Gas exchange

Gas exchange is a biological process through which different gases are transferred in opposite directions across a specialized respiratory surface. Gases are constantly required by, and produced as a by-product of, cellular and metabolic reactions, so an efficient system for their exchange is extremely important. It is linked with respiration in animals, and both respiration and photosynthesis in plants.

In respiration, oxygen (O
2) is required to enter cells, while waste carbon dioxide (CO
2) must be removed; the opposite is true for photosynthesis, in which CO
2 enters plants and O
2 is released. The exchange of gases essentially occurs as a result of diffusion down a concentration gradient: gas molecules moving from an area of high concentration to low concentration.

Diffusion

Diffusion follows Fick’s Law. It is a passive process (no energy is required) affected by factors such as the surface area available, the distance the gas molecules must diffuse across and the concentration gradient.