4.1.1—Fluid mosaic membrane model

The fluid mosaic model describes a cell membrane as a phospholipid bilayer containing different proteins. It is a flexible, moving interface rather than a static wall: the bilayer forms the basic boundary and the components together control exchange and communication.

• Bilayer arrangement: Hydrophilic phosphate heads face the aqueous environments on either side, while hydrophobic fatty-acid tails face inward to form the membrane core.
• Fluid: Phospholipids and many membrane proteins can move laterally within the membrane, although some proteins may be held in position.
• Mosaic: Different proteins are distributed through the bilayer in a scattered pattern, giving a mosaic appearance; their hydrophilic and hydrophobic regions determine where they sit.
• Selective barrier: The hydrophobic core makes passage difficult for many polar substances and ions, while membrane proteins can provide controlled routes for exchange or communication.
• Compartmentalisation: Bilayers can enclose the cell or internal organelles, separating reaction environments without making the membrane immovable.

Amphipathic phospholipids self-organise into a bilayer, the hydrophobic core limits unassisted passage, and embedded or surface-associated proteins add selective functions. Because components can move and be arranged differently, the membrane can remain continuous while adapting its exchange and communication roles.

“Fluid” does not mean that the membrane dissolves or that every component moves freely in every direction. “Mosaic” does not mean that the arrangement is meaningless or wholly random. Selective permeability is produced by lipid chemistry plus the position and function of membrane proteins.