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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, What Antibiotics Do, and MRSA
Total: 12 marks
(a)[3]
State the definition of a drug, and use your definition to explain why an antibiotic prescribed at the Norbury Health Centre counts as 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]
an antibiotic is taken into the body and changes the chemical reactions going on there, and the definition does not require a drug to be harmful or illegal [1]
⚠ If you missed marks here: The commonest wrong opening is “a drug is a harmful substance”. Cambridge’s definition is deliberately wide — nothing in it says harmful, illegal or medicine — and the two marks are for the two halves of the wording, so learn it as one sentence.
(b)[3]
State what antibiotics are used to treat, state what they do to the organisms causing that kind of infection, and state their effect on a virus.
Model Answer — 1b
antibiotics are used for the treatment of bacterial infections [1]
they kill bacteria (some stop bacteria reproducing) [1]
they have no effect on viruses [1]
⚠ If you missed marks here: “Kill germs” and “cure infections” appear on no mark scheme — the word is bacteria. And an antibiotic does not “work less well” on a virus; it has no effect at all, which is a different statement and the one that earns the mark.
(c)[3]
Explain how using antibiotics only when they are essential can limit the development of resistant bacteria.
Model Answer — 1c
every use of an antibiotic kills the bacteria that are not resistant and leaves the resistant ones alive — it selects for resistance [1]
using antibiotics less often means less of this selection, so the resistant bacteria stay a small minority of the population [1]
in practice: prescribe them only for bacterial infections and never for viral ones, do not use them routinely in farm animals, and keep some in reserve [1]
⚠ If you missed marks here: Notice the verb in the question: limit, not prevent. Resistant bacteria already exist whether or not anyone takes an antibiotic, so no policy can stop resistance appearing. What restraint changes is how big a share of the population the resistant ones end up being.
(d)[3]
State what kind of organism MRSA is, explain why an infection with it is difficult to treat, and correct the statement “a patient who has had many antibiotics becomes resistant to them”.
Model Answer — 1d
MRSA is a bacterium — methicillin-resistant Staphylococcus aureus — and is not a virus [1]
it is resistant to several antibiotics at once, so the antibiotics that would normally be used no longer kill it [1]
it is the bacterium that is resistant, never the person; the patient’s own body has not changed at all [1]
⚠ If you missed marks here: Two errors are examined here at once. MRSA is not a virus — if it were, no antibiotic would ever have worked on it and the word resistant would be meaningless. And a body does not “build up resistance”: resistance is a property of the bacterium, so it is the bacteria a patient carries that have changed, not the patient.
Question 2 — Testing Three Antibiotics on a Plate
Total: 12 marks
A student at Norbury College investigated three antibiotics, J, K and L. She spread the same species of bacterium evenly over the agar in a plate to make a lawn, then placed on it one paper disc soaked in antibiotic J, one soaked in K, one soaked in L, and one soaked in sterile water. The plate was incubated at 25 °C for 48 hours. She then measured the diameter of each clear zone in mm, across its centre. She repeated the whole investigation three times.
Disc
Zone diameter, repeat 1 / mm
Zone diameter, repeat 2 / mm
Zone diameter, repeat 3 / mm
antibiotic J
18
19
18
antibiotic K
11
30
12
antibiotic L
22
23
24
sterile water
0
0
0
(a)[3]
Identify the independent variable, the dependent variable, and one variable that the student controlled.
Model Answer — 2a
independent variable: which antibiotic is on the disc (the type of antibiotic) [1]
dependent variable: the diameter of the clear zone, in mm [1]
any one controlled variable: the species of bacterium / the concentration and volume of solution on each disc / the size of the discs / the incubation temperature, 25 °C / the incubation time, 48 hours / the same agar [1]
⚠ If you missed marks here: A variable has to be something that could have been different. “The bacteria” and “the plate” are not variables; the species of bacterium and the incubation temperature are. The dependent variable is always the thing measured, so it has to carry a unit.
(b)[2]
The disc soaked in sterile water is the control. Explain what its result tells the student.
Model Answer — 2b
the water disc contains no antibiotic but is identical in every other way, so it shows what happens when the antibiotic is the only thing removed [1]
it gave no clear zone (0 mm), so any clear zone round J, K or L must be caused by the antibiotic itself and not by the paper disc, the liquid or the handling [1]
⚠ If you missed marks here: A control is not the same thing as a repeat. Repeats tell you whether your measurements are reliable; the control tells you what the result would have been with no antibiotic, which is the only way to know the antibiotic caused the zone.
