Membrane Structure

Our current view of membrane structure is based on the fluid mosaic model (Figure 5.1). The model pro-poses that membranes are not rigid with molecules locked into place but rather consist of proteins that move around within a mixture of lipid molecules that has the consistency of olive oil. The lipid molecules of all biological membranes exist in a double layer called a bilayer that is less than 10 nm thick. By comparison, this page is approxi-mately 100 000 nm thick. The lipid molecules of the bilayer vibrate, flex back and forth, spin around their long axis, move sideways, and exchange places within the same bilayer half. Only rarely does a lipid mole-cule flip-flop between the two layers. Exchanging places within a layer occurs millions of times a second, making the lipid molecules in the membrane highly dynamic. As we will discuss later, maintaining the membrane in a fluid state is critical to membrane function. The mosaic aspect of the fluid mosaic model refers to the fact that most membranes contain an assortment of types of proteins. This includes pro-teins involved in transport and attachment, signal transduction, and processes such as electron trans-port (Figure 5.1). Because they are larger than lipid molecules, proteins move more slowly in the fluid environment of the membrane. As well, a small number of membrane proteins anchor cytoskeleton filaments to the membrane and do not move. As also shown in Figure 5.1, a number of the lipid and protein components of some membranes have carbohydrate

groups linked to them, forming glycolipids and
glycoproteins. The relative proportions of lipid and protein
within a membrane vary considerably depending on the type of membrane. For example, membranes that contain protein complexes involved in electron trans-port, such as the inner mitochondrial membrane, con-tain large amounts of protein (76% protein and only 24% lipid), whereas the plasma membrane contains nearly equal amounts of protein and lipid (49% and 51%, respectively). Myelin, which is a membrane that functions to insulate nerve fibres, is composed mostly of lipids (18% protein and 82% lipid). An important characteristic of membranes, illus-trated in Figure 5.1, is that the proteins and other com-ponents of one half of the lipid bilayer are different from those that make up the other half of the bilayer. This is referred to as membrane asymmetry, and it reflects differences in the functions performed by each side of the membrane. For example, a range of glyco-lipids and carbohydrate groups are attached to proteins on the external side of the plasma membrane, whereas components of the cytoskeleton bind to proteins on the internal side of the plasma membrane. In addition, hormones and growth factors bind to receptor proteins that are found only on the external surface of the plasma membrane.