209+Chapter+3 - Chapter 3 Outline I INTRODUCTION A A cell...

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Chapter 3 Outline I. INTRODUCTION A. A cell is the basic, living, structural, and functional unit of the body. B. Cytology is the study of cell structure, and cell physiology is the study of cell function. II. PARTS of a CELL A. A generalized view of the cell is a composite of many different cells in the body as seen in Figure 3.1 . No single cell includes all of the features seen in the generalized cell. B. The cell can be divided into three principal parts for ease of study. 1. Plasma (cell) membrane 2. Cytoplasm a. Cytosol b. Organelles (except for the nucleus) 3. Nucleus III. THE PLASMA MEMBRANE A. The plasma membrane is a flexible, sturdy barrier that surrounds and contains the cytoplasm of the cell. 1. The fluid mosaic model describes its structure (Figure 3.2). 2. The membrane consists of proteins in a sea of lipids. B. The Lipid Bilayer
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1. The lipid bilayer is the basic framework of the plasma membrane and is made up of three types of lipid molecules: phospholipids, cholesterol, and glycolipids (Figure 3.2). 2. The bilayer arrangement occurs because the lipids are amphipathic moleeules. They have both polar (charged) and nonpolar (uncharged) parts with the polar “head” of the phospholipid pointing out and the nonpolar “tail” pointing toward the center of the membrane. C. Arrangement of Membrane Proteins 1. The membrane proteins are divided into integral and peripheral proteins (Figure 3.2) a. Integral proteins extend into or across the entire lipid bilayer among the fatty acid tails of the phospholipid molecules. b. Peripheral proteins are found at the inner or outer surface of the membrane and can be stripped away from the membrane without disturbing membrane integrity. 2. Integral membrane proteins are amphipathic. a. Those that stretch across the entire bilayer and project on both sides of the membrane are termed transmembrane proteins. b. Many integral proteins are glycoproteins. 3. The combined glycoproteins and glycolipids form the glycocalyx which helps cells recognize one another, adhere to one another, and be protected from digestion by enzymes in the extracellular fluid. D. Functions of Membrane Proteins
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1. Membrane proteins vary in different cells and functions as channels, , transporters (carriers), receptors, enzymes, cell-identity markers, and linkers (Figure 3.3 ). 2. The different proteins help to determine many of the functions of the plasma membrane. E. Membrane Fluidity 1. Membranes are fluid structures, rather like cooking oil, because most of the membrane lipids and many of the membrane proteins easily move in the bilayer. 2. Membrane lipids and proteins are mobile in their own half of the bilayer. 3. Cholesterol serves to stabilize the membrane and reduce membrane fluidity. F. Membrane Permeability 1. Plasma membranes are selectively permeable, meaning that some things can pass through and others cannot.
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