AQA A-LEVEL BIOLOGY PAPER 1 2025 MARK SCHEME

AQA A- kj




LEVEL BIOLOGY PAPER 1 2025 MARK SC kj kj kj kj kj kj




HEME

AQA A-LEVEL BIOLOGY
kj kj




Paper 1: Multiple Choice Examination
kj kj kj kj




Time allowed: 1 hour
kj kj kj




Materials: You will need a ruler and a scientific calculator.
kj kj kj kj kj kj kj kj kj




Instructions:

• Answer all questions. kj kj




• For each question, choose the one answer you consider to be correct.
kj kj kj kj kj kj kj kj kj kj kj




• Record your answers clearly by writing the letter (A, B, C, or D) next to the question number
kj kj kj kj kj kj kj kj kj kj kj kj kj kj kj kj kj kj




on your answer sheet.
kj kj kj




Question 1 kj




Which of the following is the correct order of size, from smallest to largest?
kj kj kj kj kj kj kj kj kj kj kj kj kj




A) Ribosome → Nucleotide → Protein → Chloroplast
kj kj kj kj kj kj kj




B) Nucleotide → Protein → Ribosome → Chloroplast
kj kj kj kj kj kj kj




C) Protein → Nucleotide → Ribosome → Chloroplast
kj kj kj kj kj kj kj




D) Nucleotide → Ribosome → Protein → Chloroplast
kj kj kj kj kj kj kj




Question 2 kj




A student observed a eukaryotic cell under an electron microscope and identified a structure surro
kj kj kj kj kj kj kj kj kj kj kj kj kj kj




unded by a double membrane, with the inner membrane folded into cristae. What is the primary fu
kj kj kj kj kj kj kj kj kj kj kj kj kj kj kj kj




nction of this organelle? kj kj kj




A) Protein synthesis
kj kj




B) The synthesis of ATP during aerobic respiration
kj kj kj kj kj kj kj




C) The modification and packaging of proteins
kj kj kj kj kj kj




D) The hydrolysis of phagocytic cells
kj kj kj kj kj




Question 3 kj




The induced-kj




fit model of enzyme action describes the interaction between an enzyme and its substrate. Which
kj kj kj kj kj kj kj kj kj kj kj kj kj kj kj




of the following statements is correct about this model?
kj kj kj kj kj kj kj kj

,A) The active site is a rigid shape that perfectly complements the substrate.
kj kj kj kj kj kj kj kj kj kj kj kj




B) The enzyme changes shape slightly to accommodate the substrate, putting strain on the substra
kj kj kj kj kj kj kj kj kj kj kj kj kj kj




te's bonds. kj




C) The substrate changes shape to perfectly fit the active site.
kj kj kj kj kj kj kj kj kj kj




D) The active site is denatured upon binding with the substrate.
kj kj kj kj kj kj kj kj kj kj




Question 4 kj




Which of the following describes a function of a second messenger in cell signalling?
kj kj kj kj kj kj kj kj kj kj kj kj kj




A) To activate a G-protein on the cell surface membrane.
kj kj kj kj kj kj kj kj kj




B) To directly bind to DNA and initiate transcription.
kj kj kj kj kj kj kj kj




C) To amplify the signal by activating multiple protein kinase enzymes.
kj kj kj kj kj kj kj kj kj kj




D) To transport the hormone across the cell membrane via facilitated diffusion.
kj kj kj kj kj kj kj kj kj kj kj




Question 5 kj




In the light-
kj kj




dependent reaction of photosynthesis, photolysis of water produces protons (H+), electrons, and
kj kj kj kj kj kj kj kj kj kj kj kj




oxygen. What is the role of the electrons produced?
kj kj kj kj kj kj kj kj




A) To reduce carbon dioxide to glycerate-3-phosphate (GP).
kj kj kj kj kj kj kj




B) To replace electrons lost from the reaction centre of photosystem II.
kj kj kj kj kj kj kj kj kj kj kj




C) To combine with oxygen to form water.
kj kj kj kj kj kj kj




D) To generate a proton gradient across the thylakoid membrane.
kj kj kj kj kj kj kj kj kj




Question 6 kj




Sickle cell anaemia is caused by a single base substitution mutation in the gene that codes for the b
kj kj kj kj kj kj kj kj kj kj kj kj kj kj kj kj kj kj




eta-
globin chain of haemoglobin. Which of the following best describes the resulting change in the pri
kj kj kj kj kj kj kj kj kj kj kj kj kj kj kj




mary structure of the protein?
kj kj kj kj




A) A different amino acid is coded for, changing the sequence.
kj kj kj kj kj kj kj kj kj kj




B) The polypeptide chain is significantly shortened due to a premature stop codon.
kj kj kj kj kj kj kj kj kj kj kj kj




C) The amino acid sequence remains the same due to the degenerate nature of the genetic code.
kj kj kj kj kj kj kj kj kj kj kj kj kj kj kj kj




D) A different amino acid is coded for, but it does not affect the protein's structure.
kj kj kj kj kj kj kj kj kj kj kj kj kj kj kj




Question 7 kj




The movement of water through a plant from the roots to the leaves is explained by the cohesion-
kj kj kj kj kj kj kj kj kj kj kj kj kj kj kj kj kj




tension theory. Which property of water is most directly responsible for the process of transpiratio
kj kj kj kj kj kj kj kj kj kj kj kj kj kj




n pull?
kj




A) Water is a solvent for many ions and molecules.
kj kj kj kj kj kj kj kj kj




B) Water has a high specific heat capacity.
kj kj kj kj kj kj kj




C) Water molecules are cohesive due to hydrogen bonding.
kj kj kj kj kj kj kj kj




D) Water is less dense as a solid than as a liquid.
kj kj kj kj kj kj kj kj kj kj kj




