AQA GCSE Biology Biology Paper 1 (Higher), June 2025: Question 6

✅ Acceptable Answers (1 Mark)

Any precise definition stating movement down a concentration gradient:

• The movement (or spreading out) of particles from an area of higher concentration to an area of lower concentration.
• Spreading of particles / substances down a concentration gradient.

💡 Key Knowledge

Diffusion is a passive process driven by the random motion of particles in liquids or gases. It requires no energy input from respiration.

❌ Common Errors

• Saying "against the concentration gradient" (this describes active transport and loses the mark!).
• Forgetting to mention where they move from and to (e.g., just writing "particles spreading out").

🧠 Exam Technique

Learn this definition verbatim! You don't need to mention membranes here, but if you do, do not confuse it with osmosis (osmosis is water only across a partially permeable membrane).

✅ Full Mark Breakdown (Any 4 of the following):

• Mark 1 (Surface Area): (Many gill filaments / gills) provide a large surface area for (maximum) diffusion.
• Mark 2 (Concentration Gradient): Has a good blood supply / dense capillary network to maintain a steep concentration gradient.
• Mark 3 (Diffusion Distance): Filaments / capillaries have thin walls (or are one cell thick) for a short diffusion distance / time.
• Mark 4 (Proximity / Flow): Water is in close proximity to the blood supply (short diffusion distance) OR counter-current flow of water and blood to maintain concentration gradient.

🧠 The "Feature + Reason" Rule

Notice how every adaptation pairs a structural feature with its functional purpose:

• Large surface area ➔ increases rate of diffusion.
• Thin walls (one cell thick) ➔ short diffusion pathway.
• Rich capillary network ➔ maintains steep concentration gradient.

Always write both halves to guarantee the marks!

💡 Fick's Law of Diffusion

Rate of Diffusion ∝ (Surface Area × Concentration Difference) ÷ Thickness of Membrane.

Gills maximise diffusion by making Surface Area large, Concentration Difference large (via blood and water movement), and Thickness minimal.

❌ Common Misconceptions

• "Cell walls": Animal cells DO NOT have cell walls! Saying "the filaments have thin cell walls" is rejected. Say "thin walls that are one cell thick".
• Vague vessels: Do not use "arteries" or "veins" when explaining exchange surfaces—oxygen enters via capillaries or blood vessels.
• Writing only the feature without the mechanism (e.g. just writing "large surface area" with no reference to why).

✅ Complete 4-Step Chain of Explanation:

1. Point 1: Swimming involves increased movement. [1 mark]
2. Point 2: This requires increased / more muscle contraction. [1 mark]
3. Point 3: Which requires more energy (provided by respiration). [1 mark]
4. Point 4: Therefore, more respiration must take place (increasing overall metabolic rate). [1 mark]

🧠 Building an Unbroken Logic Chain

Examiners love cause-and-effect chains for exercise questions. Link them step-by-step:

Movement ➔ Muscle Contraction ➔ Needs Energy ➔ More Respiration

If you miss any link in the chain, you miss a mark.

❌ Major Examiner Traps

• "Energy is produced / created / made": According to the law of conservation of energy, energy cannot be created. Respiration transfers or releases energy. Writing "energy is produced" loses the mark!
• "Energy for respiration": Respiration does not need energy—it supplies energy for muscle contraction.
• Forgetting to explicitly mention muscle contraction. Swimming is just the observable action; the physiological cause is contracting muscles.

💡 What is Metabolic Rate?

Metabolic rate is the rate at which all the chemical reactions in the body are carried out. Since respiration is a continuous cellular chemical reaction, an increase in respiration directly increases the metabolic rate.