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Question 1 — Four Substances Leaving a Body, and Only Three of Them Excreted
Total: 12 marks
This paper covers the whole of Topic 13. Like a real Cambridge paper it ranges across every sub-topic — 13.1 what excretion is, 13.2 the urinary system and the kidney, 13.3 the nephron and 13.4 the liver and urea — and it mixes them inside single questions. All three Topic 13 papers do; they differ in the angle they come at it from, not in what they cover.
Table 1.1 lists four substances that leave the body of a healthy person.
substance
where it leaves the body
carbon dioxide
in exhaled air
cellulose from vegetables
in faeces
urea
dissolved in urine
excess sodium ions
dissolved in urine
(a)[3]
Define excretion, define egestion, and state the single question that distinguishes them.
Model Answer — 1(a)
excretion is the removal from organisms of the waste products of metabolism and substances in excess of requirements [1]
egestion is the removal of undigested food from the body as faeces [1]
the test is whether the substance was ever inside a cell, or produced by one [1]
⚠ If you missed marks here: The definition of excretion has two halves and most candidates give only the first. If you gave only “waste products”, notice that it would not cover excess water or excess ions, which are not waste at all — there is simply more of them than the body needs.
(b)[4]
For each of the four substances in Table 1.1, state whether its removal is excretion or egestion, and give the reason.
Model Answer — 1(b)
carbon dioxide — excretion: it is a waste product of respiration in cells [1]
cellulose — egestion: it is never digested or absorbed, so it never enters a cell [1]
urea — excretion: it is a waste product made by deamination in the liver [1]
excess sodium ions — excretion: they were absorbed into the blood and are present in excess of requirements [1]
⚠ If you missed marks here: The sodium ions row is the one that separates a good answer from a complete one. Ions are useful substances, so calling them “waste” is wrong — the reason they qualify is the second half of the definition, and the word excess has to appear.
(c)[2]
Carbon dioxide and urea are both excreted, but they leave the body by different routes. Name the organ that excretes each, and name the organ in which urea is produced.
Model Answer — 1(c)
carbon dioxide is excreted through the lungs; urea is excreted through the kidneys [1]
urea is produced in the liver [1]
⚠ If you missed marks here: The liver makes urea but nothing leaves the body there, so it is not an excretory organ. Keep the three verbs apart: the liver makes, the kidney removes, the bladder stores.
(d)[3]
Explain the importance of excretion.
Model Answer — 1(d)
urea is formed continuously from excess amino acids, which cannot be stored [1]
urea is toxic [1]
so if it were not removed it would accumulate in the blood and damage cells, disrupting the enzyme-controlled reactions of metabolism [1]
⚠ If you missed marks here: Answers that talk about the body staying “clean” or removing “toxins” score nothing, because neither phrase names a substance. The syllabus limits this explanation to the toxicity of urea, so that word must appear.
Question 2 — Inside a Kidney, and the Tubes Outside It
Total: 12 marks
Fig. 2.1 shows a kidney that has been cut in half lengthways, together with one of the tubes attached to it. Nothing has been named.
(a)[3]
Name the regions A and B and the structure C on Fig. 2.1.
Model Answer — 2(a)
A — the cortex [1]
B — the medulla [1]
C — the ureter [1]
⚠ If you missed marks here: Cortex outside, medulla inside — the same order every time, in this organ and in others you will meet later. C cannot be the urethra: that is the single tube below the bladder, and it is never attached to a kidney.
(b)[3]
Compare the ureter with the urethra. In your answer state what each carries, how many of each there are, and one difference in the consequence of a blockage.
Model Answer — 2(b)
a ureter carries urine from a kidney to the bladder, whereas the urethra carries urine from the bladder to the outside of the body [1]
there are two ureters, one per kidney, whereas there is only one urethra [1]
a blocked ureter affects only one kidney, whereas a blocked urethra prevents any urine leaving the body [1]
⚠ If you missed marks here: The command word is compare, so every sentence must mention both structures — build them around the words whereas or than. Describing the ureter accurately on its own answers a different question and can score as little as one mark out of three.
(c)[3]
A student writes that the bladder makes urine and then concentrates it by reabsorbing water. Identify the errors and give the correct account.
Model Answer — 2(c)
urine is formed in the kidney, not in the bladder [1]
no water is reabsorbed after the kidney — the urine is complete before it enters the ureter and nothing is added to it or taken from it afterwards [1]
the function of the bladder is simply to store urine until it is convenient to release it [1]
⚠ If you missed marks here: The reabsorption half of the claim is the dangerous one because it sounds plausible — a stretchy bag full of liquid feels like a place where water could be recovered. Ask what would collect the water: there is no arrangement of capillaries for it, and the nephron has already done that job with great care.
