Protoplasm (proton, 'first,' plasma, 'formed substance') is a technical name for living matter.
The term was first applied (1846) by the botanist Hugo von Mohl to the 'slimy, granular, semi-fluid' contents of vegetable cells, but before that Rôsel von Rosenhof (1755) had studied the amœba, which is a unit-mass of relatively pure living matter, Robert Brown and other botanists had watched the rotation of the living substance inside the cells of some plants, and Dujardin (1835) had described the 'sarcode' of Foraminifera as 'a glutinous, transparent, living jelly.' After Dujardin and Von Mohl had thus directed attention to 'sarcode' and 'protoplasm,' observations on both gradually accumulated, the idea began to be mooted that the two substances were essentially the same, and in 1861 Max Schultze defined the cell as a nucleated mass of living matter or protoplasm. We cannot indeed say that the protoplasm is the same in the cells of plants and animals, for the precise nature of living matter defies our analysis; but we do know that 'the physical basis of life' has in all cases some common characteristics of structure and behaviour, diverse as are the ways in which its inherent activity may be manifested.
Protoplasm may be conveniently studied in the unicellular Protozoa—e.g. Amœbæ and Foraminifera; in the colourless cells of blood; in the ova of animals—e.g. of frog and pond-snail; in young vegetable shoots; or in the cells of a simple plant, like Chara or Spirogyra. When we submit the living matter in its natural state to microscopic examination we usually see a clear semi-fluid substance, sometimes obscured by granules, sometimes with numerous bubbles or vacuoles, sometimes with hints of a fine network traversing the whole. This vacuolated and reticular structure is much more easily demonstrated after the cells have been 'fixed' and stained, and, if necessary, 'sectioned' according to the practice of microscopic technique. In this state the network-like appearance of the cell-substance has been demonstrated in a great number of cases, and we may fairly regard it as characteristic (see CELL).
As the students of structure have been led with increasing carefulness of microscopic analysis to distinguish between the netted framework and a more fluid stuff in its meshes, so many physiologists distinguish the framework as the acting part, which lives and is relatively stable, from the content which is acted on, and is in a state of physical and chemical change. It is clearly necessary to discriminate between protoplasm in the strict sense and the substances with which the genuinely living matter is associated—food-stuffs about to be or being utilised, and waste-products which result from the vital activity. The food-granules and the waste-products we can analyse—they may be respectively glycogen and uric acid; the living matter we cannot analyse, for it dies at the moment our analysis begins.
All physiologists are agreed that waste-products are formed when work is done or while life lasts, and that living organisms have a characteristic power of repair. They are ever changing, and yet they remain more or less the same. Streams of matter and energy pass into the organism; they are somehow incorporated into the living capital, work is done and waste is given off, and the organism continues from day to day, or from year to year, relatively intact. For while 'the transfer of energy into any inanimate material system is attended by effects retardative to the transfer and conducive to dissipation,' the secret of protoplasm, as expressed by Joly in the language of physics, is that 'the transfer of energy into any animate material system is attended by effects conducive to the transfer and retardative of dissipation.'
So far we have stated facts; speculation begins when we try to express the precise relations of the protoplasm to the waste and repair of the organism. Two somewhat different views must be considered. We may regard protoplasm as a complex substance or mixture of substances, which shares directly in the constant chemical and physical changes or metabolism of the organism. It is the climax of an ascending series of constructive or synthetic steps, by which food-material becomes more and more complex and unstable; it is subject as the organism lives to constant disruptive or analytic changes, which result in the liberation of energy and in the formation of simpler and simpler waste-products. Thus protoplasm is regarded as the changeful central substance in metabolism; it is continually being nmade, breaking up, and wasting as it lives; it is continually being made by the constructive processes of repair. We call the repairing or constructive process anabolism, and its chemically discernible steps anastates; we call the discharging or disruptive process katabolism, and its chemically discernible steps katastates.
But, on the other hand, we may regard protoplasm as a kind of ferment which influences the material round about it without itself being so directly affected as the previous conception implies. It is the relatively stable cause of metabolism, acting on less stable material of a less complex nature, acting upon it so that constructive anabolic processes or disruptive katabolic processes predominate for the time.
Furthermore, while all are agreed that in the life of organisms there is a characteristic alternation or antithesis between waste and repair, between discharge and restitution of energy, between katabolism and anabolism, there is difference of opinion as to the character of these antagonistic processes. The English physiologist Gaskell, prompted by his researches on the functions of nerves, some of which command activity while others induce rest, was led to regard what he called anabolism and katabolism as processes which bear to protoplasm a relation similar to that which sleep and wide-awake life bear to the organism. The 'winding-up' process of anabolism or restitution goes on (autonomically) of itself; the 'running-down' process of katabolism or discharge is determined by stimulus. Anabolism is comparable to the self-loading, katabolism to the stimulated firing of a gun. But the German physiologist Hering, prompted by his researches on colour-sensations, was led to regard what he called assimilation and dis-assimilation as two antagonistic kinds of activity, both dependent on stimuli which differ in their direction and results.
Apart from the precise biological problems which are raised when we seek to define the limits of our analytic knowledge of living matter, there is the great difficulty of forming any conception of the relation between life and its physical basis. We may cite Huxley's famous address on The Physical Basis of Life and Hutchison Stirling's essay As Regards Protoplasm as pre-eminent types of the numerous endeavours which have been made to secure accurate thinking about this supreme problem. Suffice it to say that in two ways we gain some knowledge of protoplasm or living matter. On the one hand, we know it as it is presented to our senses in living organisms, and the result of our analysis of this presentation leads us to recognise in protoplasm a marvellously subtle kind of matter and motion, or ultimately of motion. On the other hand, we have an intimate knowledge of protoplasm in our own brains, where its activity is manifested in thought. That we need not attempt to give an explanation of ultimate realities like protoplasm and thought, that thought is only a function of protoplasm, that protoplasm is only a form of thought, that thought and protoplasm are different aspects of one reality, are the respective conclusions of the agnostic, the materialist, the idealist, and the monist philosophers who have theorised about living matter.
See BIOLOGY, CELL, PHYSIOLOGY. The technical literature on protoplasm is not readily accessible, but references to researches since 1886 will be found in the annual Zoological Record; while some of the older investigations are cited by Prof. Geddes in the article 'Protoplasm,' Ency. Brit. The student will find the best introduction to modern speculations, such as those of Gaskell and Hering, in Prof. Michael Foster's article 'Physiology,' Ency. Brit., in Prof. Burdon Sanderson's presidential address to the Biological Section of the British Association (Report Brit. Assoc. 1889), and Nature, xl. (September 1889).