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Question 1 — Naming the Plumbing, and the Word That Is Not Excretion
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 shows the urinary system of a human. Nothing on it has been named. Study it carefully before you begin.
(a)[4]
Name the structures labelled A, B, C and D on Fig. 1.1.
Model Answer — 1(a)
A — the kidney [1]
B — the ureter [1]
C — the bladder [1]
D — the urethra [1]
⚠ If you missed marks here: Ureter and urethra differ by one letter and cost a whole mark. Do not try to recall the spelling under pressure — read the position instead. The two tubes running down from the kidneys are ureters; the single tube leaving the underside of the bladder is the urethra. Counting works too: two ureters, one urethra.
(b)[3]
Define excretion, and explain why the removal of faeces from the body is not an example of it.
Model Answer — 1(b)
excretion is the removal from organisms of the waste products of metabolism [1]
and of substances in excess of requirements [1]
faeces consist of undigested food which was never absorbed into a cell and is not a product of metabolism, so its removal is egestion [1]
⚠ If you missed marks here: The definition has two halves and candidates routinely give only the first. The second half is what allows excess water and excess ions to count — they are not waste at all, there is simply more of them than the body needs. And an answer that lists faeces among excretory products tells an examiner that excretion and egestion have not been separated, which colours the marking of everything that follows.
(c)[2]
State the substance excreted through the lungs and state where in the body it is produced.
Model Answer — 1(c)
carbon dioxide [1]
produced by respiration in cells throughout the body, then carried to the lungs in the blood [1]
⚠ If you missed marks here: The lungs are the exit, not the source — exactly as the kidney is the exit for urea rather than the organ that makes it. The same error appears at both organs, so spotting the pattern once protects you twice.
(d)[3]
Explain the importance of excretion.
Model Answer — 1(d)
urea is produced continuously in the liver from excess amino acids [1]
urea is toxic [1]
if it were not removed its concentration in the blood would rise, damaging cells and interfering with the enzyme-controlled reactions of metabolism [1]
⚠ If you missed marks here: The syllabus limits this explanation to the toxicity of urea, so an answer about the body staying “clean” or being rid of “toxins” scores nothing. Name the substance, say it is toxic, then say what accumulation would do. Three sentences, three marks.
Question 2 — One Nephron, Three Sentences
Total: 12 marks
Fig. 2.1 shows one nephron with the blood vessels associated with it. The collecting duct has been named for you; nothing else has.
(a)[4]
Name the structures labelled W, X, Y and Z on Fig. 2.1.
Model Answer — 2(a)
W — the glomerulus, a knot of capillaries [1]
X — the capsule surrounding the glomerulus [1]
Y — the tubule of the nephron [1]
Z — the capillary network (blood capillaries) surrounding the tubule [1]
⚠ If you missed marks here: Z is the one people misname. It is a blood vessel, not part of the nephron, and blood and filtrate stay in two entirely separate systems all the way along — no vessel ever carries urine and no duct ever carries blood. If you wrote “collecting duct” for Z, notice that the figure had already named that structure for you.
(b)[3]
State what is filtered out of the blood at W, and state what remains in the blood. Explain why those substances remain.
Model Answer — 2(b)
filtered out: water, glucose, urea and ions [1]
remaining in the blood: blood cells and proteins [1]
because their molecules or cells are too large to pass through the filter [1]
⚠ If you missed marks here: Candidates leave glucose off the filtered list because it seems wrong for the body to discard something useful. The filter separates by size only and cannot recognise anything — which is precisely why a second, selective stage exists further along the nephron.
(c)[3]
State what is reabsorbed as the filtrate passes along Y, and where it goes.
Model Answer — 2(c)
all of the glucose [1]
some of the ions [1]
most of the water — all of it passing back into the blood in the capillaries around the tubule [1]
⚠ If you missed marks here: The three marks here are the words all, some and most, not the three substances. Each one explains an observation: all is why there is no glucose in urine, some is why there are still ions in it, and most is why 180 dm³ of filtrate becomes about 1.5 dm³ of urine. An answer without the quantity words scores one.
(d)[2]
State the composition of the liquid arriving in the collecting duct.
Model Answer — 2(d)
urea [1]
plus excess water and excess ions — that is, urine [1]
⚠ If you missed marks here: Three components, and the word excess belongs to two of them, because water and ions are not waste products — only the surplus is removed. An answer of “urea and water” drops the ions, and an answer of “waste substances” names nothing and scores nothing.
Question 3 — Five Rows, and Two Zeros That Are Not the Same Zero
Total: 12 marks
Table 3.1 shows the concentration of five substances, in g per 100 cm³, in the blood plasma entering a kidney, in the filtrate formed at the glomerulus, and in the urine leaving the kidney.
substance
blood plasma
filtrate
urine
water
90.00
90.00
95.00
protein
8.00
0.00
0.00
glucose
0.10
0.10
0.00
urea
0.03
0.03
2.00
ions
0.72
0.72
1.50
(a)[2]
Explain the values shown for protein.
