10 - Chapter 10. THE ARCHITECTURE OF CELLS plasmalemma....

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Chapter 10. THE ARCHITECTURE OF CELLS Cells are functional masses of protoplasm , or living matter, that contain or have previously contained a nucleus and are limited peripherally by a membrane. The surface membrane ( plasmalemma ) protects the interior from the immediate external environment of the cell, but it does much more. The plasmalemma is a site of anchorage for the internal scaffolding of the cell, called the cytoskeleton, and related specializations such as the contractile apparatus of muscle cells. The interior of the cell receives communication from other cells and such signals are often first received, recognized and interpreted at the plasmalemma. There is enormous transport of materials across this membrane and, in some cases, this gives rise to specialized electrochemical events that triggers depolarization or regulates critical internal activities of the cell. On the external face of the membrane, specialized molecules provide the basis for cell-cell and cell-matrix attachment along with "signature molecules" that convey information about the identity of the cell. Because the plasmalemma plays such an important role in the everyday business of the cell, Chapter 11 is devoted to the nature and properties of membranes. A. The cytoskeleton is a dynamic scaffolding. An internal latticework of protein fibers, or filaments, plays an important role in organizing and supporting the internal organelles of cells. Generically called the cytoskeleton, these filaments are classified into several groups: microfilaments, intermediate filaments, and microtubules. Combinations of these components can be assembled in various ways to provide additional specialized structures. For example, much of the contractile apparatus that is so evident in skeletal muscle fibers is highly organized assemblies of actin (a microfilament) and myosin (an intermediate filament). The spindle apparatus, that determines the movement of sister chromatids during mitosis (mentioned earlier in Chapter 9), is made up of polymers of tubulin , the dominant component of microtubules. 1. Networks of actin (microfilaments) determine the shape of cells Actin filaments form a mesh-like network that lies in close proximity to the plasmalemma and is traversed perpendicularly by additional filaments that may distend the plasmalemma into spike- shaped or larger fold-like extrusions (Figure 10-1). The surface topography changes when actin filaments contract so that the surface may become featureless. It will be seen later (Chapter 20) that some stretch- sensitive membrane channels are activated by just such a mechanism. More extensive contraction of the radial filaments gives rise to invaginations of the plasmalemma and this is most profound when the membrane is drawn inward to achieve 2007 version – page 70
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Figure 10-1. Actin networks in cells. Actin filaments are important in maintaining the internal and superficial structure of cells. In A.
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This note was uploaded on 03/23/2009 for the course ANSCI 1110 taught by Professor Brucecurrie during the Fall '08 term at Cornell University (Engineering School).

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10 - Chapter 10. THE ARCHITECTURE OF CELLS plasmalemma....

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