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This paper covers the whole of Topic 16. Like a real Cambridge paper it ranges across every sub-topic — 16.1 asexual reproduction, 16.2 sexual reproduction, gametes and fertilisation, 16.3 sexual reproduction in plants, 16.4 sexual reproduction in humans, 16.5 sexual hormones and the menstrual cycle, and 16.6 sexually transmitted infections — and it mixes them inside single questions. All three Topic 16 papers do; they differ in the angle they come at it from, not in what they cover.
Question 1 — One Tube, Two Jobs
Total: 12 marks
Fig. 1.1 shows the human male reproductive system seen from the side. Nothing has been named for you. Work from the position and the shape of each structure. One of the labelled structures is not part of the reproductive system at all.
(a)[3]
Name the structures labelled B, C and E on Fig. 1.1.
Model Answer — 1(a)
B — the prostate gland [1]
C — a sperm duct [1]
E — a testis (one testis, two testes) [1]
⚠ If you missed marks here: The commonest loss is writing scrotum for E. The scrotum is the bag of skin, labelled F; the testis is the organ inside it that actually makes the sperm and the hormone testosterone. Second trap: C and D are both tubes and they are easy to swap. C runs from the testis and joins the tube coming down from the bladder; D is the single tube that carries everything out through the penis. Finally, “testicle” is everyday English — the mark scheme wants testis.
(b)[2]
Sperm are made in the structure labelled E. Using only the letters on Fig. 1.1, write down in order the structures a sperm passes along or through, from where it is made until it leaves the body.
Model Answer — 1(b)
E → C → D → G
route begins at E (testis) and passes next into C, the sperm duct [1]
then D (the urethra) and out through G (the penis), in that order, with no other letters included [1]
(B, the prostate gland, adds fluid to the sperm to make semen, but the sperm do not travel through the prostate gland, so it is not on the route)
⚠ If you missed marks here: Almost everyone who loses this mark puts A in the route. Sperm never enter the bladder. What is shared is the urethra, not the bladder — the sperm duct joins the urethra below the bladder, and a ring of muscle closes the bladder off while semen is passing. If you added B, re-read the question: it asks what the sperm passes along or through, and the prostate gland empties into the tube rather than carrying the sperm.
(c)[3]
Name the structure labelled A. State which body system it belongs to, and explain why it is drawn on a diagram of the male reproductive system even though it is not part of that system.
Model Answer — 1(c)
A is the bladder [1]
it belongs to the excretory (urinary) system — it stores urine, which is a waste product, not a gamete [1]
it is drawn because in the male the two systems share one tube: the urethra, D, carries urine at some times and semen at others, so you cannot show where the sperm duct joins the urethra without showing the bladder above it [1]
(the two never travel together — a ring of muscle closes the bladder off while semen passes)
⚠ If you missed marks here: “It is drawn because it is nearby” earns nothing — the examiner wants the shared urethra. A second common slip is saying the bladder stores urine and semen; it does not, and semen is not stored in the urethra either. Worth noticing for later: in the female the tube carrying urine and the tube of the reproductive system are completely separate, so the female diagram does not need the bladder in the same way.
(d)[4]
After leaving G, a sperm has to travel through the female reproductive system as far as the oviduct, where fertilisation may take place. Explain how three adaptive features of a sperm cell suit it to that journey. Then state whether the nucleus of a sperm is haploid or diploid, and explain why that matters.
Model Answer — 1(d)
it has a flagellum, which beats and drives the cell forward, so the sperm can swim the whole length of the uterus and into the oviduct [1]
it has many mitochondria packed in behind the head, where aerobic respiration releases the energy the flagellum needs to keep beating for hours [1]
it has an acrosome containing enzymes that digest a path through the jelly coat of the egg cell so the sperm nucleus can reach the egg nucleus [1]
the nucleus is haploid; it fuses with the equally haploid nucleus of the egg cell, so the nucleus of the zygote is diploid — the normal number is restored rather than doubled in every generation [1]
(accept also, for one of the three features: very small and streamlined, so it moves quickly and cheaply)
⚠ If you missed marks here: Three separate traps. Tail is not the word — Cambridge wants flagellum. Naming mitochondria is only half a mark’s worth of thinking: say that respiration in them releases energy for the flagellum. And the acrosome does not “break the egg open” or let the sperm “eat through” — it releases enzymes that digest the jelly coat. Last, haploid describes the nucleus; a sperm is not “half a cell”.