Model Answer — 3(a)
protein molecules are too large to be filtered at the glomerulus, so the filtrate value is zero [1]
since none enters the nephron, none can appear in the urine, and the plasma concentration is unchanged as the blood leaves the kidney [1]
⚠ If you missed marks here: If you wrote that protein is reabsorbed, look at the filtrate column again: it reads 0.00, so there was never anything there to reabsorb. Protein is kept out by size at the very first step, and that is why protein appearing in urine means a damaged filter.
(b)[2]
Explain the values shown for glucose.
Model Answer — 3(b)
glucose is small enough to be filtered, so the plasma and filtrate values are identical at 0.10 [1]
all of it is then reabsorbed into the blood along the tubule, so the urine value is zero [1]
⚠ If you missed marks here: Quoting the two identical figures is worth a mark on its own and takes five seconds. The commonest wrong answer here is the size argument, which belongs to protein — and if you give it for glucose you will also give the wrong diagnosis when glucose turns up in a patient’s urine.
(c)[3]
The urea concentration in the urine is about 67 times that in the plasma. Explain how this is possible.
Model Answer — 3(c)
urea is filtered freely, so the plasma and filtrate values are the same at 0.03 [1]
urea is not reabsorbed along the tubule [1]
most of the water around it is reabsorbed, so the same amount of urea ends up dissolved in far less liquid and its concentration rises [1]
⚠ If you missed marks here: If your answer said the kidney adds urea to the filtrate, you have explained a change in concentration by inventing a change in amount. Nothing is added along the tubule; substances only move the other way. Separate amount from concentration before writing, and this question answers itself.
(d)[2]
The ions concentration rises about twofold, far less than the rise for urea. Explain why.
Model Answer — 3(d)
some of the ions are reabsorbed back into the blood, whereas none of the urea is [1]
so although both are concentrated by the reabsorption of water, the amount of ions remaining is smaller and the rise is much less steep [1]
⚠ If you missed marks here: This part is the quantity word some being tested as data. If every substance were treated identically by the nephron, every row would rise by the same factor — and the fact that they do not is the evidence that reabsorption is selective.
(e)[3]
A person produces 1.40 dm³ of urine in a day, with the urea concentration shown in Table 3.1. Calculate the mass of urea they excrete in that day. Show your working.
Model Answer — 3(e)
convert the concentration: 2.00 g per 100 cm³ = 20.0 g per dm³ [1]
mass = concentration × volume = 20.0 × 1.40 [1]
= 28.0 g [1]
⚠ If you missed marks here: The whole difficulty is the unit conversion, and an answer of 2.8 g or 2800 g means the factor of ten or of a thousand went the wrong way. Write the units next to every number as you go: g per 100 cm³, then g per dm³, then g. If the units do not cancel to give grams, the arithmetic is wrong however neat it looks.
Question 4 — What the Liver Does With an Amino Acid
Total: 12 marks
(a)[2]
Describe the role of the liver in the assimilation of amino acids.
Model Answer — 4(a)
amino acids absorbed from the small intestine reach the liver first, in the hepatic portal vein [1]
those the body needs are converted into proteins, including proteins found dissolved in the blood plasma [1]
⚠ If you missed marks here: Digestion, absorption and assimilation are three consecutive events with three names, and Cambridge sets questions that use one of them precisely. Assimilation is the point at which an absorbed molecule is built into the body; describing absorption instead answers a different question and scores nothing.
(b)[3]
Describe deamination and state where it takes place.
Model Answer — 4(b)
it takes place in the liver [1]
the nitrogen-containing part of excess amino acids is removed [1]
forming urea [1]
⚠ If you missed marks here: Three words carry the three marks: liver, nitrogen-containing part and urea. Writing “the kidney breaks down excess protein to make urine” gets all three wrong at once — wrong organ, wrong molecule and wrong product — and it is the most frequently seen version of this answer.
(c)[2]
State what happens to the remainder of the amino acid molecule after deamination.
Model Answer — 4(c)
the remainder is essentially a carbohydrate [1]
which is respired to release energy, or stored as glycogen [1]
⚠ If you missed marks here: Most candidates stop as soon as they have written the word urea and lose both marks here. Only the nitrogen-containing part is excreted — the carbon is kept, which is why a diet high in protein and low in carbohydrate can still maintain blood glucose.
(d)[2]
Explain why the breakdown of carbohydrates and fats does not produce urea.
