Phospholipids are the major structural components of most membranes. These molecules form a bi-layer (a double-layer of material) on the surface of the cell with their long, hydrophobic hydrocarbon chains pointing inward to the center of the bi-layer and their hydrophilic phosphate groups facing outwards.
Within this bi-layer of lipid float various kinds of proteins, rather like ships in a lipid sea. Some proteins remain on the surface and are called extrinsic proteins, whereas intrinsic proteins are partially submerged or extend right through the phospholipid bi-layer.
Protein and lipid constituents of membranes are not fixed in any one location, but can move and locate themselves at different points on the cell surface as required. Some, having carbohydrate or polysaccharide molecules attached to them, are complex glycoprotein and glycolipid macromolecules that play roles in recognition between cells and act as receptors for molecules such as hormones.
The physical state of membranes is dynamic, and rarely static. For example, when a cell adds extra cholesterol to a membrane, this changes the fluidity and converts the membrane from a liquid-like state to a more viscous gel-like state. Components may be added or taken away as the cell changes, grows or becomes a specialist, and the modern picture of the cell membrane is a dynamic one of constant change, movement, modification and adaptation.