References only, not question text. Open Official paper ↗ where Cambridge publishes the paper,
or see the official Cambridge past-papers page; for other papers, ask your
school. “Paper 42” means Paper 4, variant 2.
Practice questions
Original questions written by HomiLabs in the style of the exam, with answers and mark schemes. They are not
Cambridge past paper questions and have not been reviewed by Cambridge. Try each one before opening the answer.
Q1Diseases, pathogens & transmission · Core · Paper 1 style · 1 mark
(a)Which row correctly matches the disease with the type of causative organism?[1]
Show answer
(a) B tuberculosis – bacterium; HIV/AIDS – virus
TB is caused by the bacterium Mycobacterium tuberculosis (or M. bovis) and HIV/AIDS by a virus. Malaria is caused by the protoctist Plasmodium and cholera by the bacterium Vibrio cholerae, so A, C and D each contain an error.
Q2Diseases, pathogens & transmission · Core · Paper 1 style · 1 mark
(a)After heavy monsoon floods, health officials want to reduce the spread of cholera in a camp for displaced families. Which measure would be most effective?[1]
Show answer
(a) C supplying chlorinated water and building proper latrines
Cholera is spread by the faecal–oral route in water or food contaminated with Vibrio cholerae, so clean water and sanitation break transmission. A targets malaria; B and D target TB, which is airborne.
Q3Diseases, pathogens & transmission · Core · Paper 2 style · 5 marks
Floods in Sindh were followed by many cases of acute watery diarrhoea, some of which were confirmed as cholera.
(a)Name the causative organism of cholera.[1]
(b)Explain why cholera outbreaks often follow floods.[2]
(c)Suggest two ways in which the spread of cholera could be reduced in flood-affected villages.[2]
Show answer
(a) Vibrio cholerae
B1 Vibrio cholerae (correct spelling of genus).
(b) Floods mix sewage with drinking-water supplies, so water is contaminated with faeces containing Vibrio cholerae; people drink the water or eat food washed in it.
B1 sewage / faeces contaminate drinking water (pipes and latrines overflow); B1 bacteria ingested in water or food (faecal–oral route).
(c) Any two of: boiling or chlorinating drinking water; providing clean bottled/tanker water; building latrines away from water sources; handwashing with soap; cooking food thoroughly; oral vaccination; treating sewage.
1 mark each for any two valid measures.
Q4Diseases, pathogens & transmission · Core · Paper 2 style · 5 marks
(a)State the type of causative organism of HIV/AIDS and two ways in which HIV is transmitted.[3]
(b)Suggest why screening donated blood for HIV is important in a hospital blood bank.[1]
(c)Explain why it is difficult to stop the spread of HIV by giving the antibiotics that are used to treat TB.[1]
Show answer
(a) Virus (retrovirus). Any two of: unprotected sexual intercourse; sharing needles; transfusion of unscreened blood; from mother to baby across the placenta, at birth or in breast milk.
B1 virus; 1 mark each for any two transmission routes.
(b) Infected blood would transmit HIV directly into the recipient's bloodstream.
B1 transfusion of infected blood passes the virus straight into the recipient.
(c) Antibiotics do not affect viruses: HIV has no cell wall or bacterial ribosomes / enzymes for the antibiotic to act on.
Q5Diseases, pathogens & transmission · Core · Paper 2 style · 5 marks
In one year a city with a population of 2 400 000 recorded 6 360 new cases of tuberculosis.
(a)Calculate the number of new TB cases per 100 000 people in this city in that year.[2]
(b)Suggest why health departments give the number of cases per 100 000 people rather than the total number of cases.[1]
(c)Suggest two reasons why TB is difficult to eradicate in large cities.[2]
Show answer
(a) 265 per 100 000
M1 6360/2400000 × 100 000; A1 = 265.
(b) It allows comparison between places or years with different population sizes.
B1 allows fair comparison between populations of different sizes.
(c) Any two of: overcrowding; poor ventilation; people do not complete the long course of treatment; multidrug-resistant strains; HIV co-infection; poverty / malnutrition; people may carry the bacteria without symptoms (latent TB).
1 mark each for any two.
