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Challenge Prep: Reproduction

IGCSE Biology 0610 — Topic 16 — Extended

Almost every mark in Topic 16 is a precise word in the right place. Pollination and fertilisation are two different events, and swapping them is the single commonest error in the whole syllabus. Fertilisation happens in the oviduct; implantation happens in the lining of the uterus. The two bloodstreams at the placenta never mix. A sperm has a flagellum. Germination needs water, oxygen and a suitable temperature — and not light. Haploid and diploid describe nuclei, not half-cells. HIV is the pathogen; AIDS is the condition it may lead to. Twelve traps, six data-led 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 at all.

⚠️ Common Traps & Misconceptions

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Twelve traps that cost marks on Topic 16 challenge papers, spread across all six sub-topics. Every one is an answer that sounds right and that mark schemes refuse.

⚠️ TRAP
Trap 1: Using “pollination” and “fertilisation” as if they meant the same thing
The Trap“The insect pollinates the flower, so the ovule is fertilised.” Or, worse, “fertilisation is when pollen lands on the stigma.” The two words describe two events that happen days apart, in two different places, to two different things — and Cambridge asks you to separate them more often than it asks anything else in this topic.
The TruthPollination is the transfer of pollen grains from an anther to a stigma. It moves a whole grain, on the outside of the carpel, and it can be done by an insect or by the wind. Fertilisation is the fusion of a pollen nucleus with a nucleus in an ovule, deep inside the ovary, after a pollen tube has grown all the way down the style. Pollination is a delivery; fertilisation is a fusion of nuclei.
Why It MattersThe definitions are worth a mark each and they are given away free. It also means that pollination can happen and fertilisation can still fail — if the pollen is from the wrong species, or the tube does not reach an ovule. Any question that gives you a plant with plenty of pollen on the stigma and no seeds is testing exactly that gap.
Example Question“Define pollination and fertilisation in a flowering plant, and state where each takes place. [4]”
⚠️ TRAP
Trap 2: Letting gametes into an answer about asexual reproduction
The Trap“In asexual reproduction the plant makes gametes but they do not join with another plant’s.” It sounds like a reasonable halfway house, and it destroys the definition, because the absence of gametes is the whole point.
The TruthAsexual reproduction is a process resulting in genetically identical offspring from one parent. There are no gametes and there is no fusion of nuclei. Sexual reproduction is defined by the fusion of the nuclei of two gametes to form a zygote, and it produces offspring that are genetically different from each other.
Why It MattersBoth definitions are marked phrase by phrase. “One parent” earns nothing on its own; “genetically identical offspring from one parent” earns the mark. And a question that shows you a potato tuber, a runner, a bacterium dividing or a fungal spore is really asking you to notice that no nuclei fused anywhere in the process.
Example Question“Define asexual reproduction and state two ways in which it differs from sexual reproduction. [4]”
⚠️ TRAP
Trap 3: Treating a haploid nucleus as “half a cell”
The Trap“A sperm is haploid, so it is half a cell and it becomes a whole cell when it joins the egg.” The picture is of two halves being stuck together, and it is wrong about what is being halved.
The TruthHaploid and diploid describe the nucleus, not the cell. A sperm is a complete cell with a membrane, cytoplasm and mitochondria — but its nucleus is haploid, meaning it contains one set of chromosomes instead of two. An egg cell is a whole cell too, and a very large one. At fertilisation the two haploid nuclei fuse to give a zygote with a diploid nucleus.
Why It MattersThe word Cambridge wants in the answer is nucleus. “The haploid sperm joins the haploid egg to make a diploid zygote” is loosely worded and often only half-credited; “the haploid nucleus of the sperm fuses with the haploid nucleus of the egg cell to form a zygote with a diploid nucleus” is the full answer. Notice that the egg cell is far bigger than the sperm and yet contributes exactly the same amount of nuclear material.
Example Question“Explain, using the terms haploid and diploid, what happens at fertilisation. [3]”
⚠️ TRAP
Trap 4: Believing that self-pollination produces clones
The Trap“Self-pollination uses pollen from the same plant, so the offspring are genetically identical to the parent.” It is a very tempting sentence, because “same plant” sounds exactly like asexual reproduction — and it is wrong.
The TruthSelf-pollination is still sexual reproduction: two gametes, two haploid nuclei, one fusion. The offspring are therefore genetically different from the parent and from each other. What is true is that self-pollination produces less variation than cross-pollination, because all the genetic material comes from a single plant rather than two. Less variation is not no variation.
Why It MattersThis is the Supplement discussion in 16.3, and the whole discussion turns on how much variation, not on whether there is any. A population that self-pollinates is less able to respond to a change in the environment such as a new disease, but it does not need pollinators and can reproduce when it is isolated or when insects are scarce. Both sides earn marks; “they are clones” earns none.
Example Question“Discuss the effects of self-pollination on a population of a wild plant species. [4]”
⚠️ TRAP
Trap 5: Giving a wind-pollinated flower large bright petals
The TrapBeing asked how a wind-pollinated flower is adapted and writing about brightly coloured petals, scent and nectar — because that is what a flower looks like in everybody’s head.
The TruthA wind-pollinated flower has small, green or dull petals (often no petals at all), no scent and no nectar — it is not advertising to anything. Its anthers hang outside the flower on long filaments so the wind can shake the pollen out, and its stigmas are large and feathery to catch pollen from the air. The pollen is small, light, smooth and produced in enormous quantities. Insect-pollinated pollen is the opposite: larger, heavier, sticky or spiky so it clings to an insect, and produced in far smaller amounts.
Why It MattersEvery adaptation must be tied to the job. “The stigma is feathery” is half an answer; “the stigma is large and feathery, giving a big surface area to catch pollen blown past in the air” is the whole one. The pollen comparison is a favourite question with a photograph or an electron micrograph, where you are being asked to read the surface of the grain.
Example Question“Fig. 2.1 shows two pollen grains. State which is from a wind-pollinated flower and give two reasons. [3]”
⚠️ TRAP
Trap 6: Putting light, soil or nutrients on the list of things a seed needs to germinate
The Trap“A seed needs water, warmth, light and soil.” Plants need light, so seeds must need light. Except that a seed germinating under 3 cm of soil has never seen any.
The TruthGermination needs exactly three things: water, oxygen and a suitable temperature. Water is needed to activate the enzymes and to transport the dissolved food to the growing regions; oxygen is needed for aerobic respiration, which releases the energy for growth; a suitable temperature is needed because germination depends on enzymes, which work too slowly when it is cold and are denatured when it is too hot. Light is not required, and neither is soil or added mineral nutrients — the food store inside the seed supplies everything until the first leaves open.
Why It MattersAlmost every germination experiment on a challenge paper includes a light-versus-dark pair for exactly this reason, and the pair that answers it is the one where nothing but light differs. If you have already decided seeds need light, you will misread your own results.
Example Question“Using the results in Table 3.1, state whether light is needed for germination and give the evidence. [3]”
⚠️ TRAP
Trap 7: The oviduct — the wrong name, and the wrong job
The TrapTwo errors that travel together. First, calling it the “fallopian tube”, which is the name used in most textbooks and websites and is not the word on this syllabus. Second, writing that fertilisation happens in the uterus, because that is where the baby ends up.
The TruthThe tube is the oviduct. Fertilisation happens in the oviduct, usually in its upper part, within a day or so of the egg cell being released from the ovary. The zygote then divides as it travels down, and by the time it reaches the uterus it is an embryo — a ball of cells — which implants into the lining of the uterus. Two events, two places, several days apart.
Why It Matters“Fallopian tube” will often be tolerated, but there is no reason to gamble a mark on it when the right word is one syllable longer. The site question is not tolerated: a diagram question asking “label the structure in which fertilisation occurs” and “label the structure in which implantation occurs” is asking for two different labels, and giving the same one twice scores one at best.
Example Question“On Fig. 5.1, label with an X the site of fertilisation and with a Y the site of implantation. [2]”
⚠️ TRAP
Trap 8: Letting the mother’s and the fetus’s blood mix in the placenta
The Trap“The mother’s blood flows into the baby through the umbilical cord, carrying the food and oxygen.” It is the picture most people carry, and it is refused wherever it appears.
The TruthThe placenta is an exchange surface, not a join. The fetal capillaries lie in spaces filled with the mother’s blood, separated from it by a thin barrier, so dissolved substances cross by diffusion (and some, such as certain nutrients and ions, by active transport) while the two bloodstreams remain completely separate. Oxygen, glucose, amino acids, water and some antibodies pass to the fetus; carbon dioxide and urea pass to the mother.
Why It MattersIt explains why the placenta has to have a huge surface area and a very thin barrier at all — if the blood mixed, none of that would be needed. It also matters because if the bloodstreams did mix, the mother’s blood pressure would destroy the fetal capillaries and the mother’s immune system could attack the fetus. Examiners hand out marks for “the blood does not mix” because it is the sentence that proves you understand the whole structure.
Example Question“Explain how substances are exchanged at the placenta without the two bloodstreams mixing. [4]”
⚠️ TRAP
Trap 9: Making the amniotic fluid feed the fetus
The Trap“The amniotic fluid surrounds the fetus and supplies it with food and oxygen.” It is a liquid, the fetus is floating in it, so it looks like the obvious supply line.
The TruthThe amniotic fluid protects. It cushions the fetus against physical damage and against sudden movements, supports its weight so it can grow evenly, and keeps its temperature and surroundings constant. It carries no significant nutrition. Everything the fetus receives arrives through the umbilical cord from the placenta, and everything it excretes leaves the same way. The amniotic sac is the membrane that encloses the fluid.
Why It MattersFour structures — placenta, umbilical cord, amniotic sac, amniotic fluid — and four separate functions, which is usually four separate marks. Merging any two of them, or giving the fluid the cord’s job, costs a mark that requires no reasoning at all to earn.
Example Question“State the functions of the placenta, the umbilical cord, the amniotic sac and the amniotic fluid. [4]”
⚠️ TRAP
Trap 10: Writing that the sperm has a tail, and stopping there
The Trap“The sperm is adapted because it has a tail to swim with and a streamlined head.” One correct idea, expressed in the wrong word, with the other two adaptations missing.
The TruthThe syllabus names exactly three adaptive features of a sperm, and expects all three: a flagellum (never a “tail”), which beats so the cell can swim to the egg cell; many mitochondria, which release the energy by aerobic respiration that the flagellum needs; and enzymes in the acrosome, which digest a path through the jelly coat of the egg cell so the sperm nucleus can enter. The egg cell has its own two: large energy stores in its cytoplasm to supply the embryo before implantation, and a jelly coat that changes at fertilisation so that no second sperm can get in.
Why It Matters“Flagellum” is a stated syllabus term, and “tail” is exactly the kind of everyday word Cambridge writes mark schemes to exclude. The mitochondria mark is the one most often missed, and it is the one that connects this topic to respiration: a cell that swims needs an energy supply, so it carries the organelles that provide it.
Example Question“Explain how the structure of a sperm cell is adapted to its function. [3]”
⚠️ TRAP
Trap 11: Blaming the wrong hormone for the loss of the uterus lining
The Trap“At the end of the cycle the oestrogen level falls, so the lining of the uterus breaks down.” Also common: “the cycle begins with ovulation”, and “the ovaries release FSH”.
The TruthDay 1 of the cycle is the first day of bleeding, and ovulation is around day 14. Before ovulation, FSH stimulates a follicle in the ovary to develop, and that follicle produces oestrogen, which repairs and thickens the lining of the uterus and triggers a surge of LH. LH causes ovulation. The remains of the follicle become the corpus luteum, which produces progesterone. It is progesterone that maintains the thickened lining, so when the corpus luteum breaks down and the progesterone concentration falls, the lining breaks down and menstruation begins. If the embryo implants, the corpus luteum is maintained, progesterone stays high, the lining is kept, and later in pregnancy the placenta takes over producing progesterone.
Why It MattersCambridge names four hormones here and asks you to say what each does and where it is produced. Almost every graph question on the cycle is answered by two rules: the hormone that thickens is oestrogen and the hormone that maintains is progesterone; the two spikes just before day 14 are oestrogen then LH.
Example Question“Explain what happens to the lining of the uterus between day 21 and day 28 if the egg cell is not fertilised. [4]”
⚠️ TRAP
Trap 12: Treating HIV and AIDS as the same thing, and reaching for antibiotics
The Trap“AIDS is a virus that is spread by sexual contact and is treated with antibiotics.” Three errors in one sentence, all of them extremely common.
The TruthHIV is the pathogen — a virus. AIDS is the condition that HIV infection may lead to, in which the immune system has been so damaged that the person can no longer fight off other infections. A person can be infected with HIV for years without having AIDS. And antibiotics kill bacteria; they have no effect on viruses, so they do nothing to HIV. They may still be prescribed to a person with AIDS — but to treat the bacterial infections that a damaged immune system cannot handle, which is a different thing entirely.
Why It MattersAn STI is an infection transmitted through sexual contact, and HIV is one. But HIV is also transmitted by infected blood — shared needles, transfused blood that has not been screened — and from an infected mother to her child across the placenta, during birth or through breast milk. Control of spread therefore means education, testing and contact tracing, screening donated blood, not sharing needles, and treating infected people so that they are far less likely to pass the virus on.
Example Question“Explain the difference between HIV and AIDS, and explain why antibiotics are not used to treat HIV infection. [4]”

