Surface area : volume and Fick’s law

A single-celled organism exchanges everything across its surface. As an organism gets bigger, its volume (and so its metabolic demand) rises with the cube of length while its surface area rises only with the square. The SA:V ratio therefore falls, and simple diffusion becomes hopelessly slow.

Hence every specialised exchange surface has: a large surface area (alveoli, villi, gill lamellae, root hairs), a short diffusion pathway (a single flattened epithelial layer), and a mechanism to maintain a steep concentration gradient (ventilation, blood flow, counter-current).

Also note: small mammals have a large SA:V, so they lose heat rapidly and must have a high metabolic rate, a high breathing rate and a high heart rate to supply the oxygen needed.

Your turn — SA:V ratio

The human gas exchange system

Air passes trachea → bronchi → bronchioles → alveoli. The alveolar epithelium is a single layer of flattened cells, and the capillary endothelium is another, so the diffusion pathway is only about 0.5 µm. There are roughly 300 million alveoli, giving an enormous surface area, and the constant flow of blood plus ventilation keeps the concentration gradient steep.

• Inspiration (active): the external intercostal muscles contract and the diaphragm contracts and flattens. Thoracic volume increases, so pressure in the lungs falls below atmospheric pressure and air flows in.
• Expiration (largely passive at rest): these muscles relax, the elastic tissue of the lungs recoils, volume falls, pressure rises above atmospheric, and air flows out. Forced expiration also uses the internal intercostal muscles — the two intercostal sets are antagonistic.

Correlation is not causation. Data linking smoking to lung disease are correlational: a positive correlation plus a plausible mechanism plus a dose–response relationship builds the case, but a single study cannot prove cause. Diseases you should be able to explain mechanistically: fibrosis (scar tissue → thicker diffusion pathway, less elastic → reduced tidal volume), emphysema (alveolar walls destroyed → smaller surface area), asthma (bronchiole smooth muscle contracts, mucus secreted → narrower airway → reduced flow of air).

Your turn — pulmonary ventilation rate

Fish, insects and plants

Fish — counter-current exchange. Water flows over the gill lamellae in the opposite direction to blood flow. Because of this, blood always meets water with a slightly higher oxygen concentration than itself, so a diffusion gradient is maintained along the entire length of the lamella. Roughly 80% of the oxygen can be extracted. With parallel flow, equilibrium would be reached halfway along and only ~50% could be absorbed.

Insects — a tracheal system. Air enters through spiracles, passes down tracheae (supported by chitin rings) into fine tracheoles that deliver oxygen directly to the respiring tissues — no blood is involved in gas transport. Diffusion is helped by a concentration gradient (oxygen used up in tissue), mass transport from abdominal pumping, and, during flight, the withdrawal of tracheal fluid, which exposes more surface area.

Plants — stomata and the compromise. Gas exchange occurs through stomata, controlled by guard cells, into the air spaces of the spongy mesophyll. But an open stoma also loses water. Xerophytes resolve this with a thick waxy cuticle, sunken stomata, hairs that trap humid air, rolled leaves, and reduced needle-like leaves — all of which trap water vapour and reduce the water potential gradient out of the leaf.

Counter-current logic

Hydrolysis in the gut and how products get in

Digestion is simply hydrolysis of large insoluble polymers into small soluble monomers that can be absorbed.

• Carbohydrates: salivary and pancreatic amylase hydrolyse starch to maltose. Membrane-bound disaccharidases on the ileum epithelium (maltase, sucrase, lactase) finish the job at the point of absorption.
• Proteins: endopeptidases hydrolyse internal peptide bonds (creating many more ends); exopeptidases remove terminal amino acids; dipeptidases, membrane-bound, split the final dipeptides.
• Lipids: bile salts from the liver emulsify lipids into tiny droplets, increasing the surface area for lipase. The products (monoglycerides and fatty acids) associate with bile salts to form micelles, which carry them to the epithelium; they then diffuse straight through the membrane because they are lipid-soluble.

Absorption: glucose and amino acids enter by co-transport with Na⁺ (see 3.2). Inside the epithelial cell, monoglycerides and fatty acids are re-formed into triglycerides in the smooth ER, packaged with proteins by the Golgi into chylomicrons, and released by exocytosis into the lacteals (lymph), not the blood capillaries.

Common error: micelles are not absorbed. They are a delivery vehicle; they break up at the membrane and release the lipid-soluble products, which diffuse in.

