How do guard cells regulate the rate of photosynthesis?

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

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 and the atmosphere.

The regulation of the stomatal opening is crucial for photosynthesis, the process by which plants convert light energy into chemical energy. During photosynthesis, plants take in carbon dioxide from the atmosphere through the stomata and release oxygen as a by-product. Therefore, the opening and closing of the stomata by the guard cells directly influences the rate of photosynthesis.

The guard cells control the stomatal opening in response to various environmental cues. For instance, in the presence of light, the guard cells absorb water and become turgid, causing the stomata to open. This allows for the intake of carbon dioxide, which is used in the light-dependent reactions of photosynthesis. Conversely, in the absence of light or when the plant is under water stress, the guard cells lose water and become flaccid, leading to the closing of the stomata. This prevents water loss through transpiration and also limits the intake of carbon dioxide, thereby reducing the rate of photosynthesis.

Furthermore, the guard cells also respond to changes in the internal carbon dioxide concentration. When the internal carbon dioxide concentration is high, indicating that the rate of photosynthesis is low, the guard cells trigger the stomata to open to allow more carbon dioxide in. Conversely, when the internal carbon dioxide concentration is low, suggesting that the rate of photosynthesis is high, the guard cells cause the stomata to close to prevent excessive carbon dioxide intake.

In summary, guard cells play a vital role in regulating the rate of photosynthesis by controlling the stomatal opening for gas exchange in response to environmental and internal cues.