The stomata are openings in leaf surfaces that:

• A. Are sites of photosynthesis
• B. Allow carbon dioxide to enter the leaf
• C. Store water
• D. Produce sugars

Answer: (B)

Stomata are the gateways that control gas exchange between a leaf and the air. They are small openings surrounded by guard cells that can open or close to regulate what goes in and out. The essential function is to let carbon dioxide diffuse into the leaf, where it is used in photosynthesis to build sugars. When stomata are open, oxygen, a byproduct of photosynthesis, and water vapor can also leave the leaf, so the plant balances carbon uptake with water loss through this opening. Photosynthesis itself happens in the chloroplasts inside leaf cells, not at the stomata, so stomata aren’t sites of photosynthesis, storage for water, or producers of sugars. The best statement is that stomata allow carbon dioxide to enter the leaf.