Guard cells open and close the stomata by changing their turgidity. The process depends on the movement of ions and water.Opening (in light)
• Light stimulates guard cells to use ATP to actively pump hydrogen ions (H+) out of the cell.
• This makes the inside more negative, so potassium ions (K+) move into the guard cells through channel proteins. Starch is also converted to malate ions.
• The build-up of K+ and malate lowers the water potential of the guard cells.
• Water enters by osmosis from neighbouring epidermal cells.
• The guard cells swell and become turgid. Because the thick inner wall stretches less than the thin outer wall, the cells bend outwards and the pore opens.Closing (in the dark or when water is short)
• K+ ions leave the guard cells.
• The water potential of the guard cells rises and water leaves by osmosis.
• The guard cells become flaccid, straighten, and the pore closes.
When a plant is short of water, its roots and leaves produce the hormone abscisic acid (ABA). ABA causes K+ ions to leave the guard cells, so the stomata close even in light. This reduces water loss by transpiration, although it also reduces carbon dioxide uptake and so photosynthesis.Most plants therefore open their stomata during the day, when light allows photosynthesis, and close them at night, when carbon dioxide is not needed.
• Light stimulates guard cells to use ATP to actively pump hydrogen ions (H+) out of the cell.
• This makes the inside more negative, so potassium ions (K+) move into the guard cells through channel proteins. Starch is also converted to malate ions.
• The build-up of K+ and malate lowers the water potential of the guard cells.
• Water enters by osmosis from neighbouring epidermal cells.
• The guard cells swell and become turgid. Because the thick inner wall stretches less than the thin outer wall, the cells bend outwards and the pore opens.Closing (in the dark or when water is short)
• K+ ions leave the guard cells.
• The water potential of the guard cells rises and water leaves by osmosis.
• The guard cells become flaccid, straighten, and the pore closes.
When a plant is short of water, its roots and leaves produce the hormone abscisic acid (ABA). ABA causes K+ ions to leave the guard cells, so the stomata close even in light. This reduces water loss by transpiration, although it also reduces carbon dioxide uptake and so photosynthesis.Most plants therefore open their stomata during the day, when light allows photosynthesis, and close them at night, when carbon dioxide is not needed.