IB Biology HL B3.1 Gas exchange Notes

Gas exchange is the exchange of respiratory gases between an organism and its environment. Animals take in oxygen for aerobic respiration and release carbon dioxide. Small organisms can often rely on diffusion across the body surface, but larger or more active organisms need specialized exchange surfaces because surface area-to-volume ratio decreases and demand for oxygen increases.

Efficient gas-exchange surfaces all solve the same diffusion problem. They are permeable so gases can cross, thin so the diffusion path is short, moist so gases dissolve, large so more gas can cross at once, and supplied with a steep concentration gradient.

Use the picture to connect each surface feature to a faster diffusion step.

Diffusion slows if the gradient disappears, so gas-exchange systems keep gradients steep. Ventilation refreshes air or water at the exchange surface, while continuous blood flow carries oxygen away and brings carbon dioxide toward the surface. Dense capillary networks add exchange area and keep red blood cells close to the surface.

Mammalian lungs are adapted for fast gas exchange. Many small alveoli provide a large surface area; type I pneumocytes and capillary walls are thin for a short diffusion path; dense capillary beds maintain gradients and bring red blood cells close; surfactant reduces surface tension; elastic fibres support ventilation and recoil.

Read the alveolus as a set of linked diffusion and ventilation adaptations, not as a label list.

Ventilation changes thoracic volume and pressure. During inspiration, the diaphragm and external intercostal muscles contract, increasing thoracic volume and lowering pressure, so air moves in. During expiration, thoracic volume decreases; relaxed breathing is mostly passive, while internal intercostals and abdominal muscles help forced exhalation.

Track muscle action first, then volume, then pressure, then airflow direction.

Spirometry records breathing patterns, ventilation rate, and lung volumes. Tidal volume is the air moved during normal relaxed breathing. Vital capacity equals tidal volume plus inspiratory reserve volume plus expiratory reserve volume. During exercise, tidal volume and ventilation rate usually increase to meet oxygen demand and remove carbon dioxide.

Leaves must exchange gases for photosynthesis and respiration while limiting water loss. Stomata allow carbon dioxide, oxygen, and water vapour to diffuse. Guard cells open and close stomata by changing turgor. Waxy cuticle, lower epidermal stomata, air spaces, mesophyll, and veins help balance gas exchange with transpiration.

A leaf must let carbon dioxide in for photosynthesis while limiting uncontrolled water loss.

A dicot leaf cross-section has a functional distribution of tissues. Waxy cuticle and epidermis protect the leaf; palisade mesophyll is packed for photosynthesis; spongy mesophyll has air spaces for diffusion; vascular bundles contain xylem for water transport and phloem for assimilate transport; stomata connect internal air spaces with the atmosphere.

Use tissue position to explain both diffusion path and transport support.

Transpiration is water loss from leaves: water evaporates from mesophyll cell walls, then water vapour diffuses out through stomata. Temperature, humidity, wind, and light change the rate by affecting evaporation, water-vapour gradients, air movement, and stomatal opening. Potometers estimate transpiration indirectly by measuring water uptake.

Stomatal density is the number of stomata per unit leaf area. Higher stomatal density can increase carbon dioxide uptake but also increases potential water loss. Leaf casts or micrographs allow stomatal counts; reliability improves by sampling multiple areas or leaves and calculating a mean.

HL begins with why haemoglobin is needed. Oxygen is poorly soluble in plasma, so haemoglobin greatly increases oxygen transport. Each haemoglobin molecule has four subunits with haem groups that bind oxygen reversibly. Cooperative binding means binding of one oxygen increases the affinity of the remaining subunits, helping loading in high pO2 and unloading in low pO2. Foetal haemoglobin has higher oxygen affinity than adult haemoglobin, helping oxygen transfer across the placenta.

Use the structure to explain why haemoglobin can both load and unload oxygen efficiently.

The Bohr shift explains oxygen release in actively respiring tissues. More respiration produces more carbon dioxide. Increased carbon dioxide lowers blood pH, which reduces haemoglobin affinity for oxygen. The oxygen dissociation curve shifts to the right, so haemoglobin releases more oxygen at the same pO2 where tissues need it.

Oxygen dissociation curves plot percentage saturation of haemoglobin against partial pressure of oxygen. The adult haemoglobin curve is sigmoid because of cooperative binding. Curve position shows affinity: a left shift means higher affinity and easier loading; a right shift means lower affinity and easier unloading. Foetal haemoglobin lies left of adult haemoglobin because it has higher oxygen affinity.

Use the same pO2 on both curves to compare affinity and unloading directly.

Core gas-exchange answers link exchange surfaces to diffusion gradients. For animals, exchange surfaces are explained by diffusion properties, ventilation, and blood flow. For plants, leaves allow carbon dioxide entry and oxygen/water vapour exit while controlling water loss through stomata. Spirometry, transpiration, and stomatal density data provide evidence of gradient and surface-area effects.

Use this for core questions on gas-exchange surfaces, alveoli, ventilation, spirometry, leaves, transpiration, and stomatal density.

Haemoglobin increases oxygen transport because oxygen is poorly soluble in plasma. Reversible and cooperative binding allow loading at high pO2 and unloading at low pO2. High carbon dioxide lowers pH and causes the Bohr shift, reducing affinity and promoting oxygen release in active tissues. Dissociation curves show these changes through sigmoid shape and left/right shifts.

Use this for HL questions on haemoglobin, Bohr shift, oxygen dissociation curves, exercise, and foetal haemoglobin.