(c)[3]
One of the results for antibiotic K is anomalous. Identify it, calculate the mean zone diameter for antibiotic K leaving it out, and suggest one cause of the anomaly.
Model Answer — 2c
the anomaly is 30 mm in repeat 2, which is far larger than the 11 mm and 12 mm of the other two repeats [1]
mean = (11 + 12) ÷ 2 = 11.5 mm
mean zone diameter for K = 11.5 mm [1]
any one sensible cause: the wrong disc was used, e.g. a disc soaked in L instead of K / the disc was soaked in a more concentrated solution / two discs were placed too close together so the zones ran into one another / the diameter was not measured across the centre [1]
⚠ If you missed marks here: An anomaly is left out of the mean but never quietly deleted — you say which value it is and why you excluded it. And “human error” earns nothing on its own: name the actual mistake that would produce a zone nearly three times too wide.
(d)[2]
Suggest two improvements to this investigation.
Model Answer — 2d
do more repeats, for example five, so that the mean is more reliable and a single anomaly has less effect on it [1]
any one further improvement: measure each zone in two directions across the centre and take a mean of those / check that every disc carries the same volume and concentration of solution / use aseptic technique throughout / space the discs further apart so the zones cannot overlap [1]
⚠ If you missed marks here: “Be more careful” and “use better equipment” score nothing anywhere on this syllabus. An improvement has to name the step being changed and say what it fixes — reliability of the mean, or a variable that was not properly controlled.
(e)[2]
The student then looked at the bacteria under a microscope and made a drawing of one of them. Her drawing is 42 mm long. The bacterium is actually 3 µm long. Calculate the magnification of her drawing. Show your working.
Model Answer — 2e
convert so that both lengths are in the same unit: 42 mm × 1000 = 42 000 µm [1]
⚠ If you missed marks here: Dividing 42 by 3 gives 14, which is the answer to a question nobody asked — the units have to match before you divide. One millimetre is 1000 micrometres, so you multiply by 1000 going from mm to µm. Magnification is a ratio, so it has no unit at all.
Question 3 — Where Resistance Comes From
Total: 12 marks
In any large population of bacteria there is variation: a few of the bacteria are already resistant to an antibiotic before that antibiotic has ever been used. The diagram shows a population of 30 bacteria in a patient at Norbury General Hospital, before, during and after a course of one antibiotic.
(a)[3]
Describe what each of the three panels of the diagram shows.
Model Answer — 3a
before: no antibiotic has been used; there are 30 bacteria, of which 28 are sensitive and 2 are already resistant [1]
during: the antibiotic is present; the 28 sensitive bacteria are killed and the 2 resistant ones survive [1]
after: the 2 survivors have reproduced, so the population is back up in number but is now made up entirely of resistant bacteria [1]
⚠ If you missed marks here: The whole point of the first panel is the word already. If your description of the middle panel says the bacteria became resistant when the antibiotic arrived, you have described a different diagram from the one printed. A describe question also wants the numbers: 28 and 2, not “most” and “a few”.
(b)[3]
Explain why the proportion of this population that is resistant rises from 2 in 30 to all of them.
Model Answer — 3b
the resistant bacteria are not killed by the antibiotic, so they survive [1]
the bacteria that are not resistant are killed, so they leave no offspring and the survivors are left with no competition [1]
the survivors reproduce and pass the resistance on to their offspring, so the proportion of the population that is resistant rises [1]
⚠ If you missed marks here: Watch the word proportion. Straight after the antibiotic there are only 2 bacteria left, far fewer than before — the number has fallen while the proportion that is resistant has gone from 2 in 30 to 2 in 2. Bacteria divide very quickly, which is why the number climbs back within days.
(c)[2]
A student writes: “The antibiotic made these bacteria resistant.” Explain why this is wrong.
Model Answer — 3c
the 2 resistant bacteria were already resistant before the antibiotic was used — the variation was there first [1]
the antibiotic does not cause resistance; it selects for it, by removing every bacterium that is not resistant [1]
⚠ If you missed marks here: The forbidden verbs are learn, adapt, get used to and make. A single bacterium never changes during this story; what changes is which bacteria are still alive. Say selects and the mark is yours.
(d)[2]
The resistance in these bacteria is carried on a plasmid. State what a plasmid is and where in the bacterial cell it is found.
