Guard cells help a plant regulate the process of photosynthesis as its environment changes. They increase and decrease in size as needed to retain or release gases essential to cellular function.
Through photosynthesis, plants convert water and sunlight into chemical energy stored as carbohydrates. Photosynthesis is a complex and intricate process, and few natural environments feature completely consistent levels of solar radiation and precipitation. Plants must therefore adapt to environmental fluctuations in order to survive. Guard cells play a critical role in these adaptations.
A plant's outermost cellular layers contain microscopic pores, called stomata, that absorb sunlight and carbon dioxide and release oxygen. Stomata must be open for photosynthesis to take place, but the open stomata allow water vapor to escape. To maintain a state of equilibrium, pairs of guard cells surrounding each stoma (the singular form of stomata) react to environmental changes. When water and sunlight are plentiful, the guard cells accumulate potassium ions, which draw water through the cell membranes. The inward flow of water causes the guard cells to expand, which in turn exposes the stomata to the outside environment. In dry conditions, the process reverses; the contracting guard cells release water back into the plant, closing off the stomata to prevent water vapor loss.