How do guard cells regulate water loss in plants?

Guard cells regulate water loss in plants by controlling the opening and closing of stomata on leaf surfaces.

Guard cells are specialised cells in the epidermis of leaves, stems and other organs that are used to control gas exchange. They are shaped like two sausages joined at the ends and enclose an opening known as a stoma. The primary function of the guard cells is to control the opening and closing of stomata, which in turn regulates the amount of water that is lost through transpiration.

The opening and closing of the stomata are regulated by the turgidity of the guard cells. When the guard cells are turgid (swollen with water), the stomata are open. Conversely, when the guard cells are flaccid (not swollen), the stomata are closed. This turgidity is controlled by the movement of potassium ions in and out of the guard cells. When potassium ions move into the guard cells, water follows by osmosis, making the cells turgid and opening the stomata. When potassium ions move out of the guard cells, water also moves out, making the cells flaccid and closing the stomata.

The regulation of water loss is crucial for plants as it helps to prevent them from drying out. In hot, dry or windy conditions, the rate of transpiration can increase, leading to excessive water loss. By closing their stomata, plants can reduce water loss and prevent dehydration. However, this also reduces the amount of carbon dioxide that can enter the leaf for photosynthesis. Therefore, plants must balance the need to conserve water with the need to photosynthesise.

In summary, guard cells play a vital role in regulating water loss in plants. By controlling the opening and closing of stomata, they help to maintain the plant's water balance and ensure its survival in different environmental conditions.