Biochemistry, 4th Edition P29. Continuing Garrett and Grisham's innovative conceptual and organizing framework, "Essential Questions," BIOCHEMISTRY guides students through course concepts in a way that reveals the beauty and usefulness of biochemistry in the everyday world. Streamlined for increased clarity and readability, this edition also includes new photos and illustrations that show the subject matter consistently throughout the text. New end-of-chapter problems, MCAT practice questions, and the unparalleled text/media integration with the power of CengageNOW round out this exceptional package, giving you the tools you need to both master course concepts and develop critical problem-solving skills you can draw upon. | What Are the Chemical and Physical Properties of Membranes 243 FIGURE Electron micrographs of several different membrane structures a Plasma membrane of Menoid-ium a protozoan. b Two plasma membranes from adjacent neurons in the central nervous system. c Golgi apparatus. d Many membrane structures are evident in pancreatic acinar cells. D. W. Fawcett Photo Researchers Inc. or in the membrane and the transport of ions sugars and amino acids across membranes and organizes and directs hundreds of cell signaling events. The Composition of Membranes Suits Their Functions Biological membranes may contain as much as 75 to 80 protein and only 20 to 25 lipid or as little as 15 to 20 protein. Membranes that carry out many enzyme-catalyzed reactions and transport activities the inner mitochondrial membrane chloroplast membranes and the plasma membrane of Escherichia coll for example are typically richer in protein whereas membranes that carry out fewer protein-related functions myelin sheaths the protective coating around neurons for example are richer in lipid. Cellular mechanisms adjust lipid composition to functional needs. Thus for example the lipid makeup of red blood cell membranes is consistent across species whereas the lipid complement of different specialized membranes within a particular cell type rat liver Figure reflects differences of function. Plasma membranes are enriched in cholesterol but do not contain diphosphatidylglycerol I I Phosphatidylcholine I I Phosphatidylethanolamine I I Sphingolipids L2I Phosphatidylinositol I I Phosphatidylserine I I Cardiolipin I I Minor lipids I I Cholesterol FIGURE The lipid composition of rat liver cell membranes in weight percent. Adapted from Andreoli T. E. 1987. Membrane Physiology 2nd ed. Chapter 27 Table II and Daum G. 1985. Lipids of mitochondria. Biochimica et BiophysicaActa 822 1-42. 244 Chapter 9 Membranes and Membrane Transport cardiolipin whereas mitochondria contain considerable amounts of .
