Gas Exchange in Insects, Fish and Leaves

Air enters through spiracles and travels through branching tracheae into fine tracheoles close to tissues. Oxygen diffuses down a gradient into respiring cells and carbon dioxide moves out. The short final pathway means the insect’s blood is not the main oxygen-delivery system in the usual terrestrial insect model.

Spiracles can close to reduce evaporative water loss, but closure also restricts exchange. Body movements can ventilate the system and help renew gradients during high demand. The arrangement is a compromise between supply and water conservation, not a system that prevents water loss completely.

Gill filaments carry many lamellae, providing a large area. Thin exchange layers and a good blood supply support diffusion over short distances. In counter-current flow, water and blood move in opposite directions along a lamella, so water can retain a higher oxygen concentration than the adjacent blood throughout much of the exchange pathway.

This maintains a favourable gradient along the surface and permits more extensive oxygen uptake than simple parallel flow that rapidly approaches equilibrium. Water leaving the gill can have a lower oxygen concentration than blood leaving because those two outputs are not necessarily adjacent during exchange. Compare neighbouring positions, not just the two outlet values.

In a dicot leaf, stomata allow gases to diffuse between the atmosphere and internal air spaces. Spongy mesophyll air spaces and moist cell surfaces support movement to photosynthesising cells. Carbon dioxide commonly enters during active photosynthesis, while oxygen can leave; the direction depends on the balance of photosynthesis and respiration.

A xerophytic plant may have a thick waxy cuticle, rolled leaves, sunken stomata or hairs that trap humid, still air near stomata. These reduce water-vapour gradients or exposed area, but can also restrict carbon dioxide entry. Explain the specific mechanism and trade-off rather than assuming every drought adaptation increases gas exchange.

Try each question before opening the worked response. Use precise biological terms, connect each cause to its effect, and check units and assumptions when interpreting data.