How do Guard Cells Control the Size of Stomata?

Question 1

How do guard cells control the size of stomata? (during a normal sunny day)

Answer 1

• in the presence of sunlight, rate of photosynthesis is high
• glucose concentration in guard cells increase, causing water potential of cell sap in guard cells to decrease
• there is a higher water potential in the cell sap of surrounding mesophyll cells compared to guard cells
• water enters guard cells by osmosis
• guard cells become more turgid and more curved
• this causes stomata to open

Question 2

How do guard cells control the size of stomata? (during a hot sunny day)

Answer 2

On a hot sunny day, excess evaporation of water causes the guard cells to become flaccid. This causes stomata to close, preventing excessive loss of water by the leaf