How do guard cells regulate gas exchange for photosynthesis?

Guard cells regulate gas exchange for photosynthesis by controlling the opening and closing of stomata on plant leaves.

Guard cells are specialised cells in the epidermis of leaves, stems and other organs that are used to control gas exchange. They are produced in pairs with a gap between them that forms a stomatal pore. The primary function of the guard cells is to control the opening and closing of the stomata, which in turn regulates the exchange of gases, particularly carbon dioxide and oxygen, between the plant's internal tissues and the external environment.

The regulation of gas exchange is crucial for photosynthesis, the process by which plants convert light energy, usually from the sun, into chemical energy in the form of glucose. Carbon dioxide is one of the key inputs in this process, and it enters the plant through the stomata. Oxygen, a by-product of photosynthesis, exits the plant in the same way.

Guard cells control the size of the stomatal pore by changing shape in response to environmental conditions. When the plant has ample water, the guard cells fill with water and become turgid, causing the stomata to open. This allows for the intake of carbon dioxide for photosynthesis. Conversely, when the plant is under water stress, the guard cells lose water and become flaccid, causing the stomata to close and limit water loss through transpiration.

The opening and closing of stomata also helps to regulate the plant's temperature, as the evaporation of water from the leaf surface can have a cooling effect. Therefore, by controlling the stomata, guard cells play a vital role in maintaining the plant's overall health and survival, as well as facilitating the process of photosynthesis.