🔍 Step-by-Step Walkthroughs

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Six challenge-level questions with real data, worked through in the order you should actually think about them. Try each part before revealing the next step.

Walkthrough 1 — A Menstrual Cycle Chart With Nothing Named 17142128 Fig. 8.1 Events and hormone concentrations over one 28-day menstrual cycle. Day 1 is the first day of bleeding. events in the ovary hormone P hormone Q hormone R hormone S thickness of the uterus lining day of the cycle shaded band day 14 V W The four hormone curves are drawn on the same arbitrary scale and are not directly comparable in concentration.

(a) Name the event happening on day 14 and give the evidence from the chart. [2] (b) Identify hormones P and Q, and give the evidence for Q. [3] (c) Identify hormones R and S and state where each is produced. [4] (d) Explain what happens to the lining of the uterus after day 24, and how the chart would look different if the egg cell had been fertilised. [4]

1

Find menstruation and ovulation before you look at a single curve

The shaded band covers roughly days 1–5, and the caption tells you day 1 is the first day of bleeding, so that band is menstruation — the lining of the uterus breaking down. The bottom track agrees: the lining is at its thinnest there. The dashed line at day 14 is ovulation, and the ovary track shows it directly: a follicle (V) enlarges up to day 14, an egg cell is released at the dashed line, and the structure left behind (W) appears after it. Every hormone can now be identified by where it sits relative to those two events.

2

Q is a single narrow peak sitting exactly on day 14

Q is low and flat for the whole cycle except for one sudden, very sharp peak immediately before day 14, and ovulation happens at day 14. A hormone whose only activity is a spike that triggers an event is LH: LH causes ovulation. P is different — it is already fairly high at day 1, falls through the first week, shows a small bump near day 14 and rises again at the very end of the cycle. That is FSH, which stimulates a follicle in the ovary to develop, and which starts rising again at the end as the next cycle begins.

3

R rises before ovulation; S only exists after it

R climbs steadily through the first half of the cycle and peaks just before the LH spike. It is produced by the developing follicle (V) in the ovary, and it is oestrogen — look at the lining track underneath and you can see it thickening as R rises. S is flat until day 14 and then rises to a broad peak around day 21, which is exactly the life of the corpus luteum (W). S is progesterone. So: oestrogen from the follicle in the first half, progesterone from the corpus luteum in the second half, and both are produced in the ovary.

4

Progesterone maintains the lining, so the lining follows progesterone down

After about day 24 the corpus luteum breaks down, so the progesterone concentration falls. Progesterone is what maintains the thickened lining, so once it falls the lining can no longer be maintained and it breaks down — which is day 1 of the next cycle. If the egg cell had been fertilised and the embryo had implanted, the corpus luteum would have been maintained, S would have stayed high instead of falling, the lining track would have stayed thick, and there would be no shaded band at the end. Later in pregnancy the placenta takes over the production of progesterone.

Full Mark-Scheme Answer(a) Ovulation — the release of an egg cell from the ovary [1]; evidence: the ovary track shows a follicle enlarging up to day 14 and a different structure after it, and there is a sharp hormone peak at that point [1]. (b) P is FSH [1]; Q is LH [1]; evidence for Q: it shows a single sharp peak immediately before day 14, and LH is the hormone that causes ovulation [1]. (c) R is oestrogen, produced by the developing follicle in the ovary [2]; S is progesterone, produced by the corpus luteum in the ovary [2]. (d) The corpus luteum breaks down so the concentration of progesterone falls [1]; progesterone maintains the thickened lining, so the lining is no longer maintained and breaks down, giving menstruation [1]. If the egg cell had been fertilised, the corpus luteum would have been maintained and progesterone would have stayed high [1], so the lining would have remained thick and there would have been no menstruation [1].
Walkthrough 2 — A Germination Experiment With Five Sets of Conditions Table 4.1 100 cress seeds were placed in each of five sets of conditions and left for six days. setwateroxygentemperaturelightseeds germinated / % Apresentpresent20 °Clight92 Bpresentpresent20 °Cdark90 Cabsentpresent20 °Clight0 Dpresentremoved by boiling20 °Clight3 Epresentpresent4 °Clight8 In set D the water was boiled to drive off dissolved oxygen and then covered with a layer of oil.

