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This paper covers the whole of Topic 15. Like a real Cambridge paper, the seven questions range across every sub-topic — what a drug is and what antibiotics do, resistance and why antibiotics do nothing to viruses, and using antibiotics well — and about half the marks apply them to material from earlier topics, the way Cambridge examines this one. All three Topic 15 papers do.
Question 1 — What a Drug Is, and What an Antibiotic Actually Does
Total: 12 marks
(a)[2]
State what is meant by a drug. Then explain why a tablet taken to treat an illness counts as a drug under this definition.
Model Answer — 1a
a drug is any substance taken into the body that modifies or affects chemical reactions in the body [1]
the definition asks only what the substance does to the chemical reactions of the body; it does not require the substance to be harmful or illegal, so a medicine that changes the rate of a reaction in the body is a drug [1]
⚠ If you missed marks here: The commonest wrong answer is “a substance that harms the body”. Cambridge does not say harmful anywhere in this definition. Learn the two halves — taken into the body and modifies or affects chemical reactions in the body — because both are marked.
(b)[3]
A patient with a bacterial throat infection is prescribed an antibiotic. Two days later her sore throat is much better. Explain what the antibiotic has done, and explain why it is wrong to describe it as a painkiller.
Model Answer — 1b
antibiotics are used for the treatment of bacterial infections: they kill the bacteria (some stop the bacteria reproducing) [1]
the antibiotic does not act on the pain, or on any other symptom, directly [1]
the sore throat improves because the number of bacteria falls — the improvement in symptoms is a consequence of that, not the mechanism [1]
⚠ If you missed marks here: “It relieves the symptoms” is the trap, and it is a real belief, not a careless answer — people feel better and assume that is what the tablet did. The chain is always kills bacteria → fewer bacteria → symptoms fade, and the middle step is the one that earns the mark.
(c)[3]
Her brother has influenza. He asks for the same antibiotic, saying that it will boost his immune system. Explain why the doctor refuses, and explain what is wrong with his description of what an antibiotic does.
Model Answer — 1c
influenza is caused by a virus, and antibiotics have no effect on viruses [1]
an antibiotic does not boost or strengthen the immune system [1]
what it does is reduce the number of bacteria, so that the body’s own defences can finish the job [1]
⚠ If you missed marks here: “Boosts the immune system” appears on no mark scheme and is worth watching for in your own writing — it sounds scientific and says nothing. The white blood cells you met in Topic 10 are unchanged by the drug; all that changes is how many bacteria they have left to deal with.
(d)[4]
A second patient is told that the bacteria causing her infection are resistant to the antibiotic she was given. State what this means, explain why it matters, and explain why it would be wrong to say that the patient has become resistant.
Model Answer — 1d
a resistant bacterium is not killed by that antibiotic — it survives a normal dose [1]
so the infection is not cleared: some bacteria are resistant to antibiotics, and this reduces the effectiveness of antibiotics [1]
resistance is a property of the bacterium, never of the person [1]
nothing has changed in the patient’s own cells; what has changed is the population of bacteria infecting her, which is why a different antibiotic may still work [1]
⚠ If you missed marks here: “The body builds up resistance to antibiotics” is the single most common error in this topic, and it is fatal here because it makes the last two marks impossible. Say the word bacterium out loud before you write the sentence.
Question 2 — Two Flasks, One Antibiotic
Total: 12 marks
Technicians at Bellingham Hospital tested an antibiotic on a bacterium taken from a patient. Two identical flasks of nutrient broth were inoculated with the same bacterium and kept at 30 °C. The antibiotic was added to flask 2 at 4 hours; nothing was added to flask 1. The number of bacteria in each flask was found every hour by counting colonies on agar plates.
Two things are given to you so that you do not have to assume them. First, the bacteria respire and use the nutrients in the broth for growth, and no more broth is added during the 12 hours. Second, in any large population of bacteria there is variation, and a few bacteria are already resistant to a given antibiotic before it is ever used.
Time / hours
0
2
4
6
8
10
12
Flask 1 (no antibiotic) / thousand bacteria per cm³
10
28
96
290
640
850
900
Flask 2 (antibiotic at 4 h) / thousand bacteria per cm³
10
24
62
40
11
3
2
(a)[3]
Describe the change in the number of bacteria in flask 1 over the 12 hours. Use figures from the data.
