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Question 1 — A Nephron From Top to Bottom
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.
Fig. 1.1 is a diagram of a single nephron and its blood supply, drawn with the tubule running down the page. Nothing has been named.
(a)[4]
Name the structures labelled P, Q, R and S on Fig. 1.1.
Model Answer — 1(a)
P — the glomerulus [1]
Q — the capsule [1]
R — the tubule of the nephron [1]
S — the collecting duct [1]
⚠ If you missed marks here: P is a knot of blood capillaries and Q is the cup that catches what is filtered out of it, so they are not two names for the same thing. If you labelled the thin grey line as part of the nephron, remember that blood and filtrate travel in two entirely separate systems: no blood vessel ever contains filtrate or urine.
(b)[3]
State which labelled structure is the site of filtration, name the substances that are filtered out of the blood there, and state what remains in the blood.
Model Answer — 1(b)
filtration happens at P, the glomerulus [1]
the substances filtered out are water, glucose, urea and ions [1]
blood cells and proteins remain in the blood because they are too large to be filtered [1]
⚠ If you missed marks here: Filtration happens once, at one place; reabsorption happens along the whole length of R. Confusing where each process occurs is a quick way to lose a mark that costs nothing to get right.
(c)[3]
State what is reabsorbed as the liquid passes along R, giving the proportion of each, and state where these substances go.
Model Answer — 1(c)
all of the glucose [1]
some of the ions [1]
most of the water — all of these passing into the blood capillaries surrounding the tubule [1]
⚠ If you missed marks here: Three marks, three quantity words. Note that the destination matters too: reabsorbed substances go back into the blood, which is why a capillary network runs the whole length of the tubule instead of stopping at the glomerulus.
(d)[2]
State the composition of the liquid passing down S, and name that liquid.
Model Answer — 1(d)
it contains urea, excess water and excess ions [1]
this liquid is urine [1]
⚠ If you missed marks here: No glucose and no protein appear in this list, and for two different reasons: all the glucose was reabsorbed, while protein was never filtered at all. Keeping those two reasons apart is worth marks in every paper on this topic.
Question 2 — Where Things Leave, and What That Is Called
Total: 12 marks
(a)[3]
List, in order, the structures through which urine passes from the place where it is formed to the outside of the body. State how many of each structure a person has.
Model Answer — 2(a)
kidney → ureter → bladder → urethra, in that order [1]
two kidneys and two ureters [1]
one bladder and one urethra [1]
⚠ If you missed marks here: Learning the chain as a single string settles the order and the two similar words at once. Counting is the safety net: if you have written two urethras or one ureter, you have them the wrong way round, whatever the spelling looks like.
(b)[3]
Define excretion, and state which substance each of the two excretory organs named in the syllabus removes.
Model Answer — 2(b)
excretion is the removal from organisms of the waste products of metabolism and substances in excess of requirements [1]
the lungs excrete carbon dioxide [1]
the kidneys excrete urea, excess water and excess ions [1]
⚠ If you missed marks here: Both halves of the definition are needed for the first mark, and the word excess is needed for the third. Water and ions are not waste products; they are removed because there is more of them than the body requires, and the amount removed changes from hour to hour.
(c)[3]
Explain the importance of excretion.
Model Answer — 2(c)
urea is produced continuously in the liver from excess amino acids, which cannot be stored [1]
urea is toxic [1]
so without excretion it would accumulate in the blood and damage cells, disrupting the enzyme-controlled reactions of metabolism [1]
⚠ If you missed marks here: The syllabus limits this explanation to the toxicity of urea, so the mark scheme is looking for that word and for what accumulation would do. Vague answers about “toxins” or staying “healthy” score nothing at all.
(d)[3]
Explain why the removal of faeces is not excretion. Sweat contains a little urea; explain why this does not change the answer you gave for part (b).
Model Answer — 2(d)
faeces consist of undigested food that was never absorbed and never entered a cell, so it is not a product of metabolism — its removal is egestion [1]
sweat does contain urea, so the substances in it do satisfy the definition of excretion [1]
but only a very small proportion of urea is lost this way, and the syllabus names the kidneys as the excretory organ for urea [1]
⚠ If you missed marks here: The sweat part is a test of honesty rather than of recall. Do not deny the biology to protect a list — say that sweat does carry a trace of urea, and then explain why the kidneys are still the answer. Denying it outright would be as wrong as leaving the kidneys out.
