AQA A-Level Biology Paper 1, June 2025: Question 9

✅ Correct Answer

• Cells of one type: Tissue
• Cells of more than one type: Organ

💡 Key Knowledge

The hierarchy of multicellular organisation:

• Specialised cells: Differentiated to carry out a specific function.
• Tissue: An aggregation of similar cells working together to perform a specific function (e.g., palisade mesophyll tissue).
• Organ: A coordinated group of different tissues performing coordinated physiological functions (e.g., a leaf contains epidermal, mesophyll, and vascular tissues).

🧠 Exam Technique

Read the order carefully! Marks are commonly lost simply because candidates reverse the two words or write "organ system" instead of "organ".

❌ Common Errors

• Writing organelle instead of tissue.
• Confusing organ system with organ.
• Inverting the order (writing organ first, then tissue).

✅ Correct Answer

• Q: Starch (grain) OR lipid / fat (droplet / plastoglobule)
• R: Stroma

💡 Key Knowledge

• Q: Appears as rounded, distinct, dense electron-opaque or translucent inclusions within the plastid matrix. Plants store photosynthetic products as insoluble starch grains or neutral lipid droplets (plastoglobules).
• R: The fluid-filled matrix surrounding grana and lamellae. Contains enzymes for the light-independent stage (Calvin cycle), chloroplast 70S ribosomes, and circular DNA.

❌ Common Errors

• Stoma vs Stroma: Writing "stoma" (the microscopic pore in a leaf epidermis) gets an instant REJECT. Always double-check spelling!
• Labeling Q as a "vacuole" or "nucleus" — remember, you are inside a single chloroplast organelle.

🧠 Exam Technique

Whenever asked to identify structures within a TEM image of an organelle, remind yourself of the scale. At ×11,000, you are observing internal ultrastructure, not cellular-level features.

✅ Correct Answer

• Observation (Mark point 1): Mutant chloroplast has fewer grana / thylakoids / lamellae (or they are thinner / smaller).
• Consequence (Mark point 2): (So) less chlorophyll and less light absorbed, resulting in less photosynthesis (or fewer light-dependent reactions).

💡 Key Knowledge

• Thylakoid membranes provide a large surface area for light-harvesting photosystems, electron transport chains, and ATP synthase.
• Fewer stacked grana directly reduces membrane surface area → fewer embedded chlorophyll molecules → reduced photoionisation of chlorophyll.

🧠 Exam Technique

The command prompt states: "predict how photosynthesis could be affected... Justify your answer." Always follow a 2-step structure:

Observation from image → Biochemical mechanism that impacts the rate

❌ Common Errors

• Writing just "photosynthesis decreases" without pointing to the missing/reduced grana or thylakoid membranes.
• Using undefined abbreviations: the mark scheme explicitly states "Ignore LDR" — always spell out light-dependent reaction in full!

✅ Correct Answer

To secure all 3 marks, you must provide both descriptions of the data and an explanation:

• MP1 (Description): The mutant has less chlorophyll (9 vs 45 µg cm⁻³) but a higher rate of photosynthesis (5 vs 1 arbitrary units).
• MP2 (Description of ratio): The mutation increased the chlorophyll a to chlorophyll b ratio (from 2:1 to 6:1 / relatively more chlorophyll a).
• MP3 (Explanation): (Chlorophyll a absorbs different light wavelengths, so) mutant absorbs more light OR absorbs a greater range of wavelengths.

📐 Interpreting the Table Data

Break down the figures in Table 6 systematically:

• Total Chlorophyll: Drops dramatically from 45 → 9 µg cm⁻³ (an 80% reduction).
• Ratio (a : b): Triples from 2:1 → 6:1. Chlorophyll a is now the vastly dominant form.
• Rate of Photosynthesis: Increases 5-fold (1 → 5 arbitrary units), directly contradicting the initial hypothesis from 09.3!

❌ Common Errors & Examiner Traps

• Absolute vs Relative Amounts: Stating the mutant has "more chlorophyll a" in absolute quantity will be REJECTED! Total chlorophyll is 9 µg cm⁻³ in the mutant vs 45 µg cm⁻³ in the healthy plant. The mutant has a higher proportion / ratio of chlorophyll a, not a higher absolute mass.
• Only Describing: Stating the numbers without explaining why this enhances the photosynthetic rate (MP3 requires mentioning light absorption / range of wavelengths).
• Ignoring the prompt: The question asks you to describe AND explain. A description gives you MP1 and MP2, but the explanation is essential for MP3.

🧠 Exam Technique: "Describe and Explain"

Structure your answer in two clear paragraphs:

1. Describe: Compare both variables from the table directly (chlorophyll level & ratio vs photosynthetic rate). Quote the direction of change.
2. Explain: Chlorophyll a is the primary reaction centre pigment. A higher proportion of a:b alters the absorption spectrum, allowing the mutant to absorb light energy more efficiently or across broader usable wavelengths per unit pigment.