Lipids are fatty acid esters, a class of relatively water-insoluble organic molecules, which are the "basic" components of biological membranes. There are three forms of lipids: phospholipids, steroids. and triglycerides.
Lipids consist of a polar or hydrophilic (attracted to water) head and one to three nonpolar or hydrophobic (repelled by water) tails (Fig. 1). Since lipids have both functions, they are called amphiphilic. The hydrophobic tail consists of one or two (in triglycerides, three) fatty acids. These are unbranched chains of carbon atoms (with the correct number of H atoms), which are connected by single bonds alone (saturated fatty acids) or by both single and double bonds (unsaturated fatty acids). The chains are usually 14-24 carbon groups long.
Figure 1: Basic lipid structure. A lipid consists of a polar head group (P) and a nonpolar tail (U for unpolar). The lipid shown is a phospholipid (two tails). The image on the left is a zoomed version of the more schematic image on the right, which will be used from now on to represent lipids with one, two, or three chains.
For lipids present in biological membranes, the hydrophilic head is from one of three groups:
- Glycolipids, whose heads contain an oligosaccharide with 1-15 saccharide (sugar) residues.
- Phospholipids, whose heads contain a positively charged group that is linked to the tail by a negatively charged phosphate group.
- Sterols, whose heads contain a planar steroid ring, for example, cholesterol (only in animals).
Figure 2: Self-organization of lipids.
Driven by hydrophilic and hydrophobic forces, the nonpolar tails of lipids (U) tend to cluster together, forming a lipid bilayer (1) or a micelle (2). The polar heads (P) face the aqueous environment.
Lipid bilayers form the foundation of all biological membranes.