B3.1 Gas exchangeIB Biology HL: Revision notes

As organisms grow, surface area-to-volume ratio falls and the distance from centre to surface rises, so large organisms need specialised gas-exchange surfaces. These are permeable, thin, moist and have a large surface area. In animals, gradients are kept steep by dense blood vessel networks, continuous blood flow and ventilation (air in lungs, water over gills).

Adaptations: branched bronchioles, huge alveolar surface area, thin alveolus walls, extensive capillary beds, and surfactant to stop alveoli sticking together.

Inhalation: external intercostals raise the ribs; diaphragm contracts and flattens; volume up, pressure down, air in. Forced exhalation: internal intercostals lower the ribs; abdominal muscles push the diaphragm up.

Vital capacity = tidal volume + inspiratory reserve + expiratory reserve.

Leaf tissues from top: waxy cuticle, upper epidermis, palisade mesophyll, spongy mesophyll with air spaces, lower epidermis with stomata and guard cells; veins carry xylem and phloem. Transpiration is a consequence of gas exchange: open stomata let CO₂ in and water vapour out. Rate rises with temperature, light, air movement and lower humidity. Stomatal density = mean count ÷ πr²; repeat counts because biological material varies.

Each haemoglobin molecule has four polypeptides, each with a haem group that binds one O₂. Binding of the first O₂ changes the molecule's conformation, raising the affinity of the others: cooperative binding. Releasing one O₂ likewise makes the rest leave more easily.

Foetal haemoglobin has a higher affinity for oxygen than adult haemoglobin, so in the placenta it loads oxygen that maternal haemoglobin is releasing.

An oxygen dissociation curve plots percentage saturation of haemoglobin against partial pressure of oxygen. It is S-shaped (sigmoid) because of cooperative binding: a shallow start (first O₂ binds with difficulty), a steep middle, and a plateau at high partial pressures.

• At the lungs (about 13 kPa) haemoglobin is almost fully saturated.
• At respiring tissues (steep part) a small fall in partial pressure releases a lot of oxygen.
• A curve to the left (foetal) means higher affinity; to the right means lower affinity.

Carbon dioxide binds to haemoglobin at allosteric sites (not the haem), changing its shape and lowering its affinity for oxygen. So in tissues with high CO₂, the curve shifts to the right and more oxygen dissociates: the Bohr shift. The benefit is that actively respiring tissues, which produce the most CO₂, receive the most oxygen.