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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 — Two Events, Several Days Apart
Total: 12 marks
Fig. 1.1 shows an insect-pollinated flower that has been cut in half from top to bottom. Nothing has been named for you. Work from the position and the shape of each structure.
(a)[4]
Name the structures labelled B, E and G on Fig. 1.1, and name the group of structures marked by the bracket K.
Model Answer — 1(a)
B — the anther; it makes and releases the pollen grains [1]
E — the style; the pollen tube grows down through it [1]
G — an ovule; it contains the female gamete and becomes the seed [1]
K — the carpel (stigma + style + ovary together, the female part of the flower) [1]
⚠ If you missed marks here: The bracket is the part people lose. K encloses the stigma, the style and the ovary, so a single-structure answer such as “ovary” cannot be right — a bracket always means a group. The bracket on the other side, J, is the stamen = anther + filament. Note also that G is an ovule, not an ovary: the ovary is the whole chamber (F), the ovules are the small round structures inside it. One ovule becomes one seed; the ovary becomes the fruit.
(b)[3]
Pollination and fertilisation are two different events. Describe what happens in each one, and state which letter on Fig. 1.1 marks the structure where each takes place.
Model Answer — 1(b)
pollination is the transfer of pollen grains from an anther to a stigma — so it takes place at D, the stigma [1]
fertilisation is the fusion of the nucleus of a pollen grain with a nucleus in an ovule [1]
fertilisation therefore takes place at G, inside an ovule, and it happens some time after pollination, once a pollen tube has grown down through E [1]
⚠ If you missed marks here: This is the single commonest error in the whole topic, so be ruthlessly precise. Pollination moves a whole grain and stops at the stigma. Fertilisation fuses two nuclei and happens deep inside the ovule. Writing “pollination is when pollen fertilises the egg” scores nothing at all — it merges the two events and names neither. They also happen in two different places and at two different times.
(c)[3]
A single pollen grain lands on D. Describe how this leads to fertilisation.
Model Answer — 1(c)
a pollen tube grows out of the pollen grain and grows down through the style (E) [1]
the pollen nucleus travels down inside the pollen tube [1]
the tube enters the ovule (G) and the pollen nucleus fuses with the nucleus of the female gamete inside it, forming a zygote [1]
⚠ If you missed marks here: Two things sink most answers. First, the pollen grain does not travel down the style — a tube grows down and carries the nucleus. Second, “the pollen tube reaches the ovule” is only two-thirds of the story; nothing has been fertilised until you say the nuclei fuse. Cambridge does not want any detail about what happens after that inside the seed, so do not waste time on it.
(d)[2]
In some plants the pollen from a flower can only ever reach a stigma on the same plant. Suggest why a population of such plants is at greater risk than a population in which pollen is carried between different plants of the same species.
Model Answer — 1(d)
pollen and ovule come from the same plant, so the offspring are genetically very similar to each other and to the parent — there is very little variation in the population [1]
so if the environment changes, or a new disease arrives, the whole population is affected in the same way; there are unlikely to be any individuals with features that let them survive, so the population could be wiped out [1]
⚠ If you missed marks here: Do not write “they would be clones”. Self-pollination still involves two gametes fusing, so the offspring are similar, not identical — that word belongs to asexual reproduction only. The mark for the second point needs the consequence spelled out: little variation matters because no individual happens to have the feature that would survive the change. “They would all die” on its own explains nothing.
Question 2 — The Field That Was Too Alike
Total: 12 marks
A farmer grows the same species of root crop in two fields. In field 1 the plants are grown from pieces of tuber cut from last year’s plants. In field 2 the plants are grown from seed produced after pollen had been carried from one plant to another. The crop was recorded for three years. Table 2.1 shows the results.
field 1 (grown from tubers)
field 2 (grown from seed)
mean mass of crop per plant / kg
4.2
3.6
range of mass of crop per plant / kg
4.0 to 4.4
1.8 to 5.9
plants infected by a new fungal disease in year 1 / %
2
2
plants infected by the same disease in year 3 / %
96
31
(a)[3]
Describe the difference between the two fields in the range of mass of crop per plant, quoting figures from Table 2.1, and explain this difference in terms of how each crop was produced.