Model Answer — 3d
a plasmid is a small circular piece of DNA, separate from the main circular DNA of the bacterium [1]
it is found in the cytoplasm of the bacterial cell [1]
⚠ If you missed marks here: The trap is answering “in the nucleus”. A bacterium has no nucleus — its structures are limited to cell wall, cell membrane, cytoplasm, ribosomes, circular DNA and plasmids, and both lots of DNA sit loose in the cytoplasm.
(e)[2]
Explain what the situation shown in the third panel means for the doctor treating this patient.
Model Answer — 3e
this antibiotic no longer kills the bacteria in the patient, so its effectiveness is reduced and continuing with it will not clear the infection [1]
a different antibiotic, one to which this bacterium is still sensitive, would have to be used instead [1]
⚠ If you missed marks here: “The antibiotic stops working” is close, but the mark scheme phrase is reduces the effectiveness of the antibiotic. Note too that the antibiotic is unchanged — it is still perfectly good against every other bacterium that is sensitive to it.
Question 4 — An Antibiotic That Blocks a Bacterial Enzyme
Total: 12 marks
Antibiotic Z works by binding to the active site of an enzyme that the bacterium uses to build its cell wall. Human cells do not have this enzyme.
(a)[3]
Explain, using the idea of the active site, how antibiotic Z stops this enzyme working.
Model Answer — 4a
an enzyme has an active site with a shape complementary to its substrate, which is why the enzyme is specific [1]
antibiotic Z is shaped to fit that active site, so it binds there and occupies it [1]
the substrate can then no longer enter the active site, so no enzyme–substrate complexes form, the reaction is not catalysed and the cell wall cannot be built [1]
⚠ If you missed marks here: Cambridge marks complementary and refuses “the same shape”. Also be careful not to say the antibiotic denatures the enzyme — nothing here changes the shape of the active site, it is simply blocked, and that is a different mechanism with a different marking word.
(b)[2]
Explain why antibiotic Z can be taken by a patient without affecting the reactions in the patient’s own cells.
Model Answer — 4b
enzymes are specific: each has an active site of its own particular shape, and antibiotic Z is not complementary to the active sites of human enzymes, so it cannot bind to them [1]
the patient does not have this enzyme at all, because human cells have no cell wall to build — so the reaction being blocked happens only in the bacterium [1]
⚠ If you missed marks here: The answer is not “the dose is too small to hurt you”. An antibiotic is safe because of what it targets, not how much of it there is: it attacks something a bacterial cell has and a human cell has not, which is why the cell wall and the bacterial ribosomes are such useful targets.
(c)[4]
The activity of this bacterial enzyme was measured at six temperatures, once with antibiotic Z present and once without it. Describe and explain the results.
Temperature / °C
10
20
30
40
50
60
Enzyme activity without antibiotic Z / arbitrary units
8
20
38
52
24
2
Enzyme activity with antibiotic Z / arbitrary units
2
5
9
12
6
1
Model Answer — 4c
without antibiotic Z the activity rises from 8 units at 10 °C to a maximum of 52 units at 40 °C, then falls steeply to 2 units at 60 °C [1]
with antibiotic Z the curve is the same shape, peaking at 40 °C, but the activity is much lower at every temperature — 12 units instead of 52 at the peak [1]
the rise up to 40 °C is because the molecules have more kinetic energy, so enzyme and substrate collide more often and more enzyme–substrate complexes form [1]
above 40 °C the enzyme is denatured: the shape of the active site changes, so the substrate no longer fits and the activity falls [1]
⚠ If you missed marks here: Enzymes are not alive, so they are never “killed” — the word is denatured, and the mark is for saying that the active site changes shape. Notice also that antibiotic Z does not move the optimum: both peaks are at 40 °C. That tells you the antibiotic is reducing how much enzyme is available to work, not changing the enzyme itself.
(d)[3]
A patient with a viral infection asks for antibiotic Z. Explain why it would do nothing at all for that infection, whatever the dose.
Model Answer — 4d
a virus is not a cell: its features are limited to a protein coat and genetic material [1]
so it has no cell wall, no ribosomes of its own and none of the enzymes antibiotic Z is shaped to block — there is nothing for the antibiotic to bind to [1]
a virus carries out no chemical reactions of its own and reproduces only inside a host cell, so raising the dose cannot help; antibiotics have no effect on viruses [1]
⚠ If you missed marks here: The reason is structural, not a matter of strength. “A higher dose would eventually work” is the misconception being examined: if there is no target, no amount of a drug can hit it. Taking the antibiotic anyway would give the patient no benefit at all — and would still select for resistance among the bacteria they are carrying.
