In biology, the membrane fluidity refers to the viscosity of the lipid bilayer of a cell membrane. The membrane phospholipids incorporate fatty acids of varying length and saturation. Shorter-chain fatty acids, and ones with greater unsaturation, are less stiff, less viscous and have lower melting points. Changes in membrane-dependent functions, such as phagocytosis and cell signalling, are hypothesized to depend upon the cell-membrane fluidity. This hypothesis lost favor, but has re-emerged recently with the discovery of discrete lipid domains, dubbed 'lipid rafts' in cellular membranes