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Many people are lactose intolerant. These people cannot digest milk products because their bodies do not produce enough of the enzyme lactase to digest lactose, the sugar found in milk. You will learn what enzymes are and what function they serve in the body. Amino Acids ◦ What is the general structure of an amino acid? An amino acid is any compound that contains an amino group (—NH2) and a carboxyl group (—COOH) in the same molecule. ◦ Amino acids have a skeleton that consists of a carboxyl group and an amino group, both of which are covalently bonded to a central carbon atom. The remaining two groups on the central carbon atom are hydrogen and an R group that constitutes the amino acid side chain. Peptides ◦ Which functional groups are always involved in amide bonds between amino acids? A peptide is any combination of amino acids in which the amino group of one amino acid is united with the carboxyl group of another amino acid. The amide bond between the carboxyl group of one amino acid and the nitrogen in the amino group of the next amino acid in the peptide chain is called a peptide bond. ◦ The amide bonds between amino acids always involve the central amino and central carboxyl groups. The side chains are not involved in the bonding. Before the invention of electric lighting, wax candles were a major source of lighting in homes. In this section, you will read more about waxes and similar compounds that make up the class of biomolecules known as lipids. Triglycerides ◦ What physical property distinguishes lipids from other classes of biological molecules? Fats, oils, and other water-insoluble compounds are called lipids. Natural fats and oils exist as triesters of glycerol with fatty acids, which are long-chain carboxylic acids (C12 through C24). This form of lipid is known as a triglyceride. ◦ Lipids tend to dissolve readily in organic solvents, such as ether and chloroform, rather than in highly polar solvents such as water. This property sets them apart from most biological substances such as carbohydrates and proteins. The hydrolysis of oils or fats by boiling with an aqueous solution of an alkali-metal hydroxide is called saponification. A typical saponification reaction is shown below. Saponification is used to make soap.