The cell membrane is a fluid matrix made of a phospholipid bilayer. Each lipid molecule has a hydrophilic, phosphorylated "head" and a hydrophobic, fatty acid "tail". The hydrophobic parts of the molecules on the bilayer face each other, whereas the hydrophilic parts form the inner and outer surfaces of the cell membrane. Small, hydrophobic molecules can traverse the cell membrane via simple diffusion.
Integrated into the membrane are protein molecules which may span the whole bilayer (and are thus known as transmembrane proteins) or just one layer. Some of these proteins act as channels for transport of ions and other particles that can not simply diffuse across the membrane.
