This topic contains less machinery than any other in the syllabus, and that is exactly what makes a challenge paper on it difficult: there is nothing to hide behind. Every mark comes down to using one word rather than another, or to reading a set of numbers in the right order. Faeces are egested, never excreted. The liver makes urea; the kidney only removes it. Glucose is filtered and then taken back; protein is never filtered at all. A urea concentration rises because water left, not because urea arrived. And a raised blood urea points at the kidneys while a lowered one points at the liver. Twelve traps, six walkthroughs, six lookalike pairs, a concept map and ten full practice questions below — every one aimed at a place where a sensible-sounding sentence earns nothing.
Twelve traps that cost marks on Topic 13 challenge papers. Every one is an answer that sounds right and that mark schemes refuse.
Six challenge-level questions worked through in the order you should actually think about them. Try each part before revealing the next step.
X is the only row that reads zero in the filtrate. That means it never got through the filter at all, and the only substance here that is too large to be filtered is protein. Notice how much work that single cell does: it identifies the row without you looking at anything else.
Y reads 0.10, 0.10, 0.00. It passed the filter unchanged and then disappeared, which is the fingerprint of complete reabsorption. That is glucose — all of it is taken back. If you had learned only “neither glucose nor protein appears in urine” you could not have separated X from Y, which is exactly why the question is set this way.
W rises from 0.03 to 2.00, a factor of about 67. Z rises from 0.72 to 1.50, roughly double. W is not reabsorbed at all, so it is concentrated by everything the water reabsorption can do: W is urea. Z is partly reabsorbed — some of the ions go back — so it rises far less: Z is ions. The three quantity words are doing the identification for you.
The plasma column tells you almost nothing — everything is present there. The filtrate column separates protein from everything else, and the urine column then separates the fully reabsorbed from the partly reabsorbed from the not reabsorbed at all. Two columns, four identifications, no memory required.
2200 cm³ = 2.20 dm³ and 350 cm³ = 0.35 dm³, because there are 1000 cm³ in a dm³. The concentration is given per dm³, so the volume must be in dm³ too. Converting before you reach for the calculator is the whole of the first mark.
Person A: 0.90 × 2.20 = 1.98 g. Person B: 5.60 × 0.35 = 1.96 g. Write both to the same number of decimal places; a comparison question is easier to mark and easier to think about when the figures line up.
The masses differ by 0.02 g, about one per cent — effectively the same. The amount of urea excreted depends on how much the liver made, which depends on the protein each person ate, and has almost nothing to do with how much they drank. What water intake changes is the volume it is dissolved in, and therefore its concentration.
Person A has drunk far more water than the body needs, so there is a large excess; less water is reabsorbed along the nephron and a large volume of dilute urine is produced. Person B has lost water in sweat and has no excess at all, so most or nearly all of the filtered water is reabsorbed and only a small volume of concentrated urine leaves. The urea still has to go either way, so in B it goes out in very little liquid and the urine is dark.
Every organ does two things to blood: whatever it is for, and using oxygen because it is made of living cells. Urea belongs in the first pile. Oxygen and carbon dioxide belong in the second. Doing this sort before writing stops the commonest error in the question, which is explaining the oxygen row as excretion.
Urea is filtered out of the blood at the glomerulus and is not reabsorbed, so it leaves in the urine and the blood flowing out contains much less of it. Quote the figures: a fall from 30 to 4 units is very often a mark on its own. Note also that it does not fall to zero, because not all the blood passing through is filtered in a single pass.
Kidney cells respire aerobically, so they use oxygen (19 to 13) and release carbon dioxide (40 to 46). Nothing is being excreted here. In fact the carbon dioxide has gone up, which is worth saying out loud, because it is the one row where the blood leaves with more of something than it arrived with.
Glucose falls very slightly, from 90 to 88, because it is filtered but all of it is reabsorbed; the small fall is the glucose respired by the kidney cells themselves. Protein does not change at all because it is too large to be filtered and never leaves the blood. An answer that gives one reason for both rows loses a mark, and it is a mark most candidates lose.
“The blood urea concentration rises steeply from 30 units on day 0 to 48 units on day 4, then levels off at about 50 to 52 units from day 6 onwards.” That is two marks: one for the direction and shape, one for quoting figures with their days. A description that contains no numbers usually scores half.
More protein is digested, so more amino acids are absorbed and carried to the liver in the hepatic portal vein. The body still needs the same number for building proteins, so a much greater proportion is in excess — and amino acids cannot be stored. The surplus is therefore deaminated in the liver, forming more urea, which is released into the blood.
The amount of urea filtered out each minute depends on the concentration in the blood arriving at the kidney. As the concentration rises, more urea is filtered and excreted per minute. Eventually the rate of removal equals the rate of production, and the concentration stops changing — a new steady level, higher than before but stable.
Only the nitrogen-containing part of each amino acid becomes urea. What is left is essentially a carbohydrate, which is respired to release energy or stored as glycogen. So blood glucose is maintained, or rises slightly, even though the diet contains no extra carbohydrate. Most candidates stop as soon as they have written the word urea and lose both marks here.
There are only three stages in this topic where something can go wrong: production of urea in the liver, filtration at the glomerulus, and reabsorption along the tubule. Every one of these four patients is a failure of exactly one of them, and naming the stage first makes each answer almost write itself.
P: urea is being produced normally but not removed, so the kidneys are failing. Q: the kidneys are removing it normally, so less must be arriving — less urea is being made, which points at the liver. This pair is set together deliberately, because a candidate who thinks the kidney makes urea will give the same answer twice.
R: protein is normally never filtered, so finding it means the filter at the glomerulus is damaged and is letting large molecules through. S: glucose is normally filtered and then completely reabsorbed, and the blood glucose is normal, so the fault must be in reabsorption. The urea figures for both are normal, which tells you the rest of the kidney is fine.
None of these observations identifies a disease, and you are not expected to name one. Each answer needs the stage plus the evidence from the numbers. For S there is a second acceptable answer — that blood glucose had been too high for all of it to be reabsorbed — but the stem rules it out by telling you the blood glucose is normal, and noticing that is worth a mark in itself.
Desert: 4 cm³ = 0.004 dm³; 24 × 0.004 = 0.096 g. Semi-aquatic: 62 cm³ = 0.062 dm³; 1.4 × 0.062 = 0.087 g. Again the two masses are similar, and again the difference is entirely in the volume of water the urea is dissolved in.
Urea is filtered and not reabsorbed in either animal. The desert species reabsorbs a greater proportion of the filtered water back into the blood, so the same amount of urea is left in a much smaller volume of liquid, giving a far higher concentration. That is the complete answer, and it does not require a single fact beyond the three sentences of 13.3.
It conserves water, which is scarce in a desert, so the animal loses much less water while still excreting the urea it must get rid of. “It is adapted to the desert” restates the question and scores nothing; naming the substance being saved is the mark.
No protein means the filter in both species is intact, holding back molecules that are too large. No glucose means reabsorption is complete in both species — all the filtered glucose is being recovered. Two separate conclusions from two separate mechanisms, and a question that says “explain what this tells you” expects both.
Six pairs that look almost identical and have different answers. The distinction is where the marks live.
Click each node to see how the whole topic connects into one story: something is made that cannot be kept, a machine removes it in two stages, and a different organ made it in the first place.
Six real student answers. Find the fault before you reveal it.
Ten Cambridge-style challenge questions. Write your answer first, then reveal the model answer and the examiner’s notes.