Paper 2 (topics 5–8)

Describe what happens during photoionisation and photolysis in the light-dependent reaction.

Explain how ATP is produced in the thylakoid membrane by chemiosmosis.

Isolated chloroplasts were added to blue DCPIP solution. The DCPIP decolourised in the light but not in the dark. Explain these results.

Describe the link reaction.

How many molecules of reduced NAD are produced in the Krebs cycle from one molecule of glucose? Explain your answer.

Explain why anaerobic respiration produces much less ATP per glucose than aerobic respiration.

Describe how a Pacinian corpuscle produces a generator potential when pressure is applied.

Explain why rod cells give greater sensitivity to light but cone cells give greater visual acuity.

Distinguish between a taxis and a kinesis.

Explain how indoleacetic acid (IAA) causes a shoot to grow towards light.

Explain how heart rate increases during exercise.

Explain what is meant by cardiac muscle being myogenic.

Explain the importance of negative feedback in homeostasis.

Explain why having separate mechanisms to correct increases and decreases from the norm gives more control.

ABO blood groups are controlled by multiple alleles: Iᴬ and Iᴮ are codominant and Iᴼ is recessive. A man of genotype IᴬIᴼ and a woman of genotype IᴮIᴼ have children. Give the possible genotypes and phenotypes of the children and their ratio.

In a dihybrid test cross the expected ratio was 1 : 1 : 1 : 1, but the offspring were 45 : 5 : 6 : 44 (data are illustrative, written for this paper). Explain these results.

In a population, 9% of individuals show a recessive phenotype. Use the Hardy–Weinberg equations to calculate the percentage of heterozygotes.

In a mark–release–recapture study, 40 beetles were marked. Later 50 were caught, of which 8 were marked. Estimate the population size.

Give two assumptions made when using mark–release–recapture.

Explain how the deletion of a single base can change the structure of a protein.

Give two examples of mutagenic agents.

Explain how mutations in tumour suppressor genes and in proto-oncogenes can lead to cancer.

Explain how hypermethylation of a tumour suppressor gene can contribute to cancer.

Explain why sequencing the genome of a bacterium allows scientists to determine its proteome, but this is harder for humans.

Suggest how knowing the proteome of a pathogen could help develop a vaccine.

Describe how a DNA probe is used to screen for a mutant allele.

Explain how screening results can be used in personalised medicine.

Explain the role of mycorrhizae in nutrient uptake by plants.

Explain how leaching of fertiliser can lead to eutrophication.

Grassland has gross primary production of 20 000 kJ m⁻² year⁻¹ and respiratory losses of 12 000 kJ m⁻² year⁻¹ (data are illustrative, written for this paper). Calculate the net primary production.

Comprehension passage (written for this paper): Gene drives against malaria. (1) Malaria is spread by female Anopheles mosquitoes. (2) Scientists have engineered a 'gene drive': a DNA sequence that includes the gene for a Cas9 enzyme and a guide RNA. (3) In mosquitoes carrying one copy, Cas9 cuts the matching sequence on the homologous chromosome, and the cell copies the gene drive into the cut. (4) As a result, more than 95% of offspring inherit the gene drive instead of the 50% expected. (5) One gene drive disrupts a gene needed for female fertility. (6) In caged populations, the gene drive spread to all mosquitoes within 8 to 12 generations and the populations collapsed. (7) Critics worry about effects on food webs and about genes spreading to other species. Explain how the gene drive DNA (sentence 2) could be inserted into mosquito cells.

Explain why the gene drive spreads through a population much faster than a normal allele (sentences 3 and 4).

Evaluate the use of gene drives to control malaria (sentences 5–7).

The gene drive must be inserted into cells that produce gametes. Explain why inserting it into body cells of adult mosquitoes would not work.

A cage started with 1 000 mosquitoes, 10% of which carried the gene drive. If 95% of offspring inherit the drive each generation from carriers, explain why the number of carriers would rise only slowly in the first generation.