Model Answer — 2a
the number rises from 10 to 900 thousand bacteria per cm³ over the 12 hours [1]
the rise is slow at first — only 96 thousand per cm³ by 4 hours [1]
it is fastest between about 4 and 8 hours (96 to 640 thousand per cm³), then levels off after about 10 hours (850 to 900) [1]
⚠ If you missed marks here: “It goes up” is a one-mark answer at best. Describe means quote figures and say where the shape changes; the levelling off at the end is a separate marking point from the rise, and candidates who write only about the middle of the graph lose it.
(b)[2]
Calculate the mean rate of increase in the number of bacteria in flask 1 between 4 hours and 8 hours. Show your working and give the unit.
Model Answer — 2b
increase = 640 − 96 = 544 thousand per cm³
correct increase of 544 thousand bacteria per cm³ over the 4 hours [1]
rate = 544 ÷ 4 = 136
136 thousand bacteria per cm³ per hour, with the unit given [1]
⚠ If you missed marks here: A rate is a change divided by the time it took, so dividing by 8 instead of 4 is the usual slip — the interval is 4 hours long, not 8. The unit carries a mark of its own, and “per hour” on the end of the number is what makes it a rate.
(c)[2]
Explain why flask 1 is described as the control, and state one conclusion that could not have been drawn without it.
Model Answer — 2c
flask 1 was treated in exactly the same way — same bacterium, same broth, same temperature, same time — except that no antibiotic was added, so it shows what happens without the one factor being tested [1]
without it you could not conclude that the fall in flask 2 was caused by the antibiotic rather than by the broth running out of nutrients [1]
⚠ If you missed marks here: “To compare” is not enough — the mark is for saying compare with what. A control differs in exactly one thing, and here flask 1 rising to 900 is the evidence that the broth was still good and that only the antibiotic can explain flask 2.
(d)[3]
The number of bacteria in flask 2 falls after the antibiotic is added, but it does not reach zero: from 11 hours onwards it stays at 2 thousand per cm³. Explain what this suggests about those bacteria, and explain what would happen to the population if the antibiotic were now removed.
Model Answer — 2d
a small number of bacteria were not killed by the antibiotic; a few in the population were already resistant to it before it was added [1]
the antibiotic killed the non-resistant bacteria and the resistant ones survived — the antibiotic selected them, it did not make them resistant [1]
if the antibiotic were removed they would reproduce and the population would grow again, but now almost all of it would be resistant, so that antibiotic would no longer be effective against it [1]
⚠ If you missed marks here: The tempting sentence is “the survivors got used to the antibiotic”. Bacteria do not adjust and do not learn; the resistant ones were there from the start. Write survive and reproduce and you have said the whole thing.
(e)[2]
Suggest why the number of bacteria in flask 1 levels off after about 10 hours, even though no antibiotic was ever added to it.
Model Answer — 2e
the flask is a closed container and no more broth is added, so the nutrients are used up — they are respired to release energy and used for growth [1]
so the bacteria can no longer divide as quickly and the number levels off; waste products also build up in the broth [1]
⚠ If you missed marks here: This is the part that makes the control matter, so read it alongside (c). Note that levelling off is not the same as falling: the bacteria are not dying in large numbers, they have simply stopped dividing as fast.
Question 3 — A Virus Is Not a Cell
Total: 12 marks
A patient at the Bellingham walk-in clinic has a common cold, which is caused by a virus. He asks for an antibiotic. The diagram compares a bacterium with a virus.
(a)[2]
State the two features to which a virus is limited. Then name two structures found in a bacterial cell that a virus does not have.
Model Answer — 3a
a protein coat and genetic material [1]
any two of: cell wall, cell membrane, cytoplasm, ribosomes, circular DNA, plasmids [1]
⚠ If you missed marks here: The syllabus word is limited to, so adding a third feature such as a nucleus or a membrane loses the mark rather than earning extra. A virus has no nucleus at all — its genetic material is not enclosed in one.
(b)[4]
Explain fully why an antibiotic has no effect on a virus.
Model Answer — 3b
antibiotics kill bacteria but have no effect on viruses [1]
the reason is structural: a virus is not a cell [1]
it has no cell wall and no ribosomes of its own, so the structures an antibiotic acts on are simply absent [1]
it carries out no chemical reactions of its own — it reproduces only inside a host cell — so there is nothing for an antibiotic to interfere with [1]
⚠ If you missed marks here: “Antibiotics work less well on viruses” is wrong in a way that costs every mark: the effect is none, not reduced. Notice how the whole answer follows from four words — a virus is not a cell.
