Structure and Function of Stomata

Stomata are tiny, microscopic pores present primarily on the epidermis (surface) of leaves. In most terrestrial (land) plants, there are significantly more stomata on the lower surface of the leaf than on the upper surface to prevent excessive water loss from direct sunlight.

A single stoma consists of three main parts:

1. The Stomatal Pore: The actual microscopic hole or opening through which gases pass.
2. Guard Cells: The pore is bordered by two kidney-shaped (or bean-shaped) specialized cells called Guard Cells. In grasses, these guard cells are dumbbell-shaped.
3. Chloroplasts: Unlike normal epidermal cells, guard cells contain chloroplasts, allowing them to perform photosynthesis.
4. Subsidiary Cells: Surrounding the guard cells are specialized epidermal cells that support their movement.

The guard cells act like the doors of the stomatal pore. They control the opening and closing of the pore through turgor pressure (water pressure).

• Opening: When water flows into the guard cells, they swell up (become turgid), curve outward, and pull the pore open.
• Closing: When the plant loses water, the guard cells shrink (become flaccid), become straight, and close the pore.

• Gas Exchange: Taking in Carbon Dioxide (CO₂) for photosynthesis and releasing Oxygen (O₂).
• Transpiration: The loss of excess water in the form of water vapour from the plant.