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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 Does
Total: 12 marks
(a)[2]
State the definition of a drug.
Model Answer — 1a
a drug is any substance taken into the body [1]
that modifies or affects chemical reactions in the body [1]
⚠ If you missed marks here: This is a quotable one-line definition and it is worth learning word for word. The commonest loss is writing “affects the body” and stopping there — the mark scheme is paying for chemical reactions, which is the phrase that ties a drug to enzymes and to the rest of biology.
(b)[3]
The definition you have just written does not contain the word harmful, the word illegal or the word medicine. Explain why the definition is deliberately this wide, and give two examples of substances that are drugs by this definition even though they help the person taking them.
Model Answer — 1b
the test is only whether the substance changes chemical reactions in the body — whether the change is helpful or harmful is not part of the definition [1]
first example: an antibiotic taken to treat a bacterial infection is a drug [1]
second example: insulin injected by a person with diabetes is a drug (or a painkiller such as paracetamol) [1]
⚠ If you missed marks here: Everyday language uses “drug” to mean something harmful, and that is exactly the trap. Cambridge’s definition is about effect on chemical reactions, nothing else, so a medicine that saves a life is still a drug.
(c)[3]
State what antibiotics are used to treat, what they do to the organism causing that illness, and what effect they have on viruses.
Model Answer — 1c
antibiotics are used for the treatment of bacterial infections [1]
they kill bacteria (some stop the bacteria reproducing) [1]
they have no effect on viruses [1]
⚠ If you missed marks here: Three words are refused by mark schemes here: germs, infections as the thing killed, and less effective applied to viruses. Antibiotics kill bacteria, and against viruses the effect is none at all, not a smaller one.
(d)[2]
A patient says: “I have taken this antibiotic several times before, so my body has become resistant to it.” Identify the error in this statement and write a corrected version.
Model Answer — 1d
the error: it is the bacterium that is resistant, never the person or the person’s body [1]
corrected: some of the bacteria infecting the patient are resistant to that antibiotic, so it no longer kills them (the patient’s own cells are not involved) [1]
⚠ If you missed marks here: “The body builds up resistance” is the single commonest wrong sentence in this topic and it scores zero every time. Before you write the word resistant, check which organism the sentence has made the subject.
(e)[2]
State what is meant by saying that some bacteria are resistant to an antibiotic, and state the effect this has on that antibiotic.
Model Answer — 1e
resistant means the bacterium is not killed by that antibiotic and can still grow and reproduce in its presence [1]
this reduces the effectiveness of the antibiotic [1]
⚠ If you missed marks here: “Reduces the effectiveness” is Cambridge’s own wording and is worth using exactly. The antibiotic has not stopped existing or gone off — it still kills every non-resistant bacterium it meets.
Question 2 — The Disc Diffusion Plate
Total: 12 marks
A microbiologist at Ashcombe Hospital spreads an even lawn of one species of bacterium over an agar plate. She places four identical paper discs on the lawn, each soaked in a different antibiotic, A, B, C and D, and incubates the plate. The result is shown below.
(a)[2]
Using the zone diameters, place the four antibiotics in order, starting with the one to which this bacterium is most sensitive. Quote the diameters in your answer.
Model Answer — 2a
order of the three that gave zones: C (34 mm), then A (26 mm), then B (14 mm) [1]
D last — it gave no clear zone at all [1]
⚠ If you missed marks here: The question said quote the diameters, so an order with no figures in it throws away a mark that costs nothing to earn. Check the direction of the order too: most sensitive means the largest zone first.
(b)[2]
Describe what has happened in the region of agar inside the clear zone around disc A, and state what the 26 mm zone around A tells you compared with the 14 mm zone around B.
Model Answer — 2b
inside the zone no bacteria have grown — the bacteria there have been killed by the antibiotic (a zone of inhibition) [1]
A gave the larger zone, so this bacterium is more sensitive to A than to B [1]
⚠ If you missed marks here: A larger zone says the bacterium is more sensitive. It does not say the bacterium is more dangerous, and it does not say the bacterium is stronger — both of those are standard distractors.
(c)[3]
Explain, using what you know about diffusion, why each clear zone is circular and why the zone has a definite edge rather than fading away across the whole plate.
