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rav65819_ch05_085-104 - *84 Chapter 5 Membranes...

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***84 Chapter 5 Membranes introduction AMONG A CELL’S MOST IMPORTANT ACTIVITIES are its interactions with the environment, a give-and-take that never ceases. Without it, life could not persist. Living cells are encased within a lipid membrane through which few water-soluble substances can pass; but at the same time, the membrane contains protein passageways that permit specific substances to move into and out of the cell and allow the cell to exchange information with its environment. Eukaryotic cells also contain internal membranes like those of the mitochondrion and endoplasmic reticulum pictured here. We call the delicate skin of lipids with embedded protein molecules that encase the cell a plasma membrane. This chapter examines the structure and function of this remarkable membrane. concept outline 5.1 The Structure of Membranes The fluid mosaic model shows proteins embedded in a fluid lipid bilayer Cellular membranes consist of four component groups Electron microscopy has provided structural evidence 5.2 Phospholipids: The Membrane’s Foundation Phospholipids spontaneously form bilayers The phospholipid bilayer is fluid Membrane fluidity can change 5.3 Proteins: Multifunctional Components Proteins and protein complexes perform key functions Structural features of membrane proteins 5.5 Active Transport Across Membranes Active transport uses energy to move materials against a concentration gradient The sodium–potassium pump runs directly on ATP Coupled transport uses ATP indirectly 5.6 Bulk Transport by Endocytosis and Exocytosis
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Bulk material enters the cell in vesicles Material can leave the cell by exocytosis 5.4 Passive Transport Across Membranes Transport can occur by simple diffusion Proteins allow membrane diffusion to be selective Osmosis is the movement of water across membranes 85 5.1 The Structure of Membranes The membranes that encase all living cells are sheets of lipid only two molecules thick; more than 10,000 of these sheets piled on one another would just equal the thickness of this sheet of paper. Biologists established the components of membranes— not only lipids, but also proteins and other molecules— through biochemical assays, but the nature of the membrane structure remained elusive. We begin by considering the theories that have been advanced about membrane structure. We then look at the individual components of membranes more closely. The fluid mosaic model shows proteins embedded in a fluid lipid bilayer The lipid layer that forms the foundation of a cell’s membranes is a bilayer formed of phospholipids (figure 5.1). For many years, biologists thought that the protein components of the cell membrane covered the inner and outer surfaces of the phospholipid bilayer like a coat of paint. An early model por- trayed the membrane as a sandwich; a phospholipid bilayer between two layers of globular protein. This
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rav65819_ch05_085-104 - *84 Chapter 5 Membranes...

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