ORGANIZATION OF CELLULAR COMPARTMENTS
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Cytoskeleton Eukaryotic cells have a variety of internal compartments, or organelles, that are membrane bound and carry out distinct cellular functions. The cell’s organelles are not free to float around haphazardly in the cytoplasmic “soup”; rather, they are elaborately organized by a protein network called the cytoskeleton (Fig. 3.10). The cytoskeleton maintains the cell’s shape, allows cell movement, and directs the trafficking of substances within the cell. Three principal types of protein filaments make up the cytoskeleton: actin filaments, microtubules, and intermediate filaments.
All three types of filaments consist of small proteins that can assemble (polymerize) into filaments of varying length. The filament structures are dynamic and can be rapidly disassembled and reassembled according to the changing needs of the cell. Actin filaments play a pivotal role in cell movement. As one might expect, muscle cells are packed with actin filaments, which allow the cell to perform its primary function of contraction. However, nonmuscle cells also possess actin filaments that are important for complex movements of the cell membrane, such as cell crawling and phagocytosis. Such movements of the cell membrane are mediated by dense networks of actin filaments that cluster just beneath the plasma membrane and interact with specific proteins embedded in it. Actin and some of the other cytoskeletal proteins make specific contacts with and through the plasma membrane and are involved in information transfer from the extracellular environment to signaling cascades within the cell.
Organization of the cytoplasm and its organelles is achieved primarily by microtubules. In animal cells, microtubules originate at the cell center, or centrosome, near the nucleus and radiate out toward the cell perimeter in fine lacelike threads. Microtubules guide the orderly transport of organelles and vesicles in the cytoplasm, as well as the equal distribution of chromosomes during cell division. Intermediate filaments, so named because their size is between that of microtubules and actin filaments, are strong, ropelike, fibrous proteins. A variety of intermediate filaments that differ from tissue to tissue have been identified. In addition to the
Extracellular fluid
Intracellular fluid