Model Answer — 2c
the antibiotic diffuses out of the disc into the agar, by net movement down a concentration gradient [1]
it diffuses equally in all directions from the disc, so the region it reaches is a circle [1]
its concentration falls with distance from the disc; at the edge of the zone the concentration has fallen too low to kill the bacteria, so beyond that line they grow normally [1]
⚠ If you missed marks here: The word diffuses has to be there — “the antibiotic spreads out” is a description, not an explanation. The third mark is about a threshold concentration, which is why the edge is sharp instead of blurred.
(d)[2]
State what the result for disc D shows about this bacterium, and explain what would happen if a patient infected with it were prescribed antibiotic D.
Model Answer — 2d
this bacterium is resistant to antibiotic D — it has grown right up to the disc, so D has not killed it [1]
the bacterial numbers would not fall, so the patient would not improve; the effectiveness of D against this bacterium is reduced to nothing [1]
⚠ If you missed marks here: No zone does not mean the disc failed or that the antibiotic was left out; it is a result, and the result is resistance. Notice also that the patient is not resistant to anything — the bacterium is.
(e)[3]
The microbiologist adds a fifth disc as a control. Describe what she should put on this disc, explain what the control shows, and name two variables she must keep the same for all five discs.
Model Answer — 2e
an identical paper disc soaked in sterile water, or a disc with no antibiotic on it [1]
it shows that any clear zone is caused by the antibiotic and not by the paper disc, the water or the act of placing a disc on the lawn [1]
any two of: same species of bacterium; same volume of liquid on each disc; same concentration of each antibiotic; same size of disc; same agar; same incubation temperature; same incubation time [1]
⚠ If you missed marks here: A control has to be identical apart from the one thing being tested, so an empty patch of agar with no disc on it is not a control. Naming variables also earns nothing unless they are variables that could actually change a zone diameter.
Question 3 — Where Resistance Comes From
Total: 12 marks
Read this information carefully. In any large population of bacteria there is variation: the bacteria are not all identical, and a few of them are already resistant to a particular antibiotic before that antibiotic has ever been used on them. The resistant feature is often carried on a plasmid. Use this information in your answers below.
(a)[4]
A patient is infected with a large population of one species of bacterium and is treated with an antibiotic. Describe, in order, what happens to that bacterial population, and state the effect on the proportion of resistant bacteria.
Model Answer — 3a
there is variation in the population and a few bacteria are already resistant before the antibiotic is used [1]
when the antibiotic is given, the non-resistant bacteria are killed [1]
the resistant few survive and reproduce, passing the resistance on to their offspring [1]
so the proportion of resistant bacteria in the population rises [1]
⚠ If you missed marks here: The four marks are four separate stages, and candidates who write one flowing sentence usually land only two of them. The last mark needs the word proportion — the number of resistant bacteria may fall at first; what rises is their share of what is left.
(b)[2]
A student writes: “The antibiotic made the bacteria become resistant to it.” Explain why this sentence is wrong, and write the correct relationship between the antibiotic and the resistant bacteria.
Model Answer — 3b
the resistant bacteria were already present before the antibiotic was used, so the antibiotic cannot have made them resistant [1]
the antibiotic selects them: it kills the rest and leaves the resistant ones to survive and reproduce — it does not create or cause the resistance [1]
⚠ If you missed marks here: Watch for the same idea in disguise: “the bacteria got used to it”, “the bacteria adapted to survive” and “the bacteria learned to resist it” are all the same error. Bacteria do not change themselves in response to being attacked.
(c)[2]
The stem states that the resistant feature is often carried on a plasmid. State what a plasmid is, and explain how having the feature on a plasmid means the resistance is passed on when the bacterium reproduces.
Model Answer — 3c
a plasmid is a small circle (ring) of DNA found in a bacterial cell, separate from the main circular DNA [1]
when the bacterium divides to make new cells, copies of the plasmid go into the new cells, so every one of its offspring carries the resistance too [1]
⚠ If you missed marks here: A plasmid is not the nucleus and it is not the main circular DNA — a bacterial cell has no nucleus at all. Naming the structure earns the first mark; the second mark is only for the passing on, which is what makes the resistance spread through the population.
(d)[2]
Explain why the process you described in part (a), repeated over many years, reduces the effectiveness of that antibiotic for everybody, not only for the patient who took it.
Model Answer — 3d
a greater and greater proportion of the bacteria in circulation are resistant, and these are passed between people [1]
so the antibiotic clears fewer and fewer infections — its effectiveness is reduced for every patient who is later infected by those bacteria [1]
⚠ If you missed marks here: The point of this part is that the bacteria travel between people even though the resistance never travels from the bacterium to the person. Keep those two sentences apart and this becomes easy.
