🍃 Carbon dioxide
Plant cells respire all the time, day and night. Respiration makes carbon dioxide as a waste product.
Excretion » Excretion in Plants and Humans
What you'll learn this session
Study time: 30 minutes
Edexcel spec: 2.70, 2.71
- What excretion means and how it differs from egestion
- Why oxygen and carbon dioxide can be waste products in a plant
- How these gases leave a leaf through the stomata
- The excretory products of the lungs, kidneys and skin
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Unlock This CourseWhat is excretion?
Every living cell makes waste. Cells are always carrying out chemical reactions, and some of those reactions make substances the body does not need. If these built up, they would harm the organism. So they must be removed. You met excretion as one of the life processes in the lesson Response to Environment and Excretion. Here we look at exactly what is removed, and where it leaves from.
Key terms:
- Excretion: the removal of the waste products of metabolism, and of substances in excess of requirements, from the organism.
- Metabolism: all the chemical reactions that happen in the cells of an organism.
- Egestion: the removal of undigested food (faeces) from the gut.
- Organ of excretion: an organ that removes excretory products from the body.
Excretion is not egestion
Faeces are mostly food that was never digested and absorbed. It passed through the gut but never entered the body's cells, so it was never part of metabolism. That is why removing faeces is egestion, not excretion. Excretory products, like carbon dioxide and urea, are made inside cells.
Waste gases in plants
Plants make waste too, even though they do not move. Two gases are involved, and each comes from a different reaction in the leaf cells.
☀️ Oxygen
In the light, leaf cells photosynthesise. Photosynthesis makes oxygen as a waste product.
Which gas is the waste depends on the light.
- In the day: photosynthesis is fast. A plant uses the carbon dioxide from its own respiration for photosynthesis, and needs more from the air. The oxygen from photosynthesis is more than the cells need, so the extra is waste and leaves the leaf.
- At night: there is no light, so no photosynthesis. Respiration carries on, so carbon dioxide is the waste gas that leaves the leaf, and oxygen is taken in.
The day and night gas exchange is explained in the lesson Day and Night Gas Exchange. Here you only need to know which gas is the waste product at each time.
How the gases leave a leaf
The waste gases leave by diffusion. Inside the leaf the gas builds up in the air spaces, so there is a higher concentration inside than outside. The gas moves down this concentration gradient and out through the stomata, the tiny pores in the leaf surface. The stomata are controlled by guard cells, as you saw in the lesson Stomata Function and Control.
Worked example
Question: Explain how oxygen made in a leaf on a sunny day leaves the plant.
Answer: Oxygen is a waste product of photosynthesis. It builds up in the air spaces in the leaf, so its concentration is higher inside the leaf than in the air outside. It leaves by diffusion, moving down the concentration gradient through the stomata.
Excretion in humans
In humans, different organs get rid of different waste. The spec names three organs of excretion: the lungs, the kidneys and the skin.
💨 Lungs
The lungs excrete carbon dioxide, made in respiration, and water vapour. Both leave in the air we breathe out.
💧 Kidneys
The kidneys excrete urea, excess water and excess ions. These leave dissolved in urine.
✋ Skin
The skin excretes sweat. Sweat is mostly water, with some salts and a small amount of urea.
Key terms:
- Urea: a waste substance made in the body that contains nitrogen. It leaves in urine and, in small amounts, in sweat.
- Urine: the liquid made by the kidneys. It contains urea, excess water and excess ions.
- Sweat: the salty liquid made by the skin, mostly water.
Notice that water and ions count as excretory products when there is too much of them. That is the "substances in excess of requirements" part of the definition. How the kidney does this is covered in later lessons, and sweating to cool down is in the lesson Skin and Temperature Regulation.
| Organ | Excretory products |
|---|---|
| Lungs | Carbon dioxide, water vapour |
| Kidneys | Urea, excess water, excess ions (in urine) |
| Skin | Water, salts, a little urea (in sweat) |
Common mistakes
- Calling faeces an excretory product. Faeces are egested, because they are undigested food that never took part in metabolism.
- Saying the lungs excrete oxygen. In humans the lungs take oxygen in. They excrete carbon dioxide and water vapour.
- Saying that oxygen is always waste in plants. Oxygen is only waste when it is made by photosynthesis in the light and is not all used by respiration.
- Saying the kidneys make carbon dioxide leave the body. Carbon dioxide leaves through the lungs.
- Writing that sweat is only for cooling. It does remove some waste too, but it is mostly water.
Exam-style question
(a) Define the term excretion. [2 marks]
(b) Name two excretory products found in urine. [2 marks]
(c) Explain why oxygen is a waste product of a leaf during the day. [2 marks]
Model answer
(a) The removal of the waste products of metabolism (1) and of substances in excess of requirements (1).
(b) Any two from: urea (1), excess water (1), excess ions (1).
(c) Oxygen is made by photosynthesis (1), and it is more than the leaf cells need for respiration, so the extra is lost through the stomata by diffusion (1).
Exam tip
When you define excretion, include both parts: waste products of metabolism and substances in excess. Do not mention faeces, which are egested.