Q6Diseases, pathogens & transmission · Standard · Paper 1 style · 1 mark
(a)Which cells are infected and destroyed by HIV, leading to AIDS?[1]
Show answer
(a) B helper T-lymphocytes
HIV infects helper T-lymphocytes (which carry the CD4 receptor), so their number falls and the immune system cannot respond to other pathogens. D is wrong: plasma cells are not the main target, although antibody production falls because helper T-cells are lost.
Q7Diseases, pathogens & transmission · Standard · Paper 2 style · 7 marks
(a)Describe how tuberculosis (TB) is transmitted between people.[2]
(b)Explain why TB spreads more easily in crowded, poorly ventilated housing.[2]
(c)State how people can be infected with Mycobacterium bovis, and one way of preventing this.[2]
(d)Explain why people who are HIV positive are more likely to develop active TB.[1]
Show answer
(a) By airborne droplets: an infected person coughs or sneezes droplets containing Mycobacterium tuberculosis which are inhaled by others, especially in close contact.
B1 droplets / aerosol released when infected person coughs, sneezes or talks; B1 inhaled by uninfected person.
(b) People are close together for long periods, so droplets reach others; poor ventilation means droplets stay in the air longer, so more bacteria are inhaled.
B1 close contact / many people sharing small rooms; B1 droplets not dispersed / remain in the air.
(c) Drinking unpasteurised milk (or eating undercooked meat) from infected cattle; prevent by pasteurising or boiling milk / testing and slaughtering infected cattle.
B1 unpasteurised / raw milk or meat from infected cattle; B1 pasteurise / boil milk or test cattle herds.
(d) HIV destroys helper T-cells, so the immune system cannot control the TB bacteria.
B1 fewer helper T-cells / weakened immune system so cannot fight M. tuberculosis.
Q8Diseases, pathogens & transmission · Challenge · Paper 2 style · 7 marks
(a)Name the vector of malaria and describe how it transmits the disease.[3]
(b)Explain why it has been difficult to develop a fully effective vaccine against malaria.[2]
(c)Suggest why malaria cases in parts of Pakistan rise sharply in the months after the monsoon.[2]
Show answer
(a) The female Anopheles mosquito; when she takes a blood meal from an infected person she takes in Plasmodium gametocytes (sexual stages); they develop in the mosquito and the infective stages move to her salivary glands; she injects them in saliva when she bites another person.
B1 female Anopheles mosquito; B1 takes up Plasmodium in blood from an infected person; B1 parasites injected (with saliva / anticoagulant) into the blood of the next person bitten.
(b) Plasmodium is a eukaryote with many stages in its life cycle, each with different antigens; it spends time inside liver cells and red blood cells, hidden from the immune system; antigens vary between strains.
1 mark each for any two of: different antigens at each stage of life cycle; parasite lives inside host cells (liver, red blood cells) so not exposed to antibodies; antigenic variation / many strains; short time in the blood before entering cells.
(c) Standing water after the rains provides breeding sites for mosquitoes, and warm temperatures speed up mosquito and parasite development, so the vector population and transmission increase.
B1 stagnant water / pools → more mosquito breeding sites (larvae); B1 warm and humid conditions → more mosquitoes / faster development of Plasmodium in the mosquito.
Q9Diseases, pathogens & transmission · Challenge · Paper 2 style · 5 marks
A town introduced a piped, chlorinated water supply. In the two years before the project there were 1840 reported cases of cholera; in the two years after it there were 412.
(a)Calculate the percentage decrease in the number of cases of cholera after the project. Give your answer to 1 decimal place.[2]
(b)Cases did not fall to zero. Suggest two reasons for this.[2]
(c)A health worker said the fall in cases proves that the water project caused the decrease. Give one reason why this might not be correct.[1]
Show answer
(a) 77.6 %
M1 (1840 − 412) / 1840 × 100; A1 77.6 %.
(b) Any two of: some people still drank untreated water from wells/canals; poor hygiene or contaminated food spread infection; infected visitors / carriers without symptoms; pipes damaged so supply contaminated; some households not connected.
1 mark each for any two valid reasons linked to cholera transmission.
(c) Other factors could also have changed, e.g. a vaccination campaign, hygiene education, weather (fewer floods), or improved reporting.
B1 another variable changed at the same time / correlation does not show causation (with example).