(a) State the conclusion about light, naming the two sets that support it. [3] (b) Explain the result for set E. [3] (c) Set D was expected to give 0 %. Suggest why three seeds germinated. [2] (d) State one variable, not listed in the table, that must be kept the same in all five sets, and explain why. [2]

1

A and B differ in light and in nothing else

Compare the rows, not the numbers. A and B have the same water, the same oxygen and the same temperature; the only difference is light against dark. They germinated 92 % and 90 % — a difference of 2 seeds in 100, which is well within the ordinary variation between two batches of seeds. So light is not required for germination. Naming the pair is worth as much as the conclusion, because it is the evidence.

2

Cold slows enzymes; it does not destroy them

At 4 °C the seeds had water and oxygen, so nothing was missing — the process was simply too slow. Germination depends on enzymes, which digest the stored food in the seed. At a low temperature the molecules have less kinetic energy, so there are fewer successful collisions between enzyme and substrate and the rate of reaction is very low. Note the wording carefully: the enzymes are not denatured by cold — that happens at high temperature — so those seeds would still germinate if they were warmed up.

3

A small non-zero result usually means the variable was not fully removed

Boiling and an oil layer reduce the oxygen but do not guarantee that none is left; a little oxygen could have dissolved back in before the oil was poured, or been trapped as a bubble against a seed. Three seeds in a hundred is the sort of number you get from an incomplete removal, not from a different biology. An alternative worth a mark: for a very short time a seed can survive on anaerobic respiration, which releases far less energy, so a very few seeds might just begin to germinate before stopping.

4

Name the variable, then say what it would otherwise ruin

Good answers: the same species and variety of seed, and seeds of the same age — old seeds germinate poorly, so a difference between sets could be caused by the seeds rather than by the conditions. Also the same volume of water, the same number of seeds (already 100 each, which is why percentages can be compared), and the same time allowed. Never write “keep everything the same”: name one variable and give its consequence.

Full Mark-Scheme Answer(a) Light is not needed for germination [1]; evidence from sets A and B [1], which differ only in light and gave almost the same percentage, 92 % and 90 % [1]. (b) Water and oxygen were both present, so the low result is caused by the low temperature alone [1]; germination depends on enzymes, and at 4 °C the enzymes work very slowly because there is less kinetic energy and fewer successful collisions [1]; so very few seeds had germinated within six days — the enzymes are not denatured and the seeds would germinate if warmed [1]. (c) The oxygen was not completely removed — some dissolved back into the water or was trapped as a bubble [1]; so a few seeds received enough oxygen for aerobic respiration (accept: a few seeds began germinating using anaerobic respiration) [1]. (d) Any one of: same species/variety and age of seed; same volume of water; same time allowed [1]; because otherwise a difference in germination could be caused by that variable instead of by the condition being tested [1].
Walkthrough 3 — Four Substances at the Placenta Table 6.1 Concentrations measured on each side of the placenta, in arbitrary units. substance in the mother’s bloodin the placenta in the fetal bloodarriving at the placenta oxygen9540 glucose9060 amino acids1218 urea2545 Fetal blood arriving at the placenta is the blood travelling along the umbilical cord towards the mother.

(a) State the direction of net movement of oxygen, glucose and urea, and name the process responsible. [4] (b) The amino acid figures are the other way round. Explain how amino acids can still move into the fetal blood. [3] (c) Explain why the placenta must have a large surface area and a thin barrier. [2] (d) State one substance in the mother’s blood that can cross the placenta and harm the fetus. [1]

1

Higher to lower, and say which side is which

Oxygen: 95 in the mother, 40 in the fetal blood — higher in the mother, so the net movement is from the mother to the fetus. Glucose: 90 against 60 — also mother to fetus. Urea: 25 against 45 — higher in the fetal blood, so the net movement is from the fetus to the mother, which is exactly what you would expect for an excretory product the fetus cannot deal with itself. All three are moving down a concentration gradient, so the process is diffusion.

2

18 in the fetus against 12 in the mother means uphill

Amino acids are already more concentrated in the fetal blood, so moving more of them into the fetus means moving them against the concentration gradient. Diffusion cannot do that. The process is active transport: protein carriers in the cell membranes of the placenta move the amino acids from a lower to a higher concentration, using energy released by respiration. That is why the cells of the placenta contain many mitochondria — a detail worth adding, because it is evidence rather than assertion.

3

Same argument as every other exchange surface you have met

A large surface area means more diffusion can happen at the same time, so the rate of exchange is high enough to supply a growing fetus. A thin barrier gives a short diffusion distance, so exchange is rapid. The placenta is folded into villi for exactly the reason the small intestine and the lung are — and the fetal capillaries lie very close to the mother’s blood spaces without the two ever mixing. If the bloodstreams mixed, none of this structure would be necessary, which is the quickest way to remember that they do not.

4

A barrier thin enough for oxygen is thin enough for other things

Some antibodies cross from the mother, which is useful — they give the newborn temporary passive immunity. But some pathogens and toxins cross too and can damage the fetus: HIV and rubella virus are examples of pathogens, and nicotine, alcohol and some drugs are examples of toxins. It is the price of a surface built for exchange, and it is a stated part of the syllabus rather than an aside.

Full Mark-Scheme Answer(a) Oxygen: mother to fetus [1]; glucose: mother to fetus [1]; urea: fetus to mother [1]; all three by diffusion, down a concentration gradient [1]. (b) The amino acid concentration is already higher in the fetal blood, so they move against the concentration gradient [1] by active transport [1], using protein carriers in the membrane and energy released by respiration [1]. (c) A large surface area means more diffusion can take place at once, giving a high rate of exchange [1]; a thin barrier gives a short diffusion distance so exchange is rapid [1]. (d) Any one of: nicotine, alcohol, a named drug, HIV, rubella virus [1].
Walkthrough 4 — Three Ways of Raising the Same Crop Table 5.1 A grower raised 200 tomato plants in each of three ways and recorded two years of results. group how the plants were produced mean yield in year 1/ kg per plant plants killed by a newdisease in year 2 Pgrown from seed after cross-pollination by insects3.422 Qgrown from seed after self-pollination in bagged flowers2.661 Rgrown from side shoots cut from one high-yielding plant4.1196 All three groups were grown in the same soil, in the same greenhouse, with the same water and mineral supply.

(a) Name the type of reproduction used for group R and state what this means about the offspring. [2] (b) Explain the year 2 result for group R. [3] (c) Explain why group Q lost more plants than group P. [3] (d) Suggest why the grower might still choose method R next year. [2]

1

Side shoots from one plant means one parent and no gametes

Group R was produced from side shoots of a single plant: one parent, no gametes and no fusion of nuclei, so this is asexual reproduction and every plant in group R is genetically identical to the parent and to each other. Groups P and Q both came from seeds, so a pollen nucleus fused with an ovule nucleus in each: both are sexual reproduction, and both give offspring that are genetically different. The bagging in Q forced self-pollination; it did not turn Q into asexual reproduction.

2

No variation means no survivors when conditions change

Because every plant in R is genetically identical, they all have the same resistance, or the same lack of resistance, to the new disease. If the pathogen can infect one of them it can infect all of them, so almost the whole group was killed. Nothing about the disease was unusual — the vulnerability came from the absence of variation in the group, which is the standard disadvantage of asexual reproduction both in the wild and in crop production.

3

Both are sexual; one draws on two plants and the other on one

In group P the pollen came from a different plant of the same species, so the offspring combine genetic material from two parents and show more variation. In group Q the pollen came from the same plant, so all the genetic material came from one individual and there is less variation — though still some, because Q is sexual reproduction. More variation means a greater chance that some plants happen to be resistant and survive, which is exactly what the numbers show: 22 lost from P against 61 from Q.

4

Yield, certainty and speed against vulnerability

R gave the highest yield, 4.1 kg per plant, and because the plants are genetically identical to a plant already known to be good, the grower knows in advance that every plant will have that desirable feature — the crop is uniform and ripens together, which makes harvesting and selling easier. It is also faster and does not depend on pollinators. The disadvantage is the one shown in year 2, and a sensible grower manages it by not planting only one variety.

Full Mark-Scheme Answer(a) Asexual reproduction [1]; the offspring are genetically identical to the parent plant and to each other — there are no gametes and no fusion of nuclei [1]. (b) All the plants in R are genetically identical [1], so they all have the same susceptibility to the new disease [1]; if one can be infected, all can, so almost all were killed [1]. (c) Group Q was self-pollinated, so all the genetic material came from one plant and there is less variation [1]; group P was cross-pollinated, so genetic material came from two different plants and there is more variation [1]; more variation makes it more likely that some plants are resistant and survive [1]. (d) It gave the highest yield (4.1 kg per plant) [1]; and because the plants are genetically identical to a known good parent, the grower can be certain every plant will carry that desirable feature and the crop will be uniform (accept: it is quicker and does not depend on pollinators) [1].
Walkthrough 5 — Four Samples From a Fertility Clinic Fig. 6.1 — two human cells, drawn to the same scale 50 µm A B C D Fig. 6.2 — one of the cells from Fig. 6.1, at a higher magnification E F G H The scale bar applies to Fig. 6.1 only. Table 6.2 Semen samples from four men attending a fertility clinic. man sperm count/ million per cm³ sperm showing forwardmovement / % sperm with normalstructure / % W625814 X71912 Y66115 Z68553 Typical reference values: at least 15 million per cm³, at least 40 % moving forward, at least 4 % with normal structure.

