The shape of the lesson
By the end of the lesson, learners can
- What the body excretes: Respiration in every cell makes carbon dioxide, and the breakdown of excess amino acids makes urea.
- Amino acids in the liver: Extended.
- Why urea must go: Extended.
- Lungs: excrete carbon dioxide from respiration.
- Kidneys: excrete urea, excess water and excess ions as urine.
- Route: kidney, ureter (two), bladder, urethra (one).
- Extended: liver assimilates amino acids into proteins; excess are deaminated and the nitrogen-containing part forms urea, which is toxic.
One picture every idea returns to
Excretion is the removal of the waste products of metabolism and substances in excess of requirements. Carbon dioxide is excreted through the lungs, and the kidneys excrete urea and excess water and ions as urine, which passes down the ureters to the bladder and out through the urethra. Extended: the liver joins amino acids into proteins (assimilation); excess amino acids are deaminated, their nitrogen-containing part being removed to form urea, which is toxic and so must be excreted.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Getting rid of what the body makes.. Every cell in your body is busy with chemical reactions, and some of their products are poisonous if they build up. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: What the body excretes | Respiration in every cell makes carbon dioxide, and the breakdown of excess amino acids makes urea. Project tab 1 of the lab (The human excretory organs) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Amino acids in the liver | Extended. Project tab 4 of the lab (The liver: assimilation and deamination) and let a student drive it while the class predicts. | Pairs |
| 21 to 30 min | Teach and check: Why urea must go | Extended. | Mix-Pair-Share |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 21 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Why urea must go, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Mix-Pair-Share
Students mix around the room until you call pair; they pair with the nearest classmate, discuss the prompt, then mix again. Three prompts of rising difficulty, three different partners.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Do not mix up the ureter and the urethra. | There are two ureters, each running from a kidney to the bladder; there is one urethra, running from the bladder to the outside. Remember the order: kidney, ureter, bladder, urethra. |
| Urea is made in the liver, not in the kidneys. | The kidneys only remove urea from the blood and excrete it. Deamination removes the nitrogen-containing part of the amino acid; it does not destroy the whole molecule. |
| Egestion is not excretion. | Faeces are mostly undigested food that never entered the body cells, so removing them is egestion. Excretion removes substances made by metabolism, such as carbon dioxide and urea. |
Excretion and the liver: worksheet
6 questions, 21 marks. Written in the style of the papers; no past paper question is reproduced.
State the waste product excreted through the lungs, and the substances excreted by the kidneys.
A diagram of the human excretory system shows structures P, Q, R and S. P is a bean-shaped organ that forms urine; Q is a tube leaving P; R is a muscular bag that stores urine; S carries urine out of the body. Name P, Q, R and S.
Extended. Describe what happens in the liver to amino acids absorbed from the small intestine.
The table shows the mass of urea excreted per day by an adult on diets containing different masses of protein.
| Protein eaten / g per day | Urea excreted / g per day |
|---|---|
| 40 | 12 |
| 80 | 25 |
| 120 | 37 |
(a) Describe the relationship shown. (b) Extended. Explain the relationship.
Extended. A person's kidneys stop working. Explain why this is life-threatening, with reference to urea.
A student writes: "Excretion is the removal of faeces, and urea is made in the kidneys." Identify the two errors and correct them.
Answer key and marking notes
- The lungs excrete carbon dioxide (produced by respiration).
- The kidneys excrete urea.
- The kidneys also excrete excess water and excess ions (salts).
- P: kidney.
- Q: ureter.
- R: bladder.
- S: urethra.
- Some amino acids are assimilated: the liver uses them to make proteins (for example plasma proteins such as fibrinogen).
- Excess amino acids cannot be stored.
- They are deaminated: the nitrogen-containing part of each amino acid is removed.
- The nitrogen-containing part is used to form urea, which passes into the blood and is carried to the kidneys; the rest of the molecule can be respired for energy.
- (a) As the mass of protein eaten increases, the mass of urea excreted increases; it roughly triples when protein triples (approximately proportional).
- (b) Protein is digested to amino acids, which are absorbed into the blood.
- The more amino acids absorbed beyond what the body needs to make proteins, the more are deaminated in the liver.
- So more urea is formed, and the kidneys excrete more urea in the urine.
- The liver continues to deaminate excess amino acids and make urea.
- Without working kidneys, urea is not removed from the blood, so its concentration rises.
- Urea is toxic, so it damages / poisons cells; without treatment such as dialysis or a transplant the person would die.
