Excretion: Getting Rid of What You Made

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 2.70 — Carbon dioxide and oxygen as waste products of plant metabolism, lost through stomata
• 2.71 — The excretory products of the lungs, kidneys and skin

Specification section 2(h) Transport — also taught in Why Transport Systems Exist, and the Two Plant Vessels, Water Uptake and Transpiration, Blood, Immunity and Clotting.

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

Start with the distinction the whole topic turns on, because almost every mark lost here is lost on it. EXCRETION is getting rid of something your own body MADE — a waste product of your own chemistry, which was inside your cells and had to be removed. EGESTION is getting rid of something that merely passed through: undigested food that was never absorbed, and so was never part of you at all. Faeces are egested. Carbon dioxide, urea and excess water are excreted. The test is simple — did your body manufacture it?

The three human excretory organs, and what each one removes

1. LUNGS — carbon dioxide, made by respiration in every cell, and water vapour along with it. Every breath out is an act of excretion.
2. KIDNEYS — urea, made by the liver, plus excess water and excess mineral ions, removed together as urine.
3. SKIN — sweat, which contains water, mineral ions and a small amount of urea. But note carefully what sweating is FOR: it cools you. The excretion is a side effect of a temperature-control mechanism, not its purpose.
4. That last point is worth holding on to. The lungs and kidneys exist partly to excrete; the skin excretes almost by accident.

Where urea comes from — the chain that earns the marks

1. You eat protein, which is digested to amino acids and absorbed.
2. Amino acids cannot be stored. Any surplus beyond what the body needs for building proteins has to be dealt with immediately.
3. In the LIVER, the nitrogen-containing part is removed from each surplus amino acid — this is deamination.
4. That produces ammonia, which is extremely toxic and could not be allowed to accumulate for a moment.
5. So the liver immediately converts the ammonia into urea, which is far less toxic and can be carried safely.
6. Urea dissolves in the blood plasma and travels to the kidneys, which remove it in the urine. Notice the division of labour: the liver MAKES urea, the kidneys EXCRETE it, and confusing those two costs marks constantly.

Plants have the same problem and solve it far more simply, which is worth understanding rather than just accepting. Their waste products are gases: respiration produces carbon dioxide, and photosynthesis produces oxygen — which, in a plant in daylight, genuinely is a waste product, since it makes far more than it can use. Both simply diffuse out through the stomata, down their concentration gradients, using the same pores that let carbon dioxide in.

Why a plant needs no excretory organ and you do

1. A plant's waste is gaseous, so it can leave by diffusion through pores already present in every leaf — no organ, no vessel, no energy required.
2. A plant also makes far less waste: it does not eat protein, so it never has surplus amino acids and never produces any nitrogenous waste at all.
3. What waste it does accumulate in solid form can simply be stored in leaves — and then shed when the leaves fall.
4. An animal produces urea continuously, in quantity, and it is toxic. It cannot be stored, cannot be shed, and will not diffuse away through the skin fast enough.
5. So an animal needs a dedicated organ, a blood supply to deliver waste to it, and a route out. That organ is the kidney, and it is the subject of the next lesson.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Learn this definition · Excretion

The removal from the body of the waste products of metabolism — substances made by the body's own chemical reactions — together with substances in excess of requirements.

Excretion against egestion — the distinction that decides the marks

A. EXCRETION removes waste the body MADE, such as carbon dioxide from respiration or urea from the breakdown of surplus amino acids. It was inside the cells and had to be removed.

B. EGESTION removes undigested food as faeces. That material passed through the gut without ever being absorbed, so it was never part of the body and is not a waste product of it.

The three organs and their excretory products — learn all of them

Lungs

carbon dioxide from respiration, and water vapour

Kidneys

urea, excess water and excess mineral ions, removed together as urine

Skin

water, mineral ions and a little urea, in sweat — though sweating exists to cool the body, not to excrete

Plants (through the stomata)

carbon dioxide from respiration, and oxygen produced in excess by photosynthesis

How urea is made and removed

1. Excess amino acids cannot be stored by the body.
2. In the liver, the nitrogen-containing part is removed from each one — deamination.
3. This produces ammonia, which is highly toxic.
4. The liver converts the ammonia into urea, which is much less toxic.
5. Urea dissolves in the blood plasma and is carried to the kidneys, which excrete it in the urine.

Model answer [4 marks]

Explain the difference between excretion and egestion, giving one example of each. [4]

Excretion is the removal of waste products made by the body's own metabolic reactions, together with substances in excess of requirements — for example carbon dioxide produced by respiration and removed by the lungs. Egestion is the removal of undigested food as faeces. That material passed through the gut without ever being absorbed into the body, so it is not a product of metabolism and its removal is not excretion.

