Organisms exchange substances with their environment.

HookWhy a shrew eats all day and an insect can't be a dog

ModelSurface area to volume ratio — the geometry that dictates everything

The rate of exchange a body can achieve by diffusion depends on its surface area, while its demand for oxygen and nutrients depends on its volume. As an organism grows, volume (a cube of its size) outpaces surface area (a square of its size), so the surface area to volume ratio falls. Small organisms have a high ratio and short diffusion distances, so simple diffusion across the body surface suffices. Large organisms have a low ratio and thick bodies, so they need specialised exchange surfaces and a mass transport system to move substances in bulk.

The efficiency of any exchange surface is summed up by Fick's law: the rate of diffusion is proportional to the surface area multiplied by the concentration difference, divided by the diffusion distance. Every good exchange surface therefore does three things — maximises surface area (folds, villi, alveoli, gill lamellae), keeps the diffusion path short (walls one cell thick), and maintains a steep concentration gradient (a blood supply or ventilation constantly renewing the supply). Whenever a question asks you to explain an adaptation, these three are the marks.

The ratio also governs heat and metabolism. A small mammal's high surface area to volume ratio means rapid heat loss, so it needs a high metabolic rate — and therefore a high oxygen demand — to stay warm, which is exactly the shrew's problem.

MechanismGas exchange — four solutions to one problem

Single-celled organisms such as Amoeba exchange gases straight across their cell-surface membrane: a high surface area to volume ratio and a diffusion distance of a fraction of a micrometre make it fast enough with no special structures.

Insects pipe air directly to their tissues through a network of tracheae and finer tracheoles, opening to the outside at valved pores called spiracles. Body movements ventilate the system, and during activity fluid at the tracheole tips is withdrawn to expose more surface. The catch is water: an open airway loses water vapour, so spiracles close between breaths, and a small surface area to volume ratio plus a waterproof exoskeleton limit the loss — a permanent trade-off between gas exchange and desiccation.

Fish use gills, whose stacked filaments and lamellae give a vast surface area, and a brilliant refinement called counter-current flow: water flows across the lamellae in the opposite direction to the blood inside them. Because the two never reach equilibrium, a diffusion gradient is maintained along the entire length of the lamella, so blood can extract far more oxygen — around 80% — than the roughly 50% a same-direction (parallel) flow would allow before the concentrations equalised and diffusion stopped.

Plants exchange gases through stomata on the leaf, opened and closed by guard cells; the spongy mesophyll's air spaces give a large internal surface and the thin, flat leaf keeps diffusion paths short. Plants face the insect's dilemma — open stomata lose water — so xerophytes like marram grass evolve sunken stomata, hairs, rolled leaves and thick waxy cuticles to cut transpiration. Humans solve it with millions of alveoli: huge surface area, walls one flattened cell thick, a dense capillary network to keep the gradient steep, and ventilation to refresh the air.

MechanismDigestion and absorption — hydrolysis, then transport

Digestion is hydrolysis run at scale, breaking large insoluble polymers into small soluble monomers that can be absorbed. Carbohydrates are attacked first by amylase (from salivary glands and pancreas), which hydrolyses starch to maltose; then membrane-bound disaccharidases on the ileum epithelium — maltase, sucrase and lactase — finish the job, producing glucose, fructose and galactose.

Lipids need an extra trick because they are not water-soluble. Bile salts from the liver emulsify them into tiny droplets, massively increasing the surface area for lipase to hydrolyse them into monoglycerides and fatty acids. These products cluster with bile salts into micelles that ferry them to the epithelial cell membrane; being lipid-soluble they diffuse across, are reassembled into triglycerides in the smooth endoplasmic reticulum, packaged with protein into chylomicrons, and released into the lacteals of the lymph.

Proteins are cut by two classes of enzyme working together: endopeptidases hydrolyse peptide bonds in the middle of the chain, creating many more ends, and exopeptidases then remove amino acids from those ends, with membrane-bound dipeptidases splitting the final dipeptides — a division of labour that speeds the whole process. The monosaccharides and amino acids are finally absorbed by co-transport with sodium ions (the mechanism from 3.2.3), while villi and microvilli give the ileum the enormous surface area the job demands.