- Removal of faeces is egestion, not excretion: faeces are mainly undigested food that was never part of the body's metabolism.
- Excretion is the removal of the waste products of metabolism (such as carbon dioxide and urea) and substances in excess (such as water and ions).
- Urea is made in the liver (from excess amino acids by deamination), not in the kidneys; the kidneys excrete it.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Which of these is an example of excretion?
- ACarbon dioxide from respiration being breathed out
- BUndigested fibre leaving the body in faeces
- CInsulin being released into the blood
- DGlucose being absorbed into the blood
Which tube carries urine from the bladder to the outside of the body?
- Athe urethra
- Bthe ureter
- Cthe renal vein
- Dthe renal artery
The diagram shows part of the human excretory system. Which labelled structure stores urine until it is released?
- AC
- BA
- CB
- DD
Compared with blood in the renal artery, blood in the renal vein contains:
- Aless urea and less oxygen
- Bmore urea and less oxygen
- Cless urea and more oxygen
- Dmore urea and more oxygen
On a hot day a person sweats heavily and drinks little. Compared with a cool day, the kidneys are likely to produce:
- Aa smaller volume of more concentrated urine
- Ba larger volume of more dilute urine
- Ca smaller volume of more dilute urine
- Dthe same volume, as urine volume never changes
Which statement describes deamination?
- ARemoval of the nitrogen-containing part of amino acids to form urea
- BJoining amino acids together to make proteins in the liver
- CBreaking down proteins into amino acids in the small intestine
- DFiltering urea out of the blood in the kidneys
A person changes from a low-protein diet to a high-protein diet. What change would you expect?
- AMore urea is made in the liver and more is excreted in urine
- BMore urea is made in the kidneys and more is excreted in urine
- CLess urea is made, because the extra amino acids are stored
- DNo change, because all amino acids are made into proteins
People whose kidneys are failing are often advised to eat less protein. What is the best explanation?
- AFewer excess amino acids are deaminated, so less toxic urea builds up
- BProtein molecules block the tubes of the nephrons and stop filtration
- CLess protein means the kidneys can excrete more carbon dioxide
- DProteins are converted into glucose, which poisons the kidneys
Quiz key
Carbon dioxide is a waste product of respiration, a metabolic reaction, and it is excreted through the lungs. Fibre in faeces was never absorbed into body cells, so its removal is egestion. Releasing insulin is secretion of a useful hormone, not removal of a waste. Absorbing glucose takes a useful substance into the body rather than removing a waste.
The single urethra carries urine from the bladder out of the body. The two ureters carry urine from the kidneys to the bladder. The renal vein and renal artery are blood vessels: the artery brings blood to the kidney and the vein takes it away; neither carries urine.
C is the bladder, a muscular bag that stores urine. A is a kidney, which forms urine but does not store it. B is a ureter, a tube carrying urine from the kidney to the bladder. D is the urethra, which carries urine out of the body.
The kidney removes urea from the blood, so the renal vein carries less urea. Kidney cells respire, using oxygen, so the renal vein also carries less oxygen. Urea is not added in the kidney (it is made in the liver), so more urea is wrong. The kidney does not add oxygen to the blood, so more oxygen is wrong.
Much water is lost in sweat, so there is little excess water for the kidneys to excrete: they produce a small volume of urine, and the same urea in less water makes it more concentrated. A larger, more dilute volume is what happens after drinking a lot. A smaller volume containing the same urea cannot be more dilute. The kidneys do change urine volume to keep the water content of the blood constant.
Deamination happens in the liver: the nitrogen-containing part of excess amino acids is removed and used to form urea. Joining amino acids into proteins is assimilation. Breaking proteins into amino acids in the gut is digestion. Filtering urea from the blood is the work of the kidneys, which excrete urea but do not make it.
More amino acids are absorbed than are needed to make proteins. The excess cannot be stored, so the liver deaminates them and makes more urea, which the kidneys excrete. Urea is made in the liver, not the kidneys. Amino acids are not stored, so the excess is not kept. Only the amino acids that are needed are assimilated into proteins; the rest are deaminated.
Less protein in the diet means fewer excess amino acids to deaminate, so the liver makes less urea. Failing kidneys cannot remove urea well, and urea is toxic, so making less of it limits the build-up in the blood. Dietary protein is digested to amino acids, so whole proteins do not reach the nephrons from the diet. Carbon dioxide is excreted by the lungs, not the kidneys. Excess amino acids are not converted into a poisonous form of glucose; the harmful product is urea.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.