This mini-lesson covers OCR Module 3 — Exchange and transport: surface area : volume and the need for exchange surfaces; the mammalian lung and ventilation (with spirometer measures); gas exchange in fish and insects; transport in animals — the heart, the cardiac cycle, ECG traces, haemoglobin and the Bohr effect, and tissue fluid; and transport in plants — xylem, transpiration and phloem mass flow.
Work through each screen, answer the questions as you go (some are extended-recall, some are calculations) and collect ⭐ stars. This is A-level content — expect quantitative work and mechanism-level detail. Press Start when you are ready.
As an organism grows, its volume (and therefore its metabolic demand for oxygen and nutrients) increases with the cube of its linear dimensions, while its surface area increases only with the square. The SA:V ratio falls, and diffusion across the body surface becomes hopelessly inadequate.
Hence every specialised exchange surface shows the same three features: a large surface area, a short diffusion pathway (usually a single layer of flattened epithelium), and a means of maintaining a steep gradient — ventilation, a circulation, or counter-current flow.
Metabolic consequence: a shrew has a very large SA:V, so it loses heat rapidly. To compensate, it has a very high metabolic rate — and therefore a very high breathing rate and heart rate. Elephants have the opposite problem, and large ears to solve it.
Air passes trachea → bronchi → bronchioles → alveoli. The alveolar epithelium is a single layer of squamous (flattened) cells, and the capillary endothelium is another, so the total diffusion distance is only about 0.5 µm. Roughly 300 million alveoli give a colossal surface area, and the dense capillary network plus constant ventilation keep the concentration gradient steep.
Fish — counter-current exchange. Water is forced over the gill filaments and their lamellae in the opposite direction to the flow of blood. This means blood always meets water with a higher oxygen concentration than its own, so a diffusion gradient is maintained along the entire length of the lamella and around 80% of the oxygen can be extracted. If the flows were parallel, the two would equilibrate about halfway along and only ~50% could be absorbed.
Insects — a tracheal system. Air enters through spiracles (which can close to limit water loss), passes into tracheae held open by chitin rings, and then into fine, fluid-filled tracheoles that carry oxygen directly to the respiring tissues. There is no oxygen transport in the blood at all. Oxygen delivery is enhanced by the diffusion gradient, by abdominal pumping (mass transport of air), and, during flight, by the withdrawal of tracheal fluid into the tissues, exposing more surface area for gas exchange.
The insect trade-off: the tracheal system is superb at delivering oxygen but is also a route for water loss. Spiracles are therefore kept closed as much as possible, and many insects have a waterproof exoskeleton and hairs around the spiracles to trap humid air.
A mammal has a double circulatory system: blood passes through the heart twice per circuit. This keeps the pulmonary circuit at a low pressure (so the delicate alveolar capillaries are not damaged) while the systemic circuit runs at high pressure — hence the much thicker wall of the left ventricle.
Myogenic control: the sinoatrial node (SAN) in the right atrium initiates a wave of depolarisation across both atria. The non-conducting collagen layer stops it passing straight to the ventricles. It reaches the atrioventricular node (AVN), which imposes a short delay — ensuring the atria have emptied before the ventricles contract — then passes down the bundle of His and the Purkyne fibres to the apex, so the ventricles contract from the bottom up, pushing blood upwards into the arteries.
Reading an ECG: the P wave is atrial depolarisation; the QRS complex is ventricular depolarisation; the T wave is ventricular repolarisation. Tachycardia = fast (over 100 bpm at rest); bradycardia = slow (under 60); fibrillation = disordered, ineffective contraction; ectopic beats = extra beats out of rhythm.
Haemoglobin has quaternary structure — four polypeptides, each with an iron-containing haem group — so it carries four O₂ per molecule.
Carbon dioxide transport: most CO₂ is carried as hydrogencarbonate ions. CO₂ diffuses into the red blood cell, where carbonic anhydrase catalyses CO₂ + H₂O → H₂CO₃, which dissociates into H⁺ and HCO₃⁻. The HCO₃⁻ diffuses out into the plasma and Cl⁻ moves in to maintain electrical neutrality — the chloride shift. The H⁺ is buffered by haemoglobin (forming haemoglobinic acid), and it is this H⁺ that causes the Bohr shift.