Model Answer — 4(d)
carbohydrates and fats contain only carbon, hydrogen and oxygen; they contain no nitrogen [1]
so there is no nitrogen-containing part to remove, and they can be broken down completely to carbon dioxide and water [1]
⚠ If you missed marks here: This is a Topic 4 fact about elements being used as an explanation, and it is the cleanest way to show you understand why only protein leads to urea. An answer saying carbohydrates are “stored instead” misses the point: they are broken down too, they just leave nothing awkward behind.
(e)[3]
A patient has badly damaged liver cells but healthy kidneys. State and explain what would happen to the concentration of urea in their blood, and explain why this is not a sign of good health.
Model Answer — 4(e)
the blood urea concentration would fall [1]
because less deamination takes place, so less urea is formed, while healthy kidneys continue to remove what there is [1]
it is not a sign of health because excess amino acids are still arriving and cannot be stored, so the nitrogen from them is no longer being dealt with [1]
⚠ If you missed marks here: Kidney failure raises blood urea and liver failure lowers it, because the two organs sit on opposite sides of the same substance. Anyone who believes the kidney makes urea gets both cases wrong, so fix the sentence now: the liver makes urea, the kidney removes it, the bladder stores the urine.
Question 5 — Two People, the Same Amount of Urea
Total: 10 marks
On the same hot day two students of similar mass ate similar meals. Student A drank 3.0 dm³ of water; student B drank almost nothing and worked outdoors. Their urine over 24 hours was collected and analysed.
student
volume of urine / cm³
urea concentration / g per dm³
appearance
A
2200
0.90
very pale
B
350
5.60
dark
(a)[2]
Calculate the mass of urea excreted by each student. Show your working.
⚠ If you missed marks here: The concentration is given per dm³ and the volume in cm³, so the conversion has to happen before the multiplication. Multiplying 2200 by 0.90 gives an answer a thousand times too large, and an examiner cannot award a mark for a number that is out by that much even if the method was right.
(b)[2]
Comment on the two masses you have calculated.
Model Answer — 5(b)
the two masses are almost identical, differing by only about 0.02 g [1]
so the amount of urea excreted depends on how much was produced by deamination in the liver — that is, on protein intake — and not on how much water was drunk [1]
⚠ If you missed marks here: A “comment” question wants you to say what the numbers mean, not to repeat them. The whole point of this data set is the contrast: the amount of urea is fixed by the liver, while the volume of urine is fixed by how much water is in excess.
(c)[4]
Explain the difference between the two students in the volume and the concentration of their urine.
Model Answer — 5(c)
student A has drunk far more water than the body needs, so there is water in excess of requirements [1]
less water is reabsorbed in the nephrons, so a larger volume of urine is produced and it is dilute [1]
student B has lost water in sweat and has no excess [1]
so almost all of the filtered water is reabsorbed, leaving the same amount of urea in a much smaller volume, which is therefore concentrated and dark [1]
⚠ If you missed marks here: Four marks means both students explained, and both halves of each explanation given — the cause (excess or no excess) and the consequence (how much water is reabsorbed). Answers that describe only student B, or that say the kidney “concentrates” the urine without saying that water was reabsorbed, typically reach two.
(d)[2]
Water is not a waste product of metabolism. Explain why the kidneys remove it at all.
Model Answer — 5(d)
excretion includes the removal of substances in excess of requirements, not only waste products [1]
so only the surplus water is removed, and the amount removed varies with how much has been taken in or lost [1]
⚠ If you missed marks here: This is the half of the definition that candidates leave out, and it explains something the rest of the topic cannot: why urine volume changes from hour to hour while the urea content does not. Note also that respiration genuinely does produce some water, so a little of it qualifies under the first half as well.
Question 6 — In One Vessel, Out Another
Total: 12 marks
Table 6.1 compares blood entering a kidney with blood leaving the same kidney. All values are in arbitrary units.
substance
blood entering the kidney
blood leaving the kidney
urea
30
4
oxygen
19
13
carbon dioxide
40
46
glucose
90
88
plasma protein
75
75
(a)[2]
Name the vessel carrying blood into the kidney and the vessel carrying blood away from it, and name the two regions seen when a kidney is cut in half.
Model Answer — 6(a)
in by the renal artery, out by the renal vein [1]
the two regions are the cortex (outer) and the medulla (inner) [1]
⚠ If you missed marks here: Cortex outside, medulla inside — a pair of words that always keeps the same order. Note that the syllabus limits the internal structure of the kidney to these two regions, so a labelling question offering more lines than that is asking for the ureter and the blood vessels, not for a third region.
(b)[3]
Explain the change in the urea value shown in Table 6.1.
