OCR A-Level Biology Module 3 Exchange and transport: exchange surfaces, the heart, blood and plants

What Module 3 actually demands

Exchange and transport is where OCR A-Level Biology A moves from cells and molecules to whole-organism physiology. The module starts from a single idea, that surface-area-to-volume ratio falls as size increases, and builds out to the exchange surfaces and transport systems that large, active organisms need. You must describe and compare gas exchange in mammals, fish and insects, work through the structure and control of the mammalian heart, relate each blood vessel to its function, explain how oxygen and carbon dioxide are carried in the blood, and explain how water and sugar move through plants.

This guide ties together the five dot-point pages for Module 3 and sets out the exam patterns OCR repeats. The examiners reward precise terminology (counter-current, cohesion-tension, hydrostatic versus oncotic pressure) and the ability to apply it to graphs, calculations and unfamiliar contexts.

Exchange surfaces and gas exchange

Substances cross surfaces by diffusion, and the rate follows Fick's law (rate is proportional to surface area times concentration difference, divided by diffusion distance). As organisms enlarge, their SA:V ratio falls and diffusion distances rise, so they need specialised exchange surfaces: large area, thin barrier, a maintained concentration gradient and permeability.

Mammals use alveoli (squamous epithelium one cell thick, dense capillaries, ventilation). Bony fish use a counter-current system in the gills, where water and blood flow in opposite directions so a diffusion gradient is maintained along the whole lamella. Insects use a tracheal system delivering air directly to the tissues through spiracles, tracheae and tracheoles, without using the blood to carry oxygen.

The mammalian heart and the cardiac cycle

The heart is a double pump; the left ventricle is thicker because it pumps to the whole body at high pressure. The cardiac cycle runs atrial systole, ventricular systole and diastole, with every valve opened or closed by the pressure difference across it. The heartbeat is myogenic: the SAN paces the atria, the AVN delays the wave so the atria empty first, and the bundle of His and Purkyne tissue spread it through the ventricles from the apex up. On an ECG, the P wave is atrial depolarisation, the QRS complex ventricular depolarisation and the T wave ventricular repolarisation.

Blood vessels and tissue fluid

Each vessel suits its pressure. Arteries have thick elastic walls that recoil to maintain high pressure; capillaries are one endothelial cell thick for a short diffusion distance; veins have wide lumens and valves for low-pressure return. Tissue fluid forms at the arterial end of a capillary because the blood's hydrostatic pressure exceeds the oncotic pressure of the plasma proteins, forcing water and small solutes out. At the venous end hydrostatic pressure has fallen below oncotic pressure, so most water is reabsorbed by osmosis; the excess drains into the lymphatic system.

Transport of oxygen and carbon dioxide

Haemoglobin binds oxygen cooperatively, giving the sigmoidal dissociation curve: it loads oxygen in the lungs and unloads it in the tissues. The Bohr effect shifts the curve right when carbon dioxide is high, releasing more oxygen to active tissue; fetal haemoglobin has a higher affinity (curve to the left) so it takes oxygen from the mother. Carbon dioxide is carried mostly as hydrogencarbonate ions formed in red blood cells by carbonic anhydrase, with the chloride shift balancing the charge as hydrogencarbonate leaves the cell.

Transport in plants

Water moves up the xylem (dead, lignified vessels) by the cohesion-tension theory: transpiration from the leaves creates tension that pulls a continuous, cohesive column of water up, drawing water into the roots down a water potential gradient. Transpiration rises with light, temperature and wind, and falls with humidity. Sucrose moves through the phloem (living sieve tubes plus companion cells) from source to sink by the mass flow hypothesis. Xerophytes reduce water loss with thick cuticles, sunken stomata, hairs and rolled or reduced leaves.

How Module 3 is examined

A typical OCR profile for Exchange and transport:

• Multiple choice and short answer. Identifying vessels or chambers, naming the stages of the cardiac cycle, classifying an exchange surface adaptation.
• Maths. SA:V ratio, heart rate from an ECG, potometer rate, and reading oxygen dissociation and cardiac pressure graphs.
• Applied and data questions. Predicting the effect of fibrosis on gas exchange, oedema from low plasma proteins, or a Bohr shift in exercising muscle.
• Level-of-Response extended answers. The cardiac cycle with valve pressures, the cohesion-tension theory, and the formation and return of tissue fluid are all predictable.

Check your knowledge

A mix of recall and application questions covering the whole of Module 3. Attempt them under timed conditions, then check against the solutions.

1. Explain how the counter-current system in fish gills increases oxygen uptake compared with parallel flow. (4 marks)
2. Describe the events of ventricular systole, including the valve actions and the pressure changes that cause them. (4 marks)
3. Explain how the structure of an artery is adapted to maintain a high, steady blood pressure. (3 marks)
4. Explain how tissue fluid is formed at the arterial end of a capillary bed. (3 marks)
5. Explain why the oxygen dissociation curve is S-shaped. (3 marks)
6. Describe how carbon dioxide is transported as hydrogencarbonate, including the chloride shift. (4 marks)
7. Explain water transport up the xylem using the cohesion-tension theory. (4 marks)
8. Describe two xerophytic adaptations and explain how each reduces water loss. (4 marks)