(e)[2]
Under good conditions this species of bacterium divides once every 30 minutes. Calculate the number of bacteria that one surviving resistant bacterium would give rise to after 5 hours, and use your answer to explain why a bacterial population can change over days rather than years.
Model Answer — 3e
5 hours = 300 minutes, so 300 ÷ 30 = 10 divisions; number of bacteria = 210 = 1024 [1]
2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 = 1024
a single survivor produces over a thousand resistant bacteria in one afternoon, so a whole population can be replaced by resistant bacteria within days [1]
⚠ If you missed marks here: Count the divisions, not the hours — 5 hours is ten divisions, not five. And 10 divisions is not 10 × 2; each division doubles what is already there, so the answer grows very fast indeed.
Question 4 — Two Patients at the Same Clinic
Total: 12 marks
Two patients are seen on the same morning at the Ashcombe Clinic. Patient 1 has a throat infection caused by a bacterium. Patient 2 has influenza, which is caused by a virus. Both feel equally unwell and both ask for antibiotics.
(a)[2]
Name the type of pathogen infecting each patient, and state the two features to which a virus is limited.
Model Answer — 4a
patient 1: a bacterium; patient 2: a virus — both are pathogens, that is, disease-causing organisms [1]
the features of a virus are limited to a protein coat and genetic material [1]
⚠ If you missed marks here: Two items only, and “a coat” is not enough — the syllabus wording is a protein coat. Adding a cell membrane, cytoplasm or ribosomes to the list loses the mark, because a virus is not a cell.
(b)[3]
State which patient an antibiotic will help, and explain fully why an antibiotic can do nothing for the other patient at any dose.
Model Answer — 4b
only patient 1, the bacterial infection — antibiotics kill bacteria [1]
antibiotics have no effect on viruses, so patient 2 gains nothing however large the dose [1]
because a virus is not a cell: it has no cell wall, no ribosomes of its own and carries out no chemical reactions of its own, so there is nothing for the antibiotic to attack [1]
⚠ If you missed marks here: “Antibiotics do not work as well on viruses” scores nothing; the effect is none. The third mark is the one that separates a grade C answer from an A* answer, and it is a structural reason, not a strength-of-drug reason.
(c)[3]
While patient 2 waits for the illness to pass, the body’s own defences are dealing with the virus. Describe what the two types of white blood cell do, naming the molecules involved.
Model Answer — 4c
phagocytes engulf and digest pathogens [1]
lymphocytes produce antibodies [1]
an antibody has a shape complementary to an antigen on the pathogen, so it binds to it; the pathogen is then destroyed or marked for destruction by phagocytes [1]
⚠ If you missed marks here: This is the part of the answer that explains why patient 2 recovers anyway. An antibiotic never “boosts the immune system”; the defences described here do all of the work on a virus and most of the finishing on a bacterium.
(d)[2]
The doctor refuses to prescribe an antibiotic for patient 2. Explain why prescribing one would give patient 2 no benefit at all, yet would still do harm.
Model Answer — 4d
no benefit: the antibiotic has no effect on the virus, so the illness lasts exactly as long as it would have done [1]
harm: the antibiotic still kills the non-resistant bacteria the patient is carrying, leaving the resistant ones to survive and reproduce — so it selects for resistance for no gain [1]
⚠ If you missed marks here: The harm mark is the interesting one: the antibiotic never touches the influenza virus, but it still acts on every other bacterium in that patient. This is why “it cannot hurt to try” is wrong, and it is the heart of the Supplement objective.
(e)[2]
A student at the clinic suggests giving patient 2 a vaccine now, to cure the influenza. Explain why a vaccine would not cure this patient’s present illness.
Model Answer — 4e
a vaccine puts weakened pathogens or their antigens into the body to stimulate lymphocytes to make antibodies and memory cells, which takes time [1]
so it gives protection against a future infection; it does not remove a virus that is already multiplying in the patient [1]
⚠ If you missed marks here: A vaccine and an antibiotic do completely different jobs, and confusing them is a favourite examiner trap. One prevents in advance by using the patient’s own lymphocytes; the other treats a bacterial infection that is already there.
