Membrane fluidity

In biology, membrane fluidity refers to the viscosity of the lipid bilayer of a cell membrane or a synthetic lipid membrane. The particular types of lipids present within the membrane can influence how the lipids and other membrane-associated molecules pack together and interact with each other, and thus the fluidity of the membrane. Viscosity of the membrane can in turn affect the rotation and lateral diffusion of proteins and other biomolecules within the membrane, thereby affecting their functions.

Membrane fluidity is strongly affected by the fatty acid substituents of the lipid molecules, particularly by whether the fatty acid chains are saturated or unsaturated. Saturated fatty acids have no double bonds in their hydrocarbon chains, which decreases fluidity. Unsaturated fatty acids have at least one double bond, creating a "kink" in the chain which increases fluidity. While the addition of double bonds raises the lipid's melting temperature, research conducted by Xiaoguang Yang supports the notion that the presence of four or more double bonds is directly correlated with increased membrane fluidity. Membrane fluidity is also notably affected by cholesterol, which in appropriate concentrations can variously make the cell membrane fluid or rigid.