membrane fludity

Membrane fluidity:
The interior of a lipid bilayer is normally highly fluid.
In the liquid crystal state, hydrocarbon chains of phospholipids are disordered and in constant motion.
At lower temperature, a membrane containing a single phospholipid type undergoes transition to a crystalline state in which fatty acid tails are fully extended, packing is highly ordered, & van der Waals interactions between adjacent chains are maximal.
Kinks in fatty acid chains, due to cis double bonds, interfere with packing in the crystalline state, and lower the phase transition temperature.
Cholesterol, an important constituent of cell membranes, has a rigid ring system and a short branched hydrocarbon tail
Cholesterol is largely hydrophobic.
But it has one polar group, a hydroxyl, making it amphipathic.
Cholesterol inserts into bilayer membranes with its hydroxyl group oriented toward the aqueous phase & its hydrophobic ring system adjacent to fatty acid chains of phospholipids.
The OH group of cholesterol forms hydrogen bonds with polar phospholipid head groups.
Interaction with the relatively rigid cholesterol decreases the mobility of hydrocarbon tails of phospholipids.
But the presence of cholesterol in a phospholipid membrane interferes with close packing of fatty acid tails in the crystalline state, and thus inhibits transition to the crystal state.
Phospholipid membranes with a high concentration of cholesterol have a fluidity intermediate between the liquid crystal and crystal states.
Two strategies by which phase changes of membrane lipids are avoided:
? Cholesterol is abundant in membranes, such as plasma membranes, that include many lipids with long-chain saturated fatty acids.
In the absence of cholesterol, such membranes would crystallize at physiological temperatures.
The inner mitochondrial membrane lacks cholesterol, but includes many phospholipids whose fatty acids have one or more double bonds, which lower the melting point to below physiological temperature
Lateral mobility of a lipid, within the plane of a membrane, is depicted at right and in an animation.
High speed tracking of individual lipid molecules has shown that lateral movements are constrained within small membrane domains.
Hopping from one domain to another occurs less frequently than rapid movements within a domain.
The apparent constraints on lateral movements of lipids (and proteins) has been attributed to integral membrane proteins, anchored to the cytoskeleton, functioning as a picket fence. See the website of the Kusumi laboratory.
Many proteins have a modular design, with different segments of the primary structure folding into domains with different functions.
Some cytosolic proteins have domains that bind to polar head groups of lipids that transiently exist in a membrane.
The enzymes that create or degrade these lipids are subject to signal-mediated regulation, providing a mechanism for modulating affinity of a protein for a membrane surface.