Question 3 — Four Substances With Their Names Removed
Total: 12 marks
Table 3.1 gives the concentration of four substances, in g per 100 cm³, in three liquids taken from a healthy person. The substances are labelled J, K, L and M.
substance
blood plasma
filtrate
urine
J
0.72
0.72
1.50
K
8.00
0.00
0.00
L
0.03
0.03
2.00
M
0.10
0.10
0.00
(a)[4]
Identify substances J, K, L and M as protein, glucose, urea or ions.
Model Answer — 3(a)
J — ions [1]
K — protein [1]
L — urea [1]
M — glucose [1]
⚠ If you missed marks here: Read down the filtrate column first, not the plasma column. The single zero there identifies protein immediately; then the urine column separates the substance that disappears entirely (glucose, all reabsorbed) from the one that rises about 67-fold (urea, not reabsorbed) from the one that merely doubles (ions, partly reabsorbed).
(b)[3]
Explain how the values for K and for M identify them, making clear that the two substances are absent from urine for different reasons.
Model Answer — 3(b)
K reads zero in the filtrate, so it never passes the filter: its molecules are too large to be filtered [1]
M is present in the filtrate at the same value as in the plasma, so it is filtered [1]
M is absent from the urine because all of it is reabsorbed back into the blood along the tubule [1]
⚠ If you missed marks here: Two zeros in the urine column, two entirely different mechanisms. This distinction is not a technicality: protein in a patient’s urine means a damaged filter, while glucose in a patient’s urine means failed reabsorption, and giving one reason for both makes the diagnosis impossible.
(c)[2]
Calculate how many times more concentrated substance L is in urine than in blood plasma. Show your working and give your answer to the nearest whole number.
Model Answer — 3(c)
2.00 ÷ 0.03 [1]
= 67 times (66.7 rounded) [1]
⚠ If you missed marks here: A “how many times” question is always a division, and it is always the larger value divided by the smaller. If your answer came out below 1 you divided the wrong way round, and the units cancel, so the answer has no unit at all.
(d)[3]
Predict how the urine column would differ if the same person drank 3 dm³ of water an hour before the samples were taken. Explain your prediction.
Model Answer — 3(d)
the urine values for J and L would be lower (the urine would be more dilute) [1]
because the person now has water in excess of requirements, so less of the filtered water is reabsorbed and a larger volume of urine is produced [1]
the amounts of urea and ions removed would be little changed — only the volume of liquid they are dissolved in has increased; the values for K and M would remain zero [1]
⚠ If you missed marks here: The third mark is the one that separates a good answer from a complete one: it is the concentration that falls, not the amount, and the two zeros do not move at all. A prediction that changes every row shows the mechanism has not been understood.
Question 4 — Two Patients and One Substance
Total: 12 marks
Table 4.1 shows blood urea concentrations for two patients. The normal range is 25–35 arbitrary units.
patient
blood urea / units
protein in urine
notes
V
68
absent
eating a normal diet
W
7
absent
eating a normal diet
(a)[3]
Describe deamination and state where it takes place.
Model Answer — 4(a)
it takes place in the liver [1]
the nitrogen-containing part of amino acids that are in excess of requirements is removed [1]
forming urea [1]
⚠ If you missed marks here: Three marks and three things that go wrong: the organ (not the kidney), the molecule (amino acids, not proteins — those were digested in the gut) and the product (urea, a chemical, not urine, a liquid). Getting all three right takes one carefully built sentence.
(b)[2]
Describe the role of the liver in the assimilation of amino acids.
Model Answer — 4(b)
amino acids that the body needs are converted into proteins [1]
these include proteins found dissolved in the blood plasma, which the liver makes [1]
⚠ If you missed marks here: Assimilation and deamination are the same organ working in opposite directions, and Cambridge sets them as consecutive parts of one question for exactly that reason. Giving a version of the same sentence for both scores once.
(c)[2]
State what happens to the remainder of an amino acid after deamination, and explain why nothing is wasted.
Model Answer — 4(c)
the remainder is essentially a carbohydrate, and is respired to release energy or stored as glycogen [1]
so only the nitrogen-containing part is excreted; the carbon in the molecule is still used by the body [1]
⚠ If you missed marks here: Candidates who stop at the word urea lose these marks and also lose the ability to explain why a high-protein, low-carbohydrate diet still maintains blood glucose. The nitrogen is the problem; the carbon never was.
