Chapter 25: Amino Acids, Peptides, and Proteins

The chapter then explores lipid structure and function, detailing how fatty acids serve as components of more complex lipids including triacylglycerols, which store energy through ester bonds, and phospholipids, which form cellular membranes. Steroid molecules are presented as lipids with distinct cyclopentanoperhydrophenanthrene ring systems that give them unique biochemical properties despite their lipid classification. The protein section provides comprehensive coverage of amino acid composition, emphasizing how the twenty standard amino acids are linked through peptide bonds to form polypeptide chains. The chapter develops the hierarchical levels of protein organization, from primary structure determined by amino acid sequence through secondary structures like alpha helices and beta sheets, tertiary structure shaped by interactions between amino acid side chains, and quaternary structure arising from associations between multiple protein subunits. Throughout the chapter, organic reaction mechanisms including acyl transfer, hydrolysis reactions, and redox transformations are applied to explain how these macromolecules undergo biological modifications and transformations. The unifying theme connects fundamental principles of organic chemistry—molecular structure, functional group reactivity, and mechanistic reasoning—to the biological context where these molecules perform essential life functions. This integration reinforces that biochemical phenomena emerge directly from underlying organic chemical principles, making this chapter essential for understanding how molecular structure enables biological activity.