AQA AS Level Biology Paper 1, June 2025: Question 7

✅ Correct Answer

Sucrose

💡 Key Knowledge

• Why sucrose? Glucose produced in photosynthesis is converted into sucrose because sucrose is a non-reducing disaccharide. It is chemically less reactive and less likely to be metabolised during mass flow transport.
• Starch is completely insoluble and cannot enter or flow through sieve tube elements.

❌ Common Errors

• Writing glucose (glucose is produced during photosynthesis, but translocated as sucrose).
• Writing starch (insoluble storage form, not transport form).

✅ Correct Points (Any Three)

• Insoluble (in water): so does not affect water potential (Ψ) / no osmotic movement into cells.
• Coiled / helical / spiral: so it is compact / packs many molecules into a small volume.
• Branched / has many ends: allows rapid hydrolysis by enzymes to quickly release glucose.
• Polymer of α-glucose: provides readily accessible glucose units for cellular respiration.
• Large molecule: cannot cross the cell-surface membrane / cannot leave the cell.

🧠 Exam Technique: "Feature + Consequence"

Because this question asks you to explain, writing down the feature alone scores 0 marks. You must supply the biological reason using conjunctions like "so" or "because":

• "Insoluble..." → "...so no osmotic effect / doesn't alter water potential."
• "Branched..." → "...so enzymes can act simultaneously on multiple ends for rapid release."

❌ Common Errors

• Writing "it has a large surface area" for branching — examiners explicitly ignore surface area here. Focus on more ends for enzyme action.
• Writing "it is insoluble" without stating its osmotic consequence.
• Stating starch is used directly in respiration (only glucose monomers enter glycolysis).

✅ Correct Suggestions (Any One)

• Some ¹⁴C-sugars were respired by the leaf/plant cells (released as ¹⁴CO₂ gas).
• Some sugars were converted into other molecules (e.g. starch storage or cellulose cell walls) within the leaf.
• Some sugars remained in the leaf and were not translocated downwards during the 15-hour period.
• Some sugars were translocated past section E (into roots) or moved upwards toward shoot tips/buds.

💡 Biological Context

Leaves act as photosynthetic sources. Not 100% of newly fixed triose phosphate is exported into the phloem immediately. The leaf itself requires energy (ATP via respiration) and structural/storage polysaccharides.

❌ Common Errors

• Vague assertions like "it was lost" without specifying a biological destination or process (e.g. respiration, synthesis of starch/cellulose).
• Assuming radioactive decay occurred (carbon-14 has a half-life over 5,700 years; decay does not explain loss over 15 hours).

✅ Arguments In Support (Evidence For)

• Phloem concentration is higher: Radioactivity is significantly higher in phloem than xylem across all sections (e.g. Section A: 41,100 vs 24,210 cpm).
• Downward movement from source: Radioactivity is present in sections B, C, D, and E below the supplied leaf, showing downward translocation away from the source.
• Source role confirmed: Shows the leaf synthesises and exports organic compounds into vascular tissue.

❌ Arguments Against / Limitations (Evidence Against)

• Radioactivity detected in xylem: Notable counts exist in the xylem (e.g. 24,210 in A, 755 in B), indicating transport is not exclusively restricted to phloem.
• Single species tested: Investigation used only geranium plants; results might not apply to all plant species.
• No evidence of upward transport: Mass flow states transport occurs to all parts of the plant, but this setup only tested sections below the leaf (none above towards apical buds/flowers).
• Downward movement could be gravity: The observed flow downwards could be passively influenced by gravity rather than driven by hydrostatic pressure gradients.

🧠 Exam Technique: Structuring an "Evaluate" Answer

• Balance is mandatory: You must state points supporting AND points conflicting with the hypothesis. Notice the mark scheme caps points against at max 3 marks, meaning you cannot achieve 4/4 without at least one supporting point!
• Follow negative constraints: The prompt explicitly commands: "Do not refer to statistical tests or sample size". Mentioning "no standard deviations", "no stats test", or "only one plant tested" (as sample size) scores 0 marks.
• Note: Mentioning "only tested in geraniums / one species" is valid because it addresses phylogenetic generalisability, not numerical sample size.

❌ Common Traps

• Forgetting to check the xylem columns in Table 4 (assuming all counts were in phloem).
• Wasting time writing: "There are no error bars so we don't know if differences are significant" — explicitly disqualified by the question prompt.