Membrane fluidity
In biology, membrane fluidity refers to the viscosity of the lipid bilayer of a cell membrane. Viscosity of the membrane can affect the diffusion of proteins and other biomolecules within the membrane, penetration of molecules through the membrane, and cellular pressure regulation of an organism. Changes in membrane-dependent functions, such as phagocytosis and cell signalling, are hypothesized to depend on the fluidity of the cell-membrane.[1] The binding of some peripheral proteins is also likely dependent on membrane fluidity.[2]
The simplest 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 packs 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-chain fatty acids, and ones with greater unsaturation (i.e. more double bonds), are less stiff and less viscous. On the molecular level, these double bonds make it harder for the lipids to pack together. 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 cholesterol and sphingomyelin, into lipid membranes (i.e. not cell membranes) can also stiffen a membrane. Such membranes can be described as "a glass state, i.e., rigid but without crystalline order".[3] Some drugs, e.g. Losartan©, are also known to alter membrane viscosity.[3] 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.
Discrete lipid domains with differing composition, and thus membrane fluidity, can coexist in model lipid membranes; this can be observed using fluorescence microscopy. 'lipid rafts' are hypothesized to exist in cell membranes and perform biological functions.[4]
Measurement of Membrane Fluidity
Membrane fluidity can be measured via electron spin resonance, fluorescence, or deuterium nuclear magnetic resonance spectroscopy. The first two involves observing spin probes or fluorescent probes incorporated into the membrane. Parameters used to characterize membrane includes rotational correlation time, order parameter, steady-state anisotropy (or polarization, which is dependent on the molecular transition dipole moment), and partition coefficient of the probe. The last of these describe the preference a particular probe has for a membrane of a given fluidity. Note that the probes are likely not sampling the entire membrane equally. For example, they can be confined within a particular lipid domain. Characterization of particular probes is an important step in helping researchers tease out membrane properties when using these techniques. Solid state deuterium nuclear magnetic resonance spectroscopy can be used to characterize membrane fluidity of multilamellar vesicles made with deuterated lipids.
Charged Lipid Membranes
Charged lipid membranes, e.g. 1,2-dimyristoyl-sn-glycero-3-phosphoglycerol (DMPG) melting takes place over a wide range of temperature. Within this range of temperatures, these membranes become very viscous.[3]
Relevance to Biological Function
Microorganisms subjected to thermal stress are known to alter the lipid composition of their cell membrane. This is one way they can adjust the fluidity of their membrane in response to their environment.[5]
See also
References
- ^ Helmreich EJ (2003). "Environmental influences on signal transduction through membranes: a retrospective mini-review". Biophysical chemistry. 100 (1–3): 519–34. doi:10.1016/S0301-4622(02)00303-4. PMID 12646388.
- ^ T. Heimburg and D. Marsh. Thermodynamics of the Interaction of Proteins with Lipid Membranes. Biological Membranes, K. Merz, Jr. and B. Roux, ed. Birhauser, Boston (1996).
- ^ a b c T. Heimburg. Thermal Biophysics of Membranes. Wiley-VCH, 2007.
- ^ Simons K, Vaz WL (2004). "Model systems, lipid rafts, and cell membranes". Annual review of biophysics and biomolecular structure. 33: 269–95. doi:10.1146/annurev.biophys.32.110601.141803. PMID 15139814.
- ^ R. B. Gennis. Biomembranes: Molecular Structure and Function. Springer-Verlag (1989).
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