If lipids were suddenly to disappear from the Earth, all living cells would collapse in a pool of fluid, because lipids are essential to the structure and function of membranes that separate living cells from their environment.
Lipids (lip = fat) are a second major group of organic compounds found in living matter. Like carbohydrates, they are composed of atoms of carbon, hydrogen, and oxygen, but lipids lack the 2:1 ratio between hydrogen and oxygen atoms. Even though lipids are a very diverse group of compounds, they share one common characteristic: they are nonpolar molecules so, unlike water, do not have a positive and a negative end (pole).
Therefore, most lipids are insoluble in water but dissolve readily in nonpolar solvents, such as ether and chloroform. Lipids provide the structure of membranes and some cell walls and function in energy storage.
Simple lipids, called fats or triglycerides, contain an alcohol called glycerol and a group of compounds known as fatty acids. Glycerol molecules have three carbon atoms to which are attached three hydroxyl (¬OH) groups. Fatty acids consist of long hydrocarbon chains (composed only of carbon and hydrogen atoms) ending in a carboxyl (¬COOH, organic acid) group. Most common fatty acids contain an even number of carbon atoms.
A molecule of fat is formed when a molecule of glycerol combines with one to three fatty acid molecules. The number of fatty acid molecules determines whether the fat molecule is a monoglyceride, diglyceride, or triglyceride.
In the reaction, one to three molecules of water are formed (dehydration), depending on the number of fatty acid molecules reacting. The chemical bond formed where the water molecule is removed is called an ester linkage. In the reverse reaction, hydrolysis, a fat molecule is broken down into its component fatty acid and glycerol molecules.
Because the fatty acids that form lipids have different structures, there is a wide variety of lipids. For example, three molecules of fatty acid A might combine with a glycerol molecule. Or one molecule each of fatty acids A, B, and C might unite with a glycerol molecule.
The primary function of lipids is to form plasma membranes that enclose cells. A plasma membrane supports the cell and allows nutrients and wastes to pass in and out; therefore, the lipids must maintain the same viscosity, regardless of the surrounding temperature. The membrane must be about as viscous as olive oil, without getting too fluid when warmed or too thick when cooled.
As everyone who has ever cooked a meal knows, animal fats (such as butter) are usually solid at room temperature, whereas vegetable oils are usually liquid at room temperature. The difference in their respective melting points is due to the degrees of saturation of the fatty acid chains. A fatty acid is said to be saturated when it has no double bonds, in which case the carbon skeleton contains the maximum number of hydrogen atoms.
Saturated chains become solid more easily because they are relatively straight and are thus able to pack together more closely than unsaturated chains. The double bonds of unsaturated chains create kinks in the chain, which keep the chains apart from one another.
Complex lipids contain such elements as phosphorus, nitrogen, and sulfur, in addition to the carbon, hydrogen, and oxygen found in simple lipids.
The complex lipids called phospholipids are made up of glycerol, two fatty acids, and, in place of third fatty acid, a phosphate group bonded to one of several organic groups. Phospholipids are the lipids that build membranes; they are essential to a cell’s survival. Phospholipids have polar as well as nonpolar regions.
When placed in water, phospholipid molecules twist themselves in such a way that all polar (hydrophilic) portions orient themselves toward the polar water molecules, with which they then form hydrogen bonds. This forms the basic structure of a plasma membrane.
Polar portions consist of a phosphate group and glycerol. In contrast to the polar regions, all nonpolar (hydrophobic) parts of the phospholipid make contact only with the nonpolar portions of neighboring molecules. Nonpolar portions consist of fatty acids. This characteristic behavior makes phospholipids particularly suitable for their role as a major component of the membranes that enclose cells.
Phospholipids enable the membrane to act as a barrier that separates the contents of the cell from the water-based environment in which it lives. Some complex lipids are useful in identifying certain bacteria. For example, the cell wall of Mycobacterium tuberculosis, the bacterium that causes tuberculosis, is distinguished by its lipid-rich content.
The cell wall contains complex lipids such as waxes and glycolipids (lipids with carbohydrates attached) that give the bacterium distinctive staining characteristics. Cell walls rich in such complex lipids are characteristic of all members of the genus Mycobacterium.
Steroids are structurally very different from lipids. The figure shows the structure of the steroid cholesterol, with the four interconnected carbon rings that are characteristic of steroids. When an ¬OH group is attached to one of the rings, the steroid is called a sterol (alcohol). Sterols are important constituents of the plasma membranes of animal cells and of one group of bacteria (mycoplasmas), and they are also found in fungi and plants. The sterols separate the fatty acid chains and thus prevent the packing that would harden the plasma membrane at low temperatures.