Question 2 — Four Curves, No Names
Total: 12 marks
Fig. 2.1 shows the events of one 28-day menstrual cycle. The four hormones have been replaced by the letters P, Q, R and S, and two structures in the ovary have been labelled V and W. Everything you need in order to identify them is in the shapes of the curves and in what is happening above and below them.
(a)[4]
Name the hormones P, Q, R and S.
Model Answer — 2(a)
P is FSH — it is high early in the cycle, which is when the follicle V is growing, and FSH is the hormone that makes a follicle develop [1]
Q is LH — the single sharp spike on day 14 is the giveaway; nothing else in the cycle has that shape, and day 14 is ovulation [1]
R is oestrogen — it climbs through the first half as the follicle grows, and the uterus lining thickens underneath it [1]
S is progesterone — it is low in the first half and only rises after day 14, exactly matching the appearance of W, the corpus luteum, and it falls away at the end of the cycle just before the lining is lost [1]
⚠ If you missed marks here: The usual mix-up is P and Q, because both come from the pituitary gland and both are drawn on the same track. Identify them by shape, not by position: LH is a single narrow spike at ovulation, FSH is a broad rise early on with a small bump at day 14. The other frequent slip is swapping R and S: if a hormone is essentially absent for the first half of the cycle it cannot be oestrogen, because the lining is already being rebuilt in those days.
(b)[2]
State where in the body hormone P is produced, and state where hormone S is produced during the second half of the cycle shown in Fig. 2.1.
Model Answer — 2(b)
P (FSH) is produced by the pituitary gland, in the brain [1]
S (progesterone) is produced by the corpus luteum — the structure labelled W — in the ovary [1]
(if the embryo implants, the corpus luteum keeps producing progesterone, and later in pregnancy the placenta takes over as the main source)
⚠ If you missed marks here: At Extended level “the ovary” on its own is not enough for progesterone — the question asks about the second half of the cycle, and the mark is for the corpus luteum. The other error is putting FSH in the ovary. FSH and LH come from the pituitary gland and travel in the blood to the ovary; oestrogen and progesterone travel in the opposite direction.
(c)[3]
Between day 5 and day 13 the concentration of R rises steadily. Explain how that rise leads to the sharp peak in Q on day 14, and describe what happens in the ovary as a result.
Model Answer — 2(c)
the rising concentration of R (oestrogen) reaches a high level that stimulates the pituitary gland to release a surge of Q (LH) [1]
the surge of LH causes ovulation — an egg cell is released from the follicle V [1]
what is left of the follicle then develops into W, the corpus luteum, which begins to secrete S (progesterone) [1]
(oestrogen has also been repairing and thickening the lining of the uterus through those same days, which is why the lining track is climbing)
⚠ If you missed marks here: Two things get confused. Oestrogen does not release the egg cell itself — it triggers the LH surge, and it is LH that causes ovulation. And FSH is not the ovulation hormone either, even though it started the follicle growing. Also be precise about the order: the corpus luteum forms after ovulation, out of the leftover follicle, which is why progesterone can only rise in the second half.
(d)[3]
Now reason in the opposite direction. Predict what would happen to the rest of the cycle shown in Fig. 2.1 if the structure labelled W failed to form after day 14. Give three effects and explain each one.
