How do guard cells regulate transpiration?

Answer 1

quite an easy question.
transpiration refers to the loss of water from the plant surface by evaporation.
upon light illumination, since gurad cells contain chlorophyll, they are able to produce sugar, which reduces the water potential inside them. Hence, there is net water influx into them, making them turgid and thus the stomata closed. Light will drive the water to evaporate at a higer rate, right? So, if the guard cells close the stomata, the water vapour inside the air space of leaves cannot escape to the environment because of the closed channels, despite there exists a concerntration gradient of water vapour. Hence, it reduces water transpiration rate, which is crucial for the plant to retain water.

But it is noted that at night, stomata will open in a relatively larger amount. GUess why. it is because of the guard cell no longer carries out photosynthesis, hence remain open. Water can hence transpire. But since at night, the low temperature does not lead to a high transpiration rate, the transpiration does not pose great harm on the plant because of the water loss. So, get it now?

Answer 2

The guard cells are specialized dermal cells that regulate the size of the openings or stomata in the epidermis of the leaf. Each stoma is surrounded by two guard cells that either take up or release water to the surround cells.

When the guard cells release water to the surrounding epidermal cells, the guard cells become flaccid which causes the two cells to close off the stoma. This prevents water loss from the leaf. Conversely, when the quard cells take up water from the surrounding cells, the guard cells swell(become turgid) which causes then to bow out, opening the stoma. This allows gas exchange and an increase in water loss from the leaf, transpiration.

Water is not directly pumped into our out of the guard cells. Instead, the guard cells actively transport potassium ions and the water follows by osmosis.
see the link for diagramaic explanation

Answer 3

http://www.phschool.com/science/biology_place/labbench/lab9/guard.html
http://www.phschool.com/science/biology_place/labbench/lab9/guard.html

Hope these sites will help