(d)[3]
Name the vessels that carry blood into and out of a kidney, and suggest why the kidneys receive about a fifth of the blood pumped by the heart even though they make up less than one per cent of body mass.
Model Answer — 2(d)
in by the renal artery, out by the renal vein [1]
for most organs the blood is a supply, but for the kidney the blood is the material being processed [1]
so the whole blood volume must pass through repeatedly for urea to be removed as fast as the liver produces it [1]
⚠ If you missed marks here: An answer that says kidney cells “respire very fast” misses the point entirely: the kidney is not consuming that blood, it is filtering it. Recognising the difference between an organ that is a customer and an organ that is a processor makes a set of otherwise absurd figures make sense.
Question 3 — One Day of Urine Samples
Total: 12 marks
A student collected every sample of urine they produced during one day and measured its volume and its urea concentration. They drank 500 cm³ of water at 08:00 and again at 09:00, and drank nothing else all day. Table 3.1 shows the results.
time
volume of urine / cm³
urea concentration / g per dm³
08:00
210
9.0
10:00
620
3.0
12:00
300
6.2
16:00
180
10.0
20:00
150
12.0
(a)[3]
Describe the results shown in Table 3.1.
Model Answer — 3(a)
the volume rises sharply from 210 to 620 cm³ at 10:00 and then falls steadily to 150 cm³ at 20:00 [1]
the urea concentration falls from 9.0 to 3.0 g per dm³ at 10:00 and then rises steadily to 12.0 g per dm³ at 20:00 [1]
the two measurements change in opposite directions throughout: a large volume always goes with a low concentration [1]
⚠ If you missed marks here: A description question is marked on figures and on shape, not on explanation, and the third mark here is the relationship between the two columns. Quoting numbers costs a few seconds and is very often the difference between one mark and three.
(b)[4]
Explain the results shown in Table 3.1.
Model Answer — 3(b)
after drinking, there is water in excess of requirements [1]
so less water is reabsorbed in the nephrons and a larger volume of urine is produced [1]
later in the day no more water has been taken in, so there is little or no excess and most of the filtered water is reabsorbed, giving a small volume [1]
the amount of urea removed is set by how much the liver produced, so the same urea in a smaller volume gives a higher concentration [1]
⚠ If you missed marks here: The fourth mark is the one candidates miss, and it is the most useful idea in the topic: the amount of urea did not change, only the volume it was dissolved in. If your answer suggested that the liver made more urea in the evening, look again — nothing in the data says that, and the student ate no differently.
(c)[2]
Calculate the total mass of urea excreted in the 10:00 and the 20:00 samples combined. Show your working.
⚠ If you missed marks here: Notice what the two numbers show: a sample four times the volume of the other contains almost exactly the same mass of urea. That is the whole point of the table, and it is worth writing down even when the question only asks for a calculation.
(d)[3]
Predict how the results would differ if the student had spent the afternoon exercising outdoors in hot weather without drinking. Explain your prediction.
Model Answer — 3(d)
the volume of the afternoon samples would be even smaller and the urea concentration even higher [1]
because water is lost in sweat, so there is no water in excess of requirements at all [1]
so an even greater proportion of the filtered water is reabsorbed, leaving the same amount of urea in a smaller volume of liquid [1]
⚠ If you missed marks here: Predictions score for direction plus mechanism, and both must be there. Note also what would not change: the mass of urea excreted, because that depends on deamination in the liver and therefore on protein intake, not on how hot the afternoon was.
Question 4 — A Filter That Cannot Choose
Total: 12 marks
(a)[2]
Name the structure at which filtration takes place, and state the property of a molecule that decides whether it is filtered.
Model Answer — 4(a)
the glomerulus [1]
the size of the molecule — small molecules pass through and large ones do not [1]
⚠ If you missed marks here: Saying the kidney filters out “the substances the body does not need” is the misconception this part is designed to catch. A filter has no way of recognising anything; it separates by size alone, and everything else in the nephron follows from that limitation.
(b)[3]
State the four substances that are filtered out of the blood, and state the two things that remain in the blood and why.
Model Answer — 4(b)
filtered out: water, glucose, urea and ions [1]
remaining: blood cells and proteins [1]
because they are too large to pass through the filter [1]
⚠ If you missed marks here: If you left glucose off the list, you have assumed that the body would not throw away something useful. It does — and then takes it all back, which is why glucose appears in the filtrate at exactly the plasma concentration and is absent from the urine.
(c)[3]
State what is reabsorbed as the filtrate passes along the tubule. Give the proportion of each.
