Final answer:
Carbon dioxide enters leaves through stomata for photosynthesis.
Explanation:
Carbon dioxide is the gas that enters the leaf through the stomata and then diffuses into the plant cell to be used in photosynthesis. When entering the leaf, CO2 diffuses over short distances until it reaches the mesophyll cells, where it further diffuses into the stroma of the chloroplast, the site of light-independent reactions.
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