ModelMass transport in animals — haemoglobin, the heart and the vessels

Haemoglobin is a quaternary protein with four haem groups, each binding one oxygen molecule. It loads oxygen where the partial pressure is high (the lungs) and unloads it where the partial pressure is low (respiring tissues). Its oxygen dissociation curve is S-shaped because binding is cooperative: the first oxygen to bind changes the molecule's shape, making the next easier to load. The Bohr effect shifts the curve right when carbon dioxide is high (and pH low), so haemoglobin releases oxygen more readily in active tissues that need it most. Foetal haemoglobin has a higher affinity than the mother's (its curve sits to the left), letting it load oxygen across the placenta.

The heart is a double pump. The right side sends deoxygenated blood to the lungs at low pressure; the left side pumps oxygenated blood to the whole body, which is why its ventricle wall is far thicker. The cardiac cycle runs on pressure: in diastole the heart relaxes and fills; in atrial systole the atria contract; in ventricular systole the ventricles contract, closing the atrioventricular valves (the first heart sound) and forcing open the semilunar valves so blood leaves through the aorta and pulmonary artery. Every valve simply opens or closes according to the pressure difference across it.

The vessels match their jobs: arteries have thick, elastic, muscular walls to withstand and smooth high pressure; capillaries are one cell thick with a huge total surface area for exchange; veins have wide lumens and valves to return low-pressure blood to the heart. At the arterial end of a capillary, high hydrostatic pressure forces water and small solutes out to form tissue fluid, leaving plasma proteins behind; at the venous end, where pressure has fallen, the low water potential draws most of that water back in by osmosis, and the lymphatic system collects the surplus.

MechanismMass transport in plants — pulling water up and pushing sugar around

Water travels up the xylem by the cohesion-tension mechanism, powered entirely by evaporation. Water evaporates from the surfaces of mesophyll cells and diffuses out of the stomata — transpiration — lowering the water potential of those cells so they draw water from the xylem. Because water molecules hydrogen-bond to one another (cohesion) and to the vessel walls (adhesion), the water in the xylem forms a continuous column, so loss at the top puts the whole column under tension and pulls it up. Transpiration speeds up with more light (stomata open), higher temperature (faster evaporation), and air movement (which sweeps away humid air to keep the gradient steep), and slows in high humidity. It is measured, as water uptake, with a potometer.

Translocation moves the sugars — mainly sucrose — through the phloem by the mass flow hypothesis, and unlike transpiration it needs energy. At a source such as a photosynthesising leaf, sucrose is actively loaded into the sieve tubes, lowering their water potential so water follows by osmosis and raises the hydrostatic pressure. At a sink such as a growing root, sucrose is removed and used, water leaves, and pressure falls. Sap therefore flows down the pressure gradient from source to sink. The evidence is neat: ringing a stem to remove the phloem makes sugars pile up above the ring, and radioactively labelled carbon dioxide fed to a leaf later shows up as labelled sucrose in the phloem.

DataRequired practical 5 — dissecting an exchange or transport system

This practical asks you to dissect an animal or plant gas exchange or mass transport system and record its structure. Common specimens are a mammalian heart (to identify the four chambers, the major vessels and the valves, and to compare the thickness of the two ventricle walls), a fish head to expose the gill filaments and lamellae, an insect to trace the tracheal system, or a plant stem cut to reveal the vascular bundles of xylem and phloem.

Method and safety carry marks. Use a sharp scalpel and scissors on a dissecting board, always cutting away from your body and fingers; treat preserved specimens with care because fixatives such as formaldehyde are irritants, work in ventilation, and disinfect surfaces and wash hands afterwards. Record what you see as a biological drawing, and the conventions are examined: draw clean single lines with no shading or sketching, keep structures in proportion, add label lines (drawn with a ruler, not crossing, ending exactly on the structure), and give the drawing a title, the view, and a magnification or scale bar.

The observations should be explained, not just noted. The left ventricle wall is visibly thicker than the right because it generates the higher pressure needed to pump blood around the whole body, whereas the right pumps only to the nearby lungs — a structure-and-function point that turns a labelling exercise into an A-level answer. If you also set up a potometer, record the distance an air bubble moves in a set time to estimate the transpiration rate, controlling temperature, humidity and light.

VocabularyKey terms the mark scheme pays for

TrapsMisconceptions that cost marks

ExamWhat examiners want