Model Answer — 4(c)
all of the glucose [1]
some of the ions [1]
most of the water [1]
⚠ If you missed marks here: Three marks for three words. If your answer read “the useful substances are reabsorbed” it would score one at best, because it does not distinguish glucose, which disappears entirely from urine, from ions, which do not, from water, of which over 99 per cent is returned.
(d)[2]
Explain why it makes sense for the kidney to filter useful substances out of the blood and then reabsorb them, rather than filtering out only the wastes.
Model Answer — 4(d)
a filter can separate only by size, and useful and waste molecules of a similar size cannot be told apart at that stage [1]
so a second, selective step recovers the useful substances afterwards, which is simpler than building a filter that could recognise every molecule [1]
⚠ If you missed marks here: This is the question students most often ask about the kidney, and the answer is a design argument rather than a fact. It also predicts the observation in part (b): glucose must appear in the filtrate, because nothing at the filter could have kept it out.
(e)[2]
State what the urine formed by this process contains.
Model Answer — 4(e)
urea [1]
excess water and excess ions [1]
⚠ If you missed marks here: The word excess belongs to two of the three components and should be written, because it makes clear that water and ions are not waste products. Urine containing glucose or protein would be a sign of disease, so neither belongs on this list.
Question 5 — Testing Two Urine Samples
Total: 10 marks
Two patients had their urine tested, and the results are shown in Table 5.1. Neither had an abnormal blood urea concentration and both had a normal blood glucose concentration.
patient
test with Benedict’s solution
biuret test
K
stayed blue
turned purple
L
gave a brick-red precipitate
stayed blue
(a)[3]
Describe how you would carry out the two tests shown in Table 5.1 on a urine sample, and state the positive result of each.
Model Answer — 5(a)
for glucose: add Benedict’s solution to the sample and heat it in a water bath [1]
a positive result is a colour change from blue through green and orange to a brick-red precipitate [1]
for protein: add biuret solution to the sample at room temperature; a positive result is a change from blue to purple [1]
⚠ If you missed marks here: The two tests are not interchangeable and only one of them needs heat. Forgetting to heat the Benedict’s test is the standard practical error, and it produces a false negative — the sample stays blue whether glucose is present or not.
(b)[3]
Suggest, with a reason in each case, what has gone wrong in patient K and in patient L.
Model Answer — 5(b)
patient K has protein in the urine, so the filtration barrier at the glomerulus is damaged [1]
because protein is normally too large to be filtered and should be completely absent [1]
patient L has glucose in the urine with a normal blood glucose concentration, so reabsorption of glucose is failing — the filter itself is working [1]
⚠ If you missed marks here: Two similar-looking findings with opposite causes. Protein in urine means the filter has failed; glucose in urine means reabsorption has failed. The stem gives you the normal blood glucose deliberately, because it rules out the other possible cause of glucose in urine — more arriving than the nephron can take back.
(c)[2]
Explain why a healthy person has neither glucose nor protein in the urine, making clear that the two reasons are different.
Model Answer — 5(c)
glucose is filtered but all of it is reabsorbed along the tubule [1]
protein is never filtered at all because its molecules are too large, so it is absent from the filtrate as well as from the urine [1]
⚠ If you missed marks here: An answer giving one reason for both absences loses a mark and, worse, makes part (b) impossible to answer correctly. The place where the two substances behave differently is the filtrate, where glucose is present and protein is not.
(d)[2]
Suggest one control that should be included when testing these urine samples, and explain why it is needed.
Model Answer — 5(d)
test a sample of urine from a person known to be healthy using the same two tests [1]
so that the negative results can be compared with a known negative, showing that the reagents are working and that the colour change seen in the patients is genuinely caused by the substance being tested for [1]
⚠ If you missed marks here: A control is not the same as repeating the test. It has to be a comparison in which you already know the answer, so that a result can be interpreted. Accept also testing distilled water, or a solution of known glucose concentration as a positive control, provided you explain what it demonstrates.
Question 6 — Ten Days on a High-Protein Diet
Total: 12 marks
A healthy adult changed to a diet containing three times as much protein, with the same total energy content. Their blood urea concentration was 30 arbitrary units on day 0. It rose steeply to 48 units by day 4, and from day 6 to day 10 it stayed between 50 and 52 units.
(a)[2]
Describe the role of the liver in the assimilation of amino acids.
Model Answer — 6(a)
the liver receives absorbed amino acids first, in the hepatic portal vein [1]
and converts those the body needs into proteins, including proteins found in the blood plasma [1]
⚠ If you missed marks here: Assimilation, absorption and digestion are three different events with three different names. Assimilation is the point at which a molecule becomes part of the body, and describing the villus instead answers a Topic 7 question rather than this one.
