Fluid Mosaic Membranes

Definition

A fluid mosaic membrane is a flexible structure made up mostly of lipids and proteins.
Its name derives from its ability to flow (fluid) and its diverse combination of molecules (mosaic).
Membranes are dynamically rearranged by the processes they support.

This type of membrane is primarily made up of a phospholipid bilayer.
There are proteins embedded within this flexible layer, either partially or fully.
The membrane can also contain cholesterol which adds further stability and flexibility.
The location and role of each protein, cholesterol and phospholipid molecule directly contribute to the mosaic-like composition.

Proteins embedded in the membrane can be either integral or peripheral.
Integral proteins span the whole membrane and are involved in transmembrane transport.
Peripheral proteins attach to the inner or outer surface of the cell membrane and are usually involved in cell signalling and maintaining cell shape.

The primary lipids are phospholipids arranged into two layers; each layer is composed of hydrophilic ‘heads’ and hydrophobic ‘tails’.
This phospholipid arrangement allows the membrane to form a barrier between the inside and outside of a cell, while still allowing certain molecules to pass through.
Cholesterol molecules present in animal cell membranes increase fluidity and stability.

The fluid mosaic model allows the substances to move across cell membranes through different processes: diffusion, osmosis, and active transport.
The selective permeability of the membrane, due to the presence of transport proteins and the phospholipid bilayer, controls what enters and exits the cell.
The proteins present have various roles including signal transmission, transport of specific molecules across the membrane, and enzymatic functions.

The fluid nature of the membrane is due to the lipids and proteins not being stationary but instead always in motion.
Membrane fluidity is affected by temperature and fatty acid composition - the more unsaturated fatty acids, the more fluid the membrane.
Cholesterol acts as a fluidity regulator by preventing fatty acid chains from packing together and crystallising.

The diverse collection of different proteins, lipids, and other components scattered throughout the membrane provides it with its mosaic nature.
The different components of the mosaic can move and interact with each other which allows biological functions to occur.
Variations in protein, lipid, and other component types and concentrations lead to unique membrane characteristics and functions.