(c)[3]
The patient asks whether a double dose would clear his cold. Explain why it would not, and give one reason why taking the antibiotic anyway would still do harm.
Model Answer — 3c
there is no target at any dose — raising the concentration cannot create a structure or a reaction that the virus does not have [1]
so the patient gets no benefit at all, however much he takes [1]
the antibiotic still acts on the harmless bacteria he is carrying: the non-resistant ones are killed and any resistant ones survive and reproduce, so it selects for resistance for no gain [1]
⚠ If you missed marks here: The belief behind this question is that a big enough dose eventually wins. It does not, because the problem is not strength but the absence of a target. The third mark is the one candidates leave out: an unnecessary course is not merely useless, it does damage.
(d)[3]
His cold clears up after a week with no drug at all. Explain what cleared it, and state what could have prevented him catching it in the first place.
Model Answer — 3d
his own body defences dealt with it: phagocytes engulf and digest pathogens, and lymphocytes produce antibodies [1]
the antibodies have a shape complementary to the antigens on the virus, so they bind to them and the virus is destroyed or marked for destruction [1]
vaccination against that virus could have prevented the infection — an antibiotic never could [1]
⚠ If you missed marks here: This is where Topic 10 pays for itself. The word Cambridge marks is complementary, not “the same shape”. And note the pairing this whole question is built on: a virus can be prevented by a vaccine and cannot be treated by an antibiotic.
Question 4 — Cholera in the Bellingham District
Total: 12 marks
After heavy flooding, a clinic in the Bellingham district treats a large number of patients with cholera. Many arrive severely dehydrated.
(a)[2]
Name the type of organism that causes cholera, and describe how it is transmitted.
Model Answer — 4a
cholera is caused by a bacterium [1]
transmission is indirect, in contaminated water (water containing sewage) or contaminated food [1]
⚠ If you missed marks here: Cholera being bacterial is the fact the whole question turns on, and it is why an antibiotic can help here but not in a cold. Flooding is the clue to the route: sewage mixes with drinking water.
(b)[4]
Explain how the cholera bacterium produces the symptoms of the disease.
Model Answer — 4b
the bacterium produces a toxin [1]
the toxin causes chloride ions to be secreted into the small intestine [1]
water then moves into the intestine by osmosis, from a higher water potential to a lower water potential through a partially permeable membrane [1]
this causes watery diarrhoea, leading to dehydration and loss of ions from the blood [1]
⚠ If you missed marks here: The bacterium does not “eat the lining” or “push water out”. The order is toxin, then chloride ions, then water follows by osmosis, and the osmosis mark needs the words water potential and partially permeable, exactly as in Topic 3.
(c)[3]
Explain why rehydration is the main treatment for a cholera patient, and why preventing cholera in the district depends on clean water and sewage treatment rather than on antibiotics.
Model Answer — 4c
the immediate danger is the loss of water and ions, so rehydration therapy replaces them and keeps the patient alive [1]
the bacterium is taken in with contaminated water, so treating sewage and supplying clean drinking water breaks the transmission route and people are not infected at all [1]
an antibiotic can only treat somebody who is already ill; it can shorten the illness, but it would not stop that person being infected again from the same water supply [1]
⚠ If you missed marks here: Notice the difference between treating a patient and controlling a disease. People die of cholera from dehydration, not from the bacteria directly, which is why fluid comes first — and public health works by cutting the transmission route, which no drug can do.
(d)[3]
An antibiotic can shorten the illness in a cholera patient, but would be of no use at all to a patient with influenza. Explain why. A health worker then suggests giving the antibiotic to every person in the district, whether ill or not. Give one reason why this would be a poor decision.
Model Answer — 4d
cholera is caused by a bacterium, and antibiotics are used for the treatment of bacterial infections — they kill bacteria [1]
influenza is caused by a virus, and antibiotics have no effect on viruses [1]
giving it to everybody is use when it is not essential: people who are not infected gain nothing, and every use kills the non-resistant bacteria and leaves the resistant ones to survive and reproduce, so resistance would rise and the antibiotic would become less effective when it is really needed [1]
⚠ If you missed marks here: The tempting answer to the last part is “it would be too expensive”. Cost is not the biology. The reason is that mass use is a mass selection event, which is exactly what “only when essential” is designed to avoid.
