Leaves. The exchange of oxygen and carbon dioxide in the leaf (as well as the loss of water vapor in transpiration) occurs through pores called stomata (singular = stoma). Normally stomata open when the light strikes the leaf in the morning and close during the night.
Accordingly, how does gas exchange occur in a single celled organism?
Single-celled organisms are aquatic and their cell surface membrane has a sufficiently large surface area to volume ratio to act as an efficient gas exchange surface. In small and thin organisms, the distance from gas exchange surface to the inside of the organism is short enough for diffusion of gases to be efficient.
Where does gas exchange occur in plants and animals?
In animals, gas exchange follows the same general pattern as in plants. Oxygen and carbon dioxide move by diffusion across moist membranes. In simple animals, the exchange occurs directly with the environment. But with complex animals, such as mammals, the exchange occurs between the environment and the blood.