8 - B STRUCTURE AND FUNCTION AT THE CELLULAR LEVEL Cells...

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B STRUCTURE AND FUNCTION AT THE CELLULAR LEVEL 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, or plasmalemma , protects the interior from the immediate external environment of the cell and provides a measure of structural support, because many internal parts of the cell are anchored to this membrane by the cytoskeleton . The plasmalemma plays such an important role in the everyday business of the cell that some effort should be devoted to understanding its structure and composition. Chapters 11 and 12 detail the nature and some properties of membranes. Typically, the surface membrane is a mere 70 to 100 angstroms (A) thick (0.000007 to 0.00001 mm) and has very little tensile strength. Perhaps the usual small size of cells (10 to 100 μ, or 0.01 to 0.1 mm in diameter) is necessitated by the fragile nature of this limiting membrane. Within the cell and suspended in the cytoplasm , or the amorphous fluid portion, is a wide array of subcellular structures called organelles , the most obvious of which is the nucleus. The nature and functions of these organelles and the cytoskeleton upon which the organelles are organized are described in subsequent chapters of Part B. . The mechanisms of nuclear and cell division are described in detail later, in Chapter 14, and the role of the genetic machinery in directing many synthetic activities of the cell are deferred until Chapter 15. In functional and operational terms, cells represent collections of discrete "compartments" separated one from another by membranes. Usually the cells in a tissue 2007 version – page 52
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are not really independent but rather act and interact with some harmony. For this to occur, communication between the cells, or the transfer of information, is important in the same way that transfer of materials such as fuels and wastes is important. Much of Parts E and F of this text is
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8 - B STRUCTURE AND FUNCTION AT THE CELLULAR LEVEL Cells...

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