Signal Transduction Involved in Cell Volume Regulation

1.fammalian cells are surrounded by a selective permeable plasma membrane that allmvs the interior of the cell to differ in composition from the surrounding solution. The plasma membrane is formed by a bilayer of (phospho-) lipids and contains many different proteins. Hydrophobic molecules such as hydrocarbons, carbon dioxide and oxygen, rapidly dissohTc into the membrane and can cross it by diffusion. Other small molecules that are polar but uncharged, for example \\vater and ethanol, can also pass the membrane by simple diffusion. In contrast, the membrane is selectively permeable for specific ions and polar molecules that can only croSs the membrane barrier with the aid of specific membrane-spanning transport proteins. Some of these proteins form hydrophilic channels through \\"vhich specific molecules diffuse dmvn their concentration gradient (passive transport). Other transport proteins use energy (direct or indirect) to pump molecules against their concentration gradient into or out of the cell (active transport).

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