Excretion — the three organs and their products
Excretion is the removal of the waste products of metabolism (reactions in cells) from the body. Learn the three organs and their products exactly: the lungs excrete carbon dioxide (waste from respiration) and water vapour; the kidneys excrete urea, excess water and salts as urine; the skin excretes urea, water and salts in sweat. Carbon dioxide belongs to the lungs only — never carry it onto the kidney row.
Stomata — how a leaf loses its waste gases
Stomata are the tiny leaf pores (controlled by guard cells) through which a leaf loses its waste gases by diffusion — plants need no specialised excretory system. Which gas is the waste depends on the process: oxygen is the waste product of photosynthesis, carbon dioxide the waste product of respiration. In bright light photosynthesis exceeds respiration, so the net loss is oxygen; in the dark only respiration occurs, so the net loss is carbon dioxide.
Urea — nitrogen waste made in the liver
Urea is the nitrogen-containing waste made in the liver from the breakdown of excess amino acids (protein the body cannot store). It is carried in the blood to the kidneys, which remove it in the urine; a little is also lost in sweat. Because urea is made by the body's own metabolism, its removal is genuine excretion — unlike faeces, which is undigested food removed by egestion and was never made inside the cells.
Drawn from real examiner reports.
Excretion ≠ egestion
Faeces is removed by egestion, not excretion. Faeces is undigested food that passed through the gut and was never absorbed into the body's cells, so it was never made by metabolism. Excretion covers only substances made inside cells by metabolic reactions — such as carbon dioxide from respiration and urea from the liver. Calling faeces an excretory product loses the mark.
Excretion is not "getting rid of waste"
Defining excretion simply as 'getting rid of waste' earns nothing. The mark-scheme definition has two elements: it is the removal of the waste products of metabolism — the substances must be waste AND made by the body's metabolism (its cell reactions). Learn the exact phrase 'waste products of metabolism' rather than a loose paraphrase.
Leaf gases: which is waste of which process
Oxygen is the waste product of photosynthesis and carbon dioxide the waste product of respiration — students often swap these. A second error is claiming a plant 'only gives out carbon dioxide': true in the dark, but in bright light photosynthesis exceeds respiration, so the net movement is carbon dioxide in and oxygen out. Always say whether you mean day or night.
Excretion ≠ secretion
Excretion and secretion are different processes. Excretion is getting rid of the waste products of metabolism (carbon dioxide, urea) that the body no longer needs. Secretion is the release of a useful substance the body still needs — for example an enzyme from a gland or a hormone such as insulin. A useful released substance is secreted, not excreted.
Urea is made in the liver, not the kidneys
Urea is made in the liver from excess amino acids; the kidneys only remove it from the blood. A common error is to say the kidneys make urea. The kidneys are organs of excretion — they filter urea, water and salts into the urine — but the chemical production of urea happens in the liver. Keep 'made in the liver' and 'removed by the kidneys' separate.
CO₂ is a lung product, not a kidney one
Asked for the excretory products of the kidneys, the answer is urea, water and salts (mineral ions), in the urine. A frequent error is to write carbon dioxide: carbon dioxide is a real excretory product, but it is removed by the lungs, not the kidneys. Match each product to its organ — lungs: carbon dioxide and water vapour; kidneys and skin: urea, water and salts.
Learn the organ → product table cold
The recall marks are guaranteed if you learn the table cold: lungs → carbon dioxide + water vapour; kidneys → urea + water + salts (urine); skin → urea + water + salts (sweat). Write the products against the organ named, and never carry carbon dioxide onto the kidney row.
Explain, do not just describe
Examiners note candidates describe when asked to explain. For an explain mark, link the product back to the process that made it — carbon dioxide is removed by the lungs because it is the waste product of respiration. A bare list of products will not score the explanation marks.
For leaf gases, state day or night
When you name the gas lost from a leaf, fix the condition first. In bright light the net loss is oxygen; in the dark it is carbon dioxide. An answer that just says 'the plant loses carbon dioxide', with no day or night qualifier, is treated as incomplete.
Excretion — the removal of the waste products of metabolism (the chemical reactions in cells) from the body. Two elements: it is waste, and it is made by the body's own metabolism.
Egestion — the removal of undigested food (faeces) from the gut. This is NOT excretion, because faeces was never made by metabolism.
The three excretory organs and their products (learn this table exactly):
| Organ | Excretory product(s) | Where the waste comes from |
|---|---|---|
| Lungs | Carbon dioxide, water vapour | Carbon dioxide is a waste product of respiration |
| Kidneys | Urea, water, salts (as urine) | Urea is made in the liver from excess amino acids |
| Skin | Urea, water, salts (in sweat) | Same wastes as the kidney, lost while sweating (temperature control) |
Define egestion, and say why it is NOT excretion.
The lungs, kidneys and skin are all organs of excretion.
State the excretory product(s) removed by each of these three organs. (4 marks)