Fig. 6.1 and Fig. 6.2 show the two human gametes. (a) Name the cells A to D in Fig. 6.1 and the structures E to H in Fig. 6.2. [4] (b) Man X has plenty of sperm and almost none of them move. Suggest which structures are likely to be faulty and explain your answer. [3] (c) Only 3 % of Z’s sperm have a normal structure. Explain, referring to Fig. 6.2, why this reduces the chance of fertilisation. [3] (d) Compare the numbers of male and female gametes produced, and explain the difference. [2]

1

The huge one is the egg cell; the tiny one is the sperm

Fig. 6.1 is drawn to one scale, and the difference in size is the point: a human egg cell is about 0.1 mm across, while a sperm is a few micrometres wide. So the large cell is the egg cell: A is its jelly coat, B is the energy stores in its cytoplasm and C is the egg cell nucleus. D is the sperm cell. In the enlarged Fig. 6.2, E is the acrosome at the tip, F is the sperm nucleus, G is the mitochondria in the middle piece and H is the flagellum. Use the word flagellum, never “tail”.

2

Something to beat with, and the energy to beat it

Movement needs two things, so there are two sensible answers and both earn credit. Either the flagellum is faulty, so the cell has no working structure to beat and propel itself with. Or the mitochondria are faulty or too few, so aerobic respiration cannot release enough energy for the flagellum to beat. Note what the data rule out: the count is 71 million per cm³, well above the reference value, so this is not a problem of numbers. When a question gives you three measurements and only one is abnormal, the explanation has to be about that one.

3

Reaching the egg cell is not the same as getting into it

Z’s sperm are numerous and they swim, so they will reach the egg cell. But a sperm still has to get through the jelly coat, and that is the job of the enzymes in the acrosome (E), which digest a path through it so the sperm nucleus can enter and fuse with the nucleus of the egg cell. If only 3 % of the cells are structurally normal, most have a defective acrosome or head, so far fewer sperm are capable of penetrating the jelly coat and fertilisation is much less likely.

4

Size, structure, motility, numbers — learn it as four rows

Size: the egg cell is much larger. Structure: the egg cell has a jelly coat and large energy stores; the sperm has a flagellum, many mitochondria and an acrosome. Motility: the sperm is motile and swims; the egg cell is not motile and is moved along the oviduct. Numbers: sperm are produced in enormous numbers, tens of millions in a single release, while typically one egg cell is released each month. The reason is that the great majority of sperm never reach the egg cell — producing millions makes it likely that at least one does.

Full Mark-Scheme Answer(a) A = jelly coat, B = energy stores (cytoplasm containing food reserves), C = nucleus of the egg cell, D = sperm cell [2]; E = acrosome, F = nucleus, G = mitochondria, H = flagellum [2]. (b) The flagellum may be faulty, so the sperm has no working structure with which to swim [1]; or the mitochondria may be faulty or too few [1], so too little energy is released by aerobic respiration for the flagellum to beat [1]. (c) The acrosome contains enzymes [1] which digest a path through the jelly coat of the egg cell [1]; with only 3 % structurally normal, very few sperm can penetrate the jelly coat, so the sperm nucleus cannot reach and fuse with the egg cell nucleus [1]. (d) Very large numbers of sperm are produced compared with about one egg cell released each month [1]; because most sperm do not survive or do not reach the egg cell, so large numbers make it likely that one will [1].
Walkthrough 6 — A Public Health Programme That Looks Like a Failure Fig. 9.1 HIV in country Z. A national programme began in 2008. 04080120160 0246810 200020042008201220162020 year programme begins new infections each year / thousands (left axis) adults aged 15–49 living with HIV / % of that age group (right axis)

(a) Describe the two trends shown after 2008. [3] (b) A student concludes that the programme failed, because the percentage of adults living with HIV went up. Evaluate that conclusion. [3] (c) Suggest three measures the programme could have included to reduce the number of new infections. [3] (d) Explain why prescribing antibiotics to everyone would not have appeared anywhere in the programme. [2]

1

Two lines, two axes, two sentences — never one

New infections rose to a peak of about 150 thousand in 2008 and then fell steeply to about 35 thousand by 2020, a fall of roughly 77 %. The percentage of adults living with HIV rose throughout, from about 5.0 % in 2000 to about 9.5 % in 2020, but the rise became much slower after 2008 — it is nearly level from 2012 onwards. Always check which axis a line belongs to before quoting a number; the two lines here use different scales and different units.

2

How many people caught it this year, against how many are alive with it

The orange line counts new infections in one year. The blue line counts everyone currently living with HIV, as a percentage of the age group. Those are not the same measurement, and they can move in opposite directions. If people who are infected are treated and live much longer, the number of people living with HIV rises even while fewer people are being infected. A rising blue line combined with a falling orange line is exactly what a successful programme looks like, and mistaking one for the other is the trap the question is built on.

3

Evaluate means say what is right about it too

An evaluation that only attacks scores badly. The student is right that the percentage living with HIV rose, and right that a lot of people in country Z are still infected, so the problem has not gone away. The student is wrong to use that figure as a measure of success, because the measurement that shows whether transmission is being prevented is the number of new infections — and that fell sharply after 2008. Note too that the graph shows a correlation in time: the fall began when the programme began, but the graph alone cannot prove the programme caused it.

4

Interrupt each route of transmission, one at a time

HIV is transmitted by sexual contact, by infected blood (shared needles, or transfused blood that has not been screened) and from an infected mother to her child across the placenta, during birth or in breast milk. So the measures follow the routes: education about how it is transmitted; testing and contact tracing, so infected people know and can be treated; antiviral drug treatment, which reduces the amount of virus and makes transmission far less likely; screening all donated blood; providing clean needles and not sharing them; and treating infected pregnant women to protect the child.

Full Mark-Scheme Answer(a) New infections fell steeply after 2008, from about 150 thousand to about 35 thousand in 2020 [1]; the percentage of adults living with HIV continued to rise, from about 8.6 % to about 9.5 % [1]; but that rise became much slower and is almost level after 2012 [1]. (b) The student is correct that the percentage living with HIV rose [1]; but that figure counts everyone alive with the infection, and it rises when treatment keeps infected people alive longer [1]; the measure of whether new infections are being prevented is the number of new infections each year, which fell sharply, so the conclusion is not supported (accept: the graph shows correlation, not proof of cause) [1]. (c) Any three of: education about how HIV is transmitted; testing and contact tracing; treating infected people with antiviral drugs to reduce transmission; screening donated blood; providing clean needles and not sharing them; treating infected pregnant women [3]. (d) HIV is a virus, and antibiotics kill bacteria [1]; antibiotics have no effect on viruses, so they would not prevent or cure HIV infection [1].

🔍 Spot the Difference

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Six pairs that look almost identical and have different answers. In this topic the distinction is nearly always where the marks live.

