The process of gaseous exchange in plants happens in the leaves. Oxygen and water vapor leave the plant while carbon dioxide enters through pores called stomata. Plants require carbon dioxide for photosynthesis and oxygen for respiration.
Photosynthesis requires an input of water and carbon dioxide. These two react to produce carbohydrates. Oxygen is released as a waste product. Photosynthetic rates are normally higher than respiration rates. This results in a net increase in the production and usage of oxygen and carbon dioxide respectively. Therefore, on balance, plants produce oxygen and use carbon dioxide.
These two gases move in and out of plants through stomata (singular stoma). These specialized openings are located along the leaf’s lower surface. Stomata are of optimum shape and size for effective diffusion of gases to take place. A plant's hydration is determined by the opening of the stomata. Initially, water comes from the soil, enters the root through osmosis, then moves into the xylem tissues in a stem that transport it to the leaves.
When light shines on the leaves, photosynthesis begins. Oxygen produced in the process exits the plants just like water vapor, but carbon dioxide follows the reverse path. Each gas moves down a pressure or concentration gradient. Gaseous exchange takes places continually as long as the stomata remain open and the concentration or pressure gradient remains in the correct direction. Generally, stomata open whenever there is light and close in the dark.