Model Answer — 2(a)
the range in field 1 is only 0.4 kg (4.0 to 4.4) but in field 2 it is 4.1 kg (1.8 to 5.9) — about ten times wider [1]
the field 1 plants were produced asexually, from one parent and with no gametes and no fusion of nuclei, so they are genetically identical to each other [1]
the field 2 plants were produced sexually, by the fusion of the nuclei of gametes from two plants, so they are genetically different from each other [1]
⚠ If you missed marks here: The first mark is a data mark and it needs numbers, not “field 1 is more uniform”. Work the range out (largest minus smallest) rather than just copying the two values. For the explanation, the phrase Cambridge is listening for is genetically identical versus genetically different — “the same” and “different” are too loose to score, because the plants also differ in size, which is exactly what the table is about.
(b)[3]
Explain why, by year 3, 96% of the plants in field 1 were infected but only 31% of those in field 2 were infected.
Model Answer — 2(b)
the field 1 plants are genetically identical, so every plant has the same resistance to the fungus; if one plant can be infected, all of them can [1]
so once the fungus reached the field it spread through the whole crop, and there were no plants left uninfected [1]
the field 2 plants are genetically different, so some of them have features that make them harder to infect; those plants stayed healthy, which is why the figure stopped at 31% [1]
⚠ If you missed marks here: Watch the direction of the argument. The plants in field 1 are not infected because they are identical — being identical is what removes the chance that any one of them happens to be resistant. Also, do not say the fungus “became stronger” or that the plants “became weaker”: nothing changed about the fungus, only the proportion of the crop it could get into. Notice both fields started at exactly 2% — that is the examiner telling you the difference cannot be about how much fungus arrived.
(c)[2]
Suggest two advantages to the farmer, other than the mean mass of crop, of growing plants from tubers rather than from seed.
Model Answer — 2(c)
the plants are genetically identical to a parent with known, desirable features, so the crop is uniform — it ripens together and the roots are all a similar size, which makes harvesting and selling easier [1]
no pollination is needed, so the crop does not depend on insects being present or on the weather being suitable, and reproduction is more reliable [1]
accept also: only one parent plant is needed / the plants grow and mature more quickly because there is no seed stage
⚠ If you missed marks here: The question says “other than the mean mass of crop”, so any answer about yield scores nothing however true it is. Read that clause. Also, “it is faster” needs a reason attached — faster because there is no seed and no germination stage. And the pollination point is the one people forget: a crop grown from seed is at the mercy of whether pollinating insects turn up.
(d)(i)[2]
In field 2, a pollen nucleus fused with a nucleus in an ovule. State whether the nucleus of the pollen grain and the nucleus of the zygote that was formed are haploid or diploid.
Model Answer — 2(d)(i)
the pollen nucleus is haploid [1]
the zygote nucleus is diploid [1]
⚠ If you missed marks here: Both words describe a nucleus, not a whole cell and not an organism, so answer in those terms. The logic is worth holding on to: two haploid nuclei fuse and the product must therefore be diploid — that is the whole point of fertilisation. A haploid nucleus is not “half a nucleus” or a damaged one; it is a complete nucleus carrying one set of the information instead of two.
(d)(ii)[2]
A student wrote: “The field 1 plants reproduce asexually, so their gametes must all be identical.” Explain what is wrong with this statement.
Model Answer — 2(d)(ii)
asexual reproduction does not involve gametes at all, so the plants in field 1 are not producing gametes for this process and there is nothing for the student to be comparing [1]
there is also no fusion of nuclei in asexual reproduction — the new plants grow from a piece of one parent, which is why they are genetically identical [1]
(the plants can still flower and make gametes for sexual reproduction; the point is that the tubers had nothing to do with gametes)
⚠ If you missed marks here: The student has landed on the right conclusion — the offspring are identical — by completely the wrong route, and an “explain what is wrong” question is marking the route. Name the definition: asexual reproduction produces genetically identical offspring from one parent, with no gametes and no fusion of nuclei. If you find yourself writing about gametes anywhere in an asexual reproduction answer, stop and check.
