Lipids

Any biomolecule that dissolves in nonpolar solvents such as chloroform (CHCl₃), benzene (C₆H₆), or diethyl ether (CH₃CH₂OCH₂CH₃) is classified as a lipid (from the Greek lipos, "fat"). Because they are soluble in nonpolar solvents, lipids are often insoluble or only marginally soluble in water, and they often feel oily or greasy to the touch.

Fatty Acids and Carboxylic Acids

Long-chain carboxylic acids such as stearic acid [CH₃(CH₂)₁₆CO₂H] are called fatty acids because they can be isolated from animal fats. These fatty acids are subdivided into two categories on the basis of whether they contain C=C double bonds: saturated fatty acids and unsaturated fatty acids.

The common names of carboxylic acids trace back to Latin or Greek stems, which indicate a natural source of the acid. The destructive distillation of ants, for example, produces formic acid (from the Latin, formica, ant). Vinegar is a 5 to 6% solution of acetic acid in water. Acetic acid therefore takes its common name from the Latin term for vinegar: acetum. The next acid, as we build up the hydrocarbon chain, is propionic acid, which takes its name from the Greek stems protos and pion. The name literally means "first fat," because this is the simplest carboxylic acid that can be isolated from fat. The next member of this family is butyric acid, from the Latin butyrum, or butter, because it can be obtained from rancid butter. The fifth carboxylic acid is known as valeric acid, because it can be obtained from plants in the genus Valerianella, a family of perennial herbs.

From this point on, common carboxylic acids tend to have an even number of carbon atoms. The next three derivatives are all given names from the Latin term for goat, caper. The carboxylic acids with 12, 14, 16 and 18 carbon atoms are named from the Latin stem for the bay tree, laurel; the genus for the spice nutmeg, Myristica; the Latin stem for the palm tree, palma; and the Greek stem for the tallow used to make candles, stear.

The very small carboxylic acids have a sharp odor. (Formic acid has an odor that is even sharper than acetic acid.) By the time the hydrocarbon chain has grown to a total of four carbon atoms, the odor of these compounds has taken a significant turn for the worse. (Butyric acid is the source of the characteristic odor of rancid butter or spoiled meat.) As the length of the hydrocarbon chain increases further, the odor of the acid changes once again this time, becoming more pleasant.

There are four important unsaturated fatty acids. One of them is a derivative of palmitic acid, and is known as palmitoleic acid.

The other three are derivatives of stearic acid. The first has a single C=C double bond in the center of the fatty acid chain, and is known as oleic acid.

The second, which is known as linoleic acid, has another C=C double bond in the nonpolar half of the fatty acid chain.

The third linolenic acid has one more C=C double bond in the same half of the fatty acid chain.

There are several regularities in the chemistry of these unsaturated fatty acids. First, they contain cis double bonds. Second, the double bonds are always isolated from each other by a CH₂ group.

So much attention is paid to the structures of the fatty acids in discussions of the chemistry of lipids that it is easy to miss an important point: Free fatty acids are seldom found in nature. They are usually tied up with alcohols or amines to form esters (RCO₂R) or amides (RCONHR). The most abundant lipids are the triesters formed when a glycerol molecule reacts with three fatty acids, as shown in the figure below. These lipids have been known by a variety of names, including fat, neutral fat, glyceride, triglyceride, and triacylglycerol.

Most animal fats are complex mixtures of different triglycerides. As the percentage of unsaturated fatty acids in these fats increases, the melting point of the triglyceride decreases until it eventually becomes an oil at room temperature. Beef fat, which is one-third unsaturated fatty acids, is a solid. Olive oil, which is roughly 80% unsaturated, is a liquid.

The effect of unsaturated fatty acids on the melting point of a triglyceride can be understood by recognizing that the cis C=C double bond introduces a rigid 30º bend in the hydrocarbon chain, as shown in the figure below.

This bend or "kink" increases the average distance between triglyceride molecules, which decreases the van der Waals interactions between neighboring molecules. Thus, the introduction of unsaturated fatty acids into a triglyceride increases the fluidity of the lipid. The table belowcompares the relative abundance of the common fatty acids in a typical animal fat (butter) and a vegetable oil (olive oil).

Relative Abundance of Fatty Acids in a Typical Fat and a Typical Oil

Fats and oils are used by living cells for only one purpose to store energy. They are a far more efficient storage system than glycogen or starch because they give off between two and three times as much energy when they are burned. (The metabolism of glycogen releases 15.7 kilojoules per gram of carbohydrate consumed, whereas the metabolism of lipids gives approximately 40 kJ/g.) This explains why the seeds of many plants are relatively rich in oils, which provide the energy the seed needs to grow until the leaves can begin to produce energy by photosynthesis.

The average human contains enough fat (21% of the body weight for men, 26% for women) to provide the energy they need to survive for up to 3 months. But, there is only enough glycogen stored in the human body at any time to provide enough energy for one day. Thus, glycogen is only used for the short-term storage of food energy. In "times of plenty," the body stores energy in the form of fat to compensate for "times of shortage" to come.

Polar Lipids

Fats and oils are neutral compounds. When one of the fatty acids in a triglyceride is replaced by a phosphate group, a phospholipid is obtained that has two nonpolar hydrophobic tails and a charged hydrophilic head.

A variety of biochemically important molecules can be obtained by forming a second ester linkage to the phosphate group. These phosphate diesters are often called phosphatides and they contain an alcohol at the position labeled with an X in the following structure.

The most important phosphatides contain the following X groups.

Note that the phosphatidyl cholines and phosphatidyl ethanolamines are zwitterions because they simultaneously carry both positive and negative charges within a molecule that has no net charge. The phosphatidyl cholines are also known as lecithins, while the phosphatidyl ethanolamines are known as cephalins.

The phosphatidyl cholines and phosphatidyl ethanolamines are described as amphipathic, (literally, "both paths") because they contain a polar, hydrophilic head and a pair of nonpolar, hydrophobic tails. These compounds can therefore spontaneously associate to form a bilayer in which the molecules are oriented so that the nonpolar tails of adjacent phospholipids form a hydrophobic pocket and the polar heads point toward the water that surrounds both sides of the bilayer.

The best model for the structure of cell membranes involves a bilayer of amphipathic lipids approximately 8 nm wide into which various proteins are embedded. It is a common mistake to assume that this membrane is static. It is not. There is a considerable amount of mobility or flexibility in the hydrocarbon tails of the lipid molecules. However, the strong hydrophobic character of the region between the inner and outer surface of the membrane resists the passage of highly charged or polar intermediates across the membrane.