Model Answer — 2(d)
no corpus luteum means almost no S (progesterone), so the lining of the uterus is not maintained: it breaks down early and bleeding starts well before day 28 [1]
progesterone normally inhibits P and Q; without it FSH rises again straight away, a new follicle starts to develop earlier and the whole cycle becomes shorter [1]
if the egg cell released on day 14 were fertilised, the embryo could not implant in a lining that is breaking down, so a pregnancy could not begin or be maintained [1]
⚠ If you missed marks here: The most common wrong prediction is “no egg would be released”. Look at the order of events on Fig. 2.1: ovulation happens on day 14, before W exists, so removing W cannot stop it. A prediction question is marked on the reasoning, not the guess — each effect needs the word “because” and a named hormone. Remember too that progesterone works in two directions at once: forwards on the uterus lining, backwards on the pituitary gland.
Question 3 — From Anther to Seedling
Total: 12 marks
A student spent a summer watching a patch of a wild flowering plant, then collected its seeds and set up an investigation with them. Questions (a) to (d) follow that plant through one complete life cycle.
(a)[3]
The student watched a bee crawl into one flower, pick up pollen grains and carry them to a second flower, where they stuck to the stigma. He wrote: “Fertilisation has now happened.”
Explain what is wrong with that statement, and describe what must happen next before fertilisation can occur.
Model Answer — 3(a)
what the bee did was pollination — the transfer of pollen grains from an anther to a stigma [1]
fertilisation is a different event: the fusion of a pollen nucleus with a nucleus in an ovule [1]
(the two nuclei that fuse are the nuclei of the two gametes, and both are haploid, so the nucleus of the zygote formed is diploid — the definition of fertilisation is the same in a plant as it is in a human)
before it can happen a pollen tube must grow out of the pollen grain, down through the style and into the ovary, entering the ovule and carrying the pollen nucleus to the nucleus inside it [1]
⚠ If you missed marks here: This is the single most examined confusion in the whole topic, so learn the two definitions as a pair. Pollination is a journey of pollen grains; fertilisation is the fusion of nuclei. A stigma can be covered in pollen and nothing has been fertilised at all. Notice also that the pollen grain itself does not travel down the style — the grain stays on the stigma and grows a tube down it.
(b)[2]
The two flowers the bee visited were on different plants of the same species. Name this type of pollination and explain one advantage of it to a wild population of this plant.
Model Answer — 3(b)
cross-pollination [1]
the offspring are genetically different from one another, so there is more variation in the population; if the environment changes — a new disease, a colder winter — some individuals are more likely to have features that let them survive and reproduce [1]
(the trade-off: cross-pollination relies on a pollinator being present, whereas self-pollination does not)
⚠ If you missed marks here: Self-pollination does not produce clones — that is a very common and costly error. It is still sexual reproduction, still a fusion of nuclei, so the offspring are still genetically different from each other; there is simply less variation than with cross-pollination. Answers that stop at “more variation” usually score nothing: say what the variation is for.
(c)(i)[3]
The student collected the seeds and set up five tubes, each holding 20 of them, for seven days. Table 3.1 shows how he set the tubes up. The results are not given to you.
Tube
Water
Oxygen
Temperature / °C
Light
A
added
present
20
on a lit windowsill
B
added
present
20
total darkness
C
none, kept dry
present
20
on a lit windowsill
D
added, but boiled first and then covered with a layer of oil
removed
20
on a lit windowsill
E
added
present
2
on a lit windowsill
Tube A is the control and 19 of its 20 seeds germinated. Predict the result for tubes C, D and E, choosing for each one either about 0 to 2 seeds or about 18 to 20 seeds, and give the biological reason for each prediction.
Model Answer — 3(c)(i)
tube C: about 0 to 2 — the seeds have no water, and water is needed to soften the seed coat, to activate the enzymes that break down the food store, and as the solvent everything inside the seed is transported in [1]
tube D: about 0 to 2 — boiling drives dissolved oxygen out of the water and the oil layer stops any more dissolving back in, so the seeds cannot respire aerobically [1]
tube E: about 0 to 2 — 2 °C is not a suitable temperature; the enzymes inside the seed work far too slowly at that temperature for the seed to mobilise its food store and grow [1]
⚠ If you missed marks here: Predicting is only half of it — the marks are for the reason, and each reason must name the condition that has been removed. Two wording traps. Say a suitable temperature, not “warmth”: too hot would denature the enzymes, so hotter is not automatically better. And it is oxygen, not “air”, that the mark scheme wants; air is a mixture and the examiner is asking for the gas. At 2 °C the enzymes are not denatured, they are simply too slow — do not write that the cold kills the seed.
