AQA A Level Biology Paper 3 June 2025 question paper 7402/3

Describe binary fission in bacteria.

Microbiologists investigated the effect of two temperatures on the population growth of a species of bacterium.

They grew separate bacterial cultures in identical nutrient media at 20 °C and 24 °C for 8 hours.

Figure 1 shows the estimated final population size of each culture.

The bacterial population at 24 °C is larger than that at 20 °C

Calculate this difference as a percentage increase.

The actual number of bacteria can be calculated from the log₁₀ value by using the 10^x button on your calculator.

Give your answer to the nearest whole number.

Show your working.

Percentage increase =

Bedaquiline is a drug that inhibits ATP synthase in mycobacteria by stopping hydrogen ion (proton) movement.

Explain how bedaquiline can kill these bacteria.

Table 1 shows four statements and the names of four parts of a nephron.

Protein is not normally present in the urine of a person.

Explain why.

Glomerular sclerosis (GS) is a type of kidney damage that can cause protein to be present in the urine.

High-protein diets are linked to GS in adults.

Scientists investigated the development of GS in underweight, 5-day-old rats.

They fed:

• 14 rats a high-protein milk diet

• 14 rats a normal-protein milk diet.

From day 21, they fed both groups of rats the same diet of solid food. At 160 days old, they examined nephrons in the rats’ kidneys for GS.

They identified the number of nephrons with GS for each group of rats.

Rats fed a high-protein diet had a larger number of nephrons with GS than rats fed a normal-protein diet. The scientists performed a statistical test on these data and they calculated P < 0.01

A journalist reported on this investigation and stated ‘feeding high-protein milk to underweight human babies could cause kidney damage in adults’.

Use all of the information provided to evaluate this statement.

Tansy, Tanacetum vulgare, is a plant that grows in dense patches. The tansy beetle, Chrysolina graminis, feeds on, and completes its life cycle on T. vulgare.

C. graminis is 8–10 mm long and green in colour.

Two other green beetles can also be found in the same habitat.

• The dock beetle, Gastrophysa viridula, which is 4–7 mm long.

• The mint leaf beetle, Chrysolina herbacea, which is 6–10 mm long.

Figure 2 shows the life cycle of C. graminis.

How many different genera of beetles are there in the information provided?

Ecologists investigated the population size of C. graminis using the

mark-release-recapture method.

Table 2 shows the ecologists’ results.

The ecologists captured and marked the first sample of beetles during the second week of August. They identified the species of beetle before marking them.

Use the information provided to suggest:

two reasons why the first sample of beetles was captured during the second week of August

one reason why the beetles had to be identified before being marked.

Use the data provided in Table 2 to estimate the population size of C. graminis.

Show your working.

The ecologists repeated the investigation over a five–year period to monitor the population size of C. graminis.

State two precautions they would have taken each year to obtain comparable data.

Draw the general structure of an amino acid.

Which statement describing protein structure is correct?

Antibodies are proteins produced and secreted by plasma cells.

Once activated, a single B cell divides and produces 4000 plasma cells.

Each one of these plasma cells produces 2000 molecules of antibody Q per second.

A human male has a blood volume of 5.4 dm³

Calculate the total number of molecules of antibody Q per cm³ of blood 24 hours after the production of these plasma cells.

Scientists investigated the effect of age on the immune response in healthy female mice.

The scientists used the following method.

1. Infected a large number of young and old mice with the same strain of influenza virus.

2. Removed the lungs from 6 young and 6 old mice on the day of infection (day 0).

3. Repeated step 2 on 8 different days after infection.

4. Determined the mean number of B cells present per gram of lung tissue for young and old mice.

Figure 3 shows the scientists’ results.

The scientists decided that further investigations would be required to conclusively

determine the effect of age on the immune response in healthy female mice.

Using all the information provided, suggest three reasons for the scientists’ decision.

Do not refer to statistical tests, repeats or sample size in your answer.

Cleft lip and palate (CLP) is a split in the upper lip and/or the roof of the mouth. CLP is present from birth.

