CHAPTER II
THE BASIS OF LIFE
Protoplasm was defined by Huxley as “the physical basis of life.” It is the material substance which lives. There is no life in anything which does not consist of, or is not supported upon, or permeated by a system of filaments of protoplasm. Huxley’s definition indissolubly links in thought protoplasm and life. But it is doubtful whether the definition is in any sense axiomatic. The adjective “physical” has too narrow a range. If the biologist could say to the chemist, “Here is a substance which was alive. If I could restore to it the energy which it has lost, if I could impart to it the movement which I recognize as life, it would again be alive,” he would offer the chemist a substance susceptible to the methods of his science, something which he could analyse. If, approaching the physicist with a group of chemical products, he could say, “Into these protoplasm broke up on dying. I cannot assure you that while it was alive they were combined into molecules within your meaning of the term. There may be no such ‘substance’ as protoplasm in the sense in which you understand the word, but so long as this mass lived these various familiar compounds were bound together in a supermolecular form. Death was their falling apart. If I could cause them to recombine, they would be alive,” he would give the physicist a problem within the range of his methods. The physicist could devise a method for measuring these units. The science which can weigh an electron, the thousandth part of an atom, need not fear failure in its attempt to gauge the size of units of structure composed of groups of heavy molecules, albumins, globulins, and other proteins,[1] with the inclusion, perhaps, of fats, sugars, inorganic salts. But herein lies the biologist’s dilemma. He cannot assert that there exists a homogeneous substance, protoplasm. He cannot assert that there exists a definite tectonic grouping of heterogeneous substances which, so long as it is maintained, constitutes a physical basis capable, and alone capable, of exhibiting the phenomena of life. Protoplasm is still a hypothetical substance—a name. Truly, in the absence of nitrogen-containing compounds of very complicated chemical constitution there is no life. All living things yield on chemical analysis approximately the same nitrogenous substances. No one can say whether the capacity for living is dependent upon the molecular—that is to say, the chemical—constitution of the basis, or whether it is dependent upon the arrangement of its molecules, its form. It is even open to question whether instability, the capacity for incessant change, both in chemical composition and in form, be not the condition which differentiates living matter from dead. “Physical basis” is too hard a term for this elusive concept of the matter which exhibits life.
If it were possible by a process of elimination to ascertain the substances which must be present in protoplasm, the physiologist might formulate a reasonable hypothesis as to the nature of this “basis.” But there is no part of any living thing, or, at any rate, no part which is not microscopic in its dimensions, which can be pointed out as protoplasm and nothing besides. It is impossible to isolate anything which can be described as pure protoplasm. Nor is it possible, by comparing various tissues which are acknowledged to be rich in protoplasm, to ascertain what chemical substances are common to them all.
If it were feasible, by analysing a number of specimens of protoplasm, to make sure that, although x is absent from one, y from another, and z from a third, some one thing, P, is always present, then P might be regarded as the physical basis, even though it were evident that P alone was not protoplasm. Protoplasm would be P combined with either x, y, or z. Globulins and albumins and other proteins are always present, but in varying proportions; but it is impossible to make certain that either of these chemical substances is more important than the rest. Nor is it possible to assert of either that it is essential.
Chemically, protoplasm is a mixture of substances, chiefly proteid, in a condition in which it is capable of manifesting the phenomena of life. But whether it be more complex and of heavier molecule than either globulin, nucleo-protein, albumin, fibrin, or any other of the nitrogenous compounds which take its place when it is dead; or whether it be as simple as either of these, but differ from them all in its instability, in the constant flux of its atoms, which causes it at one time to incline towards one of them, at another time to another, are questions which cannot at present be answered.
