Chapter 3 Cells: The Living Units I. Overview of the Cellular Basis of Life (pp. 65–66) A. The four concepts of the cell theory state (p. 65): 1. Cells are the basic structural and functional units of life. 2. The activity of an organism depends on the activities of its cells. 3. The biochemical activities of a cell are dictated by their organelles. 4. The continuity of life has a cellular basis. B. Characteristics of Cells (pp. 65–66; Figs. 3.1–3.2) 1. Cells vary greatly in their size, shape, and function. 2. All cells are composed primarily of carbon, hydrogen, nitrogen, and oxygen. 3. All cells have the same basic parts and some common functions. 4. A generalized human cell contains the plasma membrane, the cytoplasm, and the nucleus. II. The Plasma Membrane: Structure (pp. 67–69) A. The Fluid Mosaic Model (pp. 67–68; Figs. 3.3–3.4) 1. The plasma membrane is composed of a double layer of phospholipids embedded with small amounts of cholesterol and proteins. 2. The phospolipid bilayer is composed of two layers of phospholipids lying tail to tail, with their polar heads exposed to water inside and outside the cell. 3. The inward-facing and outward-facing surfaces of the plasma membrane differ in the kinds and amounts of lipids they contain. a. Glycolipids are found only in the outer membrane. b. Lipid rafts are also found only in the outer membrane, and are assumed to function in cell signaling. 4. Integral proteins are firmly inserted into the plasma membrane. a. Most integral proteins are transmembrane proteins that span the entire width of the membrane and are involved with transport as channels or carriers. 5. Peripheral proteins are not embedded in the plasma membrane, but attach to integral proteins or to phospolipids. a. Peripheral proteins may function as enzymes or in mechanical functions of the cell. 6. The glycocalyx is the fuzzy, sticky, carbohydrate-rich area surrounding the cell. B. Specializations of the Plasma Membrane (p. 69; Fig. 3.5) 1. Microvilli are fingerlike extensions of the plasma membrane that increase the surface area of the cell. 2. Most body cells are bound together using glycolipids, specialized interlocking regions, or specialized membrane junctions. a. Tight junctions are a type of membrane junction in which integral proteins on adjacent cells fuse together to form an impermeable junction in order to prevent molecules from passing through the extracellular space between cells. b. Desmosomes are mechanical couplings that are scattered along the sides of adjoining cells that prevent their separation and reduce the chance of tearing when a tissue is stressed.
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