Lipids are diverse group of unrelated molecules which includes fats, oils, steroids and sterols, waxes, and other water-insoluble molecules. Lipids are characterized by their hydrophobic, or "water-fearing," chemical behavior, which is what makes them insoluble in water. Unlike other molecules that ionize and are dissolved by water, lipid molecules are nonpolar. They are repelled by the polar nature of water and tend to aggregate in aqueous solutions. Lipids also are used to store energy and are especially abundant in seeds because lipids contain more energy by weight than carbohydrates.
Examples of lipids commonly found in biological systems include fats and oils that are storage molecules known as triglycerides. A triglyceride consists of glycerol (a three-carbon molecule) and three fatty acid molecules, long-chain hydrocarbon molecules that are attached to each of the three glycerol carbon atoms by ester linkages.
The long chain of carbon atoms of the fatty acid can be saturated or unsaturated with respect to hydrogen content. Saturated fatty acids contain as many hydrogen atoms as allowed bonded to each carbon atom. Saturated fats tend to be solid at room temperature and include substances such as butter and lard. Unsaturated fatty acids do not have the maximum number of hydrogen atoms because some of the carbon atoms form double bonds with adjacent carbon atoms in the chain. Unsaturated fats tend to be liquid at room temperature and include substances such as corn oil and olive oil.
Plants have many lipids that are unique to them. For instance, cutin and suberin are two lipid polymers that form structural components of many plant cell walls. These two molecules form a mesh-work that secures another type of lipid polymer found in plants, wax. Waxes are long-chain lipid compounds that are integrated into the cutin and suberin meshwork and are important in preventing water loss for plants. Waxes give apple peels their characteristic shiny appearance.
Phospholipids are a type of lipid molecule that is found in all living organisms. They are structurally similar to triglycerides, except instead of having three fatty acids attached to glycerol, they have only two. Replacing the third fatty acid is a charged phosphate group. This unique structure results in one end of the molecule being hydrophilic (the phosphate end, often called the head) and the other being hydrophobic (the end with the two fatty acids, often called the tail). Consequently, phospho lipids will spontaneously form an oily layer at the water surface, orienting their charged phosphate heads toward the water and their fatty acid tails away from the water and toward the air. This is the basis for the phospholipid bilayer structure that underlies the formation of all cellular membranes. In the case of a lipid bilayer, because there is water on both sides, the two layers are tail to tail, with their heads oriented to the inside and outside of the membrane, where they come into contact with water.
See also: Organic Macromolecules, Carbohydrates, Nucleic Acids, Proteins
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