AQA A-Level Biology: Exchange and Transport — Complete Revision Guide (7402)
AQA A-Level Biology: Exchange and Transport
Exchange and transport is the third course on the AQA A-Level Biology (7402) path and the central organism-scale physiology course on the specification. It is the first place the path asks the student to think about how the cellular biochemistry of biological molecules and the membrane mechanics of cells and immunity get scaled up to multicellular organisms whose interior cells cannot exchange directly with the environment. Why does an elephant need lungs while a planarian does not? Why does a fish ventilate counter-currently while a mammal does so tidally? Why does fetal haemoglobin bind oxygen more avidly than adult? Why does sucrose move from a sugar source to a sugar sink? Every answer routes back through the geometric, structural and physiological logic built here.
Course 3 of 11 on the LearningBro AQA A-Level Biology learning path was written from scratch as a brand-new course in Phase 2 of the rationalisation. The legacy catalogue handled this content as an omnibus plus three thin stubs — gas exchange, the mammalian transport system and plant transport — which gave neither comparative respiratory physiology nor the haemoglobin allosteric story the depth examiners reward. The Phase 2 redesign builds a dedicated ten-lesson treatment that moves from the underlying surface-area-to-volume constraint, through comparative gas exchange across the five major animal and plant groups, through digestion and absorption, into the mammalian cardiovascular system in mechanistic depth, and on into the plant vascular physiology of xylem and phloem.
Guide Overview
The Exchange and Transport course is structured as ten lessons that move from the geometric constraint to the comparative gas-exchange systems, then through digestion and the mammalian cardiovascular system, and finally to the two plant vascular systems.
- Surface area to volume ratio
- Gas exchange in single-celled organisms and insects
- Gas exchange in fish and plants
- Mammalian lung structure and ventilation
- Digestion and absorption
- Mammalian heart and cardiac cycle
- Haemoglobin and oxygen transport
- Blood vessels and tissue fluid
- Xylem, water transport and transpiration
- Phloem and translocation
AQA 7402 Specification Coverage
This course covers AQA 7402 Section 3.3 in full. The specification organises exchange and transport into four sub-sections — surface area to volume, gas exchange, digestion and absorption, and mass transport — each mapped here to one or more lessons (refer to the official AQA specification document for exact wording).
| Sub-topic | Spec area | Primary lesson(s) |
|---|---|---|
| Surface area to volume ratio | 3.3.1 | Surface area to volume ratio |
| Gas exchange (single-celled, insects, fish, plants, mammals) | 3.3.2 | Gas exchange in single-celled organisms and insects; gas exchange in fish and plants; mammalian lung structure and ventilation |
| Digestion and absorption | 3.3.3 | Digestion and absorption |
| Mass transport in animals (heart, vessels, blood) | 3.3.4.1 | Mammalian heart and cardiac cycle; haemoglobin and oxygen transport; blood vessels and tissue fluid |
| Mass transport in plants (xylem and phloem) | 3.3.4.2 | Xylem, water transport and transpiration; phloem and translocation |
Section 3.3 is examined across all three AQA 7402 papers. Comparative gas-exchange items appear most often on Paper 1; haemoglobin oxygen-dissociation curves and cardiac-cycle pressure-volume traces are Paper 1 and Paper 3 staples; transpiration potometer technique appears in Paper 3 practical contexts. The mass-transport content is high-density extended-response territory — a 6 mark "compare" or "explain" item on counter-current gas exchange, the Bohr effect, or the mass-flow hypothesis is reliable on every series.
Surface Area to Volume Ratio
The surface area to volume ratio lesson develops the geometric constraint that drives every other lesson in this course. As an organism scales linearly, volume grows with the cube of length while surface area grows only with the square — so larger organisms have proportionally less surface per unit volume across which to exchange gases, nutrients and waste. A single-celled organism with a high surface-to-volume ratio and a short diffusion distance can rely on direct diffusion. A multicellular organism above a few millimetres in diameter cannot, and must evolve specialised exchange surfaces (alveoli, gills, leaf mesophyll) and mass-transport systems (blood, xylem, phloem) that bring the environment closer to every cell.
The constraint also drives metabolic-rate scaling: smaller endotherms lose heat faster relative to their volume, and so must metabolise faster per unit mass — a fact that re-enters the thermoregulation content in homeostasis.
Gas Exchange in Single-Celled Organisms and Insects
The single-celled and insect gas-exchange lesson develops the two extremes of the comparative series. Amoeba and other unicellular eukaryotes exchange directly across the cell-surface membrane, with a path length below a millimetre and no specialised structures. Insects, despite being multicellular and metabolically active, do not transport oxygen in the blood; instead they ventilate a system of branching air-filled tracheae and tracheoles that deliver gas directly to respiring tissues. Spiracles in the body wall open and close to regulate water loss, and abdominal muscular ventilation supplements diffusion in active insects. The structural trade-off — air-filled tubes are great for gas transport but require waterproofing — is examined as a structure-function reasoning item.
