Lipids are organic substances consisting mostly of carbons and hydrogen atoms. They are hydrophobic, which means that they have little or no affinity to water. All lipids are soluble (or dissolvable) in nonpolar solvents, such as ether, alcohol, and gasoline. There are three families of lipids: (1) fats, (2) phospholipids, and (3) steroids.
Fatty acids and glycerol make up the larger molecule of fats. A fatty acid consists of a long carbon skeleton of 16 or 18 carbon atoms, though some are even longer. The carbonyl group, which is a carbon atom double-bonded to an oxygen atom and single-bonded to an oxygen attached to a hydrogen (OH-C=O), is the acidic group of the fatty acids. The acidic property is determined by the ability of the hydrogen to dissociate, or break away, from the oxygen atom. The carbonyl group is followed by a long chain of carbon atoms bonded to hydrogen, which is referred to as the hydrocarbon "tail." The long hydrocarbon tail gives fatty acids their hydrophobic, or "water-fearing" property. Fats cannot be dissolved in water because fats are nonpolar (an equal distribution of electrons) and water is polar (an unequal distribution of electrons). The polarity of water is unable to form bonds and break down the nonpolar fatty acid molecule.
There are different types of fatty acids, which vary in length and the number of bonds. Saturated fatty acids have single bonds between the carbon
atoms that make up the tail. The carbon atoms are "full" or saturated, and therefore cannot take up any more hydrogen. Most animal fat, such as butter, milk, cheese, and coconut oil, are saturated. Unsaturated fatty acids have one or more double bonds between carbon atoms. A double bond is the sharing of four electrons between atoms, while a single bond is the sharing of two electrons. The double bond has the ability to lend its extra two electrons to another atom, thereby forming another bond. Monounsaturated fatty acids contain only one double bond, such that each of the carbon atoms of the double bond can bond with a hydrogen atom. An example of monounsaturated fatty acids is oleic acid, which is found in olive oil. Polyunsaturated fatty acids contain two or more double bonds, such that four or more carbon atoms can bond with hydrogen atoms. Most vegetable fats are polyunsaturated fatty acids. The double bonds change the structure of the fatty acid, in that there is a slight bend where the double bond is located.
Foods high in saturated fatty acids include whole milk, cream, cheese, egg yolk, fatty meats (e.g., beef, lamb, pork, ham), coconut oil, regular margarine, and chocolate. Foods high in polyunsaturated fatty acids include vegetable oils (e.g., safflower, corn, cottonseed, soybean, sesame, sunflower), salad dressing made from vegetable oils, and fish such as salmon, tuna, and herring.
Triglycerides are the basic unit of fat and are composed of three ("tri-") fatty acids individually bonded to each of the three carbons of glycerol. Fatty acids rarely exist in a free form in nature because they are highly reactive, and therefore make bonds spontaneously.
Jeffrey Radecki, Susan Kim, The Gale Group Inc., Macmillan Reference USA, New York,