AQA AS Level Biology Paper 2, June 2022: Question 9

10 marks · Medium difficulty · Short Answer

Describe how the structure of the insect gas exchange system provides cells with sufficient oxygen and limits water loss, and describe how humans breathe in and out.

Practise this question

Question

Question text

09.1 Describe how the structure of the insect gas exchange system:

• provides cells with sufficient oxygen

• limits water loss.

Explain your answers.

[5 marks]

09.2 Describe how humans breathe in and out.

[5 marks]

Mark scheme

Show the mark scheme

Question Marking Guidance Mark Comments

1. Spiracles, tracheae, tracheoles; Reference to these 3

structures anywhere

2. Spiracles allow diffusion (of oxygen)

in answer = 1mark

OR If whole answer refers

to MPs 1–5 only,

(Oxygen) diffusion through award MAX 4 marks

tracheae/tracheoles;

3. Accept ‘network’ or

3. Tracheoles are highly branched so large ‘large number’ for

surface area (for exchange); highly branched

4. Tracheole (walls) thin so short diffusion

distance (to cells)

3., 4. and 5. If

tracheae/tracheoles

confused, penalise

once only

Highly branched tracheoles so short diffusion

distance (to cells) 5 max

09.1

(5 x AO1) 4. Allow ‘next/close to’

OR for enter cells

Tracheoles enter cells so short diffusion

distance;

5. Tracheole permeable to oxygen/air;

6. Cuticle/chitin/exoskeleton (impermeable) so 6. Allow prevents

reduce water loss; water loss

7. Spiracles (can) close so no/less water loss

Spiracles have valves so no/less water loss;

8. Hairs around spiracles reduce water loss;

Breathing in

1. Diaphragm (muscles) contract and diaphragm 1. Accept

flattens; ‘pulled/moved down’

2. External intercostal muscles contract and for flattens.

ribcage pulled up/out;

3. (Causes) volume increase and pressure 3. and 6. Accept lungs

decrease in thoracic cavity (to below or thorax for ‘thoracic

atmospheric pressure); cavity’

5 max

09.2 Reject ‘chest’ once

(5 x AO1) 19

Breathing out

4. Diaphragm (muscles) relaxes and diaphragm 4. Accept ‘returns to

moves up; domed shape’ for

moves up

5. External intercostal muscles relax and ribcage

moves down/in; 5. Accept internal

6. (Causes) volume decrease and pressure intercostal muscles

increase in thoracic cavity (to above contract

atmospheric pressure);

How to answer it

Insect and Human Gas Exchange Systems

What this question tests

This question assesses your knowledge of multicellular gas exchange adaptations (AO1). You must be able to link structural features of an insect's tracheal system to how they satisfy oxygen demand while minimizing water loss, and describe the physical mechanics of ventilation (pressure and volume changes) in mammalian breathing.

Question 09.1 (5 marks)

Insect Gas Exchange: Oxygen Supply & Water Loss

✅ Mark Scheme Points

Must mention spiracles, tracheae, and tracheoles for 1 mark.
Spiracles/tracheae/tracheoles allow diffusion of oxygen.
Highly branched tracheoles create a large surface area.
Thin tracheole walls or tracheoles entering directly into cells provide a short diffusion distance.
Water loss prevention: Impermeable cuticle/chitin/exoskeleton, spiracles can close via valves, or hairs around spiracles.

💡 Key Knowledge

Insects lack a transport system for gases; air goes directly to tissues via tubes.
Surface area to volume ratio in insects is small due to their exoskeleton, necessitating specialized internal tubes.
Water conservation is an evolutionary priority for terrestrial insects, balanced constantly against gas exchange needs.

🧠 Exam Technique

Structure-function links: Never just state a feature (e.g., "tracheoles are branched"). You must explain the consequence ("...so they provide a large surface area").
Address both bullet points in the prompt: insufficient oxygen AND limiting water loss to access full marks.

❌ Common Errors

Confusing tracheae with tracheoles (penalized only once in the mark scheme, but loses precision marks).
Forgetting to mention the role of valves or muscles in closing spiracles to stop evaporation.
Listing more than 5 points where only max 4 marks can be awarded if structure naming (point 1) is missed.

Examiner Note: If the entire answer focuses only on oxygen supply or only on water loss, candidates cap out at a maximum of 4 marks due to unbalanced coverage. Ensure both parts of the stem are explicitly covered.

Question 09.2 (5 marks)

Human Mechanics of Breathing (Inhalation and Exhalation)

✅ Mark Scheme Points

Breathing in: Diaphragm muscles contract and flatten.
External intercostal muscles contract, pulling ribcage up and out.
Thoracic cavity volume increases, causing pressure to decrease below atmospheric pressure.
Breathing out: Diaphragm muscles relax and move up (dome shape).
External intercostal muscles relax, moving ribcage down and in.
Thoracic cavity volume decreases, causing pressure to increase above atmospheric pressure.

💡 Key Knowledge

Boyle's Law in biology: Pressure is inversely proportional to volume when temperature is constant.
Air naturally moves down a pressure gradient from high pressure (atmosphere) to low pressure (lungs) during inspiration, and vice versa during expiration.

🧠 Exam Technique

Structure your answer chronologically: clearly separate Breathing in and Breathing out headings or paragraphs.
Always pair muscle action (contract/relax) with the resulting spatial change (flatten/up/down) and the resulting pressure/volume change.

❌ Common Errors

Using the forbidden word "chest" instead of "thoracic cavity" or "lungs" (penalized once by examiners).
Stating muscles "push" bones—muscles can only pull via contraction! Always use contract or relax.
Confusing internal and external intercostal roles (remember: normal quiet breathing out is largely passive due to elastic recoil, but external intercostal relaxation is credited).

Examiner Note: Top-tier answers explicitly link muscle contraction/relaxation directly to volume alterations, and volume alterations directly to pressure gradients relative to the atmosphere. Vague references to "air getting sucked in" without pressure mechanics will lose credit.

Topics

Biology · 3.3 Organisms exchange substances with their environment

Question and mark scheme from the AQA AS Level Biology examination, Paper 2, June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.