Q10Antibiotics & resistance · Core · Paper 2 style · 5 marks
(a)State three ways in which the development of antibiotic resistance can be reduced.[3]
(b)Explain why a patient should finish the full course of an antibiotic even after feeling better.[2]
Show answer
(a) Any three of: only prescribe antibiotics for bacterial infections; complete the full course; do not sell antibiotics without prescription; use narrow-spectrum antibiotics where possible; limit antibiotics in farming / for animal growth; rotate antibiotics; improve hygiene and infection control in hospitals so fewer infections need treating.
1 mark each for any three.
(b) Some bacteria, often the more resistant ones, may still be alive; stopping early lets them survive and multiply, so the infection returns and resistant bacteria increase.
B1 not all bacteria killed / partly resistant ones survive; B1 they reproduce, increasing the proportion of resistant bacteria / infection returns.
Q11Antibiotics & resistance · Standard · Paper 1 style · 1 mark
(a)Why is penicillin not effective against measles?[1]
Show answer
(a) B Viruses have no peptidoglycan cell wall for penicillin to act on.
Penicillin stops bacteria building peptidoglycan cell walls; viruses have no cell wall or metabolism of their own, so penicillin has no target. C is wrong because penicillinase is made by some bacteria, not viruses.
Q12Antibiotics & resistance · Standard · Paper 2 style · 4 marks
(a)Explain how penicillin kills bacteria that are growing and dividing.[3]
(b)Suggest why penicillin does not harm human cells.[1]
Show answer
(a) Penicillin inhibits the enzymes that form cross-links between peptidoglycan chains in the cell wall; new wall is weak; water enters by osmosis and the weakened wall cannot withstand the pressure, so the cell bursts (lysis).
B1 inhibits enzymes (transpeptidases) that make cross-links in peptidoglycan; B1 cell wall weakened as it grows; B1 water enters by osmosis → cell bursts / osmotic lysis.
(b) Human cells do not have cell walls / peptidoglycan.
B1 human (animal) cells lack a cell wall / peptidoglycan.
Q13Antibiotics & resistance · Standard · Paper 3 style · 5 marks
Paper discs (6 mm diameter) soaked in four antibiotics, P, Q, R and S, were placed on an agar plate spread with a non-pathogenic strain of bacterium. After 24 hours at 25 °C the diameter of each clear zone (including the disc) was measured.P: 24 mmQ: 18 mmR: 6 mmS: 30 mm
(a)Calculate the area of the clear zone around antibiotic P, not including the disc. Use π = 3.142 and give your answer to the nearest mm².[2]
(b)State which antibiotic is most effective against this bacterium and which one the bacterium is resistant to. Give reasons.[2]
(c)Suggest why the size of the clear zone is not always a reliable guide to which antibiotic a doctor should use.[1]
Show answer
(a) 424 mm²
M1 π(12² − 3²) = 3.142 × 135; A1 = 424 mm².
(b) S is most effective (largest clear zone, 30 mm); the bacterium is resistant to R (no clear zone beyond the 6 mm disc).
B1 S, largest zone of inhibition; B1 R, zone diameter equals disc diameter so no bacteria killed.
(c) Antibiotics diffuse through agar at different rates (molecule size), and different starting concentrations were used; it also may behave differently in the body.
B1 any one: different diffusion rates / molecular size / concentration in disc / effect in the body (toxicity, absorption) differs from agar.
Q14Antibiotics & resistance · Challenge · Paper 2 style · 6 marks
(a)Explain how a population of bacteria becomes resistant to an antibiotic.[4]
(b)Typhoid fever caused by Salmonella Typhi strains resistant to many antibiotics has spread in parts of Pakistan. Suggest why antibiotics sold without a prescription contribute to this.[2]
Show answer
(a) A random mutation produces an allele giving resistance (e.g. an enzyme that breaks down the antibiotic); the antibiotic kills susceptible bacteria; resistant bacteria survive and reproduce by binary fission; the resistance allele is passed on so its frequency increases; plasmids carrying the gene can be passed to other bacteria by conjugation.
1 mark each for any four of: mutation (random) gives resistance gene / allele; antibiotic is a selection pressure; non-resistant bacteria killed; resistant bacteria survive and reproduce; allele passed to offspring (vertical) so frequency increases; resistance genes on plasmids transferred between bacteria (horizontal / conjugation).
(b) People take antibiotics they do not need (e.g. for viral fevers) or take incomplete courses, so bacteria are exposed to antibiotics more often, giving more selection for resistant strains.
