The cell membrane is a dynamic boundary between the internal cytoplasm and the surrounding environment. The membrane is described as a lipid-protein fluid mosaic. Lecithin, a phospholipid, is a major component of the membrane. Like all phospholipids, lecithin is both hydrophobic and hydrophilic. The lipid portion of the membrane is stabilized by the presence of cholesterol which makes the membrane less 'fluid'.
Cholesterol acts to stabilize the membrane.
The globular proteins 'float' on the surface of the membrane or extend through the full thickness of the membrane. The membrane proteins are structured so that hydrophobic R-groups are exposed to the hydrophobic center of the membrane. The cell membrane serves as the point of interface between the cell and its surroundings. Substances may cross the cell membrane either actively or passively. Only small non-polarized molecules that are both water soluble and lipid soluble can diffuse through the lipid portion of the membrane.