Exocytosis is used to release proteins from a cell. Cell surface receptors are sometimes used to direct ligands to be taken up by endocytosis into the cell. Extracellular fluid is sometimes taken up into cells by pinocytosis. The covalent bonds in water can be broken in cells. Water follows salt, i.e., water diffuses from a solution of low salt concentration to one of high salt concentration.
Water absorbs heat effectively with little change in temperature.
Water has a lower surface tension than most other ligands, causing water to have very low cohesion. Ice has a lower density than liquid water. Hydrophobic interactions hold together the plasma membrane. Some membrane proteins are held in place by attachment to the cytoskeleton. The cell membrane must be fluid to work properly.