✅ 6. Mitochondria carry out aerobic respiration. Ok yes that’s a type of respiration so the original answer wasn’t technically wrong, but you want to get maximum marks and you need to let the examiner know that you understand exactly what the mitochondria are doing. If the question was “What animal goes miaow and eats mice” you wouldn’t answer “a mammal”.