Excretion in Plants and Humans

Every living cell is constantly carrying out chemical reactions — together these are called metabolism. Some of these reactions make substances the body does not need and which would become harmful if they built up. Getting rid of them is called excretion.

Excretion is the removal of the waste products of metabolism from an organism.

The key word is metabolism — excretion only deals with waste made inside the body by its own reactions.

• Respiration makes the waste gas carbon dioxide (in plants and animals).
• Photosynthesis (plants only) makes the waste gas oxygen.
• Breaking down excess amino acids in the human liver makes urea.

Watch out — excretion is NOT the same as egestion. Egestion is removing faeces at the end of the gut. Faeces is mostly undigested food that was never absorbed into the body's cells, so it is not a product of the body's own reactions — that makes it egestion, not excretion.

Exam tip. The definition examiners reward is "removal of the waste products of metabolism". Adding "made by the body / by chemical reactions in cells" makes it watertight and keeps faeces out of the answer.

Plants do not have lungs or kidneys, but they still make waste gases that must be removed. The two waste gases come from the plant's two main reactions:

• Carbon dioxide (CO₂) is a waste product of respiration (respiration happens in plant cells all the time, day and night).
• Oxygen (O₂) is a waste product of photosynthesis (photosynthesis only happens in the light).

Both gases leave the leaf through tiny pores on the underside of the leaf called stomata (one pore is a stoma). Each stoma is opened and closed by two guard cells. Gases move in and out of the stomata by diffusion.

Which gas is the waste product depends on the time of day:

• In bright daylight, photosynthesis is much faster than respiration. The plant uses up its own respiratory CO₂ and more besides, so the net waste gas leaving the stomata is oxygen.
• In the dark (no photosynthesis), only respiration happens, so the net waste gas leaving the stomata is carbon dioxide.

Exam tip. Match the gas to the reaction: CO₂ ↔ respiration, O₂ ↔ photosynthesis, and both leave through the stomata. Do not say "leaves" or "roots" — for these two waste gases the route examiners want is the stomata.

Humans remove their metabolic waste through three organs. You must be able to name each organ and the waste it removes — this is a very common recall question.

Organ	What it excretes	Removed as / by
Lungs	Carbon dioxide and water vapour	Breathed out (exhaled)
Kidneys	Urea and water	Urine
Skin	Urea, water and salts	Sweat

A quick reminder of where each waste comes from:

• Carbon dioxide is the waste gas from respiration in every cell.
• Urea is made in the liver when excess amino acids (excess protein) are broken down. Urea is then carried in the blood to the kidneys (and a little to the skin).
• Water and salts are produced/taken in by the body; the kidneys and skin help remove any excess.

Exam tip. Learn the table as three tidy facts. The trap is forgetting water vapour from the lungs and forgetting that the skin removes urea too (not just water and salts).

The lungs. Every cell respires and makes carbon dioxide. This CO₂ diffuses into the blood, is carried to the lungs, and diffuses into the alveoli (air sacs) to be breathed out. At the same time some water leaves the lungs as water vapour — this is why your breath mists up a cold mirror. So the lungs excrete two things: carbon dioxide and water vapour.

Where urea comes from. Your body cannot store extra protein. When you eat more protein than you need, the excess amino acids are broken down in the liver. This process:

1. Removes the nitrogen-containing part of the amino acid (this part would be toxic on its own).
2. Turns it into urea, which is much less toxic.
3. The urea passes into the blood and is carried to the kidneys.

The kidneys then filter the urea (and excess water) out of the blood to form urine, which is stored in the bladder and removed from the body. A small amount of urea, water and salts is also lost through the skin as sweat.

Why excretion matters. If wastes such as CO₂ and urea were allowed to build up, they would become toxic and the body's reactions (and pH) would be disrupted. Excretion keeps the internal environment safe.

Exam tip. Questions often link the systems: excess protein → liver makes urea → blood → kidneys → urine. Knowing that urea is made in the liver, not the kidney, is a frequent discriminator.

Excretion is the same idea in plants and humans — removing the waste products of metabolism — but the products and routes are different. Use this side-by-side summary to revise:

Feature	Plant	Human
Waste from respiration	carbon dioxide	carbon dioxide (lost via lungs)
Waste from photosynthesis	oxygen	— (humans don't photosynthesise)
Nitrogen waste (urea)	—	urea (made in liver; lost in urine + sweat)
Main route(s) out	stomata (diffusion)	lungs, kidneys, skin

Notice that for both organisms, carbon dioxide is a waste product of respiration. The big extra waste in humans is urea, which plants do not make.

Exam tip. If a question asks you to "compare excretion in a plant and a human", a strong answer states (a) both remove metabolic waste, (b) both excrete CO₂ from respiration, (c) the plant also excretes O₂ from photosynthesis through stomata, and (d) the human also excretes urea through kidneys and skin.