Question A
What is pollination, and where does it happen?
The transfer of pollen grains from an anther to a stigma. It happens on the outside of the carpel, and it is carried out by an insect or by the wind.
Question B
What is fertilisation in a flowering plant, and where does it happen?
The fusion of a pollen nucleus with a nucleus in an ovule. It happens inside the ovary, after a pollen tube has grown down the style and entered the ovule.
Key DifferenceOne moves a whole grain; the other fuses two nuclei. They are separated by the growth of the pollen tube, which takes hours or days, so pollination can succeed and fertilisation can still fail. This is the commonest confusion in Topic 16, and it is worth a mark every time it appears.
Question A
Define asexual reproduction and describe the offspring.
A process resulting in genetically identical offspring from one parent. No gametes, no fusion of nuclei. Fast, needs no partner and no pollinator, and every offspring carries the parent’s desirable features.
Question B
Define sexual reproduction and describe the offspring.
A process involving the fusion of the nuclei of two gametes to form a zygote, producing offspring that are genetically different from each other. Slower, and usually needs a partner or a pollinator.
Key DifferenceVariation. Asexual offspring have none, so a population is wiped out by one new disease or one change in the environment — but a grower gets a uniform, predictable crop. Sexual offspring vary, so some may survive a change, and selective breeding is possible — but the desirable features of the parent are not guaranteed. Marks come from naming which situation you are arguing about: a population in the wild, or crop production.
Question A
What is self-pollination, and what does it do to variation?
Transfer of pollen from an anther to a stigma of the same flower, or of a different flower on the same plant. Still sexual reproduction, so there is some variation — but less, because all the genetic material comes from one plant.
Question B
What is cross-pollination, and what does it do to variation?
Transfer of pollen to a flower on a different plant of the same species. Genetic material comes from two parents, so there is more variation — but it relies on a pollinator or on the wind.
Key DifferenceBoth are sexual, and neither produces clones. The trade-off is the answer to every Supplement question here: cross-pollination gives a population a greater capacity to respond to a change in the environment, while self-pollination gives certainty in isolation and does not depend on pollinators being present. “Same plant” is not “same genes”.
Question A
What happens in the oviduct?
Fertilisation — the nucleus of a sperm fuses with the nucleus of the egg cell, forming a zygote. The oviduct also carries the egg cell from the ovary towards the uterus. Never call it a fallopian tube on this syllabus.
Question B
What happens in the uterus?
Implantation — the embryo, a ball of cells formed by division of the zygote, sinks into the lining of the uterus. The fetus then develops there, and the muscular wall of the uterus pushes it out at birth.
Key DifferenceTwo events, two places, several days apart. In between, the zygote is dividing as it travels down the oviduct, which is why what implants is an embryo and not a zygote. If a diagram question asks you to mark the site of fertilisation and the site of implantation, it wants two different labels.
Question A
Where is oestrogen produced, and what does it do?
Produced by the developing follicle in the ovary, mainly in the first half of the cycle. It repairs and thickens the lining of the uterus and causes the surge of LH that triggers ovulation. It also controls the female secondary sexual characteristics at puberty.
Question B
Where is progesterone produced, and what does it do?
Produced by the corpus luteum in the ovary after ovulation, and later in pregnancy by the placenta. It maintains the thickened lining of the uterus. When it falls, the lining breaks down.
Key DifferenceOne builds, the other maintains. On any graph of the cycle, the hormone rising through the first half is oestrogen and the hormone peaking around day 21 is progesterone. Menstruation follows the fall in progesterone, not the fall in oestrogen, and pregnancy is the case where that fall never happens.
Question A
What is HIV?
A virus, and therefore a pathogen. It is transmitted by sexual contact, by infected blood and from an infected mother to her child. It infects and destroys cells of the immune system.
Question B
What is AIDS?
A condition that HIV infection may lead to, in which the immune system is so damaged that the person can no longer fight off other infections. It is not itself a pathogen and it is not transmitted.
Key DifferenceOne is the cause, the other is the possible consequence, and a person can live with HIV for years without developing AIDS. Because HIV is a virus, antibiotics have no effect on it — they are used in a person with AIDS only to treat the bacterial infections a damaged immune system cannot handle.

🔗 Reproduction Concept Map

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Click each node. The whole topic is four stories: one parent or two, the plant route from anther to seedling, the human route from gamete to fetus, and what happens when a pathogen uses that same route.

⭐ CORE FRAMEWORK 1
One parent → identical offspring   |   two parents → two haploid nuclei fuse → variation
One Parent, No Gametes, No Fusion ▶
Asexual reproduction produces genetically identical offspring from one parent. The test to apply to any example: did two nuclei fuse? If not, it is asexual. A potato tuber sprouting, a strawberry runner, a side shoot taken as a cutting, a bacterium dividing, a fungus releasing spores — all asexual.
To a population in the wild: it is fast, needs no partner and no pollinator, and every offspring is as well suited to that place as the parent was — excellent while conditions stay the same. But there is no variation, so a new disease or a change in the environment can destroy the whole population, and there is no capacity to adapt.
To crop production: the grower gets a uniform crop with a known desirable feature, ripening together, produced quickly and without waiting for pollinators. The cost is the same absence of variation: one pathogen can wipe out the entire field, as in Walkthrough 4.
Two Gametes, One Zygote, and the Word “Nuclei” ▶
Sexual reproduction is a process involving the fusion of the nuclei of two gametes to form a zygote. Fertilisation is that fusion of nuclei — the same definition in a plant and in a human. The offspring are genetically different from each other and from the parents.
The nuclei of gametes are haploid: one set of chromosomes. The nucleus of a zygote is diploid: two sets, one from each gamete. These words describe the nucleus. A sperm is a whole cell with a haploid nucleus; an egg cell is a much bigger whole cell with a haploid nucleus. The process that makes haploid nuclei is a later topic — here you need only what they are and what happens when they fuse.
Advantages and disadvantages mirror framework 1. In the wild: variation means some individuals may survive a new disease or a changed environment, so the population can adapt — but reproduction is slower and needs a partner or a pollinator. In crop production: variation makes selective breeding possible, but the offspring do not reliably carry the parent’s desirable features, and the crop is less uniform.
Where Self-Pollination Sits Between Them ▶
Self-pollination is sexual reproduction — two gametes, one fusion, offspring genetically different. It only looks asexual because both gametes came from the same plant. Calling those offspring clones is the error this framework exists to prevent.
Line the three up in order of variation: asexual (none) → self-pollination (less) → cross-pollination (more). Every Supplement discussion question in 16.1, 16.2 and 16.3 is somewhere on that line.
And read the trade-off in both directions. More variation gives a greater capacity to respond to a change in the environment; less variation gives reliability and, for a plant, independence from pollinators — which matters if the plant is isolated or insects are scarce.
⭐ CORE FRAMEWORK 2
Anther → stigma (pollination) → pollen tube → ovule (fertilisation) → seed → three conditions → seedling
A Flower Is a Machine for Moving Pollen ▶
The male part is the stamen = anther (makes and releases pollen grains) + filament (holds the anther in position). The female part is the carpel = stigma (receives pollen grains) + style (the pollen tube grows down it) + ovary (contains the ovules). Petals and sepals are neither: petals attract, sepals protected the bud.
Insect-pollinated: large brightly coloured scented petals, nectar, anthers and stigma inside the flower where an insect must brush past them, sticky stigma, and pollen that is larger, sticky or spiky and made in small amounts — because a targeted delivery wastes little.
Wind-pollinated: small dull petals or none, no scent, no nectar, anthers hanging outside on long filaments, large feathery stigmas hanging out to catch pollen from the air, and pollen that is small, light and smooth and made in huge amounts — because scattering to the wind wastes almost all of it.
From the Stigma to the Ovule ▶
Pollination = transfer of a pollen grain from anther to stigma. That is the end of pollination and the beginning of everything else.
The grain then grows a pollen tube down through the style, towards the ovary. The pollen nucleus travels down the tube. The tube enters the ovule through a small opening.
Fertilisation = the pollen nucleus fuses with a nucleus in the ovule. After it, the ovule becomes the seed and the ovary becomes the fruit. One ovule, one seed — which is why a fruit with many seeds needed many pollen grains and many pollen tubes.
Three Conditions, Three Reasons, and Nothing Else ▶
Water — it is absorbed by the seed and activates the enzymes, which digest the stored food; it also transports the dissolved food to the growing regions. Oxygen — for aerobic respiration, which releases the energy needed for growth. A suitable temperature — because those enzymes work too slowly when it is cold and are denatured when it is too hot.
Not light. Not soil. Not mineral nutrients. The seed carries its own food store, so a seedling can push up through soil in complete darkness. Light matters only once the first leaves open and the plant must start making its own food.
This is why every germination experiment is a set of pairs differing in one factor, and why a pair that differs in light almost always gives the same result. Read the rows, find the pair, quote both numbers.
⭐ CORE FRAMEWORK 3
FSH → follicle → oestrogen → LH → ovulation → corpus luteum → progesterone → a lining that can hold an embryo
The Hormone Loop, in the Order It Happens ▶
Day 1 is the first day of bleeding. FSH stimulates a follicle in the ovary to develop. The developing follicle produces oestrogen, which repairs and thickens the lining of the uterus and causes a surge of LH.
LH causes ovulation at about day 14 — the release of an egg cell from the ovary. What is left of the follicle becomes the corpus luteum, which produces progesterone.
Progesterone maintains the thickened lining. If there is no fertilisation, the corpus luteum breaks down, progesterone falls, the lining breaks down, and that is day 1 again. If the embryo implants, the corpus luteum is maintained, progesterone stays high, the lining is kept, and later the placenta takes over producing it.
Gamete → Zygote → Embryo → Implantation ▶
Sperm are made in the testes (held outside the body in the scrotum), travel along the sperm ducts, receive fluid from the prostate gland, and leave through the urethra in the penis. The egg cell is released from an ovary into an oviduct.
A sperm swims using its flagellum, powered by energy released by aerobic respiration in its mitochondria, and digests its way through the jelly coat using enzymes in its acrosome. Its haploid nucleus fuses with the haploid nucleus of the egg cell in the oviduct: that is fertilisation, and the result is a zygote with a diploid nucleus. The jelly coat then changes so no second sperm can enter.
The zygote divides as it travels down the oviduct to form an embryo, a ball of cells, which implants into the lining of the uterus — the lining that progesterone has been maintaining. Everything in framework 3 exists to make that one moment possible.
Four Structures That Keep a Fetus Alive ▶
Placenta — the exchange surface. Large surface area, thin barrier. Dissolved nutrients (glucose, amino acids), oxygen, water and some antibodies pass to the fetus; carbon dioxide and urea pass to the mother. Mostly diffusion, but active transport where a substance must move against its gradient. The two bloodstreams never mix.
Umbilical cord — carries the fetal blood to and from the placenta, so it is the only supply and the only drain the fetus has. Amniotic sac — the membrane enclosing the fluid. Amniotic fluid — protects: cushions against physical damage, supports the fetus and keeps its surroundings constant. It does not feed the fetus.
The same thin barrier that lets oxygen through also lets some pathogens and toxins across, which can damage the fetus — HIV and rubella virus, nicotine, alcohol and some drugs. That is a stated syllabus point, and it is the bridge into framework 4.
⭐ CORE FRAMEWORK 4
A pathogen using the reproductive route → the immune system → controlling the spread
What an STI Is, and What HIV Does ▶
A sexually transmitted infection is an infection transmitted through sexual contact. HIV is a pathogen — a virus — that causes an STI, and HIV infection may lead to AIDS.
HIV infects and destroys cells of the immune system, the very cells that produce antibodies against pathogens. As their numbers fall, the body can no longer defend itself, and the person becomes ill with infections a healthy immune system would have dealt with. That state is AIDS, and it is a condition, not a pathogen.
Antibiotics kill bacteria and have no effect on viruses, so they do nothing to HIV. Antiviral drugs do not remove the virus but reduce the amount of it in the body, which keeps the person well for many years and makes them far less likely to pass it on.
Three Routes In ▶
Sexual contact with an infected person — the route that makes it an STI.
Infected blood — sharing needles, or receiving transfused blood or blood products that have not been screened.
From an infected mother to her child — across the placenta, during birth, or in breast milk. This is the route that links 16.6 straight back to 16.4, and it is why treating infected pregnant women is a public-health measure.
Every Control Measure Blocks a Named Route ▶
Education about how HIV is transmitted, so people can avoid the routes. Testing and contact tracing, so infected people know their status and can be treated.
Treating infected people with antiviral drugs, which reduces transmission as well as keeping them well. Treating infected pregnant women to protect the child.
Screening all donated blood, and not sharing needles — providing clean ones instead. Notice the pattern: name the route, then name the measure that interrupts it. A list assembled that way cannot contain a wrong item.

