The Action of guard cells.

Scattered throughout the leaf epidermis are pairs of bean-shaped cells, called guard cells . These cells contain chloroplasts, which are tiny granules filled with chlorophyll . Chlorophyll gives the leaf its green color. Chloroplasts enable leaves to carry on photosynthesis because they are able to absorb sunlight and carbon dioxide, which are essential for the food making process . In response to heat and light, each pair of guard cells pulls apart, and a tiny pore forms between …

…inside, water enters the guard cells osmotically, increasing their turgor pressure. Since the guard cells are connected to each other only at their ends, the turgor pressure causes them to swell outward, opening the stoma between them . To close the stoma, the reverse process will happen. Water flows out of the guard cells because of the concentration gradient . Eventually, the elastic inner walls return to their original shape and close up the previously opened space .