B1 overuse / unnecessary use / wrong antibiotic / incomplete course; B1 increases selection pressure so resistant bacteria survive and multiply.
Q15Antibiotics & resistance · Challenge · Paper 2 style · 5 marks
A hospital laboratory in Lahore recorded the percentage of Escherichia coli isolates from patients that were resistant to one class of antibiotic.2017: 12 %2019: 31 %2021: 54 %2023: 68 %
(a)Describe the trend shown by the data.[2]
(b)Calculate the mean increase in percentage of resistant isolates per year between 2017 and 2023.[1]
(c)Explain why hospital wards may contain a higher proportion of resistant bacteria than the general community.[2]
Show answer
(a) The percentage of resistant isolates increased every year, from 12 % in 2017 to 68 % in 2023; the largest rise was between 2019 and 2021 (23 percentage points).
B1 increase throughout; B1 data quote with years, e.g. 12 % (2017) to 68 % (2023).
(b) 9.3 percentage points per year
B1 (68 − 12)/6 = 9.3.
(c) Antibiotics are used heavily in hospitals, giving strong selection for resistant bacteria; many ill / immunocompromised patients close together allow resistant bacteria to spread between them (via hands, equipment).
B1 high antibiotic use = strong selection pressure; B1 transmission between patients / staff / equipment, patients with weak immune systems.
Q16Skill: conclusions & interpreting data · Standard · Paper 5 style · 5 marks
The percentage of households using insecticide-treated bed nets and the number of malaria cases per 1000 people in one year were recorded for five villages.Village A: bed nets 15 %; cases 41 per 1000Village B: bed nets 32 %; cases 30 per 1000Village C: bed nets 48 %; cases 22 per 1000Village D: bed nets 66 %; cases 15 per 1000Village E: bed nets 81 %; cases 9 per 1000
(a)Describe the relationship between bed-net use and malaria cases.[2]
(b)A newspaper reported that 'bed nets cause the fall in malaria'. Discuss whether the data support this claim.[3]
Show answer
(a) Negative correlation: as the percentage of households using bed nets increases, the number of malaria cases per 1000 people decreases, from 41 at 15 % to 9 at 81 %.
B1 negative correlation / inverse relationship; B1 data quoted with units.
(b) Data show a strong negative correlation, and bed nets prevent mosquito bites, so it is plausible; but correlation does not prove causation: other factors may differ between villages (e.g. distance from water, insecticide spraying, wealth, access to treatment, altitude); only five villages; one year of data.
B1 supports: strong negative correlation / biologically plausible because nets stop mosquito bites at night; B1 correlation ≠ causation; B1 named other variable not controlled or small sample / no statistics.
Q17Skill: conclusions & interpreting data · Challenge · Paper 5 style · 4 marks
A school in Peshawar ran a handwashing campaign. The mean number of episodes of diarrhoea per child per year, with standard error (SE), was recorded.before campaign: mean 3.8, SE 0.4after campaign: mean 2.1, SE 0.5
(a)The 95 % confidence intervals are mean ± 2 × SE. Calculate the 95 % confidence interval for each period.[2]
(b)Use your answers to discuss whether the campaign significantly reduced diarrhoea.[2]
Show answer
(a) Before: 3.0 to 4.6; after: 1.1 to 3.1 cases per child per year
B1 before 3.8 ± 0.8 = 3.0–4.6; B1 after 2.1 ± 1.0 = 1.1–3.1.
(b) The intervals overlap (3.0–3.1), so the difference may not be significant; the mean did fall, but we cannot be confident the decrease is real rather than due to chance.
B1 confidence intervals overlap; B1 so difference not shown to be significant (although mean is lower).
Q18Skill: planning a method · Core · Paper 5 style · 6 marks
A student wants to find out which concentration of ethanol in a hand sanitiser is most effective at killing bacteria.
(a)Describe how you would prepare 20 cm³ of each of 70 %, 60 %, 50 % and 40 % ethanol solutions from a 70 % stock solution and distilled water.[2]
(b)Describe how the student could use these solutions to compare their effect on the number of bacteria on the hands.[4]
M1 volume of stock = (required % ÷ 70) × 20; A1 all four correct (to 1 decimal place).
