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.
Factors determining membrane fluidity Membrane fluidity is affected by a number of factors. The main factors are environmental (i.e. temperature) and chemical composition. 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 ( T m {\displaystyle T_{m}} ) 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 organization behave very similarly to solid and liquid states of matter.
T < T m {\displaystyle T<T_{m}} : The membrane is in the crystalline phase, where the level of order in the bilayer is high and the fluidity is low.
T > T m {\displaystyle T>T_{m}} : The membrane is in the liquid-crystal phase, where the membrane is less ordered and more fluid. At 37 °C, the typical physiological temperature for most human cells, the cell membrane is in this liquid-crystal phase; the presence of cholesterol, however, allows for membrane stabilization and a more compact organization. The composition of the membrane also affects its fluidity. Membrane phospholipids incorporate fatty acyl chains of varying length and saturation. Lipids with shorter chains are less stiff and less viscous because they are more susceptible to changes in kinetic energy because of their smaller molecular size and because they have less surface area to undergo stabilizing London forces with neighboring hydrophobic chains. Molecules with carbon-carbon double bonds (unsaturated) are more rigid than those that are saturated with hydrogens, as double bonds cannot freely turn. As a result, the presence of fatty acyl chains with unsaturated double bonds makes it harder for the lipids to pack together by putting kinks into the otherwise straightened hydrocarbon chain. While unsaturated lipids may have more rigid individual bonds, membranes made with such lipids are more fluid because the individual lipids cannot pack as tightly as saturated lipids and thus cause the membranes to have lower melting points, such that less thermal energy is required to achieve the same level of fluidity as membranes made with lipids with saturated hydrocarbon chains. Incorporation of particular lipids, such as sphingomyelin, into synthetic lipid membranes is known to stiffen a membrane. Such membranes can be described as "a glass state, i.e., rigid but without crystalline order". Cholesterol acts as a bidirectional regulator of membrane fluidity because at high temperatures, it stabilizes the membrane and raises its melting point, whereas at low temperatures it intercalates between the phospholipids and prevents them from clustering together and stiffening. Some drugs, e.g. Losartan, are also known to alter membrane viscosity. Another way to change membrane fluidity is to change the pressure. In the laboratory, supported lipid bilayers and monolayers can be made artificially. In such cases, one can still speak of membrane fluidity. These membranes are supported by a flat surface, e.g. the bottom of a box. The fluidity of these membranes can be controlled by the lateral pressure applied, e.g. by the side walls of a box.
Heterogeneity in membrane physical property Discrete lipid domains with differing composition, and thus membrane fluidity, can coexist in model lipid membranes; this can be observed using fluorescence microscopy. The biological analogue, 'lipid raft', is hypothesized to exist in cell membranes and perform biological functions. Also, a narrow annular lipid shell of membrane lipids in contact with integral membrane proteins have low fluidity compared to bulk lipids in biological membranes, as these lipid molecules stay stuck to surface of the protein macromolecules.
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