(b)[3]
Describe deamination, and state where in the body it happens.
Model Answer — 6(b)
it happens in the liver [1]
the nitrogen-containing part is removed from amino acids that are in excess of requirements [1]
forming urea [1]
⚠ If you missed marks here: Placing deamination in the kidney is the commonest single error in this sub-topic, and it is worth knowing why it is the liver: the hepatic portal vein delivers every absorbed amino acid to the liver first, so the liver is the organ that finds out whether there is a surplus.
(c)[2]
State what happens to the part of the amino acid that is left after deamination.
Model Answer — 6(c)
it is essentially a carbohydrate [1]
so it is respired to release energy, or stored as glycogen [1]
⚠ If you missed marks here: Only the nitrogen is a problem; the carbon is perfectly useful. Candidates who stop at the word urea lose both marks here and also lose the prediction in part (e), which depends on knowing that the carbon skeleton is kept.
(d)[3]
Explain why the blood urea concentration rose after the change of diet, and why it levelled off rather than continuing to rise.
Model Answer — 6(d)
more protein was digested, so more amino acids were absorbed and a greater proportion was in excess; since they cannot be stored, more were deaminated and more urea was formed [1]
as the blood urea concentration rose, more urea was filtered and excreted per minute [1]
so the rate of removal came to equal the rate of production and the concentration stopped changing, at a new higher steady level [1]
⚠ If you missed marks here: The plateau is the interesting mark, and it is a rate argument: whenever a substance stops accumulating, a rate of removal has caught up with a rate of production. The same framing explains why blood urea rises in kidney failure — there, removal can never catch up.
(e)[2]
Predict the effect of this diet on the blood glucose concentration and explain your prediction.
Model Answer — 6(e)
blood glucose would be maintained or would rise slightly, even though carbohydrate intake has not increased [1]
because the remainder of each deaminated amino acid is a carbohydrate that can be respired or stored as glycogen [1]
⚠ If you missed marks here: This is the half of deamination that most candidates never use. It also makes a nonsense of the popular belief that eating protein instead of carbohydrate removes sugar from the body entirely — the carbon has to go somewhere, and it goes into respiration or into glycogen.
Question 7 — Two Rodents and Two Very Different Kidneys
Total: 10 marks
Two rodents of similar body mass were studied. One lives in a desert; the other lives beside a river and spends much of its time in water. Neither had glucose or protein in its urine.
species
volume of urine per day / cm³
urea concentration / g per dm³
desert species
4.0
24.0
riverside species
62.0
1.4
(a)[2]
Calculate the mass of urea excreted each day by each species. Show your working.
⚠ If you missed marks here: Small volumes make the conversion easy to get wrong: 4.0 cm³ is 0.0040 dm³, not 0.040. Writing the units beside every number as you work is the only reliable defence, and an answer out by a factor of ten cannot be given the mark.
(b)[3]
Explain how the desert species produces urine with such a high urea concentration.
Model Answer — 7(b)
urea is filtered out at the glomerulus and is not reabsorbed in either species [1]
the desert species reabsorbs a greater proportion of the filtered water back into the blood [1]
so a similar mass of urea ends up dissolved in a much smaller volume of liquid, giving a much higher concentration [1]
⚠ If you missed marks here: This can be answered entirely from the three sentences of the nephron, applied to an animal you have never studied — which is exactly what a challenge paper is testing. If you found yourself reaching for extra mechanism, you were about to leave the syllabus.
(c)[2]
Suggest one advantage of this to the desert species, and state what your calculation in (a) shows about the two animals.
Model Answer — 7(c)
it conserves water, which is scarce in a desert, while still removing the urea that must be excreted [1]
the calculation shows that the two species excrete almost the same mass of urea, so the difference between them is in water conservation and not in urea production [1]
⚠ If you missed marks here: “It is adapted to the desert” restates the question and scores nothing; naming the substance being conserved is the mark. The second mark rewards using your own calculation as evidence, which is something examiners look for and candidates rarely do.
(d)[3]
Neither species had glucose or protein in its urine. Explain what each of these absences shows, and state one limitation of using these data to draw conclusions about kidneys in general.
Model Answer — 7(d)
the absence of protein shows that the filter at the glomerulus is intact and is holding back molecules that are too large [1]
the absence of glucose shows that all of the filtered glucose is being reabsorbed in both species [1]
a limitation: only two species, and no indication of how many individuals or how variable the results were, so the comparison may not represent desert and riverside mammals generally [1]
⚠ If you missed marks here: Two absences, two different mechanisms — and an answer that gives one explanation for both loses a mark. The limitation mark is about the design rather than the biology: with one animal of each kind there is no way to tell a species difference from an individual one.
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