(b) Volunteer presses fingertips on a nutrient agar plate (before); applies a fixed volume (e.g. 2 cm³) of one concentration for a fixed time (30 s), lets hands dry, presses fingertips on a second plate (after); incubate plates at 25 °C for 48 h; count colonies; calculate percentage reduction; repeat with several volunteers/days for each concentration; tape lid, do not open plates.
1 mark each for any four of: before and after samples on agar; same volume of sanitiser and same rubbing time; same fingers / same pressure and time of contact; incubation ≤ 30 °C for a stated time; count colonies and calculate % reduction; repeats / several volunteers; safety (lids taped, plates autoclaved after).
Q19Skill: planning a method · Challenge · Paper 5 style · 7 marks
Neem (Azadirachta indica) leaves are traditionally used in Pakistan as an antiseptic.
(a)Describe how the student could investigate the effect of the concentration of neem leaf extract on the growth of the bacterium Bacillus subtilis. Include how the results would be recorded and processed.[7]
Show answer
(a) Make a stock extract (e.g. 20 g leaves blended in 100 cm³ distilled water, filtered and sterilised by filtration); dilute to five concentrations (e.g. 100, 80, 60, 40, 20 % plus 0 % control) using distilled water; spread the same volume (0.1 cm³) of the same culture on nutrient agar plates using aseptic technique; soak 6 mm sterile paper discs in each concentration for the same time and place them on the agar; incubate at 25 °C for 48 h; measure diameter of clear zone in two directions at right angles and calculate mean / area; three replicate plates per concentration; calculate mean; plot graph of zone diameter against concentration.
1 mark each for any seven of: preparation of extract and dilution method with volumes; at least 5 concentrations and suitable range; control discs soaked in distilled water; same volume / density of bacterial culture spread on each plate; same type and depth of agar; same disc size and soaking time; incubation temperature (≤ 30 °C) and time stated; measure clear zone diameter (mean of two perpendicular diameters) or area; at least 3 replicates and calculate mean; aseptic technique (flame loop, sterile equipment, lid taped not sealed); record in table and plot graph.
Q20Skill: errors, reliability & improvements · Standard · Paper 3 style · 5 marks
A student placed paper discs soaked in different antibiotics on agar plates spread with bacteria. After incubation she measured the diameter of each clear zone with a ruler. The edges of some zones were blurred and some zones were oval rather than circular.
(a)Identify two sources of error in measuring the clear zones and suggest an improvement for each.[4]
(b)Explain why each antibiotic should be tested on three separate plates.[1]
Show answer
(a) e.g. zones are not perfect circles → measure two diameters at right angles and take the mean; edge of the clear zone is blurred → use a hand lens / measure against a dark background with a fixed criterion; ruler measured only to the nearest mm → use digital calipers; discs not equally soaked → soak for a fixed time and drain on filter paper.
For each of two: B1 error relevant to this experiment; B1 improvement that would reduce it.
(b) To identify anomalous results and calculate a mean, making the results more reliable.
B1 identify anomalies / calculate mean / check repeatability.
Q21Skill: errors, reliability & improvements · Challenge · Paper 5 style · 4 marks
Researchers asked 40 families in a village whether they washed their hands with soap before eating and how many times family members had had diarrhoea in the past year. They concluded that handwashing halves the risk of diarrhoea.
(a)Suggest three limitations of this survey that could make the conclusion unreliable.[3]
(b)Suggest one improvement to the study design.[1]
Show answer
(a) Any three of: data on handwashing were self-reported (people may overstate); small sample size; households not randomly chosen; other variables (water source, sanitation, income, family size) not controlled; diarrhoea recalled from memory over a long period; cause of diarrhoea not confirmed by laboratory tests.
1 mark each for any three.
(b) e.g. larger random sample; observe handwashing directly; match households for water source; confirm cases by stool tests; record cases weekly.
B1 any valid improvement linked to a limitation in (a).
Q22Skill: variables & controls · Core · Paper 5 style · 5 marks
A student compares the antibacterial effect of four brands of mouthwash using paper discs on agar plates spread with a harmless bacterium.
(a)State the independent and dependent variables.[2]
(b)State two variables that should be standardised and how each could be controlled.[2]
(c)Describe a suitable control and explain its purpose.[1]
Show answer
(a) Independent: type of mouthwash. Dependent: diameter (or area) of the clear zone around each disc.