Question 3 — Two Places, Two Events
Total: 12 marks
Fig. 3.1 shows the human female reproductive system seen from the front. Nothing has been named for you. Two regions have been ringed and labelled X and Y.
(a)[4]
Name the structures labelled A, B, D and E on Fig. 3.1.
Model Answer — 3(a)
A — the ovary [1]
B — the oviduct [1]
D — the lining of the uterus (accept uterus lining / endometrium) [1]
E — the cervix [1]
⚠ If you missed marks here: Cambridge uses oviduct. “Fallopian tube” is the name you will hear everywhere else and it will not be credited, so train yourself out of it now. The other trap is D: the question labels the lining separately from C, the uterus, precisely to see whether you notice. If you wrote “uterus” for both C and D you have thrown a mark away, and the lining is the structure the whole of the menstrual cycle is about.
(b)[2]
Name the event that takes place in region X and the event that takes place in region Y, and describe what happens in each.
Model Answer — 3(b)
at X, in the oviduct: fertilisation — the nucleus of a sperm cell fuses with the nucleus of the egg cell to form a zygote [1]
at Y, in the lining of the uterus: implantation — the embryo, a ball of cells, sinks into the thickened lining and becomes attached to it [1]
⚠ If you missed marks here: The classic error is to put fertilisation in the uterus. It happens in the oviduct, and the embryo then takes several days to travel down before it implants — two events, two organs, days apart. Also be precise about what fertilisation is: the fusion of the nuclei, not “the sperm meets the egg”. Meeting is not fusing.
(c)[3]
Structure B on Fig. 3.1 is 110 mm long. After it is released, an egg cell is moved along the oviduct at a mean speed of 1.6 mm per hour. An egg cell can only be fertilised for about 24 hours after it is released. Calculate how long the egg cell would take to travel the whole length of the oviduct, and use your answer to explain why fertilisation can only take place in region X.
Model Answer — 3(c)
time = 110 ÷ 1.6 = 68.75 hours (accept 69 hours, or about 2.9 days) [1]
but the egg cell can only be fertilised for about 24 hours, which is roughly one third of that journey [1]
so a sperm cell must reach it while it is still in the part of the oviduct nearest the ovary — region X. Any further along and the egg cell would already have passed the point at which it can be fertilised [1]
⚠ If you missed marks here: Do the arithmetic before you write the sentence: the whole force of the answer is that 24 hours is only about a third of 69, so the egg cell is only about a third of the way down the oviduct when its chance runs out. An answer that says “because X is where fertilisation happens” is circular — the question is asking you to show why, from the numbers. Keep the unit (hours) on your answer; a bare 68.75 is not a time.
(d)[3]
Structure D thickens during each cycle. Explain why this thickening matters for the event at Y, and describe what happens to D if no embryo arrives.
Model Answer — 3(d)
the lining becomes thick and develops a rich blood supply, so the embryo has something it can sink into and attach to [1]
the embryo can then obtain oxygen and nutrients from the mother’s blood in the lining, which it needs because its own food store is nearly used up by then [1]
if no embryo implants, the level of progesterone falls, the lining breaks down and is lost through the cervix and vagina — menstruation — and the cycle begins again [1]
⚠ If you missed marks here: The third mark needs the cause, not just the event. “She has a period” describes what happens; the mark is for the lining breaking down because progesterone is no longer keeping it in place. And notice the lining is not simply a soft cushion — the reason it matters is the blood supply, because that is how the embryo is supplied before the placenta exists.
Question 4 — Three Hundred Million Against One
Total: 12 marks
Table 4.1 gives information about two cells, P and Q. Both are human cells and both are gametes.
cell P
cell Q
greatest length of the cell / µm
60
130
volume of the cell / µm³
30
1 150 000
can the cell move itself?
yes
no
number released at one time
3 × 108
1
(a)(i)[2]
Identify cell P and cell Q. For each one, give a different piece of evidence from Table 4.1 that supports your answer.