The uncertainty as to the chemical nature of protoplasm is responsible for an unfortunate irregularity in the use of the term. It is ex hypothesi the most active, the most living part of an animal cell. If the cell has a nucleus and an envelope, the protoplasm must lie in the space between the two. This part of the cell is therefore often termed, without qualification, the “cell-protoplasm.” Frequently the abuse of the word is carried still further. Young cells, leucocytes, nerve-cells, etc., which have no envelope, consist of a nucleus embedded in soft cell-substance. The latter is termed its protoplasm. The cell is described as consisting of nucleus and protoplasm, the term assuming an anatomical signification. Not only is such a use of the term bad, because it indicates a confusion of thought, but it brings with it a train of ambiguities. What are the limits of the protoplasm? If the cell-body be firmer towards its exterior than it is within, is the denser substance protoplasm, or is it not? It has not the qualities which are attributed to protoplasm in so marked a degree as has the substance which it surrounds. Hence a distinction is made. The one is “ectoplasm,” the other “endoplasm.” Within the cell-body are many collections, often in the form of granules, of substances which have not the protoplasmic attributes. They constitute the “deuteroplasm” of certain cytologists. But these enclosed substances may be as far removed from protoplasm as starch grains. It is absurd to use the termination “plasm” for such well-defined products of cell activity as these. The subject is, unfortunately, obscured by conflicting terms. Nomenclatures which were invented with the object of giving definiteness to our ideas have served but to perplex them. The term “protoplasm” should be reserved as a synonym for the substance which is most alive, the substance in which chemical change is most active, the substance which has in the highest degree a potentiality of growth. Anatomical distinctions are better expressed in anatomical terms. We shall treat of such distinctions when considering the organization of the cell.
In the meantime it may be well to consider the attributes which appear to belong to this most living substance. Its chemical composition can be inferred only from the compounds found on analysis to be present in a mass of organized substance which there is reason for thinking was rich in protoplasm while it was alive. The compounds found vary within certain limits. The quantity of water associated with these compounds is still more variable. Water is essential to the existence of protoplasm. Its power of combining with water in variable quantities is one of its characteristics. Tissue rich in protoplasm yields on an average about 75 per cent. of water. Part of the protoplasm within a cell holds more water associated with it, part less.
Closely associated with its power of holding water is its tendency to assume an architectural form. In large vegetable cells, such as those of the hairs within the flowers of Tradescantia, the protoplasm may be seen, under the microscope, arranged in threads containing granules which are incessantly streaming up and down them. The spaces between the threads are filled with water. Such mobile protoplasm cannot be said to have a structural form. But in the greater number of cells, and especially in animal cells, the protoplasm is disposed in a network, with usually a tendency for the strands of the network to set in lines. In attempting to define these very variable networks, the microscopist is obliged to speak with caution. He finds it very difficult to distinguish between appearances which he is justified in regarding as inherent in the cell-substance, whether alive or dead, and appearances which he may have induced by the action of reagents whilst preparing the tissue for examination. Rarely can he assert that he sees a network in a living cell. When examining a dead cell, he is bound to recognize that the preservatives and hardening reagents which he used may have caused the proteins to coagulate in a particular pattern. If he obtains the same pattern with several different methods, he infers that the appearance which he sees is that of a structure existing in the living cell; but he is never quite sure that it is not an arrangement produced by reagents after death.
The tendency of protoplasm to dispose itself in the form of a network or sponge-work is of the greatest interest in its bearing upon the theory of its activity in effecting chemical change. The body itself, as we shall find later, is a network of tissues enclosing lymph. The lymph in the tissue-spaces contains foods and waste products in solution. The tissues are constantly taking from it the former, and discharging into it the latter. Every cell is, microscopically, a tissue. The strands of its protoplasm are perpetually sorting foods from its cell-juice, adding to its cell-juice waste products. By diffusion, foods, including oxygen, pass from lymph to cell; waste products, including carbonic acid, pass from cell to lymph. If water be added to gum, the gum swells. The mixture is homogeneous. Diffusion takes place slowly through the mucilage. When water is taken up by protoplasm, the protoplasm swells; but the mixture is not homogeneous. The protoplasm expands as a wet sponge expands, although the relation of the enclosing reticulum to the water which it encloses is far more complicated. It is, as it were, a sponge made of gum. Some water is combined with the protoplasm; the remainder fills its spaces. There is an active surface relation between the free water and the protoplasmic threads. As water rises in a capillary tube, as it passes from the inside to the outside of a flannel shirt, so it circulates within the cell.