What are the small openings on the underside of a leaf through which oxygen and carbon dioxide can move?

• A. Stomata
• B. Guard cells
• C. Vascular bundle
• D. Cuticle

Answer: (A)

Gas exchange in leaves happens through stomata, small pores on the underside of the leaf that are formed by a pair of guard cells. These pores allow carbon dioxide to enter for photosynthesis and oxygen to exit as a waste product, while also letting water vapor escape in transpiration. The guard cells control the opening and closing of the pore by changing their turgor: when they fill with water, they swell and the opening widens; when they lose water, they shrink and the pore closes. Stomata are often more numerous on the lower leaf surface to reduce water loss since that side is cooler and less exposed to direct sunlight. The other options aren’t openings: guard cells are the cells that surround the pore and regulate it; the vascular bundle (xylem and phloem) transports water and nutrients; the cuticle is the waxy coating that limits water loss but does not create an opening.