What are guard cells used for?

These are cells found outside the stomata. There are two of them shaped rather like long balloons. They exist in the plants to control gas exchange and water loss. When the plant takes in lots of water via osmosis, the guard cells become turgid (plump) and this results in them bending slightly and the stomata opening allowing lots of gas exchange and loss of water via transpiration. When the plants lack water, the guard cells become flaccid (floppy) and this shuts the stomata, preventing further water loss.