B1 IV type of mouthwash; B1 DV clear zone diameter / area.
(b) e.g. volume of bacterial culture – use a pipette to spread 0.1 cm³; temperature of incubation – incubator at 25 °C; size of discs – cut with the same hole punch; soaking time – 10 minutes timed with a stopwatch.
1 mark each for a variable with its method of control (max 2).
(c) Disc soaked in sterile distilled water, to show that the paper disc itself does not inhibit growth.
B1 disc in sterile water and showing any clear zone is due to the mouthwash.
Q23Skill: variables & controls · Standard · Paper 5 style · 5 marks
Health researchers compared the number of cases of measles in children who had received two doses of measles vaccine and in children who had not, in a district of Balochistan.
(a)Identify the independent variable and the dependent variable in this study.[2]
(b)Suggest two variables that the researchers should try to match between the two groups and explain why matching is necessary.[3]
Show answer
(a) Independent: whether children received the vaccine (vaccinated or not). Dependent: number / percentage of children who developed measles.
B1 IV vaccination status; B1 DV incidence / percentage with measles.
(b) Any two of: age; nutrition / health status; living conditions (crowding); area / exposure to cases; access to health care; plus explanation: these also affect the chance of catching measles, so must be similar for a valid comparison.
1 mark each for any two valid variables; B1 explanation that they could affect the dependent variable / make the comparison fair.
Q24Skill: statistics (SD, SE, CI, t-test, Spearman, chi-squared) · Standard · Paper 5 style · 5 marks
Clear zone diameters (mm) around eight discs soaked in garlic extract on plates of a harmless bacterium:14.5, 16.0, 15.5, 13.0, 17.0, 15.0, 14.0, 16.5
(a)Calculate the mean diameter of the clear zone.[1]
(b)The standard deviation is 1.33 mm. Calculate the standard error, SE = s ÷ √n. Give your answer to 2 decimal places.[2]
(c)The 95 % confidence interval for a second extract was 11.2 to 12.3 mm. Calculate the 95 % confidence interval (mean ± 2 × SE) for this extract and state whether the difference between the two extracts is likely to be significant.[2]
Show answer
(a) 15.19 mm
B1 121.5/8 = 15.1875 ≈ 15.19 mm.
(b) 0.47 mm
M1 1.33 / √8; A1 0.47 mm.
(c) 14.2 to 16.1 mm; the intervals do not overlap, so the difference is likely to be significant.
B1 15.19 ± 0.94 = 14.2–16.1 mm; B1 no overlap with 11.2–12.3 so difference likely significant.
Q25Skill: statistics (SD, SE, CI, t-test, Spearman, chi-squared) · Challenge · Paper 5 style · 6 marks
The number of malaria cases in a rural health centre was recorded in each season of one year.Winter (Dec–Feb): 48; spring (Mar–May): 112; summer and monsoon (Jun–Aug): 186; autumn (Sep–Nov): 74.
(a)State a null hypothesis for this test.[1]
(b)Calculate the value of χ² using χ² = Σ (O − E)²/E. Give your answer to 1 decimal place.[3]
(c)The critical value of χ² at p = 0.05 for 3 degrees of freedom is 7.82. Explain what the result shows.[2]
Show answer
(a) There is no difference between the numbers of malaria cases in the four seasons (cases are evenly spread through the year); any difference is due to chance.
B1 no (significant) difference between observed number of cases in each season / cases distributed equally.
(b) χ² = 103.0
B1 E = 420/4 = 105 for each season; M1 (O − E)²/E values: 30.94, 0.47, 62.49, 9.15; A1 sum = 103.0.
(c) χ² (103.0) is much greater than 7.82, so the null hypothesis is rejected; the difference between seasons is significant (p < 0.05) – malaria cases are not evenly spread through the year.
B1 χ² > critical value (df = 4 − 1 = 3); B1 reject null hypothesis / significant difference between seasons, not due to chance.
Q26Skill: calculations & data processing · Standard · Paper 2 style · 5 marks
A district recorded 960 new cases of a disease in 2015, when its population was 1 200 000, and 735 cases in 2024, when its population was 1 500 000.
