AQA AS Level Biology Paper 1, November 2020: Question 5

✅ Correct Answer (Mark Scheme)

• Diaphragm muscle contracts (and flattens) AND external intercostal muscles contract (ribs move up and out).
• Causes an increase in thoracic volume and a corresponding decrease in thoracic/pulmonary pressure.
• Air moves down a pressure gradient from higher atmospheric pressure outside to lower pressure inside the lungs.

💡 Key Knowledge

Inspiration is an active process requiring ATP. Contraction of the diaphragm and external intercostal muscles alters the volume of the thorax, establishing the pressure gradient necessary for bulk flow of air.

🧠 Exam Technique

A complete 3-mark answer must follow the causal chain: Muscle contraction → Volume increase → Pressure decrease → Air movement down pressure gradient. Never state that "ribs move up and out" as a substitute for muscle contraction unless linked correctly.

❌ Common Errors

• Forgetting to state that both the diaphragm and external intercostal muscles contract.
• Stating that air is "pulled" or "sucked" in rather than explaining movement down a pressure gradient.
• Confusing inspiration with expiration mechanics.

✅ Correct Answer (Mark Scheme)

• K: Bronchiole
• L: Artery / arteriole / vein / venule (Blood vessel types)

💡 Key Knowledge

Bronchioles are small airways characterized by walls of smooth muscle and a lack of cartilage plates/goblet cells in terminal regions. Blood vessels (arteries, veins, capillaries) are identified by their lumen shape and endothelial lining.

❌ Common Errors

• Labelling structure L as a "capillary" (rejected by examiners as capillaries do not appear with distinct muscular wall structures at this magnification scale).
• Adding qualifying terms like "pulmonary" which are unnecessary and can invalidate the answer if incorrect.

✅ Correct Answer (Mark Scheme)

• Animal / lung tissue does not contain starch (so iodine would serve no purpose).
• Haematoxylin stains DNA / makes the nucleus visible (allowing cellular structures to be observed).

💡 Key Knowledge

Different stains bind to specific cellular macromolecules. Iodine stains starch (found in plant storage tissues, but absent in mammals), whereas haematoxylin is a basic dye that binds to acidic nucleic acids (DNA/RNA) in cell nuclei, giving them a characteristic blue/purple appearance.

🧠 Exam Technique

This is a "two-part why" question. Make sure you address why iodine was omitted (no starch in animals) and why haematoxylin was chosen (stains nuclei/DNA).

✅ Correct Answer (Mark Scheme)

• In support: Living with a cat or dog has a statistically significant link (P < 0.05).
• In support: Obesity has a highly significant link (P < 0.001).
• Not supported: Burning wood indoors is not statistically significant (P = 0.06, which is greater than 0.05).

💡 Key Knowledge

In biology, a probability threshold of P < 0.05 is standard for statistical significance. If P < 0.05, there is less than a 5% probability that the observed result is due to chance, meaning we reject the null hypothesis. If P ≥ 0.05 (like P = 0.06), the result is not statistically significant.

🧠 Exam Technique

When asked to "Evaluate" a statement containing multiple parts, you must provide a balanced argument addressing what data supports the claim and what data refutes it. Always explicitly cite the P values from the table to back up your reasoning.

❌ Common Errors

• Stating "results are significant" without specifying which risk factor you are referring to.
• Failing to recognise that P = 0.06 is greater than 0.05 and therefore not significant.