Carbon dioxide usually enters a leaf through tiny openings called stomata. These pores connect outdoor or indoor air with spaces inside the leaf, allowing carbon dioxide to diffuse toward cells that perform photosynthesis. Guard cells can widen or narrow each opening.
The tiny pores are called stomata
Most terrestrial plants have stomata in the leaf epidermis, often concentrated on the underside. A stoma is surrounded by two guard cells. When the pore is open, carbon dioxide can enter, while oxygen and water vapor can move outward. Not every species has the same stomatal arrangement.
What happens after carbon dioxide enters?
CO₂ moves through internal air spaces and dissolves in the moist environment around photosynthetic cells. Within chloroplasts, carbon dioxide is incorporated into sugars through biochemical reactions that use energy captured from light. Roots supply water through the plant’s vascular tissues.
Why pores don’t stay wide open all the time
Open stomata also allow water vapor to escape. During drought or extreme heat, many plants partly close their stomata to reduce water loss, but that can reduce the supply of carbon dioxide for photosynthesis. Different plant species balance this trade-off in different ways.
Frequently asked questions
Does carbon dioxide enter through the roots? In ordinary green terrestrial plants, leaves are the main site of atmospheric CO₂ uptake, though some stems also photosynthesize.
Do plants only exchange gases in daylight? Gas exchange continues to serve respiration at night, and some drought-adapted CAM plants open their stomata mainly at night to take in CO₂.
For a related illustration of plant photosynthesis, see the NASA science education resources.
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