Cell Membrane / Plasma Membrane

The cell membrane, also called the plasma membrane or biomembrane, is a thin, quasi-fluid structure. It is found both extracellularly, around the entire protoplast of the cell, and intracellularly, around most of the organelles inside a eukaryotic cell, where it separates each organelle from the surrounding cytosol.

Structure

The biomembrane is only about 75 Angstrom thick, and under the electron microscope it appears trilamellate, made up of three visible layers. Its main structural component is lipid, mostly phospholipid, arranged as a bilayer. Each phospholipid molecule has one hydrophilic (water-attracting) polar head and two hydrophobic (water-repelling) non-polar tails, making the molecule amphipathic. In the bilayer, the tails of the two layers face each other, sandwiched between the two rows of heads, so that the tails never come into direct contact with the watery surroundings on either side. The membrane also contains proteins and carbohydrates, and the ratio of proteins to lipids varies from cell to cell -- for example, human red blood cells show roughly 52% protein and 40% lipid in their membrane.

The fluid mosaic model

The most widely accepted model of membrane structure is the fluid mosaic model, proposed by Singer and Nicolson in 1972. According to this model, the membrane is made of a phospholipid bilayer in which proteins are embedded like icebergs floating in a sea of lipid -- and, importantly, these proteins are not fixed in place but can change position within the membrane. Some proteins are intrinsic (integral), occurring at different depths within the bilayer; some of these span the entire thickness of the membrane and are called transmembrane proteins, several of which form channels that allow the passage of water. Other proteins are extrinsic (peripheral), sitting only on one or the other surface of the membrane. Because the lipid component is quasi-fluid, proteins are able to move laterally within the membrane, and this capacity for movement is what is measured as the membrane's fluidity -- a property that comes chiefly from the mobile lipid bilayer itself. This model replaced an earlier idea, the sandwich model proposed by Danielli and Davson, which pictured the membrane as a rigid, static sandwich of protein layers coating a lipid core; that static picture could not account for the mobility that membrane proteins are actually observed to have, which is exactly what the fluid mosaic model explains.

Transport across the membrane …