❌ “Why Is This Wrong?” Exercises

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Six answers of the kind that read fluently and score badly. Find the fault before you reveal it.

Exercise 1: “Describe how a flowering plant reproduces sexually, from pollination to the formation of a seed. [4]”
Student’s Answer“An insect carries pollen from the anther to the stigma of another flower. This fertilises the flower. The fertilised stigma then grows down into the ovary and the ovule turns into a seed.”
The FlawThe first sentence is correct and everything after it collapses. “This fertilises the flower” uses fertilisation to mean pollination — the commonest error in the topic. Nothing is fertilised when pollen lands. And it is not the stigma that grows down: the pollen grain grows a pollen tube. As written, the answer contains one marking point out of four.
Correct Answer“An insect transfers pollen grains from an anther to a stigma — this is pollination [1]. The pollen grain then grows a pollen tube down through the style [1], and the tube enters the ovule [1]. The pollen nucleus fuses with a nucleus in the ovule, which is fertilisation, and the ovule then develops into a seed [1].”
Key RuleWrite the two words in the two places they belong: pollination on the stigma, fertilisation in the ovule. If a sentence uses one where the other belongs, the marker cannot give you either.
Exercise 2: “A gardener grows new strawberry plants from runners. Explain what type of reproduction this is and what it means for the new plants. [4]”
Student’s Answer“It is asexual reproduction because only one parent is involved, so the gametes are not mixed with another plant. The new plants will be the same as the parent, so they all have the same features and the gardener knows what he is getting.”
The FlawThe conclusion is right and the biology underneath it is wrong. There are no gametes at all in asexual reproduction, so “the gametes are not mixed” contradicts the definition it is supposed to be supporting. “The same as the parent” is also too loose: Cambridge wants genetically identical, because two plants can look the same and not be identical, and can be identical and look different if they are grown in different conditions.
Correct Answer“Asexual reproduction [1]: there is one parent, no gametes and no fusion of nuclei [1]. The new plants are genetically identical to the parent plant and to each other [1], so every one of them will carry the parent’s desirable features and the crop will be uniform — but there is no variation, so all of them are equally vulnerable to the same disease [1].”
Key RuleDefinitions are marked phrase by phrase. “No gametes and no fusion of nuclei” and “genetically identical” are the phrases; anything that hints at gametes in an asexual answer actively costs you.
Exercise 3: “Explain how the fetus obtains oxygen and glucose, and gets rid of carbon dioxide and urea. [4]”
Student’s Answer“The mother’s blood flows through the placenta and into the umbilical cord, where it mixes with the baby’s blood so the oxygen and glucose are passed over. The waste then goes back the same way, and the amniotic fluid also feeds the fetus.”
The FlawTwo of the topic’s biggest misconceptions in one answer. The two bloodstreams do not mix — they stay completely separate, and substances cross a thin barrier between them. And the amniotic fluid does not feed the fetus; it protects it. There is also no named process anywhere in the answer: “passed over” is not diffusion.
Correct Answer“At the placenta, the fetal capillaries lie very close to spaces filled with the mother’s blood, but the two bloodstreams do not mix [1]. Oxygen and glucose diffuse from the mother’s blood into the fetal blood, down their concentration gradients [1]; carbon dioxide and urea diffuse from the fetal blood into the mother’s blood [1]. All of these are carried between the placenta and the fetus in the blood vessels of the umbilical cord [1]. Substances that must move against their concentration gradient, such as some amino acids, cross by active transport.”
Key RuleName the process, name the direction, and say that the blood does not mix. Those three things are the whole question, and the last of them is the one examiners most want to see.
Exercise 4: “Explain the role of hormones in controlling the menstrual cycle. [5]”
Student’s Answer“The cycle starts when the ovary releases an egg. Oestrogen from the ovary makes the lining of the uterus thicker, and when the level of oestrogen drops at the end of the cycle the lining breaks down and the period starts. FSH is released by the ovaries to grow the egg.”
The FlawThree faults. The cycle does not start at ovulation — day 1 is the first day of bleeding, and ovulation is around day 14. The lining breaks down when progesterone falls, not oestrogen; progesterone is the hormone that maintains it, and the answer never mentions it at all. And LH, which causes ovulation, is missing too, so two of the four named hormones are absent from an answer about hormones.
Correct Answer“FSH stimulates a follicle in the ovary to develop [1]. The developing follicle produces oestrogen, which repairs and thickens the lining of the uterus [1] and causes a surge of LH. LH causes ovulation at about day 14 [1]. The remains of the follicle form the corpus luteum, which produces progesterone, and progesterone maintains the thickened lining [1]. If the egg cell is not fertilised the corpus luteum breaks down, so the progesterone concentration falls and the lining breaks down — menstruation [1].”
Key RuleFour hormones are named on the syllabus, so a five-mark answer should mention all four. Oestrogen thickens; progesterone maintains. The hormone that falls before menstruation is progesterone.
Exercise 5: “A student plans an investigation into the conditions needed for germination. Comment on the plan. [4]”
Student’s Answer“I will put ten seeds in a warm sunny place with water, and ten seeds in a cold dark cupboard with no water. The first ten will germinate because they have everything seeds need: water, warmth, light and soil nutrients. This proves germination needs light.”
The FlawThe design cannot answer the question, and the biology behind it is wrong. Three variables change at once — temperature, light and water — so if the second set fails to germinate you cannot tell which one was responsible, and you certainly cannot conclude anything about light. On top of that, light and soil nutrients are not required for germination at all, and oxygen, which is required, is never mentioned. Ten seeds per set is also a small sample.
Correct Answer“Only one variable should be changed at a time, with all others kept the same [1]. To test light, set up two identical sets with water, oxygen and the same temperature, one in the light and one in the dark [1]. The three conditions actually required are water, oxygen and a suitable temperature — light and soil nutrients are not needed, because the seed contains its own food store [1]. Use a larger number of seeds, for example 100 per set, and repeat, so that the percentage germinating is reliable [1].”
Key RuleA conclusion is only as good as the set-up that produced it. If two sets differ in more than one way, no conclusion about either difference can be drawn — and saying so is worth a mark on its own.
Exercise 6: “Explain what HIV is, how it is transmitted, and how its spread can be controlled. [5]”
Student’s Answer“HIV is the same as AIDS. It is a bacterium that is caught by sexual contact and can be cured with a course of antibiotics. To stop it spreading, people who are infected should be kept away from everybody else so they cannot pass it on by coughing or touching.”
The FlawAlmost every clause is wrong, and the last one is wrong in a way that matters outside the exam. HIV is a virus, not a bacterium, so antibiotics have no effect on it and there is no cure. HIV is not AIDS — it is the pathogen; AIDS is the condition it may lead to. And HIV is not spread by coughing or by touching; the transmission routes are sexual contact, infected blood, and mother to child. Isolating infected people is not a control measure on this syllabus and would prevent nothing.
Correct Answer“HIV is a virus, a pathogen that causes a sexually transmitted infection, and infection with it may lead to AIDS, a condition in which the immune system is badly damaged [1]. It is transmitted by sexual contact, by infected blood such as shared needles or unscreened transfused blood, and from an infected mother to her child across the placenta, at birth or in breast milk [2]. Spread is controlled by education, testing and contact tracing, screening donated blood, not sharing needles, and treating infected people with antiviral drugs, which makes them much less likely to pass the virus on [2]. Antibiotics kill bacteria and have no effect on a virus [1].”
Key RulePathogen or condition; virus or bacterium; route or measure. Sort every sentence into the right one of those pairs and this question answers itself.

