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Lipids are macromolecules grouped together for different reasons than are carbohydrates. They do not share any particularly constant chemical structure, but they do share an important physical property brought about by their basic chemical composition: lipids are biological molecules that do not dissolve appreciably in water. This is because they contain nonpolar covalent bonds, and are largely composed of hydrocarbon chains or rings. Since the body is a very watery place, lipids face a challenge, as they are not able to dissolve, and must be handled similarly with regard to their transportation and usage. Several types of lipids exist.
Triglycerides: Triglycerides are composed of one molecule of the trialcohol glycerol covalently attached to three fatty acid molecules, hydrocarbon chains of varying lengths bonded through a terminal carboxylic acid group. Traditionally called fats and oils, the major role of triglycerides in the body is long-term energy storage. Fats tend to be solid at room temperature because the fatty acid chains are saturated, which means they do not contain carbon-carbon double bonds. Chains containing double bonds are called unsaturated. The more unsaturated a fatty acid chain, the more liquid the triglyceride. Thus oils are often polyunsaturated triglycerides.


A saturated triglyceride

Phospholipids: Phospholipids are a class of related compounds that structurally resemble triglycerides. In place of one of the fatty acids bonded to glycerol, however, is a hydrophilic molecule containing a phosphate group. This gives phospholipids a chemical “split personality”. A portion of the molecule is hydrophobic, and unable to dissolve in water, while another portion is strongly charged and hydrophilic. This interesting combination of properties allows phospholipids to form the structures of plasma membranes and lipoproteins. They also act as emulsifying agents, allowing other lipids to dissolve more easily in the body.
Steroids: Steroids are lipids that do not structurally resemble triglycerides, but are composed of a series of nonpolar ringsCholesterol is the most well known and prevalent steroid compound in the body. Cholesterol plays a role in the structure of cell membranes, as well as serving as the starting compound from which many others are synthesized. Other important steroids include the sex hormones and vitamin D.


Cholesterol, a steroid lipid

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