Question 5 — Using Antibiotics Only When Essential
Total: 10 marks
(a)[3]
Explain how using antibiotics only when they are essential limits the development of resistant bacteria.
Model Answer — 5a
every use of an antibiotic kills the non-resistant bacteria and leaves the resistant ones alive, so every use selects for resistance [1]
using antibiotics less often means less selection, so the resistant bacteria stay a small proportion of the population [1]
in practice this means prescribing them only for bacterial infections and never for viral ones, not using them routinely in farm animals, and keeping some antibiotics in reserve [1]
⚠ If you missed marks here: This is the whole Supplement objective in one part, so it must be an explanation, not a list of advice. The idea that carries the marks is that an antibiotic is a selecting agent, so the way to slow resistance down is to select less often.
(b)[3]
A patient with a bacterial infection is given a seven-day course of an antibiotic. She feels completely well after three days and stops taking it. Explain why she has been told to finish the whole course.
Model Answer — 5b
the symptoms stop long before the bacteria do — feeling well means the bacterial numbers have fallen, not that they have reached zero [1]
the bacteria still alive on day three are the ones least easily killed by that antibiotic [1]
stopping early leaves exactly those bacteria alive to survive and reproduce, so the infection can return and the returning population is harder to treat [1]
⚠ If you missed marks here: The first mark is a distinction between symptoms and bacterial numbers, and it is the one candidates skip. Do not write that she “becomes resistant” by stopping — what survives is a selected group of bacteria.
(c)[2]
State what MRSA is, and explain why infections caused by it are difficult to treat.
Model Answer — 5c
MRSA is a bacterium (methicillin-resistant Staphylococcus aureus) — it is not a virus [1]
it is resistant to several antibiotics at once, so most of the usual treatments fail and very few antibiotics are left that will kill it [1]
⚠ If you missed marks here: Two traps sit in this two-mark part. MRSA is a bacterium, not a virus and not a disease caught from dirt; and the difficulty comes from resistance to several antibiotics, not from the bacterium being unusually aggressive.
(d)[2]
Explain why a resistant bacterium such as MRSA matters most in a hospital. Refer in your answer to the patients and to how the bacterium is passed on.
Model Answer — 5d
hospital patients often have broken skin (wounds, burns, drips) and lowered defences, so the bacterium can enter the body easily and cause serious infection [1]
many people are close together and antibiotics are used heavily, and the bacterium spreads by direct contact and by contaminated surfaces — which is why hand washing and isolating patients matter [1]
⚠ If you missed marks here: “Hospitals are dirty” earns nothing. The two marks come from the transmission routes you already know and from the state of the patients, whose skin barrier has usually been broken by the treatment itself.
Question 6 — Why an Antibiotic Can Harm a Bacterium and Not You
Total: 12 marks
The diagram shows a bacterial cell.
(a)[3]
Using the diagram, name the structure present in this cell that an animal cell does not have at all, state the function of the ribosomes, and state one further way in which the genetic material of this cell differs from that of an animal cell.
Model Answer — 6a
the cell wall — an animal cell has none at all [1]
ribosomes are the site of protein synthesis, that is, they make proteins [1]
the bacterial DNA is a circular loop free in the cytoplasm, with extra small rings called plasmids, and there is no nucleus; an animal cell keeps its DNA in a nucleus and has no plasmids [1]
⚠ If you missed marks here: The cell wall is the answer to the first part, not the cell membrane — every cell has a membrane. And do not describe the circular DNA as “the nucleus”; a bacterial cell does not have one.
(b)[3]
Three antibiotics act in three different ways. Antibiotic V stops the bacterium building its cell wall. Antibiotic W binds to bacterial ribosomes. Antibiotic Y blocks an enzyme that the bacterium uses. For each one, explain why the patient’s own cells are not damaged.
Model Answer — 6b
V: an animal cell has no cell wall at all, so there is nothing in the patient’s cells for V to act on [1]
W: bacterial ribosomes are not the same as human ribosomes, so W binds to the bacterial ones and not to the patient’s [1]
Y: the patient does not have that enzyme, so the reaction Y blocks does not take place in human cells at all [1]
⚠ If you missed marks here: The whole idea is that a safe antibiotic attacks something the bacterium has and you do not. Test it the other way round: a drug that damaged cell membranes would be useless as an antibiotic, because your cells have membranes too.