Question 5 — A Resistant Strain on a Hospital Ward
Total: 10 marks
Norbury General Hospital finds that a strain of MRSA is passing between patients on one of its wards.
(a)[3]
Describe two ways in which this bacterium is likely to pass from one patient to another, and state what is actually being passed between them.
Model Answer — 5a
direct contact — the bacteria are carried on the hands of staff or visitors who touch one patient and then another, with nothing in between [1]
indirectly, on contaminated surfaces or equipment — bed rails, door handles, instruments — touched by one person and then by the next [1]
what is passed is the bacterium itself; resistance is not passed to a person, and no patient becomes resistant [1]
⚠ If you missed marks here: Writing that “the resistance spreads round the ward” sounds right and is worth nothing: resistance is a property of a bacterium, so what travels from bed to bed is a living organism. Everything you learned about direct and indirect transmission applies here unchanged — MRSA is simply a bacterium that is hard to treat once it arrives.
(b)[2]
Explain how hand washing and isolating an infected patient limit the spread of this strain.
Model Answer — 5b
hand washing removes the bacteria from the hands, so they are not carried on to the next patient [1]
isolating the patient keeps the resistant bacteria away from everybody else, so fewer people become infected and fewer courses of antibiotic have to be given [1]
⚠ If you missed marks here: Hygiene is on the syllabus as part of using antibiotics well, and the second half of the isolation mark is the part people miss: fewer infections means fewer antibiotic courses, and every course avoided is one less selection event.
(c)[2]
Explain why a patient recovering from surgery on this ward is at far greater risk from this bacterium than a visitor who walks through it.
Model Answer — 5c
the skin is a physical barrier that keeps pathogens out, and it works only while it is unbroken [1]
surgery cuts through the skin, so bacteria can enter the tissues and the blood directly through the wound, whereas the visitor’s skin is intact [1]
⚠ If you missed marks here: This is a barrier question, not an immunity question. Nothing has happened to the patient’s white blood cells; a doorway has simply been opened that the visitor does not have.
(d)[3]
The hospital instructs all its doctors to prescribe antibiotics only when they are essential. Explain why this has to be a rule for everybody rather than a decision each doctor makes for each patient.
Model Answer — 5d
every course of antibiotic given anywhere in the hospital selects for resistant bacteria in the patient who takes it [1]
those resistant bacteria can then be passed to other patients, so one unnecessary prescription reduces the effectiveness of that antibiotic for everybody on the ward, not just for that patient [1]
so if only some doctors hold back, selection carries on and the resistant strain keeps its advantage; the benefit appears only when the total number of unnecessary courses falls [1]
⚠ If you missed marks here: The mistake is to answer as though the consequences stayed inside one patient. Resistant bacteria move between people, so this is a question about a population, and the phrase the mark scheme is waiting for is reduces the effectiveness of the antibiotic.
Question 6 — Two Patients, One Antibiotic
Total: 12 marks
Two patients at Norbury General Hospital had the same bacterial infection and were given the same 10-day course of the same antibiotic. One took the whole course. The other felt better on day 3 and stopped taking it. The number of bacteria in each patient was measured every day.
(a)[3]
Describe the two curves, quoting figures from the graph.
Model Answer — 6a
both start at 1000 arbitrary units on day 0 and fall together — 700 on day 1, 420 on day 2, 210 on day 3 — so the curves are identical up to day 3 [1]
the patient who took the full course goes on falling and reaches 0 by day 10 [1]
the patient who stopped on day 3 rises again from 210 and is back to 1000 by day 10 [1]
⚠ If you missed marks here: Describe means numbers. “One went down and one went up” is a drawing in words, not a description, and it also misses the most important feature of the graph: the two curves are exactly the same until the moment one patient stops.
(b)[2]
Calculate the percentage fall in the number of bacteria between day 0 and day 3. Show your working.
Model Answer — 6b
fall = 1000 – 210 = 790 arbitrary units [1]
percentage fall = 790 ÷ 1000 × 100 = 79%
percentage fall = 79% [1]
⚠ If you missed marks here: 21% is the percentage that is left, not the fall. Always divide the change by the starting value, and show the subtraction on its own line, because the method mark is awarded separately from the final number.
