Edexcel IGCSE Biology · Spec 2.70-2.71
Excretion
What excretion is, the three main excretory organs, and where the waste product urea comes from.
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Excretion
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Explained
Excretion, and the word it is constantly confused with
Excretion is the removal from the body of the waste products of metabolism, meaning substances the organism made itself through its own chemical reactions.
Egestion is the removal of undigested food from the gut. That material passed through the body without ever being absorbed into it, so nothing in the body made it and nothing about it is a waste product of metabolism.
Faeces are therefore egested, not excreted. It is the commonest error in this topic and it is entirely avoidable, because the two words are separated by a single test: did the body make it?
The three organs and what each removes
The lungs excrete carbon dioxide, produced by respiration in every cell and carried to the lungs in the blood.
The kidneys excrete urea, along with excess water and excess salts, in urine.
The skin excretes water, salts and some urea in sweat. Sweating is primarily for cooling, and the excretion is a side effect of it rather than its purpose, which is worth saying if a question asks.
The liver appears in this topic too, but not as an excretory organ. It makes the urea; the kidneys remove it. Producing a waste product and excreting one are different jobs, and the liver does only the first.
Where urea comes from
The body cannot store excess protein, and it cannot store excess amino acids either.
So surplus amino acids are broken down in the liver by deamination, which removes the amino group. What is left of the molecule can be respired or converted to fat or glycogen, so the carbon part is not wasted.
The amino group becomes ammonia, which is highly toxic, so the liver immediately converts it into urea, which is much less toxic. The urea travels in the blood to the kidneys and is removed in urine.
Reading that chain in order explains why a high protein diet produces more urea, and why liver damage and kidney damage cause different problems. A damaged liver leaves ammonia in the blood. Damaged kidneys leave urea in it.
What the mark scheme accepts and rejects
An Edexcel International GCSE Biology mark scheme asks for the function of fibre in the diet and awards one mark from six accepted answers: that it helps peristalsis, prevents constipation, helps the intestines push food, helps release faeces, helps egestion, or helps in the movement of food.
Six wordings accepted, including several quite informal ones. Then one line refusing a seventh: reject excrete faeces.
That is this whole page in a single instruction. A candidate who has understood the biology of fibre perfectly, and describes it in the ordinary way, loses the mark for one wrong verb. Faeces are egested. Nothing about them is excretion, because none of the material in them was made by the body.
Notice how the mark scheme is generous everywhere except here. Helps the intestines push food is colloquial and it is credited. Excrete faeces sounds more technical and it is refused. The mark is not for sounding scientific; it is for using the one word that carries the right meaning.
Excretion in plants
Plants excrete too, and their waste products are gases. Oxygen from photosynthesis is waste when respiration is not using it all, and carbon dioxide from respiration is waste when photosynthesis is not. Both leave through the stomata by diffusion.
Plants do not egest at all, because they have no gut and take in nothing they do not absorb. So in a plant every waste product removed is excretion by definition.
Answering the definition question
A definition of excretion needs three parts: removal, from the body, of waste products of metabolism.
Leaving out metabolism is what lets faeces slip back in, so it is the part to guard. Some mark schemes accept substances made by the body as an alternative phrasing, and either version does the work provided the idea is there.
Then, if the question asks for an example, give the substance and the organ together. Carbon dioxide from the lungs. Urea from the kidneys. A substance without an organ, or an organ without a substance, is usually half the answer.
Spec 2.70-2.71
What you need to know
- Define excretion
- Name the main excretory organs and the wastes they remove
- Explain where urea comes from
Active recall
Quick check
Answer each question before opening the answer.
Name an excretory product and the organ that removes it.
Carbon dioxide — lungs; urea — kidneys; excess water and salts — kidneys (and skin in sweat).
Where is urea made and from what?
In the liver, from the breakdown of excess amino acids (deamination).
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