Organization of Cell.—All living cells contain, or consist of, a viscous, glairy material (somewhat akin in consistence to un boiled white of egg) known as protoplasm. Protoplasm is not of course a single substance but a very complex system ; it has been described as the "physical basis of life" since life as far as we know, does not appear to exist apart from protoplasm. An analy sis of an active plant tissue, like lettuce or beet leaves, shows that it contains about 9o% water. Of the dry material we should find carbon 38.19%, hydrogen 5.1%, oxygen 30.8%, nitrogen 4.5% and ash 21.5%. Similar results would be found for most plant material in the dry state though resting materials such as the potato tuber and the wheat grain would have only 2-4% of ash. Active protoplasm, and living material generally, is charac terised by a high percentage of carbon owing to the large amount of organic matter, and in plants by a high percentage of oxygen owing to the large amount of oxygen-rich materials as carbohy drates (sugars, starch). A small proportion of nitrogen is also always present in living material. In ordinary analyses of plant material the protoplasm is only a small proportion of the mate rial, a large proportion is dead cell-wall material, the contents of the cell vacuole (see above, Cytology), and protoplasmic inclu sions. The nearest approach to an analysis of protoplasm is that which has been made of certain myxomycetes (see FUNGI) which exist as naked masses of protoplasm. Kiesel (1925) has exam ined Reticularia lycoperdon. He finds in a hundred parts of the dry weight :—protein, 29.1, other nitrogenous substances, 12.0, nucleic acid, 3.7, oil 19.1, carbohydrates generally 9.9, glycogen 15.2, also lecithin and cholesterol. This analysis is of course very incomplete; 6% of the material was not determined and a large number of substances present in small quantities only must have been overlooked. We note, however, the large amount of protein and this is a general phenomenon ; just as we find life associated with protoplasm so we find protoplasm associated with protein.
It is clear that protoplasm must be a material, or rather a system of materials of very special nature since the enormous number of chemical reactions associated with life processes go on within it, and are apparently controlled and brought by it into proper co-ordination. Protoplasm also is concerned with the hereditary process, with the handing on to the male and female cells and from cell to cell in the body of the plant or animal, of the special qualities which cause the offspring to resemble their parents. (See CYTOLOGY.) The modern theory of the special peculiarities of protoplasm associated with its life activities may be termed the physico chemical theory. In this view stress is laid not so much on any special organization of the protoplasm as on its peculiar physico chemical condition.
It is pointed out that proteins are colloidal in nature or rather exist in the colloidal state (see COLLOIDS) and that, on one hand, many of the peculiarities which distinguish chemical processes in the living organism from the same processes occurring in the laboratory, and the peculiar co-ordination of processes which marks the living cell—and indeed of the whole organism for the harmony of the working of the plant or animal body is a com monplace—are related in some way with the colloidal nature of the protoplasmic substratum in which they occur. It cannot be said, however, that the colloidal theory carries us very far. We know that material in the colloidal state exhibits an enormous extension of surface and that substances may accumulate on this extended surface (by a process known as adsorption) and react together at a much faster rate than would otherwise be the case. The theory, however, does not explain how the numerous chem ical processes going on in the cell are on the one hand kept from interfering with one another, and yet on the other hand are brought into close interrelationship and co-ordination. Still less can a colloidal theory explain the even more complicated processes of growth, of differentiation (the development of new and different parts as the plant or animal develops), and of heredity. It is clear
also that the protoplasm is not a simple colloid of two phases, such as are found in gamboge in water where the particles of gamboge are the one phase and the water surrounding them the other phase. In the protoplasm there is not only protein material hut oily material (so-called lipin material) appears tc be essen tial and probably also carbohydrates play a part. We know furthermore, that various salts (of potassium, calcium, etc.) are necessary. The physical relationship of the colloidal protoplasm must therefore be far more complex than that of any colloidal material studied in the laboratory. Any attempt to explain the peculiar relationship of the processes occurring in living organisms by reference to the behaviour of a complicated, dead, colloidal medium is largely speculation since the behaviour of such a me dium is unknown.
Enzymes.—When dealing with the colloidal organization of the cell reference must be made here to the substances known as enzymes (q.v.). These are substances of unknown nature which can be extracted from dead cells whether plant or animal, and they must certainly play a very large part in the chemical pro cesses going on in the living cell. They have three marked charac teristics; they can act as catalytic agents, i.e., they cause many chemical processes to go on at a much faster rate than would otherwise be the case; in chemical language they accelerate; they can thus cause rapid changes to occur which in their absence require a high temperature or the action of strong acids or alkalis. Secondly they act in very minute quantities and do not themselves appear to be used up in the process which they accelerate, for example invertase can break down L000,000 times its weight of cane sugar. Thus they partake of the nature of chemical catalysts. Thirdly they are usually sensitive to heat (thermolabile) their activity being usually destroyed by temperatures well below that of boiling water. It is to be noted that the processes they accel erate may be either breaking down processes or building up (synthetic) ones, and that they can accelerate both such processes. Enzymes have been very actively investigated of late years especi ally by Willstatter and his pupils, but their nature is still obscure. They seem to be a part of the mechanism of the living cell by which the rate of various chemical processes are controlled. These enzymes seem to be of colloidal nature, or to have a colloidal car rier, though apparently they are not of protein nature; and further more their action is markedly specific, that is, the enzyme which affects the rate of one chemical process is unable to affect the rate of another process of marked chemical similarity. For example, sugars closely related in chemical composition require different enzymes to bring about changes in them. This is one of the puzzling aspects of cell physiology for it seems difficult to believe that the cell has a battery of enzymes to carry on the very numerous chemical processes with which it is concerned. Enzymes can, of course, only be studied outside the cell and it may be that the con ditions in the living cell are different.
In relation to the cell organization it must be pointed out that many of the processes going on in the cell are balanced or reversi ble reactions, that is reactions which are capable of going in either direction; a good example of this is the formation of fats. These substances consist of glycerol (glycerin) combined with a fatty acid. In the presence of a special enzyme, lipase (found in castor oil seeds and in the animal body), this reaction is capable of going either way, the fat may be split up into glycerol and acid, or the acid and glycerol may be combined to form fat ; the lipase accelerates both these processes. Whether the reaction goes in the direction of synthesis or analysis depends not on the lipase but on the concentration of the substances taking part, i.e., fat, glycerol, fatty acid and water. It is probable that by controlling the amount of water available for particular reactions the cell is able to control the direction in which such a reversible process shall go.