Cell membranes contain only phospholipid and protein molecules.

Micelles are tiny droplets of phospholipids in water. In each micelle the headgroups of the phopholipids are exposed to the water and the tailgroups protected from the water in the centre of the micelle.

Integral proteins can be removed from biological membranes by mild chemical treatments that leave the lipid bilayer intact.

The lipid bilayer of biological membranes is formed by phospholipid molecules with hydrophilic fatty-acid, tailgroups and hydrophobic headgroups.

The biological membrane is described as fluid because the phospholipid molecules but not the proteins can move around in the lipid bilayer.