(a)Calculate the incidence of the disease per 100 000 people in each year.[2]
(b)Calculate the percentage change in incidence between 2015 and 2024.[2]
(c)Explain why the change in the total number of cases gives a misleading impression of the change in risk of infection.[1]
(c) The population grew, so the number of cases per person (the risk) fell more than the total number of cases suggests.
B1 population size changed, so cases per 100 000 is the fair measure.
Q27Skill: calculations & data processing · Challenge · Paper 5 style · 5 marks
A water sample from a village well was diluted in a serial dilution so that the final tube was a 10⁻⁵ dilution of the original. 0.1 cm³ of this tube was spread on nutrient agar and incubated. 47 colonies grew.
(a)Calculate the number of bacteria per cm³ in the original water sample. Give your answer in standard form.[3]
(b)State one assumption made in this calculation.[1]
(c)Explain why the plate from the 10⁻³ dilution, which had colonies that had merged into one another, was not used.[1]
Show answer
(a) 4.7 × 10⁷ bacteria per cm³
M1 colonies per cm³ of diluted sample = 47 / 0.1 = 470; M1 × dilution factor 10⁵; A1 4.7 × 10⁷ per cm³.
(b) Each colony grew from a single bacterium / all bacteria were able to grow on the agar.
B1 one colony = one cell (cells not clumped) or all viable cells grow.
(c) Overlapping colonies cannot be counted accurately.
B1 colonies merge / too many to count individually.
Q28Skill: hypotheses & predictions · Core · Paper 5 style · 3 marks
A student investigates whether honey from the Swat valley inhibits the growth of bacteria. Wells are cut in agar plates spread with bacteria and filled with honey diluted to different concentrations.
(a)State a hypothesis that could be tested in this investigation.[1]
(b)Predict the results you would expect if manuka-type honey contains an antibacterial substance in addition to its high sugar concentration, compared with a sugar solution of the same concentration. Explain your prediction.[2]
Show answer
(a) The greater the concentration of honey, the larger the area of the clear zone around the well (the fewer bacteria grow).
B1 testable statement linking honey concentration to bacterial growth / clear zone size, with direction.
(b) Honey would give larger clear zones than sugar solution of the same concentration, because both cause water loss from bacteria by osmosis, but honey also contains an extra antibacterial substance.
B1 larger zones with honey than with sugar solution; B1 osmotic effect is the same in both so the extra inhibition is due to the antibacterial substance.
Q29Skill: graphs & charts · Core · Paper 5 style · 5 marks
Set 1: diameter of clear zones around five different antibiotics.Set 2: number of new cases of dengue fever in a city each week for 12 weeks.
(a)State the type of graph that should be drawn for each data set. Explain each choice.[2]
(b)Describe how the line graph for Set 2 should be drawn.[3]
Show answer
(a) Set 1: bar chart, because the independent variable (type of antibiotic) is categoric / discontinuous. Set 2: line graph, because both variables (time and number of cases) are continuous.
B1 bar chart for categoric data; B1 line graph for continuous data.
(b) Time on the x-axis, number of new cases on the y-axis; axes labelled with units; linear scale using more than half the grid; points plotted accurately as small crosses; points joined with ruled straight lines (point to point) as intermediate values are not known.
1 mark each for any three of: independent variable (week) on x-axis; both axes fully labelled with units; even scale using most of the grid; points plotted precisely (× or ⊙); point-to-point ruled lines (no extrapolation).
Q30Skill: risk assessment · Standard · Paper 5 style · 4 marks
A student cultures bacteria from a soil sample on nutrient agar plates and tests the effect of an antibacterial cream. Loops are sterilised in a Bunsen flame and the bench is wiped with 70 % ethanol.
(a)Identify two hazards in this investigation and describe how the risk from each could be reduced.[4]
Show answer
(a) e.g. Bacteria could be pathogenic / contaminants may grow → use a non-pathogenic strain, aseptic technique, tape lid with two strips (not sealed all round), incubate at ≤ 30 °C, never open plates, autoclave after use. Flame / ethanol for sterilising loop is flammable → keep ethanol away from Bunsen, tie back hair. Hot agar → use heat-proof gloves.
For each of two hazards: B1 hazard identified; B1 appropriate precaution.
Similar questions
Questions that test the same subtopic in different years, grouped so you can work on one skill at a time.
References only: find each question in its paper (official link where available, otherwise through your school).
