AQA A-Level Biology Paper 1, June 2025: Question 5

✅ Model Answer & Mark Breakdown

💡 Key Knowledge

• Proton Gradient Establishment: Protein X acts as an ATP-driven proton pump, establishing a steep electrochemical H⁺ gradient across the tonoplast.
• Antiporter Co-transport: Protein Y is an antiport co-transporter; moving H⁺ down its concentration gradient facilitates the transport of fructose against its gradient.
• Water Potential (Ψ): Adding solute (fructose) always makes water potential more negative (decreases Ψ). Water moves down a water potential gradient from high (less negative) to low (more negative) Ψ.

🧠 Exam Technique & Diagram Navigation

The question provides explicit bullet points: you must mention ATP hydrolase, carrier protein X, and carrier protein Y. Structure your answer in chronological order following the diagram:

ATP breakdown → H⁺ pumped in (X) → H⁺ & fructose co-transport (Y) → Ψ drops → Osmosis

❌ Common Errors & Mark Losses

• "Energy is made/created/produced": Never write this! The mark scheme explicitly states Reject make/create/produce energy . Always write releases energy.
• Vague Ions: Writing "hydrogen" instead of H⁺ ions or protons loses marks.
• Incomplete Osmosis link: Stating "water enters by osmosis" without explaining why (decreasing/more negative water potential) fails to secure Mark 4.

✅ Model Answer & Mark Breakdown

💡 pH and Proton Concentration

• pH Formula: pH = -log₁₀[H⁺] . A high H⁺ concentration means low (acidic) pH; a low H⁺ concentration means high (less acidic/more alkaline) pH.
• If proton pumping into the vacuole ceases, [H⁺] in the vacuole decreases, which causes pH to increase.

🧠 Exam Technique

• State the direction of pH change clearly: it increases (or becomes less acidic). Stating merely that "pH changes" scores zero for the final mark.
• Link cause to effect: Respiration stopped → no ATP → pump X stops → fewer H⁺ in vacuole → pH increases .

❌ Common Errors

• Inverse relationship confusion: Many students write that fewer H⁺ ions makes the solution "more acidic" or that "pH decreases". Always remember: fewer H⁺ = higher pH.
• Rejecting Energy Production: Writing "no energy is produced" is rejected by examiners. Write "no ATP produced" or "no energy released".

✅ Model Answer & Mark Breakdown

💡 Key Knowledge

• Gene Expression to Protein Density: Increased transcription of mRNA and translation leads to a higher density of carrier proteins X and Y embedded in the vacuolar membrane.
• Transport Kinetics: More transport proteins increase the maximum rate ( Vmax ) of movement because carrier proteins are no longer the rate-limiting factor.

🧠 Exam Technique

• Identify the direction of expression: it must be an increase in expression / more transcription.
• Connect the protein directly to function: explicitly mention that more carrier proteins results in a faster rate of transport of H⁺ or fructose.

❌ Common Errors

• Wrong Direction of Movement: The mark scheme explicitly rejects stating that "fructose moves out". Fructose must move into the vacuole to sweeten the fruit and draw water in for ripening.
• Vague statements: Writing "it works faster" without specifying active transport of H⁺ or co-transport of fructose.