AQA GCSE Biology Biology Paper 1 (Higher), June 2022: Question 5

✅ Correct Answer

Guard cells (or guard)

💡 Key Knowledge

Guard cells surround each stoma. When water is plentiful, they become turgid and curve outward, opening the pore. When water is scarce, they lose water, become flaccid, and close the pore to prevent wilting.

✅ Correct Answers (Give any two)

• Vessel type: Transpiration involves xylem, whereas translocation involves phloem.
• Substances carried: Transpiration transports water and mineral ions, whereas translocation transports dissolved sugars / sucrose.
• Direction of flow: Transpiration moves substances in one direction (upwards only), whereas translocation moves substances in both directions (upwards and downwards / bidirectionally).
• Cell viability: Transpiration uses dead, hollow cells; translocation involves living cells.
• Energy requirement: Transpiration is a passive physical process (does not require metabolic energy), whereas translocation is an active process (requires energy).

🧠 Exam Technique: Making True Comparisons

Whenever a question asks for differences, you must explicitly mention both systems to earn the mark. Writing only "transpiration carries water" scores 0 unless you add "...whereas translocation carries dissolved sugars".

❌ Common Errors

• Stating that translocation transports glucose (it transports sucrose / dissolved sugars; glucose is converted before transport).
• Saying translocation moves substances "side-to-side" rather than bidirectionally (up and down).
• Mixing up xylem and phloem vessels.

✅ Correct Answer

Tick box 3: Warm with low humidity

💡 Scientific Explanation

• Warm temperature: Water molecules have more kinetic energy, evaporating faster from mesophyll cells into the air spaces.
• Low humidity (dry air): Maintains a steep concentration gradient of water vapour between the inside of the leaf and the surrounding atmosphere, speeding up diffusion outward.

❌ Distractor Breakdown

High humidity flattens the concentration gradient, drastically reducing diffusion and slowing transpiration, regardless of temperature.

✅ Mark Scheme Breakdown

At (mid)night:

• Stomata are (almost) closed because there is no light / less light for photosynthesis. [1 mark]
• Closing stomata reduces / prevents water loss. [1 mark]

At midday:

• Stomata open widest / wider because light intensity is at its maximum for photosynthesis. [1 mark]
• Opening wide allows the plant to take in maximum / more carbon dioxide (CO₂) for photosynthesis. [1 mark]

🧠 Top-Grade Structure: "Observation + Reason"

Divide your answer into two distinct time periods to secure all 4 marks:

1. Night state: Closed → Why? No light for photosynthesis → Advantage? Conserves water.
2. Day state: Widest → Why? Maximum sunlight → Advantage? Absorbs maximum CO₂ for photosynthesis.

❌ Where Students Lost Marks

• Writing just "it is dark" without linking it to photosynthesis.
• Merely reading numbers from the graph (e.g. "width is 75 at midday"). The mark scheme strictly instructs: ignore values for time and width.
• Saying stomata open to "let oxygen in" (photosynthesis consumes CO₂, not O₂).
• Stating stomata open to "take in CO₂" without specifying more / maximum intake or linking to higher light.

✅ Mark Scheme Breakdown

• Difference from graph: The stomata are open wider and for a longer time (or greater total area). [1 mark]
• Survival benefit: This allows the plant to take in more / sufficient carbon dioxide (CO₂) for photosynthesis (to maintain its rate of photosynthesis despite the lower concentration in the air). [1 mark]

🧠 Exam Technique: Both Dimensions of the Curve

Look at Figure 7 carefully: the dashed line (low atmospheric CO₂) has:

• A higher peak → stomata open wider.
• A broader curve → stomata stay open for a longer duration.

State both features to guarantee mark 1, then explain the physiological benefit (CO₂ uptake for glucose production) for mark 2.

💡 Plant Physiology Concept

When atmospheric CO₂ drops, the diffusion gradient into the leaf is weaker. To compensate and ensure enough CO₂ enters for the Calvin cycle / glucose production, guard cells open wider and remain open longer.