(d)[3]
Suggest, with a reason, which organ is failing in patient V and which is failing in patient W.
Model Answer — 4(d)
patient V: the kidneys — urea is still being produced normally by the liver but is not being filtered out and excreted, so it accumulates at 68 units [1]
patient W: the liver — less deamination means less urea is formed [1]
and the healthy kidneys continue to remove what urea there is, so the concentration falls to 7 units [1]
⚠ If you missed marks here: One measurement, two opposite diagnoses, decided by the direction of the change. It works only if you are certain that the liver makes urea and the kidney removes it — anyone who believes the kidney makes urea will name the same organ twice.
(e)[2]
Both patients eat carbohydrate and fat as well as protein. Explain why only the protein in their diet affects the amount of urea produced.
Model Answer — 4(e)
carbohydrates and fats contain only carbon, hydrogen and oxygen and no nitrogen [1]
so they have no nitrogen-containing part to remove, and can be broken down completely without producing urea; only amino acids contain nitrogen [1]
⚠ If you missed marks here: This is a Topic 4 fact about elements doing explanatory work. It also predicts something useful: cutting fat from the diet will not change a blood urea reading at all, while cutting protein will.
Question 5 — Designing an Investigation Into Water Intake
Total: 10 marks
A class wants to investigate how the volume of water a person drinks affects the volume and the concentration of the urine they produce. They have access to measuring cylinders, a clock and a method for measuring urea concentration.
(a)[4]
Describe how this investigation should be carried out. In your answer state the independent variable, the dependent variables, and two variables that must be controlled.
Model Answer — 5(a)
independent variable: the volume of water drunk, which should be varied over a suitable range with each volume tested on the same person on different days [1]
dependent variables: the volume of urine produced in a fixed time, and its urea concentration [1]
control the protein content of the diet, because that determines how much urea is produced [1]
control the temperature or level of activity, because sweating loses water and would change the volume of urine independently of what was drunk [1]
⚠ If you missed marks here: The two control variables are where the marks are, and they are not generic: protein intake is the one that fixes the amount of urea, and sweating is the one that competes with the kidney for water. Writing “keep everything else the same” names nothing and scores nothing.
(b)[3]
Predict the results of this investigation and explain them.
Model Answer — 5(b)
as the volume drunk increases, the volume of urine increases and its urea concentration decreases [1]
because more water is in excess of requirements, so less water is reabsorbed in the nephrons [1]
the mass of urea excreted stays about the same, since it depends on deamination in the liver, so the same urea in more liquid gives a lower concentration [1]
⚠ If you missed marks here: Predictions score for direction plus mechanism, and the third mark is the one that shows real understanding: the amount of urea is fixed by the liver, so only the volume of solvent changes. If you predicted that more water would “flush out more urea”, this is the mark you lost.
(c)[3]
Suggest two limitations of this investigation and one improvement that would address one of them.
Model Answer — 5(c)
limitation: results from a single person may not apply to others, since people differ in size and in diet [1]
limitation: it is very difficult to collect all the urine produced in the fixed time, and water is also lost in sweat and in exhaled air, which is not measured [1]
improvement: repeat with a number of people and calculate a mean, or repeat each volume on several days with the same person, so that the effect of ordinary day-to-day variation is reduced [1]
⚠ If you missed marks here: An improvement has to fix a limitation you have actually named, not just be good practice in general. “Repeat the experiment” on its own is not enough — say what repeating achieves, which here is reducing the effect of natural variation between days and between people.
Question 6 — One Kidney Doing the Work of Two
Total: 12 marks
Table 6.1 gives measurements made on blood entering and leaving one kidney of a healthy adult, in arbitrary units.
substance
entering the kidney
leaving the kidney
urea
36
5
glucose
92
90
plasma protein
74
74
oxygen
20
14
(a)[2]
Name the vessels carrying blood into and out of the kidney, and name the two regions visible when a kidney is cut in half, stating which is on the outside.
Model Answer — 6(a)
in by the renal artery and out by the renal vein [1]
the cortex is the outer region and the medulla the inner one [1]
⚠ If you missed marks here: Cortex outside, medulla inside, always the same way round. The syllabus limits the internal structure of the kidney to those two regions, so if you are trying to remember a third one you are revising a different course.
(b)[3]
Explain the change in urea shown in Table 6.1, quoting figures in your answer.
