cytoplasm(redirected from Cytoplasmic fluid)
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term once used for the fundamental material of which all living things were thought to be composed. It was studied by a number of early scientists, especially by Félix Dujardin, J. E. Purkinje, M. J. S.
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That portion of living cells bordered externally by the plasma membrane (cell membrane) and internally by the nuclear envelope. In the terminology of classical cytology, the substance in living cells and in living organisms not compartmentalized into cells was called protoplasm. It was assumed at the time that the protoplasm of various cells was similar in structure and chemistry. Results of research on cell chemistry and ultrastructure after about 1960 showed that each cell type had a recognizably different “protoplasm.” Primarily for that reason, the term protoplasm gradually fell into disuse in contemporary biology. The terms cytoplasm and nucleoplasm have been retained and are used descriptively; they are used almost synonymously with the terms cytosome (body of cytoplasm) and nucleus, respectively.
Many cells, especially the single-celled organisms or protistans, have regional cytoplasmic differentiation. The outer region is the cortex or ectoplasm, and the inner region is the endoplasm. In many cases the cortical layer is a gel made up of a meshwork of cytoskeletal fibers.
Cytoplasm contains mostly water, from 80 to 97% in different cells, except for spores and other inactive forms of living material, in which water may be present in lesser amounts. The dry mass of cells consists mainly of macromolecules: proteins, carbohydrates, nucleic acids, and lipids associated with membranes. The small molecules present in cells are mainly metabolites or metabolic intermediates. The principal ions other than the hydrogen and hydroxyl ions of water are the cations of potassium, sodium, magnesium, and calcium, and the anions chloride and bicarbonate. Many other elements are present in cytoplasm in smaller amounts. Iron is found in cytochrome pigments in mitochondria; magnesium is present in chlorophyll in chloroplasts; copper, zinc, iodine, bromine, and several other elements are present in trace quantities.
Sedimentation of cells by centrifugation shows that organelles and inclusions can be separated from the ground cytoplasm, the fluid phase of the cytoplasm in which they are suspended. The ground cytoplasm in turn has been shown to consist of a cytoskeletal network and the cytosol, the fluid in which the cytoskeleton is bathed. The cytoskeleton consists of several biopolymers of wide distribution in cells. Microtubules have been observed in electron micrographs of a vast number of different cell types. They consist of the protein tubulin, and are frequently covered by a fuzzy layer of microtubule-associated proteins. See Cytoskeleton
In most cells the smaller particles exhibit Brownian motion due to thermal agitation. In some cells lacking extensive cytoskeletal structure, particles can be moved freely around the cell by Brownian motion. In others they are restricted by their surrounding cytoskeletal elements. Particles of various types may also undergo saltatory motions which carry them farther than Brownian motion possibly could. Such excursions result from the interaction of a particle with an element of the cytoskeleton such as one or more microtubules or microfilaments. See Cell (biology)
the nonnuclear part of the protoplasm of a cell surrounded by a membrane. The term “cytoplasm” was proposed by the German scientist E. Strasburger in 1882 in contrast to the protoplasm of the nucleus (nucleoplasm). Cytoplasm contains permanent components—organoids, such as mitochondria, the Golgi apparatus, the endoplasmic reticulum, ribosomes, and plastids—which are the universal structures that perform the main functions of the cell; various temporary inclusions— deposits of specific substances, such as lipids, carbohydrates, proteins, pigments, and secretory granules; and special formations— myofibrils and tonofibrils. All the inclusions are embedded in the hyaloplasm, a colloidal solution of many molecules that is the comparatively homogeneous part of the cytoplasm. (SeeCELL and PROTOPLASM.)