Model Answer — 4(a)(i)
P is the male gamete, the sperm cell — evidence: it can move itself (it has a flagellum), or 3 × 108 are released at one time, or its volume is tiny [1]
Q is the female gamete, the egg cell — evidence: only one is released at a time, or it cannot move itself, or its volume is enormous because of its energy stores [1]
⚠ If you missed marks here: Both marks need the evidence as well as the name, and the two pieces of evidence must be different from each other. Do not be caught by the length row: cell P is only about twice as long as cell Q, because most of P’s length is a very thin flagellum. Length is the one row that makes the two cells look similar — volume is the row that tells the truth.
(a)(ii)[2]
Calculate how many times greater the volume of cell Q is than the volume of cell P. Show your working and give your answer to two significant figures.
⚠ If you missed marks here: Two marks means the working is worth one of them on its own, so write the division down even if you are confident. “Two significant figures” means 38 000, not 38 333 and not 38 333.33 — and it does not mean two decimal places. The number is worth pausing on: the egg cell has about forty thousand times the volume, and almost all of that is food store.
(b)[3]
Cell P has a flagellum, a large number of mitochondria packed immediately behind its head, and a vesicle at its tip called the acrosome. Explain how each of these three features helps cell P to carry out its function.
Model Answer — 4(b)
the flagellum beats from side to side, so the cell can swim through the fluid in the uterus and along the oviduct to reach the egg cell [1]
the mitochondria are the site of aerobic respiration, which releases the energy needed for the flagellum to keep beating over that whole journey [1]
the acrosome contains enzymes that digest a path through the jelly coat of the egg cell, so that the sperm nucleus can enter and fuse with the egg cell nucleus [1]
⚠ If you missed marks here: Say flagellum, never “tail”. The mitochondria mark is not given for “they give it energy” — name aerobic respiration and say what the energy is used for. And the acrosome does not “break the egg”: it releases enzymes that digest a route through the jelly coat, which is a controlled chemical process, not a hole punched by force.
(c)[2]
Explain two adaptive features of cell Q.
Model Answer — 4(c)
it contains large energy stores (food reserves) in its cytoplasm, which supply the zygote and then the embryo with the energy and materials it needs until it has implanted and can obtain nutrients from the mother [1]
it is surrounded by a jelly coat that changes as soon as one sperm nucleus has entered, so that no further sperm cells can get in and only one nucleus fuses with the egg cell nucleus [1]
⚠ If you missed marks here: “It is big” is not an adaptive feature; it is a description. The mark is for saying what the size is for — food reserves for the days between fertilisation and implantation, when nothing is feeding the embryo. For the jelly coat, the change happens at fertilisation, not before it: its job is not to keep sperm out generally, it is to keep the rest out once one has arrived.
(d)(i)[2]
A sperm cell in which the mitochondria are damaged still has a normal flagellum and a normal acrosome, but it never reaches an egg cell. Explain why.
Model Answer — 4(d)(i)
damaged mitochondria mean aerobic respiration cannot take place at a normal rate, so far less energy is released from glucose [1]
the flagellum cannot go on beating without that energy, so the cell cannot swim far enough to reach the egg cell in the oviduct — having the right structures is useless if there is no energy to work them [1]
⚠ If you missed marks here: The trap is to blame the flagellum: the question has already told you the flagellum is normal, so any answer about the flagellum being faulty contradicts the stem. Read the given information as a set of things you are not allowed to blame. Also say released, not “made” — respiration releases energy from glucose, it does not create it.
(d)(ii)[1]
State the term used to describe the nucleus of cell P and the nucleus of cell Q.
Model Answer — 4(d)(ii)
both nuclei are haploid [1]
⚠ If you missed marks here: One word, so give the exact one. Every gamete nucleus is haploid, whichever gamete it is — the difference between cell P and cell Q is in size, structure, motility and number, never in this. The nucleus formed when they fuse, the zygote nucleus, is diploid.
Question 5 — Five Tubes and a Wrong Assumption
Total: 10 marks
A student investigated the conditions needed for the germination of cress seeds. She set up five tubes, each containing 100 seeds on cotton wool, and left them for five days. Tube D contained water that had been boiled to drive out the dissolved oxygen and was then covered with a layer of oil to keep air out. Tube B was wrapped in aluminium foil. Table 5.1 shows her results.
tube
water
temperature / °C
oxygen available
light
seeds germinated after 5 days / %
A
yes
20
yes
yes
92
B
yes
20
yes
no
90
C
no
20
yes
yes
0
D
yes
20
no
yes
3
E
yes
2
yes
yes
5
(a)[2]
State which two tubes should be compared to find out whether light is needed for germination, and state the conclusion, quoting figures from Table 5.1.