Model Answer — 6(b)
urea falls from 30 to 4 units, a fall of about 87 per cent [1]
because it is filtered out of the blood at the glomerulus [1]
and is not reabsorbed, so it leaves the body dissolved in the urine [1]
⚠ If you missed marks here: Quoting the two figures is a mark in its own right in almost every data question, and it costs five seconds. The reasoning mark is the phrase not reabsorbed — being filtered is not by itself enough, since glucose is filtered too and none of it is lost.
(c)[3]
Explain the changes in the oxygen and carbon dioxide values. State whether these changes are examples of excretion.
Model Answer — 6(c)
the cells of the kidney are respiring aerobically [1]
so they use oxygen (19 to 13) and release carbon dioxide (40 to 46) [1]
these changes are not excretion by the kidney — they are the ordinary gas exchange of a living tissue; carbon dioxide is excreted through the lungs [1]
⚠ If you missed marks here: Treating every row of a renal artery and renal vein table as excretion is the commonest whole-question error in this topic. Before writing, sort each row into one of two piles: the special job of the organ, and ordinary respiration by its cells. That sort works for any organ, not just this one.
(d)[2]
Explain why the glucose value falls only slightly while the plasma protein value does not change at all.
Model Answer — 6(d)
glucose is filtered but all of it is reabsorbed, so almost none is lost; the small fall from 90 to 88 is glucose used in respiration by the kidney cells [1]
plasma protein does not change because its molecules are too large to be filtered and never leave the blood at all [1]
⚠ If you missed marks here: Two rows that look almost the same and have opposite mechanisms. An answer that gives one reason for both loses a mark, and it is the mark that separates a candidate who has learned the list from one who has understood the two stages.
(e)[2]
Suggest why the blood leaving the kidney still contains some urea rather than none at all.
Model Answer — 6(e)
only a proportion of the blood passing through the kidney is filtered on each pass, so some urea travels straight through in the blood [1]
this is sufficient because the blood circulates repeatedly, and the kidney only has to remove urea as fast as the liver produces it [1]
⚠ If you missed marks here: A suggest question expects reasoning from what you know rather than a recalled fact. The key idea is that the kidney is not a perfect trap and does not need to be — it needs only to balance a rate of removal against a rate of production, which is the same framing that explains why blood urea plateaus on a high-protein diet.
Question 7 — Four Patients and One Newspaper Headline
Total: 10 marks
Table 7.1 shows results for four patients. The normal range for blood urea is 25–35 arbitrary units.
patient
blood urea / units
protein in urine
glucose in urine
blood glucose
P
64
absent
absent
normal
Q
8
absent
absent
normal
R
30
present
absent
normal
S
31
absent
present
normal
(a)[4]
Suggest, for each of the four patients, where the problem lies. Give a reason in each case.
Model Answer — 7(a)
P: the kidneys — urea is still being made by the liver but is not being filtered out and excreted, so it accumulates at 64 units [1]
Q: the liver — less deamination means less urea is formed, and healthy kidneys continue to remove what there is, giving 8 units [1]
R: the filtration barrier at the glomerulus is damaged, because protein is normally too large to be filtered [1]
S: reabsorption of glucose is failing, because glucose is normally filtered and all of it reabsorbed, and the normal blood glucose rules out an excess arriving [1]
⚠ If you missed marks here: Every one of these four is a failure at exactly one of three stages — producing urea in the liver, filtering at the glomerulus, or reabsorbing along the tubule. Name the stage first and the reason follows. Notice too that the blood glucose column is in the table for a reason: it removes the alternative explanation for patient S.
(b)[3]
A newspaper reports that people eating a high-protein diet have blood urea concentrations 40 per cent above average, and concludes that high-protein diets damage the kidneys. Evaluate this conclusion.
Model Answer — 7(b)
the data show a correlation between protein intake and blood urea [1]
but a raised blood urea is exactly what would be expected in healthy people eating more protein, because more amino acids are in excess and are deaminated — so it is not evidence of damage [1]
other factors may also differ between the two groups, for example how much water they drink or how much exercise they take, and the study as described does not control for them [1]
⚠ If you missed marks here: An evaluation that simply denies the data is as weak as one that accepts the conclusion. The correlation is real; what fails is the inference. Say what the evidence does support before you say what it does not, and you will pick up the first mark before the argument even starts.
(c)[3]
Suggest one measurement that would provide better evidence about whether the kidneys of these people were damaged, and explain why it would be better.
Model Answer — 7(c)
test the urine for protein [1]
protein is normally too large to be filtered, so it should be completely absent from healthy urine [1]
its presence would therefore show that the filtration barrier itself was damaged, which a blood urea reading cannot show because urea is raised by diet alone [1]
⚠ If you missed marks here: The strongest answers to “suggest a better measurement” questions name a measurement whose normal value is zero, because any reading at all is then meaningful. Protein in urine is exactly that. Accept also a suggestion to measure both groups before and after the diet change, provided you explain that it separates the diet from any pre-existing difference.
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