AQA GCSE Combined Science: Trilogy Biology Paper 1 (Higher), 2023: Question 4

✅ Correct answer

A

💡 Key knowledge

• The upper epidermis is the outer layer at the top of the leaf.
• It is above the palisade mesophyll.
• In diagrams, it is usually shown as the top row of cells.

🧠 Exam technique

• Look for the top layer of cells.
• If the question says “tick one box”, only one answer is needed.
• Use the diagram labels carefully — the examiner wants the tissue, not the function.

❌ Common errors

• Choosing the palisade layer because it is also near the top.
• Confusing epidermis with guard cells.
• Not matching the label to the actual position in the figure.

✅ Correct answer

Any three of the following ideas score the 3 marks:

• It has large air spaces between the cells.
• This allows gases to diffuse/move through the leaf.
• This helps gases move to and from the photosynthetic palisade layer.

💡 Key knowledge

• Spongy mesophyll cells are loosely packed.
• The gaps let oxygen, carbon dioxide and water vapour move easily.
• Gas movement happens by diffusion.
• The gases need to reach the palisade mesophyll where most photosynthesis happens.

🧠 Exam technique

• Use the phrase “large air spaces” — this is the key structural feature.
• Then explain the function: “so gases can diffuse through the leaf”.
• Finish by linking to photosynthesis: “to/from the palisade layer”.
• That structure → function chain is exactly what earns marks.

❌ Common errors

• Saying only “it has air spaces” without explaining why they matter.
• Talking about chloroplasts here — that is not the main marking point.
• Writing “oxygen goes in for photosynthesis” instead of carbon dioxide.
• Missing the direction: gases move to and from the palisade layer.

This would typically gain all 3 marks because it states the adaptation, the process, and the link to function.

✅ Correct answer

Any one developed explanation from the mark scheme can score 2 marks:

• Xylem is composed of hollow tubes/cells with no cytoplasm or nucleus, so there is no obstruction to water flow.
• OR xylem contains lignin, which holds tubes open / strengthens them.
• OR xylem has no end walls, so water flows easily/continuously.

💡 Key knowledge

• Xylem transports water and mineral ions from roots to leaves.
• It is made of dead cells joined end to end to form tubes.
• Lignin is a strong waterproof material that supports the vessel.

🧠 Exam technique

• You need structure + reason for full marks.
• For example: hollow tubes → less resistance → water flows easily .
• Or: lignin → strengthens walls → keeps xylem open .
• The examiner will not award both marks for just saying “it transports water”.

❌ Common errors

• Saying only “xylem carries water” — too vague for 2 marks.
• Writing “thick walls” without mentioning lignin or why thick walls help.
• Forgetting to link the feature to easy water flow or support.
• Mixing up xylem with phloem.

Each example gives a structure and a function, which is exactly what the mark scheme wants.

✅ Correct answer

• Water enters the guard cells by osmosis.
• The cells become turgid, so they swell and curve, opening the stoma.

💡 Key knowledge

• During the day, glucose is made in guard cells by photosynthesis.
• More glucose means a lower water potential in the guard cells.
• Water moves in by osmosis from a region of higher water concentration.
• The inner wall of each guard cell is thicker/less flexible, so the cells bend apart when turgid.

🧠 Exam technique

• Use the sequence: glucose increases → water enters by osmosis → cells become turgid → stoma opens.
• If you mention the wall thickness, make it clear that this causes the cells to curve apart.
• Keep the process in the correct order — this is how marks are awarded.

❌ Common errors

• Saying “glucose opens the stomata” without explaining how.
• Forgetting osmosis.
• Saying water enters by diffusion — this is wrong here.
• Not mentioning that the guard cells swell/curve.

A clear sequence like this shows cause and effect, which is ideal for 2-mark explanation questions.

💡 What top answers did well

• Used accurate biology terms: diffuse, osmosis, turgid, lignin, hollow tubes.
• Linked structure to function clearly.
• Kept answers short but specific.

❌ Where students lost marks

• Being too vague: “helps transport” without saying how.
• Mixing up tissues and functions.
• Missing the key words that the mark scheme expects.

🧠 Best exam habit

• Always ask: What is the structure? What is the function?
• For 2 marks, try to give one structure + one explanation.
• For 3 marks, give a complete chain of reasoning.

✅ Answers in one line

04.1: A

04.2: Large air spaces → gases diffuse through leaf → to/from palisade mesophyll.

04.3: Xylem is hollow / lignified / has no end walls, so water flows easily.

04.4: Water enters by osmosis, guard cells become turgid and curve, opening the stoma.

🧠 Final tip

In GCSE Biology, the examiner is usually rewarding clear biological explanation, not long paragraphs. Use the key terms, stay focused on the question, and match the tissue to its job.