Excretion: Plants and the Human Organs of Excretion
Excretion is the removal from the body of the waste products of metabolism (the chemical reactions going on inside cells).
Do not confuse it with egestion — faeces are undigested food that never entered the cells, so egestion is not excretion.
Waste gases in a leaf
Two gases are produced as waste products of metabolism in a plant:
| Gas | Made by | When | Waste because… |
|---|---|---|---|
| Carbon dioxide | respiration in every living cell | day and night | not needed by the cell |
| Oxygen | photosynthesis in cells containing chloroplasts | daylight only | produced faster than respiration can use it |
Both gases leave the leaf by diffusion through the stomata (singular: stoma), the pores mainly on the lower epidermis, opened and closed by guard cells.
What actually comes out, and when
- In bright light: photosynthesis is faster than respiration, so there is a net loss of oxygen and a net uptake of carbon dioxide.
- In darkness: only respiration happens, so there is a net loss of carbon dioxide and net uptake of oxygen.
- At the compensation point: the two rates are equal and there is no net movement of either gas.
Water vapour is also lost through the stomata, but that is transpiration, not excretion.
Human organs of excretion
| Organ | Excretory product(s) |
|---|---|
| Lungs | carbon dioxide (and water vapour) |
| Kidneys | urea, excess water and excess ions (mineral salts), removed as urine |
| Skin | sweat — containing water, ions and a small amount of urea |
Urea is made in the liver from excess amino acids (which cannot be stored). The urea is carried dissolved in the blood plasma to the kidneys.
Common mistake: examiners repeatedly see students writing that carbon dioxide is excreted by the kidneys. It is not. Carbon dioxide is excreted by the lungs only. Kidneys excrete urea, water and ions.
The skin's main job is temperature control — but because sweat contains urea and ions, the skin still counts as an organ of excretion.
Examiner tip. When a question asks about carbon dioxide removal by a plant, say it is used up / removed by photosynthesis, not just that it 'goes into the leaf'. Vague answers that repeat the wording of the question earn nothing. Equally, never list carbon dioxide as a kidney product — that error costs a mark almost every time it appears.