AQA A-Level Biology Paper 2, June 2025: Question 2

✅ Correct Answer

Homeostasis

🧠 Exam Technique

• Ensure correct spelling: roots are homeo- (similar/unchanging) and -stasis (standing still).
• Do not confuse with haemostasis (blood clotting/stopping bleeding) — an examiners' trap!

✅ Correct Answer (2 Marks)

1. Mark 1: The high glucose concentration decreases / lowers the water potential (Ψ) of the blood (or blood water potential becomes more negative).
2. Mark 2: Water leaves / moves out of body cells into the blood by osmosis (down a water potential gradient).

💡 Key Knowledge

Glucose is an active solute. When blood glucose rises drastically above normal:

• Blood plasma has a higher solute concentration, giving it a lower / more negative Ψ than the intracellular fluid.
• Water moves down the water potential gradient, from higher Ψ (inside cells) to lower Ψ (blood) via osmosis across the cell membrane.

❌ Common Errors

• Ignoring negative constraints: Mentioning ADH, collecting ducts, or kidney filtration wastes time and gains 0 marks.
• Vague terms: Saying "water moves from high water concentration to low water concentration" is not A-Level standard; you must state water potential.
• Wrong direction: Stating that glucose enters cells and draws water in.
• Writing "WP" instead of spelling out water potential or using the Greek letter Ψ.

✅ Correct Answer

Tick box 2: Activation of cyclic AMP hydrolysis

💡 Mechanism of Glucagon (Second Messenger)

1. Glucagon binds to specific cell surface receptors on liver target cells.
2. Activates adenylate cyclase, converting ATP to cyclic AMP (cAMP).
3. cAMP activates protein kinase A.
4. Protein kinase activates enzymes that catalyse glycogenolysis (glycogen to glucose) and gluconeogenesis (amino acids/glycerol to glucose).
5. Therefore, glucagon promotes synthesis of cAMP, not its hydrolysis (breakdown).

🧠 Exam Technique

Watch out for negative phrasing: "Which is not a result..."

Glucagon causes cAMP formation, not cAMP hydrolysis (which would deactivate the cascade via phosphodiesterase).

✅ Correct Answer (Any 2 of the following)

• Mark 1 (Detection): A change / deviation in blood pH is detected by chemoreceptors (or change in pH alters stimulation of chemoreceptors).
• Mark 2 (Correction): Blood pH is returned / restored to the set point / norm / 7.35 to 7.45.
• Alternative Mark 1: Separate mechanisms/systems control increases and decreases (providing greater degree of control).

🧠 Top-Level Exam Technique

• Use the Figure: Always quote specific data given in the diagram. Here, state the normal range: 7.35 to 7.45.
• Negative feedback requires two distinct phases:
  1. Detection of deviation: Receptor senses shift away from the set point.
  2. Corrective response: Effector acts to reverse the change back towards normal.

❌ Common Errors

• Providing a purely theoretical definition without applying it to the chemoreceptor/pH loop provided in Figure 1.
• Confusing negative feedback with positive feedback (e.g. claiming the response amplifies the initial change).

✅ Correct Answer (2 Marks)

1. Mark 1 (Cause of pH decrease): Increased (aerobic) respiration produces more carbon dioxide (CO₂)
   OR Anaerobic respiration produces lactate / lactic acid.
2. Mark 2 (Figure 1 pathway): (Blood) pH decreases (below 7.35), which increases stimulation of chemoreceptors, resulting in an increased rate of ventilation.

💡 Physiological Pathway

Vigorous muscle contraction demands high rates of ATP synthesis:

• Higher CO₂ production → CO₂ dissolves to form carbonic acid (H₂CO₃) → dissociates into H⁺ and HCO₃⁻ → lowers blood pH (pH < 7.35).
• Chemoreceptors in carotid & aortic bodies and medulla oblongata detect higher [H⁺].
• Nerve impulses sent to respiratory centre increase rate and depth of ventilation to exhale excess CO₂.

❌ Common Errors & Examiner Traps

• Comparative word missing: Simply saying "respiration produces carbon dioxide" scores 0. You must state more CO₂ or increased respiration.
• Scientific inaccuracy: Writing that "anaerobic respiration produces CO₂ and lactate" will lose Mark 1 (human anaerobic respiration only produces lactate, not CO₂!).
• Irrelevant physiological pathways: Discussing heart rate, the SAN, or cardiac output is explicitly ignored by the mark scheme because the question asks about ventilation and Figure 1.