Gas Exchange in Fish and Plants
The fish and plant gas-exchange lesson develops counter-current exchange in the bony-fish gill and stomatal-controlled gas exchange in the leaf. In the fish gill, water flows across the gill lamellae in the opposite direction to blood flow within the lamellar capillaries, maintaining a diffusion gradient along the entire length of the exchange surface and allowing up to 80 percent oxygen extraction. A concurrent system would equilibrate halfway along and extract no further oxygen — a comparison examiners reliably set as a 4 mark explanation item.
In the leaf, gas exchange occurs through stomata in the lower epidermis controlled by paired guard cells whose turgor regulates pore aperture. Spongy mesophyll cells with large air spaces provide the internal exchange surface for the diffusion of carbon dioxide into and oxygen out of photosynthesising cells. The same stomatal opening that admits carbon dioxide creates a route for transpirational water loss — the trade-off central to the xylem and transpiration lesson later in the course. Xerophytic adaptations (sunken stomata, hairy leaves, rolled leaves, thick cuticle) reduce water loss in arid environments.
Mammalian Lung Structure and Ventilation
The mammalian lung lesson develops the structural hierarchy from trachea through bronchi and bronchioles to the alveoli, and the mechanics of tidal ventilation driven by the diaphragm and external intercostal muscles. The alveoli supply a vast surface area (around 70 square metres in an adult), a thin exchange epithelium (alveolar squamous epithelium plus capillary endothelium, around 0.5 micrometres total), a dense capillary network maintaining the diffusion gradient, and surfactant reducing surface tension and preventing alveolar collapse. The Fick equation — diffusion rate proportional to surface area × concentration gradient ÷ thickness — gives the quantitative framework for explaining each adaptation. Lung diseases (tuberculosis, asthma, emphysema, fibrosis) are examined through the Fick lens: each disrupts one or more terms.
Digestion and Absorption
The digestion and absorption lesson develops the enzymatic breakdown of carbohydrates, proteins and lipids along the mammalian gastrointestinal tract — amylases, proteases (endopeptidases and exopeptidases), lipases acting with bile-salt emulsification — and the absorptive mechanisms in the ileum. Glucose and amino acids enter the enterocyte by sodium co-transport (the secondary active transport mechanism introduced in transport across membranes) and exit basolaterally by facilitated diffusion into the hepatic portal venous drainage. Lipids are absorbed as fatty acids and monoglycerides via micelles, reassembled into chylomicrons in the enterocyte, and exocytosed into the lacteal lymphatic drainage. Microvilli amplify the absorptive surface area and are the canonical AQA structure-function example for digestive absorption.
Mammalian Heart and Cardiac Cycle
The mammalian heart lesson develops the four-chambered heart, the double circulation (pulmonary and systemic), the cardiac cycle (atrial systole, ventricular systole, diastole) and the pressure-volume traces that examiners ask candidates to interpret. The atrioventricular and semilunar valves open and close passively in response to pressure differences, generating the lub-dub of the auscultated heart sounds. The septum prevents the mixing of oxygenated and deoxygenated blood. The sinoatrial node initiates the heartbeat; the atrioventricular node delays it to allow ventricular filling; the bundle of His and Purkyne fibres conduct depolarisation through the ventricles. Cardiac output equals stroke volume times heart rate, and is regulated by autonomic and hormonal inputs revisited in homeostasis.
Haemoglobin and Oxygen Transport
The haemoglobin lesson is the course's pedagogical centrepiece. It develops the cooperative binding of oxygen by the haemoglobin tetramer, the sigmoidal oxygen-dissociation curve, and the Bohr effect (the rightward shift of the dissociation curve at elevated carbon dioxide partial pressure or reduced pH, increasing oxygen unloading in metabolically active tissues). Fetal haemoglobin has a higher affinity for oxygen than maternal haemoglobin (its curve sits to the left), allowing fetal blood to extract oxygen from the placental maternal supply. Comparative haemoglobins — the lugworm in its hypoxic burrow has a leftward-shifted curve to load oxygen at low partial pressure; the llama at altitude similarly — are examined as applied AO2 items.
This lesson is a direct application of the protein structure-function principle developed in protein structure and function. Haemoglobin's two alpha and two beta subunits, each with a haem prosthetic group containing a central iron, are the canonical worked example of quaternary structure and allosteric regulation in the AQA specification.
Blood Vessels and Tissue Fluid
The blood vessels and tissue fluid lesson develops the structural adaptations of arteries (thick muscular and elastic walls to withstand and smooth out the high pulsatile pressure from ventricular ejection), arterioles (smooth muscle in the wall providing resistance control for blood flow distribution), capillaries (single endothelial-cell-thick walls providing the exchange surface for tissue fluid formation), venules and veins (thin walls, large lumen, valves to prevent backflow at low pressure). Tissue fluid is formed at the arteriolar end of the capillary bed when hydrostatic pressure exceeds oncotic pressure, and is reabsorbed at the venular end as oncotic pressure exceeds hydrostatic pressure. Excess tissue fluid is returned to the circulation via the lymphatic system. Failure of this balance — in cardiac failure, in protein malnutrition, in lymphatic obstruction — causes oedema, a routine AO2 application item.