(c)(ii)[2]
Before running the investigation the student predicted that tube B, kept in total darkness, would give a result of about 0 seeds. Evaluate that prediction.
Model Answer — 3(c)(ii)
the prediction is wrong: tube B differs from the control only in having no light, and light is not one of the conditions needed for germination, so tube B should give a result close to tube A, about 18 to 20 seeds [1]
a germinating seed is living on the food store inside it and is not photosynthesising, so it has no use for light yet; light only matters once the shoot is above the soil with green leaves. In the wild almost every seed germinates buried in the dark [1]
⚠ If you missed marks here: Light is the classic wrong fourth condition, and it comes from confusing the needs of a seed with the needs of a green plant. “Evaluate” asks for a verdict plus the reasoning behind it, so “he is wrong” on its own scores nothing — say what tube B is actually testing and what the seed is living on in the meantime.
(d)[2]
Each of these seeds contains a food store. Explain what a germinating seed uses that food store for, and explain why it cannot replace the food store itself until the shoot has reached the surface of the soil.
Model Answer — 3(d)
the food store is respired to release energy for cell division, growth and the active transport of substances into the cells, and it also supplies the raw materials the seedling builds its new cells out of [1]
it cannot replace that food by photosynthesis until the shoot is above the soil, because underground there is no light and the seedling has no green leaves containing chlorophyll to absorb it — which is why a seed needs a food store big enough to reach the surface on [1]
⚠ If you missed marks here: A dry seed looks inert, so it is easy to forget it is a living organism that respires. Say what the released energy is used for — “for energy” is circular and scores nothing, and respiration does not “make” energy, it releases energy already stored in the food. The second mark is the pay-off from (c)(ii): the seedling is in a race to reach the light before its food store runs out, which is precisely why light is needed after germination and not during it.
Question 4 — Fast, Until the Year the Fungus Arrived
Total: 12 marks
A biologist counted two wild populations of the same plant species on two islands for six years. Population J reproduces only by runners — side shoots that root where they touch the ground and become new plants. Population K reproduces only by seeds. In year 3 a fungal disease reached both islands for the first time. Nothing else about the two islands differed.
Table 4.1
Year
1
2
3
4
5
6
Number of plants in population J
400
910
1850
240
60
20
Number of plants in population K
380
520
690
610
640
780
(a)[3]
Describe and explain the difference between the two populations, both before and after the disease arrived. Use figures from Table 4.1 in your answer.
Model Answer — 4(a)
before year 3, J increased far faster than K (400 to 1850, against 380 to 690) because asexual reproduction is rapid and needs no pollinator, no gametes and no fertilisation — one parent is enough [1]
after the disease arrived J collapsed, from 1850 to 20 by year 6, because every plant in J is genetically identical to its parent; if one plant has no resistance to the fungus, none of them has [1]
K kept increasing, to 780 by year 6, because its plants are genetically different from one another; some already had features that let them survive the fungus, and those plants survived and reproduced [1]
⚠ If you missed marks here: Watch the wording carefully: the plants in K did not “become resistant because the fungus arrived”. The variation was already there before year 3 — the fungus simply removed the plants that did not have it. Answers that only describe the shapes of the two lines score at most one mark; the marks are for the explanation, and the phrases the examiner is looking for are “genetically identical” and “genetically different”.
(b)[2]
A gardener reads the study and says: “This proves asexual reproduction is always better, because you get far more plants far faster.”
Evaluate this statement using Table 4.1 and Fig. 4.1.
Model Answer — 4(b)
he is right for years 1 to 3: J reached 1850 plants while K reached only 690, so asexual reproduction really is faster while conditions stay the same [1]
but “always” is wrong: after the disease J fell to 20 while K rose to 780, so the advantage of speed lasts only while the environment does not change — the same feature that makes it fast, identical offspring, is what destroys it [1]
⚠ If you missed marks here: An “evaluate” question needs both sides and a verdict, with figures. Answers that only disagree score one mark at most, because part of the statement is true and you have to say which part. The word to attack is always — a single word like that turns a reasonable claim into one you can dismantle with one counter-example.
