The Phospolipid Bilayer

The Phospolipid BIlayer is aqueous environment inside and outside a cell prevents membrane lipids from escaping from the bilayer but nothing stops these molecules from moving about and changing places with one another within the plane of the bilayer. The membrane therefore behaves as a two dimensional fluid which is crucial for membrane function. Membranes consist largely of a lipid bilayer which is a double wall of phospoholipid, cholesterol and glycolipid molecules containing chains of fatty acids. Lipids give cell membranes a fluid character with a consistency approaching that of light oil. The fatty acid chains allow many small, fat soluble molecules such as oxygen to permeate the membrane, but they repel large water soluble molecules, such as sugar and electrically charged ions such as calcium

The top and bottom layers of the membrane have their stems facing each other. Proteins can stretch through the top bottom or both layers of phospolipid bilayer. Embedded in the lipid bilayer are large proteins many of which transport ions and water soluble molecules across the membrane. Some proteins in the plasma membrane form open pores called membrane channels which allow the free diffusion of ions into and out of the cell. Others bind to specific molecules on one side of membrane and in a process that is not clearly understood transport the molecules to the other side. Sometimes one protein simultaneously transports two types of molecules in opposite directions. Most plasma membranes are about 50 percent protein by weight while the membranes of some metabolically active organelles are 75 percent
                     
Phospolipid bilayer has 5 reasons for making it possible for the cell membrane to perform its job. Used to join cells together in cell adhesion Attach the membrane to the cytoskeleton Proteins gather together as enzymes and carry out different steps of metabolic reactions that take place near the cell membrane