Biological membranesCells are surrounded by thin membranes; it is these cell membranes which separate inside from out, life from non-life. It is believed that a cell or plasma membrane similar those of today's cells defined the boundary of the first cell nearly 4 billion years ago. Since then, cells have evolved such that the plasma membranes and intracellular membranes now perform many functions:
Red blood cells (erythrocytes) are the most abundant and simplest cells in the human body. One obvious question is how red blood cells maintain their biconcave shape. It turns out that an understanding the properties of the cell membrane is necessary to understand the shape (see: The structure of red blood cells).
Structure of the cell membraneIt took nearly 100 years to uncover the structure of the cell membrane as it is now understood (see: Revealing the nature of the cell membrane). In 1972 the fluid mosaic model was proposed and is now accepted as the basis of our understanding of cell membranes. The two most important components of this mode are a lipid bilayer and protein molecules.
As the name suggests, the lipid bilayer is two layers of lipid molecules sandwiched together as shown in the very simplified diagram below. The lipid molecules are termed amphiphilic (*) meaning `loving both'. One end of the molecule is hydrophilic (`water loving', polar) and the other is hydrophobic (`water hating', nonpolar). When lipid molecules are placed in aqueous solution they can spontaneously form a bilayer structure with the hydrophilic heads on the outside (in contact with the solution) and the hydrophobic tails inside (away from the solution). Many other organizations occur, some of which are also important in biological systems .
Lipid molecules make up between 30 and 80% of biological membranes by mass. The remainder is protein (20 to 60%) and sometimes carbohydrate (0 to 10%). Here we shall consider only the protein molecules. The protein molecules are very much larger than the lipid molecules so although there may be similar masses of each, there are many more lipid molecules. The proteins are located such that they either completely penetrate the membrane (transmembrane proteins) or just one of the two bilayers in which case they may be on the inside or outside of the cell. The diagram below shows how a transmembrane protein spans the width of the membrane. Many other types of molecule that are attached to the inside and outside of the membrane have been omitted for simplicity.
The fluid membrane isn't the determining factor in shape of red blood cell, instead the crystalline spectrin network is. Thus we attempt to understand the shape of red blood cells in terms of crystalline membranes rather then than a general fluid amphiphilic membrane model.