(c)[3]
Antibiotic Y works by fitting into the active site of a bacterial enzyme. Explain, using what you know about enzymes, how this stops the reaction that the bacterium needs.
Model Answer — 6c
normally the substrate binds to the active site of the enzyme, because the two have complementary shapes [1]
the antibiotic molecule has a shape complementary to that active site, so it fits into the site and blocks it [1]
the substrate can no longer bind, so that reaction stops and the bacterium cannot make what it needs in order to grow and reproduce [1]
⚠ If you missed marks here: Cambridge marks the word complementary, not “the same shape”. The enzyme here is blocked, not denatured — denaturing is a change of shape caused by heat or extreme pH, which is a completely different mechanism.
(d)[3]
Explain why none of the three antibiotics V, W or Y would have any effect on a virus.
Model Answer — 6d
a virus is not a cell and has no cell wall, so V has nothing to act on [1]
a virus has no ribosomes of its own — it reproduces only inside a host cell — so W has nothing to bind to [1]
a virus carries out no chemical reactions of its own, so it has no enzyme for Y to block; this is why antibiotics have no effect on viruses [1]
⚠ If you missed marks here: “A virus is too small” is not the reason and scores nothing. The reason is structural: the features of a virus are limited to a protein coat and genetic material, so every target an antibiotic has is simply absent.
Question 7 — Fourteen Years of Resistance Data
Total: 10 marks
Ashcombe Hospital tested samples of one species of bacterium every year from 2010 to 2024 and recorded the percentage of samples that were resistant to each of two antibiotics, P and Q.
The hospital also recorded how many courses of each antibiotic it prescribed.
antibiotic
courses prescribed per 1000 patients, 2010
courses prescribed per 1000 patients, 2024
P
210
265
Q
6
5
(a)[3]
Describe the trend shown by each antibiotic between 2010 and 2024. Quote figures from the graph, including a comparison of the two antibiotics in 2016.
Model Answer — 7a
P rose continuously and steeply, from 4% in 2010 to 48% in 2024 [1]
Q rose only slightly and stayed almost level, from 2% to 7% over the same period [1]
in 2016 P had already reached about 20% while Q was only about 4% — P was already about five times Q [1]
⚠ If you missed marks here:Describe means read the graph and quote numbers with their units and years; it does not mean explain. An answer that says “P went up a lot and Q went up a little” is true and scores almost nothing.
(b)[2]
Calculate the increase in the percentage of samples resistant to antibiotic P between 2010 and 2024, and then calculate that increase as a percentage of the 2010 value. Show your working.
Model Answer — 7b
increase = 48 − 4 = 44 (percentage points) [1]
44 ÷ 4 × 100 = 1100%
as a percentage of the 2010 value the increase is 1100% [1]
⚠ If you missed marks here: Two different quantities are being asked for and they are easy to mix up. The rise is 44 percentage points; the rise relative to where it started is 1100%, because the starting value was tiny. Always divide by the original value.
(c)[2]
Antibiotic P was prescribed freely at Ashcombe Hospital while antibiotic Q was kept in reserve. Explain, in terms of selection, why the two curves are so different.
Model Answer — 7c
P was used very often, and every use killed the non-resistant bacteria and left the resistant ones to survive and reproduce, so the resistant proportion rose steadily [1]
Q was used rarely, so there was far less selection and the resistant bacteria remained a small proportion of the population [1]
⚠ If you missed marks here: Do not write that heavy use “made” the bacteria resistant to P. Heavy use did not create anything; it simply applied the same selection again and again, which is why the P curve climbs and the Q curve does not.
(d)[3]
A newspaper reports that these data prove that prescribing antibiotic P causes resistance to it. Evaluate this claim, and suggest what further evidence would be needed before the claim could be made with confidence.
Model Answer — 7d
the data show a correlation only — heavy prescribing of P and rising resistance to P happened together in one hospital, which does not by itself prove that one caused the other [1]
other factors could explain it: resistant bacteria may have been brought in by patients from elsewhere, P may also be widely used outside the hospital, and only one species in one hospital was tested [1]
further evidence: the same measurements from several other hospitals with different prescribing rates; and resistance figures from before and after a deliberate reduction in the prescribing of P at Ashcombe [1]
⚠ If you missed marks here:Evaluate asks you to judge how good the evidence is, so an answer that simply repeats the explanation from part (c) scores nothing here. The word the examiner is waiting for is correlation, followed by a change that would actually test it.
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