Model Answer — 5(a)
compare tubes A and B — they differ only in light; water, temperature and oxygen are the same in both [1]
92% germinated in the light and 90% in the dark: a difference of only 2%, so light is not needed for germination [1]
⚠ If you missed marks here: This is the trap in the question, and most people walk into it. Light is not one of the conditions needed for germination — the three are water, oxygen and a suitable temperature. A seed germinates underground, in the dark, using the food store inside it; it does not need to photosynthesise until the shoot is above the soil. Also, quote both figures: “A and B are about the same” does not earn the data mark.
(b)[3]
Explain the results for tubes C, D and E.
Model Answer — 5(b)
C: no water, so the food store cannot be dissolved and transported and the enzymes have no water to work in; the seed coat is not softened either, so no seeds germinate [1]
D: no oxygen, so the seeds cannot carry out aerobic respiration and almost no energy is released for growth, so only 3% germinate [1]
E: at 2 °C the temperature is far below the optimum, so the enzymes that break down the stored food work very slowly — only 5% germinate in the five days [1]
⚠ If you missed marks here: Three separate reasons are needed, so do not let them collapse into “the conditions were not right”. Take care with tube E: the enzymes at 2 °C are not denatured, they are simply working slowly because the molecules have less kinetic energy — and the proof is that the seeds are still alive and would germinate if warmed. Denaturing happens at high temperature and it is permanent. For tube D, name aerobic respiration; “they could not breathe” scores nothing.
(c)[2]
The student used 100 seeds in each tube rather than 5. Explain how this makes her conclusions more reliable, and state one other variable she must keep the same in every tube.
Model Answer — 5(c)
with 100 seeds, one or two dead or damaged seeds change the percentage very little, so a single unusual seed cannot distort the result; the percentage is a much better estimate of what the seeds of that species do [1]
any one sensible controlled variable, for example: the same variety and age of seed / the same volume of water / the same mass of cotton wool / the same five-day period / the same size of tube [1]
⚠ If you missed marks here: Say what a large sample actually does — it dilutes the effect of an anomalous seed — rather than the empty phrase “it makes it more accurate”. Reliability and accuracy are different things, and this is reliability. For the second mark, do not name light, water, temperature or oxygen: those are the variables she is deliberately changing between tubes, so they cannot be your answer.
(d)[1]
Suggest one improvement to tube D so that the student can be more confident that it was the lack of oxygen, and nothing else, that stopped the seeds germinating.
Model Answer — 5(d)
set up a further tube in which the water is boiled and cooled in exactly the same way and covered with the same layer of oil, but is then bubbled with air to put the oxygen back — then boiling and the oil are the same in both tubes and oxygen is the only difference [1]
accept also: use the same boiled and cooled water in every tube, so that any effect of boiling applies to all of them equally
⚠ If you missed marks here: The point of this part is that tube D changed three things at once: it removed the oxygen, but it also boiled the water and covered it with oil. Any of those could have killed the seeds. A good improvement therefore keeps the boiling and the oil and adds the oxygen back — that is what isolates the variable. “Repeat it” is not an answer here; repeating a flawed design just gives you the same flaw twice.
(e)[2]
Instead of growing cress from seed, a grower could produce new plants from pieces cut from a parent plant. State one advantage and one disadvantage of producing plants in this way.
Model Answer — 5(e)
advantage: the new plants are genetically identical to the parent, so every plant has the same useful features (such as flavour or size) and the crop is uniform; no pollination or seed is needed [1]
disadvantage: there is no genetic variation between the plants, so a single disease or a change in conditions could destroy the entire crop at once [1]
⚠ If you missed marks here: Answer as a grower would, with a consequence attached. “They are all the same” is not an advantage until you say why sameness is useful, and it is not a disadvantage until you say what it costs. Note that the same fact — identical offspring — is both the advantage and the disadvantage, which is exactly why Cambridge likes asking this.
