Oxygen in, carbon dioxide out. Every solution is a trade-off between surface area, water loss and structural cost.
Every sub-topic below feeds at least one of these questions.
How are multicellular organisms adapted to carry out gas exchange?
What are the similarities and differences in gas exchange between a flowering plant and a mammal?
The required syllabus content for B3.1, in order. Each card is one lesson-sized checkpoint.
Gas exchange as a vital function in all organisms
Properties of gas-exchange surfaces
Maintenance of concentration gradients at exchange surfaces in animals
Adaptations of mammalian lungs for gas exchange
Ventilation of the lungs
Measurement of lung volumes
Adaptations for gas exchange in leaves
Distribution of tissues in a leaf
Transpiration as a consequence of gas exchange in a leaf
Stomatal density
Aerobic respiration consumes oxygen and produces CO₂. Photosynthesis is the reverse. Both depend on continuous gas exchange — and large organisms can't manage by diffusion alone.
A unicellular organism has a high surface-area-to-volume ratio. Gas exchange directly across the plasma membrane is fast enough — diffusion alone supplies O₂ to and removes CO₂ from every part of the cell.
As organisms get larger, surface-to-volume ratio falls and the distance from outside to interior cells grows. Diffusion is too slow. So large organisms evolve specialised gas exchange systems (lungs, gills) and transport systems (circulatory systems) to bring respiratory gases to and from the cells.
Across every kingdom — lungs, gills, leaves, skin — gas exchange surfaces converge on the same four properties.
More molecules can exchange per unit time. Achieved by folding, branching, or sheer number of small units (alveoli, gill filaments).
Often one cell thick. Minimises diffusion distance — Fick's law: rate is inversely proportional to thickness.
Allows gases to pass through. Cell membranes are naturally permeable to O₂ and CO₂; the trick is keeping the surface thin and intact.
A film of water lets gases dissolve before crossing the membrane. Always a trade-off with water loss in air-breathing organisms.
Animals don't just have exchange surfaces — they actively maintain steep concentration gradients across them.
Fish gills also use countercurrent flow: water flows past the gill filaments in one direction while blood flows in the opposite direction inside the filaments. This maintains the concentration gradient along the entire exchange surface, extracting up to ~85% of available oxygen.
The mammalian alveolar lung is a textbook case of all four exchange-surface properties working together.
Path of air: trachea → primary bronchi (one per lung) → secondary & tertiary bronchi → terminal bronchioles → respiratory bronchioles → alveolar ducts → alveoli.
~300 million alveoli per lung. Their combined surface area — about a tennis court's worth — is folded into a chest cavity.
Type II pneumocytes secrete surfactant — a lipid-protein film that reduces surface tension in the alveolar fluid lining. Without it, surface tension would collapse the alveoli.
Each alveolus is surrounded by a dense network of pulmonary capillaries — the diffusion distance is sometimes just two cells (alveolus + capillary wall).
The repeated branching of bronchioles delivers fresh air close to every alveolus, maintaining concentration gradients throughout.
Mammalian lungs are bellows. Changing the volume of the thorax changes its internal pressure, and air moves down the pressure gradient.
Diaphragm contracts and flattens downward. External intercostal muscles contract, lifting the ribcage up and out. Thoracic volume increases → pressure inside falls below atmospheric → air flows in.
Diaphragm relaxes, returning to its dome shape. External intercostals relax, ribcage falls under gravity. Thoracic volume decreases → pressure rises → air flows out.
Forced expiration (during exercise or coughing) adds active components: internal intercostals contract to pull the ribcage further in and down; abdominal muscles contract to push the diaphragm up more forcefully.
Four named volumes you should understand and be able to measure with a spirometer.
Air in/out per normal breath at rest.
Extra inhalable beyond a normal breath.
Extra exhalable beyond a normal breath.
Tidal + insp. reserve + exp. reserve.
Vital capacity is the practical maximum you can move in or out. The lungs also retain a residual volume (~1.2 L) even after maximum exhalation — they never fully empty.
Plants need CO₂ in for photosynthesis and O₂ out as a byproduct. The leaf is structured for that exchange — while minimising water loss.
| Tissue | Function |
|---|---|
| Waxy cuticle | Waterproof outer layer — minimises evaporative water loss. |
| Upper epidermis | Transparent protective layer letting light through to mesophyll. |
| Palisade mesophyll | Tall column cells packed with chloroplasts — main photosynthesis site. |
| Spongy mesophyll | Irregularly shaped cells with large air spaces between — high surface area for gas exchange. |
| Stomata (lower epidermis) | Pores controlled by guard cells. Allow CO₂ in and O₂ out; also where water vapour escapes. |
| Veins (xylem + phloem) | Xylem brings water up; phloem distributes sugars from leaf. |
You should be able to draw a plan diagram of a leaf cross-section — showing the distribution of these tissues without drawing individual cells.
Open stomata let CO₂ in. They also let water vapour out. Transpiration is an unavoidable consequence — but rate varies with environment.
Plants open stomata to gather CO₂ for photosynthesis when light is available. More open stomata → faster transpiration.
Water molecules gain kinetic energy and diffuse faster. Evaporation from mesophyll cells increases too.
Higher water concentration outside the leaf reduces the concentration gradient across the stomata. Water leaves more slowly.
Wind sweeps water vapour away from the leaf surface, maintaining a steep concentration gradient between inside and outside.
Stomatal density is a practical measurement — and a useful indicator of the environmental conditions a plant has adapted to.
Method: use a microscope with a calibrated eyepiece graticule (or take a leaf cast with clear nail polish, peel off and view), count stomata in a known field of view area, and divide. Stomata per mm² = density.
Biological variation is large. A single count of one field of view isn't reliable. Repeat in several fields across the leaf, and across several leaves. Calculate the mean and the standard deviation (or standard error) — these tell you both the typical value and the spread.
An extra 3 sub-topics for HL — same syllabus, deeper mechanism.
Adaptations of foetal and adult haemoglobin for the transport of oxygen
Bohr shift
Oxygen dissociation curves as a means of representing the affinity of haemoglobin for oxygen at different oxygen concentrations
If you can't define one of these in a sentence, that's where to revise next.
“How do multicellular organisms solve the problem of access to materials for all their cells?”
“What is the relationship between gas exchange and metabolic processes in cells?”
“What are the advantages of small size and large size in biological systems?”