A biological membrane is not a rigid sheet. It is a dynamic assembly in which lipids and proteins can move laterally while maintaining an overall bilayer organization. This is the fluid-mosaic model.

The basic structure is a phospholipid bilayer with hydrophilic surfaces facing aqueous environments and a hydrophobic interior formed by lipid tails.

Embedded within or associated with that bilayer are proteins with several possible arrangements:

• transmembrane proteins cross the bilayer and expose different regions on its two sides;
• integral membrane proteins are strongly associated with the hydrophobic interior even if they do not cross the entire membrane;
• peripheral membrane proteins associate more loosely with the membrane surface or with other proteins.

Many transmembrane proteins contain hydrophobic amino-acid side chains where they contact the bilayer interior and polar or charged regions where they contact water.

Fluidity

Lipids and many membrane proteins diffuse laterally within the membrane. The membrane therefore behaves more like a two-dimensional fluid than a fixed crystal.

Fluidity depends on lipid composition and temperature. Cis-unsaturated hydrocarbon chains pack less tightly than straight saturated chains and generally increase fluidity. Cholesterol is an amphipathic sterol, a compact lipid whose carbon skeleton contains several fused rings. In animal-cell membranes it can buffer fluidity by restricting some lipid motion at higher temperatures while preventing overly tight packing at lower temperatures.

Membranes are asymmetric

The two leaflets of a membrane are not necessarily chemically identical. Different lipids and proteins can be enriched on one side, and membrane proteins have defined orientations.

Short sugar chains can be covalently attached to some membrane lipids or proteins and are commonly exposed on the non-cytosolic surface of the plasma membrane, where they can contribute to recognition and interactions.

This asymmetry matters because a transporter, receptor or enzyme may function only when oriented correctly.

The fluid-mosaic model therefore combines two ideas: the membrane is an organized barrier with persistent sidedness, yet many of its components remain mobile within that structure.