Question 6 — Four Lines and One Decision
Total: 12 marks
Fig. 6.1 shows the concentration of four hormones in the blood of a woman during one 28-day cycle. The hormones are labelled W, X, Y and Z. Two of them are produced in an ovary. The other two are produced by a gland in the brain and act on the ovary.
(a)[4]
Identify hormones W, X, Y and Z.
Model Answer — 6(a)
W is FSH — it comes from the brain and rises slowly over the first twelve days, which is the time during which a follicle containing an egg cell is developing in the ovary [1]
X is LH — the single sharp spike on day 13 is unmistakable; it is the surge that causes the egg cell to be released [1]
Y is oestrogen — it comes from the ovary, peaks just before the egg cell is released, and rises a second time later in the cycle [1]
Z is progesterone — it stays very low until the egg cell has been released, then rises to a peak around day 21 and falls sharply at the end of the cycle [1]
⚠ If you missed marks here: Work from the shapes, not from memory of a list. One spike, very tall and very narrow, is always LH. A hump entirely in the second half of the cycle is always progesterone. Then the two remaining lines split by which comes from where: the stem told you two hormones come from the brain, so W and X are the pair with the early activity, and Y and Z are the ovary pair. The commonest mix-up is W and Y, because both rise in the first half — separate them by the second rise, which only oestrogen has.
(b)[2]
State the day of the cycle on which the egg cell was released, and give the evidence from Fig. 6.1 for your answer.
Model Answer — 6(b)
day 14 (accept day 13 to 14) [1]
evidence: hormone X reaches a sharp peak on day 13, and it is this surge that causes the egg cell to be released; also, hormone Z only begins to rise after day 14, because it is produced by the structure left behind in the ovary once the egg cell has gone [1]
⚠ If you missed marks here: The second mark is for evidence, so name a hormone and describe what its line does. “Because it is always day 14” is not evidence, it is a memorised number, and in a different cycle length it would be wrong. Read the release of the egg cell from the graph itself: the surge comes first, the release follows it.
(c)[3]
Describe the role of hormone W, hormone Y and hormone Z in this cycle.
Model Answer — 6(c)
W (FSH) stimulates a follicle containing an egg cell to develop and mature in the ovary, and stimulates the ovary to produce oestrogen [1]
Y (oestrogen) causes the lining of the uterus to repair and thicken after menstruation, and once its level is high it triggers the surge of X [1]
Z (progesterone)maintains the thickened lining of the uterus; when its level falls at the end of the cycle the lining breaks down [1]
⚠ If you missed marks here: Every hormone needs a target and an effect: what it acts on and what that does. “It controls the cycle” earns nothing from any of the three. Keep oestrogen and progesterone apart by their jobs — oestrogen builds the lining, progesterone keeps it. A hormone travels in the blood, so it reaches the whole body and only affects the organs that respond to it; it does not travel along a nerve.
(d)[3]
In a later cycle, an embryo implants in the lining of the uterus on day 22. Describe how the concentrations of hormone Z and hormone W over the following weeks would differ from those shown in Fig. 6.1, and explain the importance of each difference.
Model Answer — 6(d)
Z (progesterone) stays high instead of falling towards day 28, and remains high throughout the pregnancy [1]
so the lining of the uterus is maintained and does not break down; menstruation does not happen and the embryo is not lost (later in pregnancy the placenta takes over producing progesterone) [1]
W (FSH) stays low throughout, so no further follicles develop and no more egg cells are released while the woman is pregnant [1]
⚠ If you missed marks here: This part is testing whether you understand the cycle as a chain of causes rather than a set of four graphs to memorise. If progesterone is what holds the lining in place, then a pregnancy that depends on that lining must keep progesterone up — the graph follows from the biology, not the other way round. Do not write that the woman “stops having a cycle because she is pregnant”; that is the observation, and the mark is for the hormone that causes it.
Question 7 — A Pathogen, a Condition, and the Difference Between Them
Total: 10 marks
Read this before you begin. HIV is a virus. A virus consists only of genetic material inside a protein coat; it is not a cell and it can only reproduce inside a living host cell. Antibiotics are drugs that kill bacteria.
(a)[2]
State what is meant by a sexually transmitted infection, and explain why HIV and AIDS are not the same thing.