(c)[2]
Explain why the second patient felt better on day 3 even though a great many bacteria were still present.
Model Answer — 6c
the symptoms are caused by the bacteria; as the antibiotic kills bacteria their number falls, and the symptoms fade with it [1]
feeling better only shows that the number has fallen far enough for the symptoms to stop — 210 units of bacteria were still alive, so the infection was not cleared [1]
⚠ If you missed marks here: An antibiotic is not a painkiller and does not relieve symptoms directly. Symptoms improve only as a consequence of the falling bacterial numbers, which is exactly why the way you feel is such an unreliable guide to whether the infection has gone.
(d)[3]
The bacteria still alive on day 3 are the ones that are least easily killed by this antibiotic. Use this idea to explain why stopping the course early is dangerous.
Model Answer — 6d
the bacteria killed first are the ones most easily killed, so the ones still alive on day 3 are the least sensitive to this antibiotic [1]
stopping the antibiotic leaves exactly those bacteria alive, with no competition from the ones that have been killed, so they reproduce and the infection returns [1]
the returning population has come from the least easily killed bacteria, so the same antibiotic will be less effective against it next time [1]
⚠ If you missed marks here: The tempting wrong answer is that the survivors are weak because they were nearly killed. They are the opposite: they are the survivors precisely because the antibiotic dealt with them least well, and stopping hands them an empty field. Note this is the same reasoning as the diagram in Question 3, running inside one patient over a week.
(e)[2]
State what the second patient should have done, and give the reason.
Model Answer — 6e
taken the whole 10-day course, carrying on even after the symptoms had gone [1]
because the full course is long enough to kill the bacteria that are still there after the symptoms stop, so none is left alive to reproduce [1]
⚠ If you missed marks here: Two answers score nothing: doubling the dose to catch up, and keeping the leftover tablets for the next time. The instruction on the syllabus is simply to complete the course, and the reason has to be about the bacteria that are left, not about the patient feeling ill again.
Question 7 — The Antibiotic and the Body’s Own Defences
Total: 10 marks
A patient comes to the Norbury Health Centre with a deep cut on the hand that has become infected with bacteria.
(a)[2]
Explain how the cut has made this infection possible.
Model Answer — 7a
the skin is a physical barrier that stops pathogens entering the body, and it does that only while it is continuous or unbroken [1]
the cut breaks the barrier, so bacteria from the skin surface or from whatever caused the cut can enter the tissues underneath [1]
⚠ If you missed marks here: Skin is one of the five body defences the syllabus names, and both marks here are about the barrier. Nothing has gone wrong with the patient’s white blood cells — a route in has simply been opened.
(b)[2]
Name the type of white blood cell that removes bacteria from the wound and describe what it does to them.
Model Answer — 7b
phagocyte [1]
it engulfs the bacteria and then digests them [1]
⚠ If you missed marks here: The other white blood cell, the lymphocyte, produces antibodies — it does not remove bacteria itself. And “eats them” is not a marking term: the pair of words wanted is engulfs and digests.
(c)[3]
The doctor prescribes an antibiotic. Explain what the antibiotic does, and what it does not do.
Model Answer — 7c
the antibiotic kills the bacteria in the wound, so their number falls [1]
it does not boost the immune system, and it does not relieve the symptoms directly [1]
it brings the bacterial population down to a level the body’s own defences can deal with, and those defences finish the job [1]
⚠ If you missed marks here: “It boosts your immune system” is the single commonest wrong sentence written about antibiotics. The white blood cells are exactly as they were; what the antibiotic changes is the size of the job they are facing, and that is a much more useful way to picture it.
(d)[3]
The doctor also tells the patient to keep the wound clean and covered. Explain why an antibiotic is not a substitute for doing this.
Model Answer — 7d
keeping the wound clean and covered stops further bacteria entering in the first place, so there is less for the antibiotic and the body to deal with [1]
every course of antibiotic selects for resistant bacteria, so antibiotics should be used only when essential — preventing an infection avoids the need for one at all [1]
if a resistant bacterium did get into the wound the antibiotic would not kill it, and hygiene would then be the only thing keeping it out [1]
⚠ If you missed marks here: “There are always antibiotics if it gets worse” is the attitude the whole of this topic is written against. Hygiene and restraint in prescribing are the two halves of the same policy: one reduces how often an antibiotic is needed, and every antibiotic not needed is one that has not selected for resistance.
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