How is section 3.3 Exchange weighted in the AQA A-Level Biology exam?

AQA does not publish fixed per-section weightings, but 3.3 is a large, content-dense section that appears on both Paper 1 and Paper 2 (and is assessable on Paper 3). Expect several multiple-choice and structured questions, at least one application question using unfamiliar data (e.g. a dissociation curve, a transpiration table, or a digestion experiment), and frequent links to 3.1 (biological molecules) and 3.2 (cells and transport across membranes). Mastering the single unifying idea - surface area to volume ratio and Fick's law - pays off across the whole section.

What is the one idea that links the whole of 3.3 together?

Surface area to volume ratio. As organisms get larger, volume increases faster than surface area, so SA:V falls and simple diffusion across the body surface is no longer enough. Every specialised exchange surface (alveoli, gills, tracheoles, villi, root hairs) and every mass transport system (blood, xylem, phloem) exists to solve that one problem - maximising surface area, minimising diffusion distance, and maintaining steep concentration gradients, exactly as Fick's law predicts.

Why is the oxygen dissociation curve S-shaped rather than a straight line?

Because oxygen binding to haemoglobin is cooperative. Haemoglobin has four binding sites; the first oxygen is difficult to load (the flat start of the curve), but binding it changes the molecule's shape so the next oxygens load more easily (the steep middle), and the last site is hard to fill as the molecule saturates (the flat top). The S-shape is ideal for transport: haemoglobin saturates fully at the high partial pressure of oxygen in the lungs, yet unloads steeply at the lower partial pressure found in respiring tissues.

What is the difference between the cohesion-tension theory and the mass flow hypothesis?

Cohesion-tension explains water movement in the xylem; mass flow explains sugar movement in the phloem. Cohesion-tension is passive and pull-driven - transpiration puts the water column under tension and cohesion between water molecules lets the whole column be pulled up. Mass flow involves an active step - sucrose is actively loaded into the phloem at the source, which draws water in by osmosis, raises hydrostatic pressure, and drives sap down a pressure gradient to the sink. Xylem moves water and minerals upward only; phloem moves organic solutes from source to sink in either direction.

How should I answer co-transport questions on glucose absorption?

Set up the sodium gradient first. A sodium-potassium pump actively transports sodium out of the epithelial cell into the blood using ATP, keeping internal sodium low. Sodium then diffuses from the lumen into the cell down its gradient through a co-transporter protein, dragging glucose (or an amino acid) in against its concentration gradient. Glucose then leaves the cell into the blood by facilitated diffusion. The key examiner point is that the energy comes from the sodium gradient (and therefore indirectly from the pump and ATP), not from glucose moving down its own gradient.

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AQA A-Level Biology 3.3 Exchange: a deep-dive overview of surface area to volume ratio, gas exchange, digestion, and mass transport

A deep-dive AQA A-Level Biology guide to section 3.3 (Organisms exchange substances with their environment). Ties together surface area to volume ratio and Fick's law, gas exchange in insects, fish and plants, digestion and co-transport, and mass transport in animals and plants, with the exam patterns AQA repeats.

Generated by Claude Opus 4.824 min readAQA-7402-3.3Updated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

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What section 3.3 actually demands
The unifying idea: SA:V and Fick's law
Gas exchange across four systems
Digestion and absorption
Mass transport in animals
Mass transport in plants
How 3.3 is examined

What section 3.3 actually demands

Section 3.3 (Organisms exchange substances with their environment) is one of the most exam-dense parts of AQA A-Level Biology (7402). It is built on a single conceptual spine - the surface area to volume ratio and Fick's law - and then applied to four contexts: gas exchange, digestion and absorption, mass transport in animals, and mass transport in plants.

The section rewards two skills. The first is precise factual recall of named structures and mechanisms (counter-current flow, micelles, the cardiac cycle, the cohesion-tension theory). The second is the ability to apply those ideas to unfamiliar data: read a dissociation curve, interpret a transpiration potometer table, explain a digestion experiment. AQA loves application questions, so train both.

The unifying idea: SA:V and Fick's law

Everything in 3.3 follows from one fact: volume increases with the cube of length, surface area only with the square. So as an organism grows, its surface area to volume ratio (SA:V) falls.

For a cube of side $l$ :

\text{SA:V} = \frac{6l^2}{l^3} = \frac{6}{l}

A small organism (high SA:V, short diffusion distance) can exchange substances across its body surface alone. A large organism cannot - its surface is too small for its volume, and the diffusion distance to the core is too long. The solution is always the same: a specialised exchange surface plus a mass transport system.

