Membrane fluidity

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.^[1]

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 increases fluidity. Unsaturated fatty acids have at least one double bond, creating a "kink" in the chain. The double bond decreases fluidity. While the addition of one double bond raises the melting temperature, research conducted by Xiaoguang Yang et. al. supports that four or more double bonds has a direct correlation to membrane fluidity. Membrane fluidity is also affected by cholesterol.^[2] Cholesterol can make the cell membrane fluid as well as rigid.

Factors determining membrane fluidity

Membrane fluidity can be affected by a number of factors.^[1] The main factors affecting membrane fluidity are environmental (ie. temperature), and compositionally.^[3] 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 ${\displaystyle T_{m}}$ 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 organizations are very similar to a solid and liquid state.

• ${\displaystyle T<T_{m}}$: The membrane is in the crystalline phase, the level of order in the bi-layer is high and the fluidity is low.
• ${\displaystyle T>T_{m}}$: 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.

The composition of a membrane can also affect its fluidity. The membrane

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.^[4] Another way to change membrane fluidity is to change the pressure.^[1] 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

Measurement methods

Membrane fluidity can be measured with

Membrane fluidity can be described by two different types of motion: rotational and lateral. In electron spin resonance, rotational correlation time of spin probes is used to characterize how much restriction is imposed on the probe by the membrane. In fluorescence, steady-state anisotropy of the probe can be used, in addition to the rotation correlation time of the fluorescent probe.^[1] Fluorescent probes show varying degree of preference for being in an environment of restricted motion. In heterogeneous membranes, some probes will only be found in regions of higher membrane fluidity, while others are only found in regions of lower membrane fluidity.^[8] Partitioning preference of probes can also be a gauge of membrane fluidity. In deuterium nuclear magnetic resonance spectroscopy, the average carbon-deuterium bond orientation of the deuterated lipid gives rise to specific spectroscopic features. All three of techniques can give some measure of the time-averaged orientation of the relevant (probe) molecule, which is indicative of the rotational dynamics of the molecule.^[1]

Lateral motion of molecules within the membrane can be measured by a number of fluorescence techniques:

Diffusion coefficients of fluorescent lipid analogues are about 10⁻⁸cm²/s in fluid lipid membranes. In gel lipid membranes and natural biomembranes, the diffusion coefficients are about 10⁻¹¹cm²/s to 10⁻⁹cm²/s.[1]

Charged lipid membranes

The melting of charged lipid membranes, such as 1,2-dimyristoyl-sn-glycero-3-phosphoglycerol, can take place over a wide range of temperature. Within this range of temperatures, these membranes become very viscous.^[4]

Biological relevance

Microorganisms subjected to thermal stress are known to alter the lipid composition of their cell membrane (see

• Annular lipid shell
• Homeoviscous adaptation
• Lipid bilayer
• Lipid bilayer phase behavior
• Liposome
• Saffman–Delbrück model

References