Main Organ for Gaseous Exchange

The leaf is the main organ for gaseous exchange in a plant. Gaseous exchange is the movement of gases between the plant and the air by diffusion — carbon dioxide, oxygen and water vapour pass through tiny pores called stomata. This is a physical process and does not require energy from the plant.

The role of stomata

Each stoma is a pore in the epidermis, mostly on the lower surface, bordered by two guard cells. When the stomata are open, gases diffuse freely between the outside air and the air spaces of the spongy mesophyll. The direction of net diffusion depends on which process dominates.

• In bright light — photosynthesis is faster than respiration. The leaf takes in carbon dioxide and gives out oxygen overall.
• In darkness — only respiration occurs. The leaf takes in oxygen and gives out carbon dioxide.

Adaptations for efficient exchange

• Thin, flat shape gives a large surface area and short diffusion distance.
• Many stomata provide many openings for gases to pass.
• Air spaces in the spongy mesophyll connect the stomata to every inner cell, so gases can reach them.
• Moist cell surfaces inside the leaf let gases dissolve before diffusing across the cell membrane.

The rate of gaseous exchange therefore depends on how wide the stomata are open. When the guard cells open the stomata fully, gases move quickly; when they close, exchange almost stops. This is why gaseous exchange, photosynthesis and transpiration are all linked through the same pores.