Protoplasm is the living contents of a cell that is surrounded by a plasma membrane. It is a general term for the cytoplasm. Protoplasm is composed of a mixture of small molecules such as ions, amino acids, monosaccharides and water, and macromolecules such as nucleic acids, proteins, lipids and polysaccharides. In eukaryotes the protoplasm surrounding the cell nucleus is known as the cytoplasm and that inside the nucleus as the nucleoplasm. In prokaryotes the material inside the plasma membrane is the bacterial cytoplasm, while in Gram-negative bacteria the region outside the plasma membrane but inside the outer membrane is the periplasm.
The word "protoplasm" comes from the Greek protos for first, and plasma for thing formed. It was first used in 1846 by Hugo von Mohl to describe the "tough, slimy, granular, semi-fluid" substance within plant cells, to distinguish this from the cell wall, cell nucleus and the cell sap within the vacuole. Thomas Huxley later referred to it as the "physical basis of life" and considered that the property of life resulted from the distribution of molecules within this substance. Its composition, however, was mysterious and there was much controversy over what sort of substance it was. By the time Huxley wrote, a long-standing debate was largely settled over the fundamental unit of life: was it the cell or was it protoplasm? By the late 1860s, the debate was largely settled in favor of protoplasm. The cell was a container for protoplasm, the fundamental and universal material substance of life. Huxley's principal contribution was to establish protoplasm as incompatible with a vitalistic theory of life. Attempts to investigate the origin of life through the creation of synthetic "protoplasm" in the laboratory were not successful.
The idea that protoplasm of eukaryotes is divisible into a ground substance called "cytoplasm" and a structural body called the cell nucleus reflects the more primitive knowledge of cell structure that preceded the development of electron microscopy, when it seemed that cytoplasm was a homogeneous fluid and the existence of most sub-cellular compartments, or how cells maintain their shape, was unknown. Today, it is known that the cell contents are structurally very complex and contain multiple organelles.
Protoplasm was said to exist in two forms: a liquid-like sol state or a jelly-like gel state.
- Cammack, Richard; Teresa Atwood; Attwood, Teresa K.; Campbell, Peter Scott; Parish, Howard I.; Smith, Tony; Vella, Frank; Stirling, John (2006), Oxford dictionary of biochemistry and molecular biology, Oxford [Oxfordshire]: Oxford University Press, ISBN 0-19-852917-1
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