Chapter 26: Lipids

Students learn how the saturation state of fatty acid chains—whether saturated or unsaturated—affects their chemical behavior and physical properties, with particular emphasis on how cis and trans geometric isomers influence biological activity and membrane organization. The chapter explores the chemical mechanisms underlying lipid metabolism, particularly the hydrolysis of ester bonds in triacylglycerols and the industrial process of saponification, which converts fats into fatty acid salts. Phospholipids are presented as amphipathic molecules critical for forming the lipid bilayer of biological membranes, with their polar head groups and nonpolar tails creating the structural foundation of cell membranes. The chapter discusses how cholesterol molecules integrate into membranes and modulate membrane fluidity by interacting with phospholipid chains. Steroid chemistry is introduced through the characteristic four-ring carbon skeleton and biosynthetic pathways that generate diverse steroid hormones and regulatory molecules. Throughout, the chapter reinforces how functional group reactivity—particularly ester formation and hydrolysis—connects organic chemistry principles to biological mechanisms, demonstrating that understanding lipid structure and reactivity is fundamental to comprehending energy metabolism, cell biology, and pharmacology.