(c)[3]
Now think about a farmer rather than a wild population. State one advantage and one disadvantage of asexual reproduction to crop production, and one advantage of sexual reproduction to crop production.
Model Answer — 4(c)
advantage of asexual reproduction: every plant is genetically identical to a parent chosen for desirable features, so the crop is uniform in size and quality, ripens at the same time and can be harvested in one operation [1]
disadvantage of asexual reproduction: there is no variation, so a single new disease or pest can destroy the entire crop, exactly as happened to population J [1]
advantage of sexual reproduction: the offspring are genetically different, so new varieties can be produced and the ones with, for example, disease resistance or a higher yield can be selected and grown [1]
⚠ If you missed marks here: The point that catches people out is that uniformity is an advantage to a farmer, even though it was a fatal weakness for the wild population in (a). The same biological fact reads differently depending on who is asking, which is precisely why Cambridge splits this objective into “to a population in the wild” and “to crop production”. “It is faster” is a weak answer here — the farmer cares about the crop, not the population size.
(d)[2]
In population K a nucleus from a pollen grain fuses with a nucleus in an ovule. State whether these two nuclei are haploid or diploid, and name the cell that is formed.
Model Answer — 4(d)
both are gamete nuclei, so both are haploid [1]
the cell formed is a zygote, and the nucleus of the zygote is diploid [1]
⚠ If you missed marks here: Haploid and diploid describe the nucleus, not the whole cell and certainly not “half a cell” — a pollen grain and an egg cell are complete cells with complete cytoplasm. Also, the cell formed is a zygote: not an embryo, not a seed. The embryo comes later, when the zygote has divided.
(e)[2]
Explain why the new plants in population J are genetically identical to their parent, while those in population K are not.
Model Answer — 4(e)
J reproduces asexually: there is one parent, no gametes and no fusion of nuclei, so each new plant is genetically identical to that parent [1]
K reproduces sexually: the nuclei of two gametes fuse, and the two gametes come from different parents, so every offspring is genetically different from the others and from its parents [1]
⚠ If you missed marks here: The marking phrase for asexual reproduction is no gametes and no fusion of nuclei — write it every time. A surprising number of answers describe runners as producing “seeds without pollen”, which is not asexual reproduction at all. And “the same” is not the same as genetically identical: two identical plants grown in different soils will not look the same, but they are still genetically identical.
Question 5 — Seven Days, Two Places
Total: 10 marks
Table 5.1 records the first twelve days of human development. Two entries in the last column have deliberately been left blank.
Time after fertilisation
What can be seen
Where it is
0 hours
the nucleus of a sperm fuses with the nucleus of an egg cell
not shown
30 hours
the single cell has divided into two cells
oviduct
3 days
a ball of about 16 cells
oviduct
4 days
a ball of about 100 cells arrives
uterus
7 days
the ball of cells sinks into the lining and becomes attached to it
not shown
12 days
the placenta begins to form
uterus
(a)[3]
Name the process taking place at 0 hours, name the cell it produces, and name the structure in which it takes place. Justify your choice of structure using Table 5.1.
Model Answer — 5(a)
the process is fertilisation — the fusion of the nuclei of a male gamete (sperm) and a female gamete (egg cell) [1]
it produces a zygote [1]
it takes place in the oviduct; the table shows the cell is already in the oviduct only 30 hours later, and it is still there at 3 days, so it cannot have started in the uterus [1]
⚠ If you missed marks here: Cambridge wants oviduct; “fallopian tube” is the everyday name and is not credited. Two more traps: fertilisation is the fusion of nuclei, not “the sperm going into the egg” — the sperm entering is only the step before it. And the cell produced is a zygote, not an embryo; the embryo is the ball of cells the zygote later becomes.
