Membrane fluidity

Membrane fluidity is the ease with which lipids and proteins move within the plane of a biological membrane. It shapes membrane flexibility, permeability, and the activity of embedded proteins.

Lipid bilayer: A double layer of amphiphilic lipids that forms the basic structure of cell membranes. Its molecular packing creates the physical environment whose fluidity is being described.
Phospholipid: An amphiphilic lipid with a phosphate-containing head group and hydrophobic tails. Its head-and-tail structure determines how neighboring membrane molecules pack.
Homeoviscous adaptation: The adjustment of membrane lipid composition to maintain suitable viscosity as environmental conditions change. It is a biological response that stabilizes membrane fluidity across changing temperatures.
Membrane viscosity: A membrane’s resistance to molecular flow or deformation. It is related to fluidity but describes resistance rather than ease of movement.
Lipid lateral diffusion: The movement of lipid molecules within the plane of a membrane. It is a direct molecular measure of membrane fluidity.
Fatty acid: A carboxylic acid with a hydrocarbon chain, often incorporated into membrane lipids. Chain length and unsaturation alter the space and motion available in a bilayer.
Membrane protein: A protein associated with or embedded in a biological membrane. Its function and movement can depend on the fluid lipid environment around it.
Membrane order: The degree of alignment and packing among molecules in a membrane. Order often tracks inversely with fluidity, but is a distinct structural property.
Cholesterol: A sterol that inserts between membrane lipids in animal cells and modifies their packing. Its effects on lipid packing depend on temperature and help buffer fluidity.
Sphingolipid: A class of lipids built on a sphingoid base, common in eukaryotic membranes. Many sphingolipids pack tightly and contribute to locally ordered membrane regions.

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