Question 8 kj




A patient with a defective immune system is unable to produce a specific antibody response again
kj kj kj kj kj kj kj kj kj kj kj kj kj kj kj

,st a pathogen. Which type of cell is most likely to be deficient in this patient?
kj kj kj kj kj kj kj kj kj kj kj kj kj kj kj




A) Phagocytes
kj




B) Cytotoxic T cells
kj kj kj




C) B lymphocytes
kj kj




D) Natural killer cells
kj kj kj




Question 9 kj




During DNA replication, the enzyme DNA helicase is responsible for which of the following functio
kj kj kj kj kj kj kj kj kj kj kj kj kj kj




ns?
A) Joining Okazaki fragments together on the lagging strand.
kj kj kj kj kj kj kj kj




B) Synthesising new DNA strands in the 5' to 3' direction.
kj kj kj kj kj kj kj kj kj kj




C) Unwinding the double helix by breaking hydrogen bonds between complementary bases.
kj kj kj kj kj kj kj kj kj kj kj




D) Adding an RNA primer to initiate replication.
kj kj kj kj kj kj kj




Question 10 kj




In a population of beetles, a mutation gives some individuals a selective advantage. Over many ge
kj kj kj kj kj kj kj kj kj kj kj kj kj kj kj




nerations, the frequency of the advantageous allele increases. This process is an example of:
kj kj kj kj kj kj kj kj kj kj kj kj kj




A) Genetic drift
kj kj




B) Natural selection
kj kj




C) Stabilising selection
kj kj




D) The founder effect
kj kj kj




MARK SCHEME & EXPLANATIONS
kj kj kj




Question 1 kj




Correct Answer: B kj kj




• Explanation:

o Nucleotides are the smallest, as they are the monomers of nucleic acids (approx. 1
kj kj kj kj kj kj kj kj kj kj kj kj kj kj




nm).

o Proteins are polymers of amino acids and are larger than a nucleotide (approx. 10 n
kj kj kj kj kj kj kj kj kj kj kj kj kj kj




m).

o Ribosomes are larger protein complexes (approx. 20- kj kj kj kj kj kj




30 nm) made of RNA and protein.
kj kj kj kj kj kj




o Chloroplasts are large organelles visible under a light microscope (approx. 5- kj kj kj kj kj kj kj kj kj kj




10 µm). kj




o Distractor A & D: Incorrectly place nucleotide after ribosome/protein.
kj kj kj kj kj kj kj kj




o Distractor C: Incorrectly places nucleotide after protein.
kj kj kj kj kj kj

, Question 2 kj




Correct Answer: B
kj kj




• Explanation:

o The description (double membrane, folded inner membrane into cristae) is a classic
kj kj kj kj kj kj kj kj kj kj kj k




description of a mitochondrion. Its primary function is the synthesis of ATP during
j kj kj kj kj kj kj kj kj kj kj kj kj kj




aerobic respiration (the Krebs cycle occurs in the matrix, and oxidative phosphoryla
kj kj kj kj kj kj kj kj kj kj kj




tion occurs on the cristae).
kj kj kj kj




o Distractor A: Protein synthesis occurs at ribosomes (which are not membrane-
kj kj kj kj kj kj kj kj kj kj




bound).

o Distractor C: Modification and packaging of proteins occurs in the Golgi apparatus.
kj kj kj kj kj kj kj kj kj kj kj




o Distractor D: Hydrolysis of phagocytic cells is a function of lysosomes.
kj kj kj kj kj kj kj kj kj kj




Question 3 kj




Correct Answer: B
kj kj




• Explanation:

o The induced- kj




fit model states that the active site is not a rigid shape. Instead, it changes its confor
kj kj kj kj kj kj kj kj kj kj kj kj kj kj kj kj




mation (shape) slightly when the substrate binds. This puts strain on the chemical b
kj kj kj kj kj kj kj kj kj kj kj kj kj




onds of the substrate, lowering the activation energy for the reaction.
kj kj kj kj kj kj kj kj kj kj




o Distractor A: This describes the old, outdated 'lock and key' model.
kj kj kj kj kj kj kj kj kj kj




o Distractor C: The enzyme changes shape, not the substrate.
kj kj kj kj kj kj kj kj




o Distractor D: The active site changes shape to fit the substrate, but it is not denature
kj kj kj kj kj kj kj kj kj kj kj kj kj kj kj




d. Denaturation involves a permanent loss of structure.
kj kj kj kj kj kj kj




Question 4 kj




Correct Answer: C
kj kj




• Explanation:

o A key feature of a second messenger (e.g., cAMP) is signal amplification. One horm
kj kj kj kj kj kj kj kj kj kj kj kj kj




one molecule binding to a receptor can activate many G-
kj kj kj kj kj kj kj kj kj




proteins, which in turn activate many adenylyl cyclase enzymes, producing many cA
kj kj kj kj kj kj kj kj kj kj kj




MP molecules. These cAMP molecules then activate multiple protein kinase A enzy
kj kj kj kj kj kj kj kj kj kj kj




mes, leading to a large cellular response.
kj kj kj kj kj kj




o Distractor A: Activating a G- kj kj kj kj




protein is the role of the first messenger (the hormone) binding to the receptor.
kj kj kj kj kj kj kj kj kj kj kj kj kj