Question 5 — Using Antibiotics Only When Essential
Total: 10 marks
You are given this, so that you do not have to assume it: in any large population of bacteria there is variation, and a few bacteria are already resistant to a given antibiotic before it is ever used. Bacteria also reproduce very quickly, so a population can change over days rather than years.
(a)[3]
Explain how using antibiotics only when essential can limit the development of resistant bacteria.
Model Answer — 5a
when an antibiotic is used, the non-resistant bacteria are killed and the few resistant ones survive [1]
the survivors reproduce and pass the resistance on, so the proportion of resistant bacteria in the population rises — the antibiotic selects resistant bacteria, it does not create them [1]
every use of an antibiotic is therefore a selection event, so fewer uses means less selection and the resistant bacteria stay a small minority [1]
⚠ If you missed marks here: Any answer containing “the bacteria become resistant because they were exposed” loses the whole question. The resistant ones were already there; the antibiotic only decides which ones are left. Survive and reproduce is the phrase to write.
(b)[3]
State three things that doctors, patients or farmers can do to make antibiotic resistance develop more slowly.
Model Answer — 5b
any three, one mark each: prescribe antibiotics only for bacterial infections, never for viral ones [1]
complete the full course — symptoms stop long before the bacteria do, and the ones still alive when you feel better are the hardest to kill [1]
do not use antibiotics routinely in farm animals; keep some antibiotics in reserve for infections nothing else will treat; use hygiene such as hand washing and isolation to stop a resistant strain spreading between patients [1]
⚠ If you missed marks here: “Stop taking them when you feel better” is the answer people actually give, and it is precisely backwards. Three marks means three separate measures — writing the same idea three ways scores one.
(c)[2]
Explain why the routine use of antibiotics in healthy farm animals is a particular problem.
Model Answer — 5c
it is a very large amount of antibiotic used when there is no infection to treat, so it is the opposite of using antibiotics only when essential [1]
it selects for resistant bacteria in the animals, and those bacteria can reach people through food or through direct contact, so the resistance is already present when a doctor needs that antibiotic for a patient [1]
⚠ If you missed marks here: The mark is not for saying it is unfair on the animals. It is for the scale of the selection and for the link back to people — resistant bacteria do not stay on the farm.
(d)[2]
State what MRSA is, and explain why an infection with it is difficult to treat.
Model Answer — 5d
MRSA is a bacterium — methicillin-resistant Staphylococcus aureus [1]
it is resistant to several antibiotics at once, so most antibiotics will not kill it and very few treatments are left [1]
⚠ If you missed marks here: Plenty of candidates write that MRSA is a virus, or a disease you catch from dirt. It is a bacterium, and it matters most in hospitals because the people there already have broken skin and lowered defences.
Question 6 — A Vaccine and an Antibiotic Do Different Jobs
Total: 12 marks
A public health team at Bellingham runs a vaccination programme and also supplies antibiotics to its clinics. A member of the public asks why both are needed.
(a)[4]
Describe how a vaccine produces long-term immunity, from the injection onwards.
Model Answer — 6a
weakened pathogens, or their antigens, are put into the body [1]
the antigens stimulate an immune response by lymphocytes [1]
the lymphocytes produce antibodies, which have a shape complementary to the antigen [1]
memory cells remain, so if the same pathogen is met later, antibodies are produced rapidly and in large amounts — long-term immunity [1]
⚠ If you missed marks here: An answer that begins “antibodies are injected” has described passive immunity instead and loses the first mark and usually the rest. The step candidates forget under time pressure is the memory cells, and it is the one that explains the word long-term.
(b)[3]
Explain why a vaccine can protect a person against a virus but an antibiotic cannot.
Model Answer — 6b
a vaccine works through the body’s own immune system, which recognises the antigens on the surface of the virus — a protein coat still carries antigens [1]
an antibiotic acts on structures that a bacterial cell has, such as its cell wall or its ribosomes [1]
a virus is not a cell and has none of them, so antibiotics kill bacteria but have no effect on viruses [1]
⚠ If you missed marks here: The comparison mark is for saying what each one acts on. A vaccine never touches the virus itself — it prepares your lymphocytes — whereas an antibiotic needs a physical target, and that is the whole difference.
(c)[2]
Explain why the vaccination programme does not remove the need for antibiotics, and why antibiotics do not remove the need for the vaccination programme.
