Cell membrane

The cell membrane or plasma membrane is the outer surface of a cell and encloses the contents of the cell. The cell membrane is composed of a phospholipid bilayer studded with proteins. The phosphlipid bilayer consists of two fatty acid chains that link to two of the three carbons of a molecule of glycerol. The third carbon is attached to a phosphate group, which is in turn often attached to an alcohol. The alcohol makes the phosphate-alcohol group, or head group, hydrophilic. The fatty acid chains, or tails, are hydrophobic. This results in the hydrophilic parts pointing towards each other and the hydrophilic parts pointing to the extra and intracellular areas. The fluid mosaic model of the plasma membrane holds that there are two types of proteins in the phospholipid bilayer. The first is called an integral protein. These proteins are generally fixed and cannot be removed without rupturing the cell. One type of integral protein called the transmembrane protein spans the membrane, so that its surface is both on the inside and outside of the cell. A second type of membrane protein is called a peripheral protein and can easily be removed from the membrane.

Membrane Permeability

An essential feature of any plasma membrane is the ability to transport matertials in and out of the cell.

Passive Transport

Semi-permeable or selectively permeable membranes allow certain types of of substances through the membrane and not others. Some forms of transport require no energy to perform- these mechanisms are called passive transport. The most common forms of passive transport are diffusion and osmosis. Diffusion occurs when a certain substance, such as an ion, is more concentrated on one side of the membrane. If the membrane allows this ion through, then ions will move from the more concentrated side to the less concentrated side until reaching equilibrium.

Osmosis is the diffusion specifically of water across a membrane.