What openings in a leaf allow gas exchange?

The openings in a leaf that allow gas exchange are called stomata. Each stoma is a tiny pore in the leaf’s epidermis, usually surrounded by two specialized guard cells that control whether it opens or closes.

When stomata are open, carbon dioxide diffuses into the leaf for photosynthesis, while oxygen produced during photosynthesis can diffuse out. Water vapor also escapes through these pores in a process called transpiration. This creates a trade-off: opening stomata helps a plant acquire carbon dioxide but can increase water loss.

Many leaves have more stomata on their lower surface, where they are less exposed to direct sunlight and wind. Guard cells change shape as water and ions move in or out of them, adjusting the pore’s size. In drought conditions, plants often close their stomata to conserve water, although this also limits carbon dioxide uptake. Stomata are not the same as guard cells: the stoma is the pore, while guard cells regulate its opening.