Why and how do guard cells open and close stomata?
Guard cells have a special structure that helps them to open and close. The aim of the process is to control the amount of water getting outside the plant in case of water stress.
When guard cells open CO2 gets in for the process of photosynthesis to take place..Water gets out the leaf from the stomata through transpiration.
So, for CO2 to get in for the plant to photosynthesize food, water gets out. As if the plant is paying water as a currency for taking in CO2.
This is why in cases of lack of water in the soil, the plan tends to close the stomata to stop the loss of water, even though this will make CO2 less available for photosynthesis.
Guard cells have an inner thick sell wall and an outer thin one. They also have radial fibers all around. They have several ion pumps and channels.
When the plant feels the water stress, it performs several physiological changes including pumping of ions so the result is moving the water outside the guard cells. This makes the inner thick walls to come near each other so the stomata close.
When the water gets into the cell, the radial fibers tend to push the cell wall out and the hence the cell curves leading to the opening of the stomata.