Gas Exchange in Animals

Q1, Sample Band 6 response (7 marks), annotated

In insects, gas exchange occurs through the tracheal system: air enters via spiracles on the body wall, moves through branching tracheae and tracheoles, and oxygen diffuses directly into cells from the tracheoles. No circulatory transport of O₂ occurs, haemolymph does not carry oxygen. In mammals, air enters via the airways and reaches the alveoli, where O₂ diffuses across a thin membrane into capillary blood, which carries it via haemoglobin to all tissues. [1, structures + gas pathway named for both groups]

A fundamental similarity is that both systems rely on diffusion as the final step, oxygen moves down a concentration gradient across a moist, thin membrane in both cases. [1, similarity with reason]

Transport of gases: Insects deliver oxygen directly to cells through tracheoles, bypassing the circulatory system. Mammals require blood + haemoglobin to carry O₂ from alveoli to distant cells. [1, transport criterion] Role of haemolymph/blood: Insect haemolymph does not transport O₂; mammalian blood is the essential O₂ carrier. Size limitation: The tracheal system cannot supply cells more than a few millimetres from a tracheole, this limits insect body size. Alveoli + circulation can supply cells anywhere in the body regardless of size. [1, size limitation criterion] Ventilation mechanism: Insects ventilate passively or via body movements that compress the tracheal tubes; mammals use diaphragm and rib movements to generate pressure differences that inflate and deflate the lungs. [1, ventilation criterion]

The mammalian system is better suited to a large, active body because: alveoli provide enormously greater total surface area than tracheoles; blood transport extends supply to all tissues irrespective of distance; and vigorous diaphragm ventilation can scale up with metabolic demand. The tracheal system is effective and efficient for a small-bodied animal but is fundamentally constrained by diffusion distance. [1, justified evaluation]

Despite these differences, both systems succeed because they all obey the same underlying principle: maximise surface area, minimise barrier thickness, and maintain concentration gradients. [1, overall linking statement]

Marking criteria.

• 1 markNames the tracheal system (spiracles, tracheae, tracheoles) and alveoli; describes the gas pathway in each.
• 1 markIdentifies diffusion as the common fundamental mechanism in both systems.
• 1 markCompares transport of gases: insects direct to cells via tracheoles; mammals via blood/haemoglobin.
• 1 markCompares body size limitation: tracheal system constrains insect size; alveoli + circulation allow large bodies.
• 1 markCompares ventilation mechanisms (or any valid third criterion, e.g. role of haemolymph, O₂ carrier molecules, membrane surface area per unit mass).
• 1 markEvaluates which system better suits a large, active body with a specific, correct reason (distance limitation of tracheoles, or scalability of lungs + circulation).
• 1 markOverall response uses precise terminology (spiracles, tracheoles, haemolymph, alveoli, capillaries, concentration gradient, diffusion) and draws a clear comparative conclusion.

Q2, Sample Band 6 response (8 marks), annotated

Student 1 is partly correct: a large surface area and a thin membrane are genuinely necessary for efficient gas exchange. Fick's law tells us that diffusion rate is proportional to (surface area × concentration gradient) and inversely proportional to diffusion distance (membrane thickness). Therefore, maximising surface area and minimising membrane thickness both directly increase diffusion rate. In alveoli, for example, the enormous combined surface area (~70–100 m² in a human) and the single-cell-thick alveolar/capillary wall together make rapid exchange structurally possible. [1, correctly identifies what is right in Student 1's claim; 1, applies Fick's law to explain why surface area and thin membrane matter]

However, Student 1's claim overlooks the third factor in Fick's law: the concentration gradient. If the gradient is not maintained, diffusion rate falls toward zero regardless of surface area or membrane thickness. [1, identifies the missing factor]

Role of ventilation: In mammalian lungs, breathing continuously brings fresh, O₂-rich air into the alveoli, keeping the O₂ concentration on the air side high. Without ventilation, O₂ would be depleted in the alveolar air and the gradient would collapse. [1, ventilation and concentration gradient on the air/water side] Role of blood flow: Blood arriving at the alveolus has low O₂ concentration (having delivered O₂ to tissues). As O₂ diffuses across and O₂-loaded blood leaves, fresh deoxygenated blood continues to arrive, keeping the gradient steep on the blood side. [1, blood flow and concentration gradient on the blood side]

