Phospholipid bilayer

Phospholipid bilayers form the plasma membrane that surrounds all cells. The bilayer consists mainly of phospholipids which are amphipathic molecules, with a hydrophillic phosphate head group and two hydrophobic fatty acid chains. As well as these, the membrane  also contains various other types of lipid such as choleserol and proteins. These molecules contribute significantly to the mass of the membrane. The bilayer is arranged so that the phospholipid heads face outwards and the fatty acid chains face inwards, with cholesterol and proteins scattered throughout the membrane. This structure is described as fluid because the phospholipids can diffuse along the membrane.