Model Answer — 7(a)
a sexually transmitted infection is an infection that is transmitted from one person to another through sexual contact [1]
HIV is the pathogen — the virus itself, which a person becomes infected with. AIDS is the condition that HIV infection may lead to, often years later, when the immune system has been damaged badly enough that the person suffers repeated serious infections [1]
⚠ If you missed marks here: Use the word pathogen for HIV; that is the term the syllabus uses and it is what separates the two ideas. A person can be infected with HIV for many years and be well, so “HIV is another name for AIDS” is not a small slip — it removes the whole distinction the question is built on. Note also that HIV is a sexually transmitted infection, not the only one.
(b)[2]
Describe two ways, other than sexual contact, in which HIV may be transmitted from one person to another.
Model Answer — 7(b)
through blood — sharing needles or syringes, or receiving a transfusion of infected blood or blood products [1]
from an infected mother to her child — across the placenta to the fetus, during birth, or in breast milk [1]
(all routes involve the transfer of body fluids containing the virus from an infected person)
⚠ If you missed marks here: Both routes must be genuine ones. HIV is not transmitted by touching, sharing cups or plates, coughing, insect bites or using the same toilet, and offering one of those loses the mark. The mother-to-child route is worth knowing precisely because it links to the placenta: the placenta allows exchange between the two bloodstreams without them mixing, but some pathogens and toxins can still cross it.
(c)[3]
HIV infects and destroys a type of white blood cell called a lymphocyte. Lymphocytes produce antibodies. Explain how the destruction of these cells leads to AIDS.
Model Answer — 7(c)
the number of lymphocytes in the blood falls steadily, so far fewer antibodies are produced [1]
antibodies normally attach to the antigens on pathogens and mark them for destruction, so without them pathogens entering the body are not destroyed and multiply freely [1]
the person therefore suffers repeated and severe infections, including ones a healthy immune system would deal with easily — this collection of illnesses is AIDS [1]
⚠ If you missed marks here: Follow the chain all the way through: fewer lymphocytes → fewer antibodies → pathogens not destroyed → repeated infections → AIDS. A common half-answer stops at “the immune system is weakened”, which restates the question. Be clear too that people with AIDS become ill from other pathogens; HIV itself does not cause the pneumonia or the other infections, it removes the defence against them.
(d)[1]
A person infected with HIV is given a course of antibiotics. Explain why the antibiotics will have no effect on the HIV.
Model Answer — 7(d)
antibiotics kill bacteria, and HIV is a virus, not a bacterium, so the antibiotic has no effect on it [1]
(extra, not needed for the mark: a virus is not a cell — it has no cell wall, no cytoplasm and no ribosomes for an antibiotic to act on, and it reproduces inside the person’s own cells)
⚠ If you missed marks here: One mark, so make the contrast explicit in one sentence: antibiotics act on bacteria, HIV is a virus. The reason people get this wrong is a general belief that antibiotics are a cure for “germs”. They are not; they are drugs with a specific target. Antibiotics may still be prescribed to someone with HIV — but for the bacterial infections their damaged immune system cannot fight off, never for the virus itself.
(e)[2]
Describe two ways in which the spread of sexually transmitted infections in a country can be reduced.
Model Answer — 7(e)
testing and treating: people are screened so that infections are found early, and infected people are treated with antiviral drugs, which lowers the amount of virus in their body fluids and makes them far less likely to pass it on [1]
contact tracing and education: the partners of an infected person are traced and offered testing, and people are taught how these infections are and are not transmitted [1]
accept also: screening all donated blood before transfusion; not sharing needles and providing clean ones; treating infected pregnant women so the virus is not passed to the fetus
⚠ If you missed marks here: This is a public-health question, so answer at the level of a country, not an individual: what does a health service actually do? Testing, treating, tracing, screening donated blood, and education. Two clearly different measures are needed — “test people” and “screen people” are the same measure written twice and will score once. Note that treating infected people is itself a way of reducing spread, which is not obvious until you think about it.
Self-Assessment
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A* : 56+
A : 48-55
B : 40-47
C : 32-39
D : 24-31
E : 16-23
U : <16
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