The function of guard cells is critical for regulating the opening and closing of stomata in plant leaves. They control gas exchange, allowing carbon dioxide to enter for photosynthesis while minimizing water loss through transpiration. When guard cells swell with water, they curve outward, opening the stomatal pore; conversely, when water is scarce, they lose turgor pressure and close the pore. This dynamic function is essential for maintaining plant water balance and overall health.