The pores on leaves through which carbon dioxide enters are called stomata. Each stoma is a microscopic opening in the leaf’s epidermis, usually controlled by a pair of specialized guard cells.
When stomata open, carbon dioxide diffuses into the leaf’s internal air spaces and eventually reaches mesophyll cells, where chloroplasts carry out photosynthesis. Oxygen produced by photosynthesis can diffuse out through the same openings. Stomata therefore function as adjustable gateways for gas exchange rather than as structures that perform photosynthesis themselves.
Opening the pores has a cost: water vapor also escapes through them in a process called transpiration. Guard cells respond to light, humidity, carbon dioxide concentration, and water stress, helping the plant balance carbon uptake against dehydration. Many plants open their stomata mainly during the day, while CAM plants typically open them at night to conserve water.
“Stomata” is the plural of “stoma.” Mesophyll refers to internal leaf tissue, thylakoids are membrane sacs inside chloroplasts, and chloroplasts are the organelles where photosynthesis occurs; none of those terms names the pores.