(b)[3]
Using Table 5.1, state what the ball of cells is called by day 4. Name the event happening at 7 days and complete the missing entry in the last column for that row. Then name the hormone that has kept the lining of the uterus thick and ready, and state where that hormone is being made at day 7.
Model Answer — 5(b)
by day 4 the ball of cells is an embryo [1]
at 7 days the event is implantation, and it happens in the lining of the uterus [1]
the hormone is progesterone, and at day 7 after fertilisation it is still being made by the corpus luteum in the ovary (the placenta only takes over later) [1]
⚠ If you missed marks here: The precise phrase is “implants into the lining of the uterus” — not “the wall”, and not “the uterus” on its own. On the hormone, oestrogen is the one that rebuilds the lining in the first half of the cycle; progesterone maintains it afterwards, and that is the one that matters here. Saying “the placenta” is a full week too early: Table 5.1 shows the placenta only starts to form on day 12.
(c)[2]
A student wrote: “The zygote travels to the oviduct and implants there, and then grows into an embryo.”
Use Table 5.1 to identify two things that are wrong with that sentence, correcting each one.
Model Answer — 5(c)
implantation does not happen in the oviduct: the table shows the ball of cells reaches the uterus at 4 days and only sinks into the lining of the uterus at 7 days [1]
it is not the zygote that implants: by day 7 the zygote has divided many times into a ball of about 100 cells, which is the embryo. The zygote is also formed in the oviduct, so it does not “travel to” it — it travels from it, towards the uterus [1]
⚠ If you missed marks here: This is the most common sequence error in the topic, and the fix is to read the “where it is” column as a journey: oviduct → uterus, one direction only. Notice too that the order of the words “implants… then grows into an embryo” is back to front — the embryo exists before implantation, which is why there is something solid enough to sink into the lining.
(d)[2]
Predict what would happen if the ball of cells arrived in the uterus at 4 days exactly as Table 5.1 describes, but the lining of the uterus was thin and poorly supplied with blood. Explain your answer.
Model Answer — 5(d)
the embryo would fail to implant, or would not stay attached: there is not enough lining for it to sink into and become anchored in [1]
and even if it did attach, a lining with few blood vessels could not supply enough dissolved nutrients and oxygen to the embryo, and could not carry its waste away, so it could not grow and the pregnancy would not continue — the placenta that forms at 12 days needs a rich blood supply on the mother’s side to exchange with [1]
⚠ If you missed marks here: It is easy to treat the thick lining as decoration. It is not — it is the site of implantation and the mother’s half of the future exchange surface, which is why the cycle spends two weeks building it before an egg cell is even released. A prediction with no “because” in it will not score, and “the baby would be unhealthy” is too vague: name what is missing (nutrients, oxygen, a place to attach).
Question 6 — A Barrier That Lets Some Things Through
Total: 12 marks
Table 6.1 shows the concentrations of five substances measured at the placenta of a healthy pregnancy: in the mother’s blood in the spaces around the placental villi, and in the fetal blood arriving along the umbilical cord. The two bloodstreams are separated by a thin barrier and never mix.
Substance
In the mother’s blood at the placenta
In the fetal blood arriving at the placenta
oxygen / arbitrary units
98
40
glucose / mg per 100 cm³
90
72
carbon dioxide / arbitrary units
41
56
urea / mg per 100 cm³
26
39
amino acids / arbitrary units
28
46
(a)[3]
Use Table 6.1 to state the direction of the net movement of oxygen, of carbon dioxide and of urea across the placenta. Name the process by which all three move, and explain what decides the direction.
Model Answer — 6(a)
oxygen moves from the mother’s blood to the fetal blood (98 against 40) [1]
carbon dioxide and urea both move from the fetal blood to the mother’s blood (56 against 41, and 39 against 26) [1]
all three move by diffusion, and the direction is decided by the concentration gradient: net movement is always from the higher to the lower concentration, and no energy from respiration is needed [1]
⚠ If you missed marks here: The direction is set by the concentration gradient, never by “what the fetus needs” — molecules do not know what anyone needs. Watch urea especially: it is an excretory product the fetus itself has made, and because a fetus cannot pass urine out of the uterus usefully, the urea has to cross to the mother, whose kidneys remove it. Losing the word net is also expensive: particles move both ways, and it is the balance that counts.
