Why does water move through a membrane?

The cells need water to maintain their metabolic functions, the way water enters through cells membranes is by osmosis.

Explanation:

The cell membrane is a lipidic bilayer composed of phospholipids and embedded proteins. Phospholipids have the characteristic of being amphiphilic which means that possess hydrophilic and lipophilic characteristics. The most common way is by osmosis, that is a mechanism that moves water from high concentration to less concentration of water.  In the cell membrane, there are is channels of proteins called aquaporins that facilitate the pass of water through the membrane. Also, as water is a very small molecule without charge, this characteristic allows passing a limit amount of water molecules through the hydrophilic characteristic of membranes.

stimulus is a change in the environment that causes an organism to react. the response is the reaction to the stimulus. some examples of stimuli for zebras are drought, to many predators, and bad feeding grounds such as dead grass or plants. zebras respond to stimuli just like all other living organisms