AQA A LEVEL BIOLOGY
PAPER 2 MARKING SCHEME
2025 ORIGINAL PAPER
Q1. Describe the role of carrier proteins in the facilitated diffusion of glucose into a cell.
(3 marks)
Answer:
● Carrier proteins bind specifically to glucose molecules.
● The protein undergoes a conformational change to transport glucose across the
membrane.
● Glucose is moved down its concentration gradient without the use of ATP.
Q2. Explain one way in which the structure of a chloroplast is adapted to its function in
photosynthesis.
(2 marks)
Answer:
● The thylakoid membranes provide a large surface area for the attachment of chlorophyll,
electron carriers, and enzymes.
● This increases the efficiency of the light-dependent reaction.
Q3. A gene has 1,500 base pairs. Calculate the number of amino acids in the polypeptide
translated from this gene. (Assume no introns or stop codons).
(2 marks)
Answer:
, ● 1,500 base pairs = 750 codons (as 3 bases = 1 codon)
● 1 codon = 1 amino acid → 750 amino acids
Q4. Describe what happens during crossing over and explain its importance.
(4 marks)
Answer:
● Homologous chromosomes pair up during meiosis I.
● Non-sister chromatids exchange segments at chiasmata.
● This produces new combinations of alleles.
● Increases genetic variation in gametes.
Q5. A student investigated the effect of temperature on the rate of photosynthesis using
pondweed. The volume of oxygen released was recorded at different temperatures.
Temperature (°C) 10 20 30 40 50
O₂ produced (cm³/hr) 2 5 8 6 1
a) Describe the trend in oxygen production between 10°C and 50°C.
(2 marks)
Answer:
● The volume of oxygen increases from 10°C to a peak at 30°C.
● After 30°C, the oxygen production decreases.
b) Explain the results between 30°C and 50°C.
(3 marks)
Answer:
● At temperatures above 30°C, enzymes involved in photosynthesis (e.g., Rubisco) begin
to denature.
, ● The structure of the active site changes, reducing catalytic activity.
● This leads to a slower rate of the light-independent reaction and less O₂ production
overall.
Q6. Explain how differences in DNA lead to genetic diversity in a population.
(6 marks)
Answer:
● Genetic diversity arises from mutations—changes in the base sequence of DNA.
● Mutations may produce new alleles.
● Meiosis introduces genetic variation via independent assortment of chromosomes.
● Crossing over during prophase I further mixes genetic material.
● Random fertilisation also contributes to genetic diversity.
● These mechanisms ensure that individuals in a population have different genotypes and
phenotypes.
Q7. Describe the process of transcription and translation in protein synthesis.
(6 marks)
Answer:
● Transcription: RNA polymerase binds to DNA at the promoter region.
● DNA unwinds, and one strand acts as a template.
● Complementary RNA nucleotides pair with DNA bases (A–U, T–A, C–G, G–C).
● mRNA is formed and leaves the nucleus via nuclear pores.
● Translation: mRNA binds to a ribosome.
● tRNA brings specific amino acids, matching codons via anticodons.
● Peptide bonds form between amino acids to build the polypeptide.
PAPER 2 MARKING SCHEME
2025 ORIGINAL PAPER
Q1. Describe the role of carrier proteins in the facilitated diffusion of glucose into a cell.
(3 marks)
Answer:
● Carrier proteins bind specifically to glucose molecules.
● The protein undergoes a conformational change to transport glucose across the
membrane.
● Glucose is moved down its concentration gradient without the use of ATP.
Q2. Explain one way in which the structure of a chloroplast is adapted to its function in
photosynthesis.
(2 marks)
Answer:
● The thylakoid membranes provide a large surface area for the attachment of chlorophyll,
electron carriers, and enzymes.
● This increases the efficiency of the light-dependent reaction.
Q3. A gene has 1,500 base pairs. Calculate the number of amino acids in the polypeptide
translated from this gene. (Assume no introns or stop codons).
(2 marks)
Answer:
, ● 1,500 base pairs = 750 codons (as 3 bases = 1 codon)
● 1 codon = 1 amino acid → 750 amino acids
Q4. Describe what happens during crossing over and explain its importance.
(4 marks)
Answer:
● Homologous chromosomes pair up during meiosis I.
● Non-sister chromatids exchange segments at chiasmata.
● This produces new combinations of alleles.
● Increases genetic variation in gametes.
Q5. A student investigated the effect of temperature on the rate of photosynthesis using
pondweed. The volume of oxygen released was recorded at different temperatures.
Temperature (°C) 10 20 30 40 50
O₂ produced (cm³/hr) 2 5 8 6 1
a) Describe the trend in oxygen production between 10°C and 50°C.
(2 marks)
Answer:
● The volume of oxygen increases from 10°C to a peak at 30°C.
● After 30°C, the oxygen production decreases.
b) Explain the results between 30°C and 50°C.
(3 marks)
Answer:
● At temperatures above 30°C, enzymes involved in photosynthesis (e.g., Rubisco) begin
to denature.
, ● The structure of the active site changes, reducing catalytic activity.
● This leads to a slower rate of the light-independent reaction and less O₂ production
overall.
Q6. Explain how differences in DNA lead to genetic diversity in a population.
(6 marks)
Answer:
● Genetic diversity arises from mutations—changes in the base sequence of DNA.
● Mutations may produce new alleles.
● Meiosis introduces genetic variation via independent assortment of chromosomes.
● Crossing over during prophase I further mixes genetic material.
● Random fertilisation also contributes to genetic diversity.
● These mechanisms ensure that individuals in a population have different genotypes and
phenotypes.
Q7. Describe the process of transcription and translation in protein synthesis.
(6 marks)
Answer:
● Transcription: RNA polymerase binds to DNA at the promoter region.
● DNA unwinds, and one strand acts as a template.
● Complementary RNA nucleotides pair with DNA bases (A–U, T–A, C–G, G–C).
● mRNA is formed and leaves the nucleus via nuclear pores.
● Translation: mRNA binds to a ribosome.
● tRNA brings specific amino acids, matching codons via anticodons.
● Peptide bonds form between amino acids to build the polypeptide.