Tissue fluid: at the arteriole end of a capillary, the hydrostatic pressure exceeds the oncotic (osmotic) pressure created by the plasma proteins, so fluid is forced out. At the venule end, hydrostatic pressure has fallen and the oncotic pressure dominates, so most of the fluid returns. The excess (about 10%) is drained by the lymphatic system. If plasma protein levels fall, or hydrostatic pressure is too high, fluid accumulates — oedema.
Tissue fluid is the fluid that bathes every cell. It is formed from plasma, and its formation is a straight tug-of-war between two pressures.
Oedema is the accumulation of tissue fluid. It follows logically: high blood pressure raises the hydrostatic pressure so more fluid is forced out; a low plasma protein concentration (from starvation, or liver or kidney disease) lowers the oncotic pressure so less returns; and a blocked lymph vessel stops the excess draining away. Each mechanism produces the same swelling by a different route.
Xylem vessels are dead, hollow, lignified tubes with no end walls — a continuous column of water from root to leaf.
The cohesion-tension theory: water evaporates from the mesophyll cell walls and diffuses out of the stomata (transpiration). This lowers the water potential of the mesophyll, drawing water from the xylem. Because water molecules are cohesive (hydrogen bonding), the whole column is pulled up under tension, and adhesion to the lignified walls helps. Evidence: tree trunks measurably shrink in diameter during the day, when transpiration and therefore tension are greatest, and the column breaks if air is introduced.
Transpiration rate increases with light (stomata open), temperature (more kinetic energy; steeper water potential gradient), and air movement (removes the humid boundary layer); it decreases with humidity. A potometer measures water uptake, which is a close proxy since about 99% of the water absorbed is transpired.
Phloem — the mass flow hypothesis. At the source, companion cells use a proton pump to actively load sucrose into the sieve tube (co-transport with H⁺). This lowers the water potential, so water enters from the xylem by osmosis and the hydrostatic pressure rises. At the sink, sucrose is removed and used or stored, water leaves, and pressure falls. The pressure gradient drives the sap — mass flow.
Adaptations: xerophytes (marram grass, cacti) have thick waxy cuticles, sunken stomata, hairs and rolled leaves — all of which trap humid air and reduce the water potential gradient. Hydrophytes (water lily) have stomata on the upper surface and large air spaces (aerenchyma) for buoyancy and gas exchange.
Tap a card, then tap the stage of the cardiac cycle or the ECG feature it belongs to.
Tap an item on the left, then its partner on the right.
SA:V: volume rises with the cube of length, surface area only with the square — so large organisms need exchange surfaces and a mass transport system
Exchange surfaces: large surface area, thin (short diffusion distance), and a mechanism to maintain a steep concentration gradient
Lungs: alveoli: squamous epithelium ~0.5 µm thick. Inspiration is active (diaphragm and external intercostals contract); expiration at rest is passive elastic recoil
Fish & insects: counter-current flow across gill lamellae maintains the gradient along the whole length; insects use spiracles, tracheae and tracheoles direct to the tissues
Heart: atrial systole → ventricular systole → diastole. Valves open and shut passively on pressure differences. Cardiac output = stroke volume × heart rate
ECG: P wave = atrial depolarisation; QRS = ventricular depolarisation; T wave = ventricular repolarisation
Haemoglobin: cooperative binding → sigmoid curve. Bohr effect: CO₂ lowers pH and affinity, shifting the curve right so more O₂ is unloaded at respiring tissue
Plants: xylem: transpiration pull + cohesion + adhesion. Phloem: sucrose actively loaded at the source lowers Ψ, water enters, hydrostatic pressure drives mass flow to the sink
That is the whole of OCR Module 3 — Exchange and transport. Press Finish to see your score.
You have worked through Exchange & transport for OCR A-level Biology A (H420). 🎉
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