Cell membranes

Cell membranes are large cellular structures that constitute the boundary of a cell or a cell organelle. In contrast to proteins or nucleic acids, membranes are not made up of polymers, but a large number of diverse, relatively small molecules that form non-covalent interactions.

Phospholipids

The principal building blocks of cell membranes are a variety of compounds that are collectively known as phospholipids. Phospholipids are composed of three different parts: the backbone, a polar head group and a fatty acid chain (Figure 2.1). Different combinations of backbone, head groups and fatty acids result in a wide variety of phospholipids. In mammalian cells, glycerol is the backbone of the most abundant class of phospholipids, termed phosphoglycerides, although sphingolipids are also abundant. Backbone moieties have three hydroxyl groups, which are available for the conjugation of the polar head group, and two fatty acid chains. The polar head group is linked to the backbone via a phosphoester bond. In addition, two fatty acids are conjugated to the remaining hydroxyl groups of the backbone. The fatty acids can be broadly divided into two groups. The saturated fatty acids do not contain double bonds and always contains an even number of carbon atoms (usually 16–20). Due to the free rotation of the single carbon–carbon bonds, these lipid chains can adopt linear configurations. By contrast, the unsaturated fatty acids contain one or more double bonds. When the groups that lie on either side of the double bond are on opposite sides of the double bond (trans configuration) they are called trans fatty acids (Chapter 5). Like unsaturated fatty acids, trans fatty acids can adopt (near) linear configurations. However, when the groups next to the double bond are on the same side of the double bond (cis configuration) the fatty acids adopt very different shapes. The cis fatty acids have characteristic bends and cannot adopt linear configurations (Chapter 5).

A characteristic feature of the phospholipids is that they are amphipathic: they are both hydrophilic (due to the polar head group) and lipophilic (due to their fatty acid chain).

The lipid bilayer

Due to their amphipathic nature, phospholipids spontaneously organise in such a way that they form two sheets, with the polar head groups facing the aqueous exterior and the fatty acid chains forming a hydrophobic core (Figure 2.2). This structure is termed the lipid bilayer. Phospholipids can diffuse freely in the lipid bilayer, which behaves as a two-dimensional fluid. The lipid bilayer is asymmetrical, because phospholipids on one side of the bilayer do not freely flip to the other side. Thus, on the outside of cell membranes, phospholipids with PC head groups are enriched, while the cytoplasmic side is enriched with PE and PS lipids. The composition of the lipid bilayer is not homogeneous. The properties of cell membranes is influenced by the properties of the locally enriched phospholipids, e.g. the length of fatty acid chains influences the thickness of the lipid bilayer, and the presence of phospholipids containing cis fatty acids reduces the density of phospholipids in the lipid bilayer.

Sterols are another class of lipid components. Cholesterol is the most abundant sterol found in cell membranes (Chapter 5). Cholesterol impacts on the fluidity of the membrane: low concentrations of cholesterol can increase fluidity, whereas high concentrations of cholesterol can have the opposite effect.