✍️ Ultra-Detailed Practice Questions

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Ten Cambridge-style challenge questions, each drawing on more than one sub-topic. Write your answer first, then reveal the model answer and the examiner’s notes.

Question 1
[7 marks]
Fig. 1.1 A flower cut in half from top to bottom. Nothing has been named for you. A B C D E F G H J K J and K are brackets: each one marks a group of structures, not a single one.

(a) Name the structures A, B, D, F and G, and state what the brackets J and K represent. [4] (b) State the function of B and the function of D. [2] (c) This flower is insect-pollinated. State one feature visible in Fig. 1.1 that would be different in a wind-pollinated flower, and explain why. [1]

Model Answer(a) A = petal, B = anther [1]; D = stigma, F = ovary [1]; G = ovule [1]; J = the stamen (anther + filament), the male part, and K = the carpel (stigma + style + ovary), the female part [1].
(b) B, the anther, makes and releases pollen grains [1]. D, the stigma, is the sticky surface that receives pollen grains at pollination [1].
(c) Any one, with a reason: the petals would be small and dull or absent, because there is no insect to attract [1]; or the anthers would hang outside the flower so the wind can blow the pollen away; or the stigma would be large and feathery and hang outside, giving a large surface area to catch pollen carried in the air.
Examiner’s NotesThe stamen and carpel marks are free if you have learned them as sums: stamen = anther + filament, carpel = stigma + style + ovary. In (c) the mark is for the reason, not the feature — “the stigma is feathery” on its own is a description, and “so it can catch pollen blown past in the air” is the biology. Here is the same flower fully named:
An insect-pollinated flower, cut in half from top to bottom Half of the petals and sepals have been cut away so you can see inside the carpel. petal large, coloured, scented — attracts insects anther makes and releases pollen grains filament holds the anther up where insects brush it stigma sticky surface that receives pollen grains style the pollen tube grows down through this ovary contains the ovules; becomes the fruit ovule contains the female gamete; becomes the seed sepal protected the flower when it was a bud stamen = anther + filament, the male part carpel = stigma + style + ovary, the female part Plural note: several stamens together are the stamens; the carpel is sometimes called the pistil, but Cambridge says carpel.
Question 2
[8 marks]
A farmer grows potatoes. Each year she keeps some tubers from her best plants and plants them the following spring; the shoots that grow from them produce her whole crop. Her neighbour grows potatoes from seed produced by cross-pollinated flowers. (a) Name the type of reproduction used by each grower and justify each answer. [3] (b) Give two advantages of the farmer’s method for crop production. [2] (c) In one year a new fungal disease reaches the area. Predict which grower loses more of the crop and explain fully. [3]
Model Answer(a) The farmer uses asexual reproduction [1], because the new plants grow from a tuber of a single parent plant — there are no gametes and no fusion of nuclei [1]. The neighbour uses sexual reproduction, because seeds are formed after a pollen nucleus fuses with a nucleus in an ovule [1].
(b) Any two of: the offspring are genetically identical to a parent already known to be good, so every plant carries the desirable features [1]; the crop is uniform and ready at the same time; it is faster; it does not depend on pollinators being present [1].
(c) The farmer loses more [1]. All her plants are genetically identical, so they all have the same susceptibility to the fungus and if one can be infected they all can [1]. The neighbour’s plants were produced sexually and are genetically different from one another, so there is variation and some plants may be resistant and survive [1].
Examiner’s NotesThe justification is where the marks are: naming the type of reproduction is one mark, and “no gametes, no fusion of nuclei” is another. In (c) do not write that the plants “become resistant” or “learn to resist” — the resistance either exists in a plant or it does not, and variation is what makes it possible for some plants to have it.
Question 3
[7 marks]
A student investigates germination. She sets up four dishes of 50 bean seeds each: dish 1 with damp cotton wool at 20 °C in the light; dish 2 with damp cotton wool at 20 °C in a dark cupboard; dish 3 with dry cotton wool at 20 °C in the light; dish 4 with damp cotton wool at 20 °C in the light, with the seeds covered by boiled, cooled water under a layer of oil. (a) State the three conditions required for germination and explain why each is needed. [3] (b) State which dish tests the need for oxygen, and explain how it does so. [2] (c) The student writes: “Dish 2 germinated well, so light must not be needed — but my seeds had no soil, so they had no nutrients, which is why fewer germinated than I expected.” Comment on her reasoning. [2]
Model Answer(a) Water — it is absorbed and activates the enzymes that digest the stored food, and transports dissolved food to the growing regions [1]. Oxygen — for aerobic respiration, which releases the energy needed for growth [1]. A suitable temperature — because germination depends on enzymes, which work too slowly when cold and are denatured when too hot [1].
(b) Dish 4 [1]: the water was boiled to remove dissolved oxygen and the oil layer stops oxygen from the air dissolving back in, while water, temperature and light are the same as in dish 1, so oxygen is the only variable that differs [1].
(c) Her first conclusion is correct: dishes 1 and 2 differ only in light and both germinated, so light is not required [1]. Her second statement is wrong: a seed contains its own food store, so it needs no soil and no mineral nutrients to germinate; nutrients matter only once the seedling begins to grow after germination [1].
Examiner’s NotesIn (a) the word enzymes should appear twice — for water and for temperature — and aerobic respiration once, for oxygen. Part (b) is really a question about controlled variables: naming the dish is not enough, you must say what stayed the same. Part (c) is a “comment” question, so credit is given for saying clearly which part is right as well as which is wrong.
Question 4
[9 marks]
Fig. 5.1 The human female reproductive system, seen from the front. Nothing has been named for you. X Y A B C D E F X and Y are regions marked with a dashed circle, not single structures.

(a) Name the structures A, B, C and E. [2] (b) Name the event that happens in region X and the event that happens in region Y, and state how many days apart they usually are. [3] (c) Describe what happens to the zygote between X and Y. [2] (d) Name the male structures in which sperm are produced and along which they travel to the urethra, and state why the testes are held outside the main body cavity. [2]

Model Answer(a) A = ovary, B = oviduct [1]; C = uterus, E = cervix [1].
(b) X: fertilisation — the haploid nucleus of a sperm fuses with the haploid nucleus of the egg cell to form a zygote with a diploid nucleus, in the oviduct [1]. Y: implantation — the embryo sinks into the lining of the uterus [1]. They are about 6 to 7 days apart (accept about a week) [1].
(c) The zygote divides repeatedly to form a ball of cells, an embryo [1], while it is moved down the oviduct towards the uterus [1].
(d) Sperm are produced in the testes and travel along the sperm ducts to the urethra [1]. The testes are held outside the body in the scrotum because sperm develop best at a temperature slightly below normal body temperature [1].
Examiner’s NotesWrite oviduct, not fallopian tube. The most expensive mistake on this question is answering (b) with “the uterus” twice: fertilisation and implantation are in different organs, and the several days in between are what turns a zygote into an embryo. Here is the same diagram fully named:
The female reproductive system, seen from the front The two circled places are the two that examiners most often ask you to distinguish. site of fertilisation in the oviduct, not the uterus site of implantation the embryo sinks into the uterus lining a few days after fertilisation ovary produces egg cells and the hormones oestrogen and progesterone oviduct carries the egg cell to the uterus uterus thick muscular wall; the fetus develops inside it uterus lining thickens and breaks down over the menstrual cycle cervix the ring of muscle at the neck of the uterus vagina receives semen; the birth canal Say oviduct, not fallopian tube. The two events happen in two different organs several days apart.
Question 5
[8 marks]
(a) Compare a human sperm cell and a human egg cell in terms of size, motility and numbers produced. [3] (b) Explain how three features of a sperm cell adapt it to its function. [3] (c) Explain the function of the jelly coat of the egg cell, both before and at fertilisation. [2]
Model Answer(a) Size: the egg cell is much larger than the sperm [1]. Motility: the sperm is motile and swims using its flagellum, while the egg cell is not motile and is moved along the oviduct [1]. Numbers: very large numbers of sperm are produced, whereas usually only one egg cell is released each month [1].
(b) A flagellum, which beats so that the cell can swim to the egg cell [1]. Many mitochondria, which release the energy from aerobic respiration needed for the flagellum to beat [1]. Enzymes in the acrosome, which digest a path through the jelly coat of the egg cell so the sperm nucleus can enter [1].
(c) Before fertilisation the jelly coat protects the egg cell [1]. At fertilisation it changes so that no further sperm can enter, which prevents more than one sperm nucleus fusing with the egg cell nucleus [1].
Examiner’s NotesSay flagellum. “Tail” is the everyday word and mark schemes are written to exclude it. The mitochondria mark is the one most often lost, and it is the one that connects this topic to respiration: a cell that has to swim needs the organelles that release the energy. Note that in (a) each comparison must mention both cells — “the sperm is small” is half a comparison.
Question 6
[9 marks]
mother’s blood Fig. 7.1 A fetus in the uterus, in section. Nothing has been named for you. Fig. 7.2 An enlarged view of part of structure P from Fig. 7.1. P Q R S T U V W green arrows: substances moving one way orange arrows: substances moving the other way Neither figure is drawn to scale.

