MEMBRANE FLUIDITY IN THE NAME OF GOD

IN BIOLOGY, MEMBRANE FLUIDITY REFERS TO THE VISCOSITY OF THE LIPID LAYER 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, THEREBY AFFECTING THE FUNCTIONS OF THESE MOLECULES.VISCOSITYLIPID LAYERCELL MEMBRANESYNTHETIC LIPID MEMBRANEDIFFUSION

FACTORS DETERMINING MEMBRANE FLUIDITY 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 composition of a membrane can also affect its fluidity. The membrane phospholipids incorporate fatty acids 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 due to their smaller molecular size and they have less surface area to undergo stabilizing van der Waals interactions with neighboring hydrophobic chains.phospholipidsfatty acidssaturation

. Lipid chains with double bonds are more fluid than lipids that are saturated with hydrogen and thus have only single bonds. On the molecular level, unsaturated double bonds make it harder for the lipids to pack together by putting kinks into the otherwise straightened hydrocarbon chain. Membranes made with such lipids have lower melting points: less thermal energy is required to achieve the same level of fluidity as membranes made with lipids with saturated chains. Incorporation of particular lipids, such as sphingomyelin, into synthetic lipid membranes is known to stiffen a membrane.melting pointssphingomyelin

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.Losartan

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.lipid domainsfluorescence microscopylipid raft

BIOLOGICAL RELEVANCE Microorganisms subjected to thermal stress are known to alter the lipid composition of their cell membrane (see homeoviscous adaptation). This is one way they can adjust the fluidity of their membrane in response to their environment. Membrane fluidity is known to affect the function of biomolecules residing within or associated with the membrane structure.homeoviscous adaptation

For example,the binding of some peripheral proteins is dependent on membrane fluidity. Lateral diffusion (within the membrane matrix) of membrane-related enzymes can affect reaction rates. Consequently, membrane-dependent functions, such as phagocytosis and cell signalling, can be regulated by the fluidity of the cell-membrane.phagocytosiscell signalling

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