Haemoglobin and the oxygen dissociation curve

Haemoglobin has quaternary structure: four polypeptides, each with a haem group containing Fe²⁺, so one molecule loads four O₂.

The curve is S-shaped (sigmoid) because of cooperative binding: the first oxygen binds with difficulty, but it changes the tertiary/quaternary structure, exposing the remaining haem groups, so the second and third bind far more easily. The fourth is again harder, as few sites remain — hence the plateau.

• At the lungs — high pO₂ → high % saturation → haemoglobin loads oxygen.
• At respiring tissue — low pO₂ → the curve is on its steep part, so a small fall in pO₂ causes a large fall in saturation → oxygen is unloaded exactly where it is needed.
• The Bohr effect — respiring tissue produces CO₂, which forms carbonic acid and lowers pH. The lowered pH changes haemoglobin’s shape, reducing its affinity for oxygen. The curve shifts right, and more oxygen is unloaded at the same pO₂.
• Different haemoglobins: a fetus, or an organism in a low-oxygen environment (e.g. a llama, or a lugworm), has haemoglobin with a higher affinity — curve shifted left — so it can still load oxygen at low pO₂. Very active animals tend to have low-affinity haemoglobin, which unloads readily.

Fetal haemoglobin must have a higher affinity than the mother’s, or it could not take up oxygen from maternal blood across the placenta, where the pO₂ is already low.

The Bohr effect

The cardiac cycle — pressure runs everything

The heart is a double pump: the right side sends deoxygenated blood to the lungs, the left (with a much thicker muscular wall) sends oxygenated blood around the body at high pressure.

• Atrial systole — atria contract; atrial pressure exceeds ventricular pressure; the atrioventricular valves are open; the ventricles fill completely.
• Ventricular systole — ventricles contract; ventricular pressure rises above atrial pressure, so the AV valves snap shut. When ventricular pressure exceeds aortic/pulmonary artery pressure, the semilunar valves open and blood is ejected.
• Diastole — ventricles relax; ventricular pressure falls below arterial pressure, so the semilunar valves close; when it falls below atrial pressure, the AV valves open and passive filling begins again.

Valves are entirely passive: they open and close because of the pressure difference across them. Reading a cardiac-cycle graph is nothing more than comparing the three pressure curves.

Atheroma → CVD: damage to the endothelium leads to plaque; the lumen narrows, raising blood pressure and risking thrombosis. If a coronary artery is blocked, cardiac muscle is starved of oxygen → myocardial infarction. Risk factors: high blood pressure, high blood cholesterol, smoking, obesity, inactivity.

Your turn — cardiac output

Cohesion-tension and mass flow

Xylem — 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 out of the xylem. Because water molecules are cohesive (hydrogen bonding), a continuous column is pulled up under tension; adhesion to the lignified xylem walls helps. Evidence: tree trunks measurably narrow during the day when tension is greatest, and if the column is broken (an air bubble), water cannot be pulled up.

Transpiration rate rises with light (stomata open), temperature (more kinetic energy, steeper gradient), wind (removes the humid layer), and falls with humidity. It is measured with a potometer, which actually measures water uptake — a close proxy, since ~99% of water taken up is transpired.

Phloem — the mass flow hypothesis. At the source, sucrose is actively loaded into the sieve tube (using the H⁺ gradient from a proton pump — a co-transport mechanism). This lowers the water potential of the sieve tube, so water enters by osmosis from the xylem, raising the hydrostatic pressure. At the sink, sucrose is removed and used or stored, water leaves, and pressure falls. The resulting pressure gradient drives the solution — mass flow — from source to sink.

Evidence for and against: a ringing experiment (removing a ring of bark and phloem) causes swelling above the ring — showing phloem carries sugars downwards. Radioactive tracers (¹⁴CO₂) show labelled sucrose in the phloem. Aphid stylets show sap flows faster than diffusion could explain, and the flow rate is greater near the source. Against: sieve plates should obstruct flow, and not all solutes travel at the same rate.

Your turn — potometer

Loading the phloem

Which exchange system?

Tap a feature, then tap the organism or organ it belongs to.

Digestive enzyme and its job

Tap an item on the left, then its partner on the right.

The big ideas to take away

That is the whole of AQA 3.3 Organisms exchange substances with their environment. Press Finish to see your score.