Chapter 12: Lipids & Cell Membranes

Lipids & Cell Membranes from Biochemistry (Eighth Edition) presents an in-depth examination of lipids and cell membranes, establishing them as dynamic, sheetlike boundaries that define cellular compartments and regulate the flow of matter and information. The discussion begins with the structural properties of fatty acids, explaining how hydrocarbon chain length and the degree of unsaturation affect melting points and the fluidity of the membrane environment. It categorizes the three major classes of membrane lipids—phospholipids (including phosphoglycerides and sphingomyelin), glycolipids, and cholesterol—and contrasts these with the unique ether-linked, branched lipids found in archaea that allow for survival in extreme conditions. A central concept is the amphipathic nature of membrane lipids, which drives the spontaneous formation of lipid bilayers and liposomes in aqueous media through hydrophobic interactions, creating robust barriers impermeable to ions and polar molecules. The text further explores the diverse roles of membrane proteins, distinguishing between integral proteins that span the bilayer and peripheral proteins that associate with the surface, while detailing structural motifs such as the alpha helices in bacteriorhodopsin and the beta strands in bacterial porins. Hydropathy plots are introduced as a computational tool for predicting transmembrane helices based on amino acid energetic considerations. The chapter elucidates the fluid mosaic model, highlighting the rapid lateral diffusion of membrane components versus the slow transverse diffusion (flip-flop), a phenomenon experimentally verified by fluorescence recovery after photobleaching (FRAP). It also covers the regulation of membrane fluidity through cholesterol and fatty acid composition, the formation of lipid rafts, and the inherent asymmetry of biological membranes, illustrated by the Na+-K+ pump. Finally, the summary encompasses the complex internal membrane systems of eukaryotic cells, including the nuclear envelope and mitochondria, and details the molecular mechanisms of receptor-mediated endocytosis and membrane fusion involving transferrin, clathrin-coated pits, and SNARE proteins.