CLP can be caused by various genetic and environmental factors.

CDH1 is a gene involved in lip and mouth development in a fetus before a baby is born.

Scientists hypothesised that CLP is caused by a ‘two-hit’ model:

• a ‘genetic hit’ through loss of CDH1 function

• an ‘environmental hit’ leading to CDH1 methylation caused when inflammation affects a fetus.

Inflammation can result from the mother having an infection or diabetes.

Scientists investigated the ‘two-hit’ model using human cells grown in vitro.

They:

• grew normal cells and cells with a loss of CDH1 function

• treated some of each type of cell with a drug to cause inflammation and left some of each type of cell untreated

• measured the percentage CDH1 methylation of the cells.

Figure 4 shows the scientists’ results.

The frequency of live births with CLP is 1 in 700

Despite knowing this frequency, the Hardy–Weinberg equation cannot be used to estimate any allele frequencies for CLP.

Suggest two reasons why.

Describe one effect methylation could have on CDH1

Suggest and explain three reasons why the data in Figure 4 might not provide evidence for the ‘two-hit’ model.

A student completed a dissection of an animal gas exchange system.

Figure 5 shows the scientific drawing he produced from his observations.

Suggest two ways the scientific drawing in Figure 5 could be improved.

Do not refer to adding labels or using a sharp pencil in your answer.

Describe the role of structures A and B in increasing the volume in the thoracic cavity.

Lung volume can be determined with a spirometer using the helium dilution method.

The lung volume (V2) is calculated using the equation:

where C1 = the initial concentration of helium in the spirometer C2 = the helium concentration in the spirometer when equal with the lungs V1 = volume of gas in the spirometer.

A student used a spirometer with a volume of 3250 cm³ and an initial concentration of 10% helium. They found C2 was 3.78%

Use the equation to calculate this student’s lung volume in dm³

Show your working.

Asbestos is a fibrous mineral. If asbestos is inhaled, it can cause a lung disease

called asbestosis.

The latency period of a disease is the time between exposure to the cause of the disease and the symptoms occurring. The latency period for asbestosis is 15 to 40 years.

Asbestosis can be diagnosed using a biopsy. This is where a sample of lung cells is removed from a patient and examined with a microscope.

Asbestosis can also be diagnosed without a biopsy. To diagnose asbestosis without a biopsy, five conditions must be met. Three of these are given below.

1. A scan of the lungs showing fibrous scar tissue.

2. Reduced gas exchange.

3. Shortness of breath.

These three conditions are also present in many other lung diseases.

Using the information provided, suggest the other two conditions that must be met before a diagnosis of asbestosis can be made.

Suggest two advantages of diagnosing asbestosis without a biopsy.

Idiopathic pulmonary fibrosis (IPF) is a lung disease similar to asbestosis. Both

diseases cause:

• the death of alveolar epithelial cells due to DNA damage

• dead cells to be replaced by inelastic, fibrous scar tissue.

Currently, there is no cure for asbestosis and no effective treatment to slow its progression. There is also no cure for IPF. However, the drugs nintedanib and pirfenidone can slow the formation of scar tissue and slow the progression of IPF.

In two separate clinical trials, doctors investigated the effect of nintedanib and pirfenidone on 263 IPF patients.

• 124 patients were treated with nintedanib. 7 of these patients had to stop treatment due to side effects.

• 139 patients were treated with pirfenidone. 14 of these patients had to stop treatment due to side effects.

The doctors recorded how many days each patient lived after treatment.

Figure 6 shows their results.

Suggest why neither clinical trial had a control group.

The doctors concluded that both drugs increased life expectancy in patients with IPF compared with survival data for previous patients with IPF.

Using all the information, evaluate whether nintedanib and pirfenidone should be considered as treatments for asbestosis.

Turn over for Section B

Section B

one question.

You are advised to spend no more than 45 minutes on this section.

Write an essay on one of the topics below.

Either

The importance of condensation and hydrolysis reactions in biological systems.

Or

The importance of responses to changes in the internal and external environment of organisms.