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.
“Could you explain this to someone else ? ”Kidneys, nephrons, ultrafiltration, ADH - Revision and understanding through self explanation
Try using these cards with a partner to revise Kidneys, nephrons, ultrafiltration, ADH, loops of henle, and collecting ducts
Instructions
Print onto card. Cut out, place face down in a pack.
Pick up a card
Explain the concept with a whiteboard or paper.
Use all of the words on each card
Then look up the concept in a textbook and correct your mistakes.
Research shows the effectiveness of self explanation ie could you explain this to someone else
Try using these cards with a partner to revise Kidneys, nephrons, ultrafiltration, ADH, loops of henle, and collecting ducts
Instructions
Print onto card. Cut out, place face down in a pack.
Pick up a card
Explain the concept with a whiteboard or paper.
Use all of the words on each card
Then look up the concept in a textbook and correct your mistakes.
There is a downloadable pdf here
If you found this useful then please like and share.
Kidney Diseases, Dialysis and Transplant Questions - Questions by Topic
A-level Biology Questions on Kidney Dialysis and Transplants - useful powerpoints/animations and videos
If you use then like and share
Kidney Disease Questions
OCR A/OCR B, Eduqas - excellent application questions on Kidney Diseases, Dialysis and Transplants
IIf you found this useful then please like and share.
Kidney Questions with mark schemes -A level Biology Questions by topic
A level Biology Questions by topic - Kidney Questions with markschemes, useful for OCR, AQA, Eduqas
Here are lots of good questions on the kidney. I expect the kidney to appear frequently in the new style papers as they are very synoptic, give opportunities to require calculation and get students to analyse and explain data.
if you use then please like and share
If you found it useful then please ❤️ (at the bottom of the page) and share, you can follow me on instagram - alevelbiologytutor
Y13 & Y12 OCR A and AQA weekly Masterclass information
Going through past papers by topic often gives a useful insight into the thoughts of the examiners and how they see a topic and the emphasis they put on different parts of a specification. I have included all the markschemes and comments from the examiners reports. PDF below.
Here are lots of good questions on the kidney. I expect the kidney to appear frequently in the new style papers as they are very synoptic, give opportunities to require calculation and get students to analyse and explain data. If you use this then please like and share
