protoplasm

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protoplasm,

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. Schultze, and Hugo von Mohl (who is credited with introducing the name), all working in the 19th cent. Many of the notions associated with the term have survived. Thus it is still accepted that all living organisms are made largely of the same classes of substances such as salts and organic molecules, that some of these are organized into structures large enough to be seen in the microscope and that water almost always is by far the most abundant material. However, the term is rarely used any more in a strictly scientific sense, although it survives in more literary usages. The unity of living matter is now most often described in terms of the cellcell,
in biology, the unit of structure and function of which all plants and animals are composed. The cell is the smallest unit in the living organism that is capable of integrating the essential life processes. There are many unicellular organisms, e.g.
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 as the unit of all living organisms (virusesvirus,
parasite with a noncellular structure composed mainly of nucleic acid within a protein coat. Most viruses are too small (100–2,000 Angstrom units) to be seen with the light microscope and thus must be studied by electron microscopes.
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, which are noncellular are at the border of life, being unable to reproduce independently outside living cells) and of the ubiquity of key biochemical molecules, especially nucleic acidsnucleic acid,
any of a group of organic substances found in the chromosomes of living cells and viruses that play a central role in the storage and replication of hereditary information and in the expression of this information through protein synthesis.
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 and proteinsprotein,
any of the group of highly complex organic compounds found in all living cells and comprising the most abundant class of all biological molecules. Protein comprises approximately 50% of cellular dry weight.
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.

Protoplasm

 

the contents of a living cell, including its nucleus and cytoplasm; the physical substrate of life, the living matter of which organisms are composed. The physical properties, chemical composition, and structural and morphological characteristics of the protoplasm of animal, plant, and microbial cells and of unicellular organisms have much in common, a fact that bears witness to the unity of living nature.

The concept of protoplasm originated and was established during the study of the structure and properties of the cell and the development of cell theory. When this theory was being elaborated, the membrane was considered to be the chief structure of the cell. The cell contents were regarded as a secondary substance, a “gum.” Thus, botanists of the 1840’s did not regard the cell contents as an essential part of the cell, just as the contents of a container are not the container itself. However, by the mid-19th century, chiefly owing to botanical studies, it became increasingly obvious that the cell contents are indeed the main substrate of life. Credit for this idea is due chiefly to the German botanist H. Mohl (1844, 1846) who used the term “protoplasm” extensively (the term was first used in 1839 by the Czech scientist J. Purkinje to designate a cambium-like substance in plants that develops into animal cells). Another approach was followed by such zoologists as the French scientist F. Dujardin, who studied the Protista (especially rhizopods) and sarcode, the jellylike substance of which they consist. The notion that the F. Leydig, and M. Schultze and the Russian botanist and microbiologist L. S. Tsenkovskii later made important contributions to the study of protoplasm. In 1925, the American zoologist E. Wilson devised the colloidal theory of protoplasm, which states that protoplasm is a multiphasic colloid in which water is the dispersion medium and proteins and lipids are the main dispersed phases.

Beginning in the 1940’s and 1950’s, extensive new data was obtained on the composition and structure of protoplasm owing to the development and application of physicochemical methods of analysis in the field of biology. New intracellular organoids were discovered and the structure of those already known was studied in detail, including that of the nucleus, mitochondria, ribosomes, Golgi apparatus, and endoplasmic reticulum. The importance of the regulatory and structural role of biological membranes became clear. It was shown, for example, that numerous cell enzymes are not distributed at random in protoplasm but are contained within different intracellular structures. During this stage in the study of protoplasm, efforts were made to examine structures in relation to their functions. The study of this unity at the level of protoplasmic ultrastructures and of their constituent biopolymers resulted in the origin and development of molecular biology. The term “protoplasm” is sometimes incorrectly used to denote the extranuclear part of the cell (the cytoplasm).

REFERENCES

Rukovodstvo po tsitologii, vols. 1–2. Moscow-Leningrad, 1965–66.
Kozlov, Iu. P. “Polimernaia priroda protoplasmy.” In Biofizika. Moscow, 1968.
Loewy, A., and P. Siekevitz. Struktura i funktsiia kletki. Moscow, 1971. (Translated from English.)
De Robertis, E., W. Nowinski, and F. Saez. Biologiia kletki. Moscow, 1973. (Translated from English.)
The Cell, vol. 2. Edited by J. Brachet and A. Mirsky. New York-London, 1961.

Z. S. KATSNEL’SON

protoplasm

[′prōd·ə‚plaz·əm]
(cell and molecular biology)
The colloidal complex of protein that composes the living material of a cell.

protoplasm

Biology the living contents of a cell, differentiated into cytoplasm and nucleoplasm