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Lipids

Lipids are large, nonpolar organic molecules. They do not dissolve in water. Lipids include

• triglycerides,
• phospholipids,
• steroids,
• waxes

Fats are lipids that store energy. These compounds are an economical form of reserve fuel storage because they yield more than twice as much energy per gram as do carbohydrates.

Triglycerides (Neutral Fats)

The most abundant lipids in living organisms are the neutral fats.A neutral fat consists of glycerol joined to one, two, or three fatty acids. Glycerol is a three-carbon alcohol that contains three hydroxyl groups. A fatty acid is a long, unbranched hydrocarbon chain with a carboxyl group at one end. About 30 different fatty acids are commonly found in lipids, and they typically have an even number of carbon atoms.

Saturated fatty acids contain the maximum possible number of hydrogen atoms. Fats high in saturated fatty acids, such as animal fat tend to be solid at room temperature.Common dietary saturated triglycerides include butter and fats in red meat.

Unsaturated fatty acids include one or more adjacent pairs of carbon atoms joined by a double bond. Therefore they are not fully saturated with hydrogen. They are usually soft or liquid at room temperature. Unsaturated triglycerides are found primarily in plant seeds where they serve as an energy and carbon source for germinating plants.

Triglycerides are formed by condensation reactions. A reaction between a hydroxyl group and a carboxyl group results in the formation of a covalent linkage known as an ester linkage.

Phospholipids

A phospholipid consists of a glycerol molecule attached at one end to two fatty acids, and at the other end to a phosphate group. The two ends of a phospholipid differ both physically and chemically.The fatty acid portion of the molecule is hydrophobic (water disliking) and not soluble in water.However, the portion composed of glycerol, phosphate, (the “head” of the molecule) is hydrophilic and water-soluble.

The cell membrane is made of two layers of phospholipids, called the lipid bilayer. The inability of lipids to dissolve in water allows the membrane to form a barrier between the inside and outside of the cell.

Waxes

A wax is a type of structural lipid consisting of a long fatty-acid chain joined to a long alcohol chain. Waxes are waterproof, and in plants, form a protective coating on the outer surfaces. Waxes also form protective layers in animals. For example, earwax helps prevent microorganisms from entering the ear canal.

Steroids

Unlike most other lipids, which are composed of fatty acids, steroid molecules are composed of four fused carbon rings with

various functional groups attached to them. Many animal hormones, such as the male hormone testosterone, are steroid compounds. One of the most familiar steroids in humans is cholesterol. Cholesterol is needed by the body for nerve and other cells to function normally. It is also a component of the cell membrane.