(b)[2]
Amino acids are at a higher concentration in the fetal blood than in the mother’s blood, and yet amino acids continue to move from the mother to the fetus. Explain how this is possible.
Model Answer — 6(b)
they are being moved against the concentration gradient, from a lower to a higher concentration, so this cannot be diffusion — it is active transport [1]
active transport uses protein carriers in the cell membranes of the placenta and energy released by respiration in those cells [1]
⚠ If you missed marks here: Diffusion cannot move anything up a gradient, ever, so the moment a table shows movement towards the higher concentration you should be writing active transport. The second mark needs both halves: carrier proteins and energy from respiration. “The blood pushes them across” and “the placenta absorbs them” are not explanations.
(c)[2]
From about week 12 of pregnancy the placenta takes over from the corpus luteum as the main source of one hormone. Name that hormone and predict, with a reason, what would happen to the pregnancy if the placenta failed to produce it.
Model Answer — 6(c)
the hormone is progesterone [1]
without it the lining of the uterus would no longer be maintained and would break down, so the placenta and the fetus would lose their attachment and blood supply and the pregnancy would end [1]
(the placenta also produces oestrogen; progesterone is the one that maintains the lining, so it is the one the mark scheme wants)
⚠ If you missed marks here: Many answers assume the corpus luteum keeps working for the whole nine months. It does not — the placenta grows large enough to take the job over, which is a neat piece of design: the structure that depends on the lining is the one that then maintains it. A prediction only scores if it is joined to a reason, so the word “because” should appear in your answer.
(d)[3]
Antibodies from the mother’s blood cross the placenta into the fetal blood. Some pathogens and toxins can cross it too. Explain the benefit of the antibodies to the baby, state one limitation of that protection, and predict one effect of a pathogen crossing the placenta.
Model Answer — 6(d)
the mother’s antibodies protect the newborn against the pathogens she is immune to, at a time when the baby’s own immune system cannot yet make antibodies of its own [1]
the limitation: the baby did not make these antibodies itself, so the protection is temporary — the antibodies are broken down over the following months and are not replaced, and they only cover the pathogens the mother happens to be immune to [1]
a pathogen crossing the placenta can infect the fetus and damage its development before birth, and a toxin can do the same; the placenta is a barrier but it is not a complete one [1]
⚠ If you missed marks here: The placenta is often described as “a filter that keeps out everything harmful”. It is not — that is exactly why this part of the syllabus exists. Second, the baby does not make antibodies “out of the mother’s blood”: the finished antibody molecules cross, already made, which is what makes the protection short-lived rather than lasting.
(e)[2]
State one job that the amniotic fluid does. Then predict what would happen if the amniotic sac ruptured in the fourth month of pregnancy, and explain why the fetus would nevertheless still be receiving food and oxygen.
Model Answer — 6(e)
the amniotic fluid cushions the fetus against knocks and jolts, supports its weight so it can move and grow evenly, and keeps its temperature steady (any one) [1]
if the sac ruptured the fluid would drain away, so the fetus would lose that cushioning and support and pathogens could enter the uterus — but food and oxygen would still reach it, because those cross the placenta and travel along the umbilical cord; the amniotic fluid never supplied them [1]
⚠ If you missed marks here: The misconception this part is built on is that the amniotic fluid feeds the fetus, or that the fetus “drinks” it for nutrition. It does not. Everything the fetus needs comes through the placenta and along the umbilical cord; the fluid is protection and support only. Keep the two structures apart: the amniotic sac is the membrane, the amniotic fluid is the liquid inside it.
Question 7 — Reading a Public Health Claim Carefully
Total: 10 marks
A newspaper column prints this sentence:
“HIV is just a bad infection. A course of antibiotics clears it up, and the only people who catch it are those who share needles.”
Every sentence in Question 7 is about evaluating a claim, not simply recalling a fact.