Xylem, Water Transport and Transpiration
The xylem and transpiration lesson develops the cohesion-tension theory of water transport in plants. Water evaporates from spongy mesophyll cell walls into the substomatal air space and diffuses out through stomata down a water-potential gradient (transpiration). The negative pressure generated by this evaporation pulls water up the continuous water columns in the xylem vessels — narrow, lignified, dead conducting cells that act as inert pipes. Cohesion of water molecules to each other (the hydrogen-bonding property established in water and inorganic ions) and adhesion of water to the xylem walls maintain the continuous columns even under tension.
This lesson owns the transpiration potometer practical, in which the rate of water uptake through a cut shoot is measured by tracking an air bubble in a graduated capillary tube. The four environmental variables — light intensity, temperature, humidity and air movement — are each tested by altering one while holding the others constant. The water-uptake rate is a proxy for transpiration rate under the assumption that lateral water storage in the shoot is negligible over the measurement window.
Phloem and Translocation
The phloem and translocation lesson develops the mass-flow hypothesis for the bidirectional transport of sucrose from sources (photosynthesising leaves, stored carbohydrate mobilised in spring) to sinks (growing tissues, developing fruits, storage organs in autumn). Sucrose is actively loaded into sieve-tube elements at the source by proton co-transport, lowering the water potential and drawing water in by osmosis; the resulting hydrostatic pressure drives bulk flow through the sieve tubes to the sink, where sucrose is unloaded and the water exits. The mass-flow model is supported by ringing experiments, radiotracer studies (carbon-14-labelled carbon dioxide tracking through phloem) and aphid stylet evidence (the cut stylet of a feeding aphid exudes phloem sap continuously under positive pressure). Limitations and alternative hypotheses (pressure-flow modifications, the role of plasmodesmata) are included for AO3 evaluation.
Synoptic Links Across the Specification
Exchange and transport is the system-physiology hub of AQA 7402. The Fick-equation reasoning developed for the alveolus generalises to the gill, the leaf, the placental membrane and the proximal convoluted tubule — every exchange surface in the specification is described by the same surface-area × gradient ÷ thickness equation. The haemoglobin allosteric story is the structural application of the protein-folding content from biological molecules, and returns when haemoglobin variants (sickle-cell, fetal) appear in the inheritance content of DNA, genes and inheritance. The cardiac-output and blood-vessel content underwrites the cardiovascular control reflexes in homeostasis and the heart-rate response in the autonomic reflexes of response to stimuli. Counter-current exchange in the fish gill is structurally analogous to counter-current multiplication in the loop of Henle, also covered in homeostasis. The water-potential vocabulary developed for plant transport is the same vocabulary used for osmosis in the transport across membranes lesson.
Required Practical Anchors
This course's principal practical-skills anchor is the transpiration potometer technique in the xylem and transpiration lesson. Although not numbered among the twelve formal AQA required practicals in the rigid sense, potometer methodology is examined frequently on Paper 3 practical items and shares the quantitative-rate-determination logic of RP1 (enzyme rate, anchored in the enzyme kinetics lesson) and RP3 (membrane permeability, anchored in the cell membrane lesson). The dissection of mammalian or fish gas-exchange and circulatory organs, a curriculum expectation, is also anchored in this course.
Revision Strategy
Exchange and transport rewards a comparative-table revision habit. Build a single landscape table whose columns are amoeba, insect, fish, plant and mammal, and whose rows are exchange surface, ventilation mechanism, transport medium, oxygen handling and characteristic adaptation. Drill it from memory each week. The same comparative discipline applies to the cardiac-cycle pressure-volume trace: sketch it from memory, identify each valve event, and label atrial systole, ventricular systole and diastole.
Practise haemoglobin dissociation curves in batches of ten — including the Bohr shift, the fetal-vs-maternal comparison and the high-altitude llama variant. Drill the cohesion-tension and mass-flow models until they can be sketched from memory with one labelled diagram and three lines of mechanistic explanation. Interleave exchange-and-transport questions with biochemistry questions, because the haemoglobin items are routinely synoptic with protein structure, and the digestion items are routinely synoptic with enzyme kinetics. Use the LearningBro practice quizzes under timed conditions.
Closing
Exchange and transport is the central organism-physiology course on the AQA 7402 path and the first place the student must integrate molecular biochemistry, membrane mechanics and whole-organism scaling. The Phase 2 redesign gives this content its own dedicated ten-lesson treatment, replacing the legacy omnibus-plus-stubs arrangement that left comparative respiratory physiology and the haemoglobin allosteric story underdeveloped. Start with the Exchange and Transport course and work through all ten lessons in sequence; treat the Fick-equation reasoning as a single transferable analytical habit, and lock the haemoglobin dissociation curves and cardiac-cycle traces early because they re-enter Paper 3 in synoptic combination with respiration, homeostasis and the autonomic-nervous-system content. The full AQA A-Level Biology learning path walks the whole sequence end-to-end.