The rate of exchange across any surface is given by Fick's law:

\text{rate of diffusion} \propto \frac{\text{surface area} \times \text{difference in concentration}}{\text{diffusion path length}}

Read every exchange surface in the section through these three terms - large area, steep gradient, short path - and you will explain its adaptations correctly every time.

Gas exchange across four systems

Single-celled organisms rely on a high SA:V - gases diffuse straight across the cell-surface membrane.

Insects use a tracheal system: air enters through spiracles (which close to limit water loss), passes along tracheae and fine tracheoles, and reaches respiring cells directly. Some insects ventilate by abdominal pumping, and during flight the tracheole tips go gaseous to speed delivery.

Fish use gills with many filaments and lamellae for a large surface area, and the standout adaptation is counter-current flow: blood and water flow in opposite directions, so a diffusion gradient for oxygen is maintained along the whole length of the lamella, allowing up to about 80 percent of the oxygen in the water to be absorbed. With parallel flow, the concentrations would equilibrate halfway and only about 50 percent would be taken up.

Dicot leaves exchange gases through stomata into a spongy mesophyll full of air spaces. This creates the central plant trade-off: stomata must open for carbon dioxide but then lose water by transpiration. Xerophytes (marram grass, cacti) limit that loss with sunken stomata, hairs, rolled leaves, a thick cuticle and a reduced SA:V.

Digestion and absorption

Large molecules are hydrolysed into small, soluble products that can cross membranes:

Carbohydrates: amylase makes maltose; membrane-bound disaccharidases (maltase, sucrase, lactase) finish the job at the absorption surface.
Proteins: endopeptidases cut internal bonds (creating more ends), exopeptidases cut terminal bonds, and membrane-bound dipeptidases release the final amino acids.
Lipids: bile salts emulsify lipids into small droplets (more surface area), lipase hydrolyses triglycerides into monoglycerides and fatty acids, and micelles ferry these to the epithelium.

The ileum maximises absorption with villi and microvilli (huge surface area), a thin epithelium and a rich blood supply. The headline mechanism is sodium co-transport: a sodium-potassium pump keeps internal sodium low, sodium then re-enters from the lumen down its gradient through a co-transporter, dragging glucose or amino acids in against their gradient. Fatty acids and monoglycerides simply diffuse in, reform triglycerides, become chylomicrons and enter the lacteal.

Mass transport in animals

Haemoglobin carries up to four oxygen molecules with cooperative binding, giving the S-shaped dissociation curve: it loads oxygen at the high partial pressure of the lungs and unloads steeply at the low partial pressure of tissues. The Bohr effect shifts the curve right when carbon dioxide rises, lowering affinity so more oxygen is released to active tissue - delivery is automatically matched to demand.

The four-chambered heart drives the cardiac cycle: atrial systole, ventricular systole, then diastole. Valves are passive - they open and close purely because of pressure differences, preventing backflow ("lub" = atrioventricular valves closing, "dub" = semilunar valves closing). Vessels match function: thick elastic arteries for high pressure, one-cell-thick capillaries for exchange, wide-lumen valved veins for low-pressure return.

Mass transport in plants

Xylem moves water and minerals upward by the cohesion-tension theory: transpiration from the leaves puts the continuous water column under tension; cohesion (hydrogen bonding) holds the column together so it is pulled up; adhesion to the lignified walls helps it resist gravity. Transpiration rate rises with light, temperature and wind, and falls with humidity - measurable with a potometer.

Phloem moves sucrose from source to sink by the mass flow hypothesis: sucrose is actively loaded at the source (the only ATP-requiring step), lowering water potential so water enters by osmosis and raises hydrostatic pressure; sap then flows down the pressure gradient to the sink, where sucrose is unloaded. Ringing experiments and aphid-stylet studies support the model.

How 3.3 is examined

A typical AQA profile for this section:

Multiple choice and short structured questions on named mechanisms: counter-current flow, co-transport, the cardiac cycle, cohesion-tension.
Application questions using unfamiliar data - the highest-value skill. Expect a dissociation curve to interpret (state the shift, give the consequence), a potometer or transpiration table, or a digestion-experiment result.
Maths skills: calculating SA:V, percentage change in transpiration rate, or rate from a potometer reading. Always show working and units.
Synoptic links back to 3.1 (proteins, enzymes, lipids) and 3.2 (transport across membranes, active transport).