(a)[3]
Identify three separate errors in the newspaper sentence, and correct each one.
Model Answer — 7(a)
antibiotics cannot treat it: HIV is a virus, and antibiotics kill bacteria and have no effect on viruses [1]
it is not “cleared up”: the person remains infected, and the infection may later lead to AIDS. HIV is the pathogen; AIDS is the condition it may cause [1]
sharing needles is only one route. HIV is also transmitted through sexual contact, in infected blood given in a transfusion, and from an infected mother to her child — across the placenta before birth, during birth, or in breast milk [1]
⚠ If you missed marks here: Use the two words precisely: HIV is the pathogen, AIDS is the condition it may lead to, and they are not interchangeable. On antibiotics, say explicitly that they act on bacteria and have no effect on viruses — “antibiotics do not work” is only half the mark. And when a claim says “only”, the fastest way to break it is to list the routes it left out.
(b)[3]
Explain why a person infected with HIV may, several years later, become seriously ill with a range of infections that a healthy person would fight off easily.
Model Answer — 7(b)
HIV infects and destroys lymphocytes, a type of white blood cell [1]
with fewer lymphocytes far fewer antibodies can be made, so pathogens entering the body are not marked out and destroyed [1]
the immune system can no longer defend the body, so the person becomes ill with infections a healthy immune system would deal with; this stage is called AIDS. It takes years because the number of lymphocytes falls slowly [1]
⚠ If you missed marks here: “It weakens the immune system” restates the question rather than answering it. Name the cell: lymphocytes, the white blood cells that make antibodies — phagocytes are the ones that engulf pathogens, and mixing the two up is a routine loss of marks. The delay of years is worth a sentence too, because it is the reason a person can pass HIV on long before feeling ill.
(c)[2]
Table 7.1 shows figures published by one country over nine years.
Year
Percentage of pregnant women tested for HIV / %
New HIV infections in babies under one year old, per 100 000 births
2010
22
310
2013
48
190
2016
71
95
2019
93
28
A health minister says: “Testing pregnant women has cut the number of babies born infected.” Evaluate this claim using Table 7.1.
Model Answer — 7(c)
the data are consistent with the claim: as testing rose from 22% to 93%, new infections in babies fell from 310 to 28 per 100 000 births — a fall of more than 90% [1]
but this is only a correlation, not proof of cause: nine years passed and other things changed too (treatment of infected mothers, health education, screening of donated blood), and testing on its own changes nothing — it is the treatment that follows a positive test that reduces transmission to the baby [1]
⚠ If you missed marks here: “The numbers went down so it worked” is a description, not an evaluation. Two moves earn the second mark every time in data questions like this: point out that two things changing together does not prove one caused the other, and name a plausible alternative explanation. Here there is also a nice biological point — a test is only a measurement; what changes the outcome is what is done afterwards.
(d)[2]
The same newspaper column ends by advising readers simply to “stay away from anyone who looks unwell”. Explain why testing and contact tracing control the spread of a sexually transmitted infection far better than that advice does.
Model Answer — 7(d)
the advice fails because most people carrying a sexually transmitted infection look and feel completely well, often for years — with HIV the number of lymphocytes falls slowly, so a person can pass the infection on long before any symptom appears. Appearance identifies nobody [1]
testing detects an infection that is producing no symptoms at all, so the infected person can be treated and knows to avoid passing it on; contact tracing then finds the people who may already have been infected by that person and tests them too, so each of them can be treated before infecting anyone else [1]
(the other measures in this syllabus work the same way: health education, screening donated blood before transfusion, not sharing needles, and treating an infected pregnant woman so the infection is not passed to her baby)
⚠ If you missed marks here: The whole answer turns on one fact: an infected person usually looks perfectly healthy. If you did not say that, you cannot explain why the advice is useless, and every control measure in this part of the syllabus stops making sense. “Keeping clean” is another answer worth nothing here — these infections are not spread by dirt.
Self-Assessment
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A : 48-55
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D : 24-31
E : 16-23
U : <16
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