AQA A-Level Biology Paper 1, June 2023: Question 2
8 marks · Medium difficulty · Short Answer
Describe three ways the structure of chitin is similar to cellulose, explain one adaptation of the insect gas exchange surface, and explain the importance of the xylem being kept open as a continuous tube.
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Question text
02.1 Chitin is a polysaccharide. The chitin monomer is a β-glucose molecule with one OH
group replaced by an NHCOCH3 group. NHCOCH3 can be represented by N(Ac).
Figure 1 shows the monomer that forms chitin and the chitin polymer.
Figure 1
Chitin has a similar structure to cellulose.
Use Figure 1 to describe three ways the structure of chitin is similar to the structure
of cellulose.
[3 marks]
02.2 Chitin keeps the tracheae open in the tracheal system of gas exchange in an insect.
Gas exchange does not occur in the tracheae.
Explain the importance of one adaptation of the gas exchange surface in the tracheal
system of an insect.
[2 marks]
02.3 Lignin is a polymer found in the walls of xylem vessels in plants. Lignin keeps the
xylem vessel open as a continuous tube.
Explain the importance of the xylem being kept open as a continuous tube.
[3 marks]
Mark scheme
Show the mark scheme
Question Marking Guidance Mark Comments
1. (Alternate) monomers/glucoses are Ignore they are both
flipped/upside down/rotated (by 180o); polysaccharides
2. (Joined by) glycosidic bonds; 4. Accept as an
additional mark point,
3. (Forms) straight/linear/unbranched (chains/
‘contains 1-4
molecules);
3 max linkages/bonds’
02.1 (3 x 4. Reject if reference
AO2) made to 1-6
5. Accept as an
additional mark point,
‘have β glucose’
Ignore both contain C,
H and O
EITHER Mark as pairs, 1 and
2 OR 3 and 4 OR 5
1. Tracheole (wall) thin/one cell thick;
and 6
2. (So) rapid diffusion (into cells)
Ignore ‘liquid in
OR tracheoles’
(So) short diffusion pathway/distance;
OR
3. Accept touch OR
3. Tracheoles enter/supply tissues/muscle fibres; push OR ‘close to’ for
enter
4. (So) diffusion direct into cells
3. Accept cells for
OR 2 max
tissues
02.2 (So) short diffusion pathway/distance (2 x
AO1)
OR
(So) rapid diffusion (into cells);
OR 5. Accept ‘large
number’ OR ‘many’
5. Tracheoles are highly branched; for highly branched
6. (So) short diffusion distance/pathway
OR 6. Ignore SA
(So) large surface area for (rapid) diffusion; 6. Ignore ‘to volume
ratio’ OR ‘:vol’
1. (Allows unbroken) water column 1. Accept idea of 7
continuous flow OR
OR
stream of water
(So) no barrier to (water) movement;
1. Ignore chain of
2. Cohesion from H bonds between (all) water water molecules
(molecules)
OR
3 If 1, 2 or 3 are not
Cohesion from (polar) attraction between (all) awarded accept a
02.3 (3 x
water (molecules); principle mark for
3. Evaporation/transpiration creates tension (in AO1)
correct reference to
column) cohesion-tension
OR causing water
Water moves from xylem (into cells) creates movement
tension
OR
(To) pull up water creates tension (in xylem);
How to answer it
Carbohydrates, Gas Exchange, and Transport Systems Study Guide
What this question tests
This exam block assesses your understanding of polymer structures (comparing chitin to cellulose), insect gas exchange adaptations (tracheoles and diffusion pathways), and plant transport mechanisms (the cohesion-tension theory in xylem vessels). It tests AO1 (knowledge and understanding) and AO2 (application of structural details to biological functions).
Comparing Chitin and Cellulose Structure
✅ Correct Answers (Any 3)
- Monomers/glucoses are flipped, upside down, or rotated by 180 degrees (alternate).
- Monomers are joined by glycosidic bonds.
- Forms straight, linear, or unbranched chains/molecules.
- Alternative accepted: Contains 1-4 linkages/bonds or has beta (β) glucose.
💡 Key Knowledge
- Chitin is a structural polysaccharide found in insect exoskeletons and fungal cell walls.
- Like cellulose, its structural integrity relies on long, straight chains running parallel to one another, forming strong microfibrils linked by hydrogen bonds.
🧠 Exam Technique
- Look closely at Figure 1: notice how adjacent monomer units alternate orientation (one right-side up, the next inverted).
- Be specific: state "straight and unbranched chains" rather than just vague shape descriptions.
❌ Common Errors
- Stating broadly that "both are polysaccharides" (this is ignored by examiners as it is too general and stated in the stem).
- Mentioning 1-6 glycosidic bonds (chitin and cellulose exclusively use 1-4 linkages).
- Stating they both contain C, H, and O (true for all carbohydrates, earns no marks).
Insect Gas Exchange Adaptations
✅ Correct Answers (Marked as pairs: 1+2, 3+4, or 5+6)
- Pair A: Tracheole wall is thin / one cell thick (1) — so rapid diffusion into cells / short diffusion pathway (2).
- Pair B: Tracheoles enter/supply tissues or muscle fibres (3) — so diffusion is direct into cells / short pathway / rapid diffusion (4).
- Pair C: Tracheoles are highly branched/numerous (5) — so short diffusion pathway / large surface area for rapid diffusion (6).
💡 Key Knowledge
- Insects rely on a tracheal system where air moves directly to tissues.
- Because gas exchange does not occur in the main tracheae, fine terminal tracheoles handle delivery directly to metabolising cells.
🧠 Exam Technique
- Always link a structural feature to its functional consequence using "so that" or "resulting in". A feature without a consequence loses the second mark of the pair.
❌ Common Errors
- Mentioning "surface area to volume ratio" for tracheoles (examiners explicitly reject this; focus on surface area alone or number of branches).
- Referring to "liquid in tracheoles" as an adaptation.
Xylem Structure and Cohesion-Tension
✅ Correct Answers (Any 3)
- Allows an unbroken water column / no barrier to water movement / maintains continuous flow.
- Cohesion from hydrogen bonds (or polar attraction) between water molecules.
- Evaporation/transpiration creates tension in the column / water moving from xylem into cells creates tension or pulls water up.
- Principle fallback: If specific points aren't hit, award 1 mark max for a correct reference to "cohesion-tension causing water movement".
💡 Key Knowledge
- Xylem vessels are dead, hollow tubes reinforced with lignin, making them rigid and waterproof.
- The continuous water column is pulled up the plant by negative pressure generated by transpiration at the leaves.
🧠 Exam Technique
- Mention key terminology precisely: use "hydrogen bonds", "cohesion", and "tension" to secure biological mechanism marks.
❌ Common Errors
- Saying lignin provides active transport or pushes water up (lignin is purely structural and dead; water movement is passive driven by transpiration pull).
- Describing a "chain of water molecules" instead of referencing hydrogen bonding or molecular continuity accurately.
Topics
Biology · 3.1 Biological molecules · 3.3 Organisms exchange substances with their environment
Question and mark scheme from the AQA A-Level Biology examination, Paper 1, June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.