Model answer [4 marks]

Explain why plants do not need excretory organs but mammals do. [4]

A plant's main waste products are gases — carbon dioxide from respiration and oxygen from photosynthesis — which diffuse out through the stomata without any special organ. A plant also produces no nitrogenous waste, because it does not take in protein and so never has surplus amino acids. A mammal produces urea continuously from the breakdown of surplus amino acids; urea is toxic and cannot be stored or allowed to accumulate, so a dedicated organ, the kidney, is needed to remove it from the blood.

Mark-losing trap. Faeces are EGESTED, not excreted. Nothing in them was ever absorbed, so the body never made it.

Mark-losing trap. The LIVER makes urea; the KIDNEYS excrete it. Saying the kidneys make urea is the most common error in this topic.

Mark-losing trap. Sweating is for cooling. Excretion through the skin is a side effect, and saying otherwise loses the mark.

Mark-losing trap. Oxygen is a waste product for a plant in daylight — it makes far more than it can use. Do not call it a product only.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · Identify [1 mark] — Which organ excretes urea from the body?
2. Grade 7 · Explain [2 marks] — Explain why the removal of faeces from the body is not an example of excretion.
3. Grade 8 · Explain [4 marks] — A person eats far more protein than their body needs. Explain what happens to the surplus. Select every statement that belongs in a full-mark explanation.
4. Grade 9 · Calculate [4 marks] — A person produces 1.6 dm³ of urine per day, containing 18 g/dm³ of urea. Their blood plasma contains 0.30 g/dm³ of urea. Calculate the volume of blood plasma that would have to be completely cleared of urea each day to remove the same mass.
5. 9+ · Analyse [5 marks] — A plant and a mammal of similar mass are compared. The mammal has kidneys, a bladder and a dedicated blood supply devoted to removing waste; the plant has no excretory organ of any kind, yet is perfectly healthy. Select every statement that belongs in a full-mark explanation.

The people behind this science

Three ways into the same idea — the one who made urea without a body, and the one who found the filters inside the kidney, and the one convinced the body was poisoning itself. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Friedrich Wöhler — the one who made urea without a body

In 1828 Wöhler heated a mineral salt and obtained urea — the substance this lesson is about — with no living thing involved at all. He wrote to Berzelius that he could make urea without needing a kidney, whether of man or of dog, and in doing so struck the first serious blow against the belief that the chemistry of living things was different in kind from any other. What his flask could never show him was the road a body takes to the same substance: the surplus amino acids, the liver, the ammonia too toxic to keep.

• “How did you come to make urea by accident?”
• “Why did it matter so much that no living thing was involved?”
• “What did people believe about the chemistry of living things before that?”
• “Did making urea tell you anything about how a body makes it?”
• “Were you as certain as your admirers that you had destroyed the vital force?”

Marcello Malpighi — the one who found the filters inside the kidney

Turning his microscope on the kidney, Malpighi found it was not the solid mass it appeared to be but was packed with tiny knots of blood vessels and tubules — structures nobody had suspected, and which are still called Malpighian corpuscles. He could not say what they did, but he showed that excretion happens in machinery far below the scale anyone had been looking at.

• “What did the inside of a kidney look like under your lens?”
• “Why did you think those tiny knots of vessels were there?”
• “How can you work out what a structure does when you can only see its shape?”
• “What did you expect a kidney to be made of before you looked?”

Élie Metchnikoff — the one convinced the body was poisoning itself

In his later years Metchnikoff became convinced that bacteria in the large intestine release substances that poison us gradually, and that this slow self-poisoning IS ageing. He was wrong about the conclusion and ran far past his evidence, but the underlying instinct is this lesson's: waste that is not removed does harm, and an organism that cannot excrete is in serious trouble.

• “Why did you think our own waste products were shortening our lives?”
• “What happens to a body that cannot remove what it produces?”
• “How did you decide the large intestine was the source of the trouble?”
• “Looking back, where did your reasoning run ahead of your evidence?”

Carry on through the course

• Previous lesson: Blood Vessels and the Circulation
• Next lesson: The Kidney and the Nephron
• Levels of Organisation — From organelle to organism — the five-step ladder that every later topic in biology is built on.
• Comparing Cells, and Stem Cells — What plant and animal cells share and where they differ — then why a cell specialising is one of the most powerful ideas in biology.
• Carbohydrates, Proteins and Lipids — The three big food molecules — what they are made of, what they are built from, and how to test for them.
• Enzymes: Catalysts With a Shape — Why a protein with a hole in it speeds up a reaction, and why heat destroys that ability permanently.
• Enzymes and pH — Why acid ruins one enzyme and is the only condition another will work in — and how to find an enzyme's best pH by experiment.
• All of Biology · Biology key terms · required practicals · Incandio Science · The 4BI1 specification