AQA AS Level Biology Paper 2, June 2022: Question 7

✅ Correct Answers

• Four continuous bands drawn: Upper epidermis, palisade mesophyll, spongy mesophyll, and lower epidermis.
• Proportional representation: Band widths must closely match the relative proportions seen in Figure 8.
• Clear labeling: One correct tissue correctly identified (e.g., palisade mesophyll, spongy mesophyll, xylem, phloem, upper/lower epidermis).

💡 Key Knowledge

• Biological drawings of tissues must never show individual cells unless explicitly requested.
• Lines must be continuous, thin, sharp, and drawn with a single sweep (no sketching or overlapping lines).
• Tissues are organized in distinct layers within a Dicotyledonous leaf cross-section.

🧠 Exam Technique

Examiners look for distinct structural layers. Ensure you draw at least 3 distinct bands corresponding to the anatomical features visible in the photomicrograph. Do not waste time drawing individual cells!

❌ Common Errors

• Drawing individual cells (automatic penalty).
• Using sketched, hairy, or overlapping lines.
• Attempting to label individual stomata or waxy cuticles instead of whole tissues.

✅ Correct Answers (Mark Scheme Breakdown)

• Point 1: Standard deviations (SD) do not overlap for mean shelf life (or mean transpiration rate).
• Point 2: Shelf life is significantly longer/different in April (16 days vs 5 days in December).
• Point 3: Standard deviations do overlap for mean number of stomata, indicating no significant difference/no link to shelf life.
• Point 4: Significant difference in mean transpiration rate means it is likely linked to shelf life.

💡 Key Knowledge

When data sets have overlapping ranges defined by standard deviations (or ±2 SD values representing 95% of data), differences between means are generally not statistically significant. Non-overlapping standard deviations strongly imply a significant difference.

🧠 Exam Technique

Always check the ± values in tables first. When asked what you can conclude, structure your answer by systematically addressing: (1) what the statistical test/SD shows, (2) the direct trend in the data, and (3) linking variables (stomata vs transpiration rate) to shelf life.

❌ Common Errors

• Vaguely stating "there is a correlation" without mentioning standard deviations or significance.
• Claiming the number of stomata affects shelf life, missing the fact that their SD values overlap significantly.

✅ Correct Answers (Choose one valid pathway)

• Pathway A (Light): Reduce light intensity → Stomata close → Decreased rate of evaporation/transpiration.
• Pathway B (Humidity/Air Flow): Increase humidity OR reduce air movement → Reduces water potential gradient between plant and air → Decreased rate of evaporation/transpiration.

💡 Key Knowledge

Cut flowers lose water via transpiration through their stomata. Minimizing water loss keeps cells turgid and extends shelf life. Factors decreasing transpiration include higher humidity, lower light intensity, and less wind/air flow.

🧠 Exam Technique

This is a multi-step causal question requiring 3 clear marks: (1) The environmental change (e.g., reduce light or increase humidity), (2) The physiological mechanism (e.g., stomata close or water potential gradient reduced), and (3) The ultimate effect (reduced transpiration/water loss).

❌ Common Errors

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• Mentioning temperature changes, which was explicitly forbidden by the question stem.
• Failing to link the environmental change to a mechanism (e.g., just saying "put them in the dark" without explaining that stomata close).