(a) Name the structures P, Q, R and S in Fig. 7.1. [2] (b) State the function of R and S. [2] (c) In Fig. 7.2, the green arrows and the orange arrows show substances moving in opposite directions. Name two substances travelling with the green arrows and two travelling with the orange arrows. [2] (d) Explain how substances are exchanged at V and W without the two bloodstreams mixing. [2] (e) State the source of progesterone in the later part of pregnancy. [1]

Model Answer(a) P = placenta, Q = umbilical cord [1]; R = amniotic sac, S = amniotic fluid [1].
(b) R, the amniotic sac, is the membrane that encloses the fluid and the fetus [1]. S, the amniotic fluid, protects the fetus — it cushions it against physical damage, supports it and keeps its surroundings constant [1].
(c) Green (to the fetus): any two of oxygen, glucose, amino acids, water, antibodies [1]. Orange (to the mother): carbon dioxide and urea [1].
(d) The fetal capillary (W) lies very close to the space filled with the mother’s blood (V), separated by a thin barrier with a large surface area, so substances cross by diffusion down their concentration gradients, and by active transport where they must move against a gradient [1]; the two bloodstreams remain completely separate and do not mix [1].
(e) The placenta [1].
Examiner’s NotesDo not give S the job of feeding the fetus — it protects, and everything the fetus receives comes through Q from P. In (c), “food” is too vague for a mark: name glucose or amino acids. The sentence “the two bloodstreams do not mix” is almost always worth a mark, so write it even when you are not sure it was asked for. Here is the same figure fully named:
mother’s blood A fetus in the uterus, drawn in section Inside the placenta A finger of fetal tissue pushed into a pool of the mother’s blood. The two bloodstreams never mix. placenta the exchange organ; see the inset umbilical cord carries blood between fetus and placenta amniotic sac the membrane enclosing the fetus amniotic fluid supports the fetus and protects it from bumps — it does not feed the fetus uterus wall thick and muscular cervix to the fetus → oxygen, glucose, amino acids, water, mineral ions, some of the mother’s antibodies ← to the mother carbon dioxide and urea Why the blood must stay separate The mother’s blood pressure would burst the fetal capillaries, and her immune system could attack fetal cells. Everything crosses the thin barrier by diffusion or active transport. The barrier is not a filter Some pathogens and some toxins cross it too and can harm the developing fetus. A large surface area, a short diffusion distance and a good blood supply on both sides — the same four features as an alveolus or a villus.
Question 7
[8 marks]
(a) Describe the changes that take place in the ovary and in the lining of the uterus during one menstrual cycle. [4] (b) Explain the roles of FSH and LH. [2] (c) A woman becomes pregnant. Explain why menstruation does not occur at the end of that cycle. [2]
Model Answer(a) In the ovary: a follicle develops during the first half of the cycle [1]; an egg cell is released at ovulation at about day 14, and the remains of the follicle form the corpus luteum [1]. In the uterus: the lining breaks down and is lost during days 1 to about 5 [1]; it is then repaired and thickened, and is maintained thick from about day 14 to day 24 before breaking down again [1].
(b) FSH stimulates a follicle in the ovary to develop (and stimulates the follicle to produce oestrogen) [1]. LH causes ovulation, the release of the egg cell [1].
(c) The embryo implants, so the corpus luteum is maintained and continues to produce progesterone [1]; progesterone maintains the thickened lining of the uterus, so it does not break down and there is no menstruation [1].
Examiner’s NotesPart (a) has two tracks and four marks — two marks for the ovary and two for the uterus lining — so an answer that describes only the bleeding cannot score more than half. Quote day numbers: they cost nothing and they prove you know the order. In (c), the marking point is progesterone stays high; an answer built on oestrogen will not score.
Question 8
[7 marks]
(a) Name the hormone that controls the development of male secondary sexual characteristics and the hormone that controls the development of female secondary sexual characteristics, and state the organ that produces each. [4] (b) State two secondary sexual characteristics that develop at puberty in a male and two that develop in a female. [2] (c) State one other role of oestrogen, in the menstrual cycle. [1]
Model Answer(a) Male: testosterone [1], produced by the testes [1]. Female: oestrogen [1], produced by the ovaries [1].
(b) Male, any two of: growth of facial, underarm and pubic hair; deepening of the voice; growth of the penis and testes; increased muscle development; growth spurt [1]. Female, any two of: development of the breasts; growth of underarm and pubic hair; widening of the hips; start of the menstrual cycle; growth spurt [1].
(c) Oestrogen repairs and thickens the lining of the uterus after menstruation (accept: it causes the surge of LH that triggers ovulation) [1].
Examiner’s NotesBoth hormones are produced by the gonads themselves — testes and ovaries — which is worth saying because candidates often invent another gland. Notice how tightly this joins to the rest of the topic: the same ovary that produces the egg cell also produces the hormone, and the same oestrogen that drives puberty also thickens the uterus lining each month.
Question 9
[8 marks]
(a) Define a sexually transmitted infection, and state the relationship between HIV and AIDS. [2] (b) Describe three ways in which HIV can be transmitted. [3] (c) Explain why a person with AIDS may die from an infection that would not seriously harm a healthy person. [2] (d) Explain why a fetus can become infected with HIV even though the mother’s and the fetus’s blood do not mix. [1]
Model Answer(a) A sexually transmitted infection is an infection transmitted through sexual contact [1]. HIV is the pathogen — a virus — and infection with HIV may lead to AIDS, which is a condition, not a pathogen [1].
(b) By sexual contact with an infected person [1]; by infected blood, for example sharing needles or receiving unscreened transfused blood [1]; from an infected mother to her child, across the placenta, during birth or in breast milk [1].
(c) HIV infects and destroys cells of the immune system [1], so the body can no longer produce enough antibodies or defend itself, and infections that a healthy immune system would destroy are able to spread [1].
(d) The placenta has a thin barrier that some pathogens can cross, so the virus can pass from the mother’s blood into the fetal blood even though whole blood does not mix [1].
Examiner’s NotesPart (a) is worth learning as two sentences. In (c) the mark is for naming the immune system cells as the target — “it makes you weak” scores nothing. Part (d) is the kind of link challenge papers love: it tests 16.6 and 16.4 at once, and the answer is the syllabus statement that some pathogens and toxins can cross the placenta.
Question 10
[9 marks]
(a) Fertilisation is defined in the same way in a flowering plant and in a human. State that definition, and then state exactly what fuses with what in each case. [3] (b) State where fertilisation takes place in a flowering plant and where it takes place in a human, and describe how the male gamete reaches the female gamete in each. [4] (c) Describe what the fertilised structure develops into in each case, up to the point at which it is able to grow independently. [2]
Model Answer(a) Fertilisation is the fusion of the nuclei of two gametes [1]. In a flowering plant, a pollen nucleus fuses with a nucleus in an ovule [1]. In a human, the nucleus of a sperm fuses with the nucleus of an egg cell; both nuclei are haploid and the zygote formed has a diploid nucleus [1].
(b) In a plant, fertilisation takes place in the ovule, inside the ovary [1]; the pollen grain lands on the stigma at pollination and grows a pollen tube down through the style and into the ovule, and the pollen nucleus travels down the tube [1]. In a human, fertilisation takes place in the oviduct [1]; the sperm swims there using its flagellum, and digests its way through the jelly coat using the enzymes in its acrosome [1].
(c) In a plant, the ovule becomes the seed (and the ovary becomes the fruit); the seed germinates when it has water, oxygen and a suitable temperature [1]. In a human, the zygote divides to form an embryo, which implants in the lining of the uterus and develops into a fetus, supplied through the placenta and umbilical cord [1].
Examiner’s NotesThis is the question that rewards seeing the topic as one idea. The definition of fertilisation does not change between a plant and an animal — only the names of the nuclei and the route the male gamete takes. Notice the parallel in (b): the plant gamete cannot swim, so it grows a tube to travel down; the human gamete can swim, so it carries mitochondria to power the journey. Two solutions to one problem, which is exactly the sort of comparison an examiner writes a nine-mark question around.
IGCSE Biology 0610 — Topic 16: Reproduction — Challenge Preparation
For examination preparation only. Not an official Cambridge Assessment resource.