The protoplasm is the essence of life. We can refer to all the constituents of the cell collectively by the term protoplasm which Huxley so aptly characterized as "the physical basis of life". Hugo von Mohl (1846) described the protoplasm and discussed its importance.
Physical properties of Protoplasm
On the basis of microscopic studies protoplasm appears as a clear, colourless, jelly-like gelatinous substance. It is a water solution of very small granules and globules of different substances. Due to this it appears as viscous, elastic and granular. It can be stretched like a thread which returns to its original position when released. It appears foamy as a result of the presence of a large number of vacuoles It responds to various external stimuli e.g. , heat, electric shock and chemicals. Irritability is an inherent characteristic of protoplasm.
Protoplasm shows following streaming movements :
(1) Cyclosis : This streaming is of two types.
(a) Rotation : The protoplasm moves in one direction only (either clockwise or anti-clockwise) around a vacuole inside a cell e.g., leaf cells of Hydrilla or Vallisneria.
(b) Circulation : The protoplasm moves in different directions around different vacuoles inside a cell. e.g. , staminal hair of Tradescantia.
(2) Ciliary movement : When an organism as a whole or zoospores or gametes move from a place to another with the help of cilia e.g., Chlamydomonas.
(3) Amoeboid movement : The protoplasm moves like Amoeba with the help of pseudopodia e.g., slime molds.
Protoplasm is a complex polyphasic colloidal system. The most important physical characteristic of protoplasm is its colloidal nature to which it owes most of its properties.