Summary
In biology, membrane fluidity refers to the viscosity of the lipid bilayer of a cell membrane or a synthetic lipid membrane. Lipid packing can influence the fluidity of the membrane. Viscosity of the membrane can affect the rotation and diffusion of proteins and other bio-molecules within the membrane, there-by affecting the functions of these things. Membrane fluidity is affected by fatty acids. More specifically, whether the fatty acids are saturated or unsaturated has an effect on membrane fluidity. Saturated fatty acids have no double bonds in the hydrocarbon chain, and the maximum amount of hydrogen. The absence of double bonds decreases fluidity, making the membrane very strong and stacked tightly. Unsaturated fatty acids have at least one double bond, creating a "kink" in the chain. The double bond increases fluidity. Membrane fluidity is also affected by cholesterol. Cholesterol can make the cell membrane fluid as well as rigid. Membrane fluidity can be affected by a number of factors. One way to increase membrane fluidity is to heat up the membrane. Lipids acquire thermal energy when they are heated up; energetic lipids move around more, arranging and rearranging randomly, making the membrane more fluid. At low temperatures, the lipids are laterally ordered and organized in the membrane, and the lipid chains are mostly in the all-trans configuration and pack well together. The melting temperature of a membrane is defined as the temperature across which the membrane transitions from a crystal-like to a fluid-like organization, or vice versa. This phase transition is not an actual state transition, but the two levels of organizations are very similar to a solid and liquid state. The membrane is in the crystalline phase, the level of order in the bi-layer is high and the fluidity is low. The membrane is in the liquid-crystal phase, the membrane is less ordered and more fluid. At 37 °C, this is the state of the membrane: the presence of cholesterol, though, allows for the membrane stabilization and a more compact organization.
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Sphingomyelin
Sphingomyelin (SPH, ˌsfɪŋɡoˈmaɪəlɪn) is a type of sphingolipid found in animal cell membranes, especially in the membranous myelin sheath that surrounds some nerve cell axons. It usually consists of phosphocholine and ceramide, or a phosphoethanolamine head group; therefore, sphingomyelins can also be classified as sphingophospholipids. In humans, SPH represents ~85% of all sphingolipids, and typically make up 10–20 mol % of plasma membrane lipids. Sphingomyelin was first isolated by German chemist Johann L.
Cell membrane
The cell membrane (also known as the plasma membrane or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of a cell from the outside environment (the extracellular space). The cell membrane consists of a lipid bilayer, made up of two layers of phospholipids with cholesterols (a lipid component) interspersed between them, maintaining appropriate membrane fluidity at various temperatures.
Lipid bilayer phase behavior
One property of a lipid bilayer is the relative mobility (fluidity) of the individual lipid molecules and how this mobility changes with temperature. This response is known as the phase behavior of the bilayer. Broadly, at a given temperature a lipid bilayer can exist in either a liquid or a solid phase. The solid phase is commonly referred to as a “gel” phase. All lipids have a characteristic temperature at which they undergo a transition (melt) from the gel to liquid phase.
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