The fish gill example reinforces this. Counter-current flow is not a structural feature of the gill lamella itself, it is a flow arrangement that maximises the concentration gradient along the full length of the gill surface. Without water movement (ventilation), the oxygen in the water adjacent to the gill surface would be depleted and exchange would stop. [1, specific example from gills or lungs supporting the argument]

Therefore, Student 2 is more accurate. The surface provides the structural capacity for exchange, but ventilation and blood flow are not optional additions, they are required to keep the concentration gradient steep and diffusion going. An exchange surface without maintained gradients is like a large door without a pressure difference: the capacity exists but no movement occurs. [1, justified conclusion]

A complete account of efficient gas exchange must integrate all four features: large surface area, thin barrier, moist surface, and maintained concentration gradient. None can be eliminated. [1, integrating all four features explicitly]

Marking criteria.

• 1 markCorrectly identifies what is right in Student 1's claim (large surface area + thin membrane both contribute, per Fick's law).
• 1 markExplicitly invokes Fick's law (or the relationship: rate = SA × gradient / distance) to explain why both factors matter.
• 1 markIdentifies concentration gradient as the third factor in Fick's law and explains that Student 1's claim ignores it.
• 1 markExplains the role of ventilation in maintaining high O₂ on the air/water side.
• 1 markExplains the role of blood flow in maintaining low O₂ on the blood side (or explains CO₂ removal).
• 1 markUses a specific named example (fish gill counter-current flow, or alveoli breathing + capillary blood flow) to support the argument.
• 1 markReaches an explicit conclusion that Student 2 is more accurate with a stated reason.
• 1 markResponse integrates all four features of an efficient exchange surface (large SA, thin, moist, maintained gradient) and identifies them as inseparable requirements.

Q3, Sample Band 6 response (6 marks)

The claim is partly correct but significantly oversimplified. [1, overall evaluative judgement]

What is correct: Gills and alveoli do solve the same fundamental problem, supplying oxygen to blood and removing CO₂, using the same underlying principle: large surface area, thin barrier, moist surface, maintained gradient. In this sense they share a common design logic. [1, correctly identifies the common element]

What is wrong or oversimplified:

• "Essentially the same organ": Gills and alveoli differ structurally and functionally in important ways. Gills are external (or near-external) folded structures where water flows over a surface; alveoli are internal air sacs where air flows into them. [1, structural/positional difference]
• Medium exchanged: Gills exchange gases with water, which has far less dissolved oxygen per litre than air. This is why fish need counter-current flow to extract sufficient oxygen, while mammals can use simpler tidal ventilation (air in, air out). [1, medium and ventilation difference]
• "Only real difference is inside vs outside": The difference in medium (water vs air), ventilation method (counter-current vs tidal), oxygen availability, and the structural forms (lamellae stacked on gill bars vs millions of spherical air sacs) are all biologically significant, not just positional. [1, additional substantive difference]

More accurate statement: "Gills and alveoli are analogous organs that solve the same fundamental gas exchange problem using the same diffusion principles, but they are adapted to their different environments, gills extract O₂ from water using counter-current exchange to overcome low O₂ availability, while alveoli exchange gases with O₂-rich air using tidal ventilation. Their structural forms, ventilation mechanisms, and physical medium differ substantially." [1, defensible reformulation citing at least two specific differences]

Marking criteria.

• 1 markStates an overall evaluative judgement (e.g. "partly correct but oversimplified").
• 1 markCorrectly identifies what is true: both organs use the same diffusion-based principles (large SA, thin, moist, maintained gradient).
• 1 markIdentifies a substantive structural or positional difference (e.g. gills are external/folded lamellae; alveoli are internal air sacs).
• 1 markIdentifies the difference in medium (water vs air) and its consequence (e.g. lower O₂ availability in water, necessity of counter-current in fish).
• 1 markIdentifies a further substantive difference (ventilation method, structural form, O₂ availability or any valid distinct point).
• 1 markReformulates the claim into a biologically defensible statement that acknowledges the shared principle but specifies at least two meaningful differences.