Model Answer — 6c
a vaccine is given before infection and prevents disease; an antibiotic is given after infection and treats it [1]
a vaccine protects only against the one pathogen whose antigens it carries, and there is no vaccine for most bacterial infections, so antibiotics are still needed; equally, no antibiotic will help against the viral diseases the vaccines prevent [1]
⚠ If you missed marks here: A common belief is that a vaccine and an antibiotic are two versions of the same thing. One is prevention and is specific to a single pathogen; the other is treatment and works only on bacteria. Say both halves.
(d)[3]
A successful vaccination programme against a bacterial disease also slows the development of antibiotic resistance. Explain how.
Model Answer — 6d
fewer people develop the infection, so fewer courses of antibiotic are prescribed [1]
each course kills the non-resistant bacteria and leaves any resistant ones to survive and reproduce, raising the proportion that are resistant [1]
so fewer courses mean less selection, and resistant bacteria such as MRSA develop more slowly — this is what using antibiotics only when essential means in practice [1]
⚠ If you missed marks here: The link runs through the number of prescriptions, not through the vaccine acting on bacteria. A vaccine does nothing to a resistant bacterium; it simply means the antibiotic is never reached for.
Question 7 — Prescribing Rates and Resistance in Six Regions
Total: 10 marks
The Bellingham prescribing study collected data from six regions of one country in the same year. For each region it recorded the number of antibiotic prescriptions issued per 1000 people per year, and the percentage of samples of one bacterium that were resistant to that antibiotic.
Region
Prescriptions per 1000 people per year
Samples resistant / %
A
380
11
B
520
17
C
610
22
D
740
29
E
890
36
F
455
33
(a)[3]
Describe the relationship between the prescribing rate and the percentage of samples that are resistant. Use figures from the table.
Model Answer — 7a
as the prescribing rate increases, the percentage of resistant samples increases — a positive correlation [1]
supported with figures, e.g. region A has 380 prescriptions per 1000 people per year and 11% resistant, while region E has 890 and 36% resistant [1]
the pattern holds for five of the six regions (A, B, C, D and E); region F does not follow it [1]
⚠ If you missed marks here: Two habits earn the extra marks every time: quote a pair of figures from each end of the range, and say openly that one point does not fit. Describing a trend as though every point obeys it is treated as a misreading of the data.
(b)[2]
Identify the region that does not fit the pattern, quoting its figures, and suggest one reason for it.
Model Answer — 7b
region F: only 455 prescriptions per 1000 people per year, yet 33% of samples resistant — close to region E on resistance but close to region A on prescribing [1]
any sensible suggestion, e.g. a resistant strain has spread from patient to patient in a hospital there because of poor hygiene; or prescribing in that region was much higher in earlier years; or resistant bacteria were brought in by people from another region; or antibiotics are used routinely in farm animals there [1]
⚠ If you missed marks here: An anomaly must be named with its numbers — “F is different” scores nothing. And a suggestion has to be a mechanism that could really raise resistance; “the data are wrong” is not one.
(c)[3]
A newspaper reports: doctors in region E have caused the resistance there by prescribing too much. Evaluate this claim using the data.
Model Answer — 7c
the data show a correlation only, and a correlation does not by itself show causation [1]
the regions differ in other ways that were not controlled — hygiene, the number of hospitals, how many patients complete the course, use in farm animals — and any of these could affect the resistance figure [1]
region F shows that a high resistance figure can occur without a high prescribing rate, which weakens the claim; but the biological mechanism (each use kills non-resistant bacteria and lets resistant ones survive and reproduce) makes it a reasonable hypothesis worth testing [1]
⚠ If you missed marks here:Evaluate means argue both ways and then judge. Answers that only write “correlation is not causation” get one mark; answers that only agree with the newspaper get one mark. The strongest point available to you is region F, because it is evidence from the table itself.
(d)[2]
State two pieces of further data that would be needed before the newspaper claim could be accepted.
Model Answer — 7d
prescribing and resistance figures for the same regions over several years, to see whether resistance rose after prescribing rose [1]
any one of: the number of samples tested in each region; confirmation that the same species of bacterium and the same antibiotic were tested everywhere; data from many more regions; figures for antibiotic use in farm animals in each region [1]
⚠ If you missed marks here: “More data” is not an answer — say which data and what it would settle. Time order is the strongest single request you can make, because a cause has to come before its effect.
Self-Assessment
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A : 48-55
B : 40-47
C : 32-39
D : 24-31
E : 16-23
U : <16
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