The stomata in a leaf open and close to control gas exchange. Explain how the structure and position of the stomata and guard cells make the leaf well-adapted for gas exchange, and how a large concentration gradient is maintained across the stomata.

Stomata are situated on the lower epidermis, reducing water loss by shielding them from direct sunlight and wind.

Each stoma is a tiny pore surrounded by two guard cells. The guard cells change turgor: when they take up water they swell, opening the pore; when they lose water they shrink, closing it. This allows the plant to control the rate of gas exchange.

The spongy mesophyll beneath the stomata contains large intercellular air spaces. These spaces give a high surface area and a short diffusion path for gases, so CO₂ can reach the chloroplasts quickly and O₂ can leave efficiently.

A large concentration gradient across the stomata is maintained because CO₂ is continually consumed in the Calvin cycle, keeping the internal CO₂ concentration low, while O₂ produced in photosynthesis diffuses out, keeping the internal O₂ concentration lower than the atmosphere. This gradient drives CO₂ in and O₂ out.