Model Answer — 6(b)
urea falls from 36 to 5 units [1]
because it is filtered out of the blood at the glomerulus [1]
and is not reabsorbed, so it passes on into the urine and leaves the body [1]
⚠ If you missed marks here: Quoting the two figures earns a mark on its own in almost every data question of this kind. The reasoning mark is not reabsorbed: being filtered is not enough by itself, since glucose is filtered too and none of it is lost.
(c)[3]
Explain the glucose and plasma protein values in Table 6.1, making clear that the two are almost unchanged for different reasons.
Model Answer — 6(c)
glucose is filtered, but all of it is reabsorbed, so almost none is lost [1]
the small fall from 92 to 90 is glucose used in respiration by the kidney cells [1]
plasma protein is unchanged because its molecules are too large to be filtered, so it never leaves the blood at all [1]
⚠ If you missed marks here: Two rows that look the same and are not. Notice also that the oxygen row in the table exists to remind you that a kidney is a living tissue: the fall from 20 to 14 units is respiration, not excretion, and explaining it as excretion is the standard whole-question error here.
(d)[2]
Explain why the blood leaving the kidney still contains urea rather than none at all.
Model Answer — 6(d)
only a proportion of the blood passing through the kidney is filtered on any single pass [1]
that is sufficient, because the blood circulates repeatedly and the kidney only has to remove urea as fast as the liver makes it [1]
⚠ If you missed marks here: A rate argument again: a system that removes a substance does not have to be perfect, only fast enough. The same framing explains why blood urea settles at a new steady level on a high-protein diet instead of rising for ever.
(e)[2]
After a person donates one kidney, their blood urea concentration rises slightly for a few weeks and then returns to normal. Suggest an explanation.
Model Answer — 6(e)
at first only half as many nephrons are available, so the rate of removal of urea is lower than the rate at which the liver produces it, and the concentration rises [1]
the higher concentration means more urea is filtered per minute by the remaining kidney, so removal comes back into balance with production and the concentration stops rising [1]
⚠ If you missed marks here: A suggest question wants reasoning from what you know, and the reasoning here is the balance of two rates. It also explains why one kidney is enough: the remaining organ does not have to work harder in any mysterious sense, it simply filters blood with a slightly higher urea concentration in it.
Question 7 — Does Drinking More Water Remove More Urea?
Total: 10 marks
A website advises readers to drink four litres of water a day because, it says, this “flushes more waste out of the body”. A student decides to examine the claim.
(a)[3]
Describe what happens in the kidneys of a person who drinks a very large volume of water, and state the effect on the volume and the concentration of their urine.
Model Answer — 7(a)
there is water in excess of requirements [1]
so less of the filtered water is reabsorbed in the nephrons [1]
a larger volume of urine is produced and it is more dilute [1]
⚠ If you missed marks here: The chain has three links and all three are needed: the excess, the reabsorption, and the result. Answers that jump straight to “you produce more urine” describe the observation without explaining it, and an explain command needs a because for every mark.
(b)[3]
Evaluate the claim that drinking more water removes more waste from the body.
Model Answer — 7(b)
the claim is not supported for urea: the mass of urea excreted depends on how much is produced by deamination in the liver, which depends on protein intake, not on water intake [1]
drinking more water changes the concentration of the urine, not the amount of urea in it [1]
there is a limited sense in which it is true — excess water and excess ions themselves are excreted — but the website is describing waste products, and those are unaffected [1]
⚠ If you missed marks here: An evaluation is not a flat contradiction. The third mark is for acknowledging the part of the claim that is defensible before rejecting the part that is not, and that is a habit worth carrying into every evaluate question you meet.
(c)[4]
Describe an investigation the student could carry out to test the claim, stating what should be measured and what result would show that the claim is wrong.
Model Answer — 7(c)
collect all the urine produced over 24 hours on two separate days, one with a normal fluid intake and one with a much higher one [1]
measure the volume of urine and its urea concentration, and multiply them to obtain the total mass of urea excreted on each day [1]
keep the diet, especially the protein content, the same on both days, since that determines how much urea is produced [1]
the claim would be shown to be wrong if the total mass of urea excreted were about the same on both days, even though the volumes differed greatly [1]
⚠ If you missed marks here: The crucial design decision is measuring a mass rather than a concentration, because concentration alone would appear to support the claim in reverse. Notice too that the control — identical protein intake — is not optional here: without it, a difference in urea could be caused by the food rather than by the water.
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