Читать книгу The Mechanism of Life - Stéphane Leduc - Страница 5

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LIFE AND LIVING BEINGS

Primitive man distinguished but two kinds of bodies in nature, those which were motionless and those which were animated. Movement was for him the expression of life. The stream, the wind, the waves, all were alive, and each was endowed with all the attributes of life—will, sentiment, and passion. Ancient Greek mythology is but the poetic expression of this primitive conception.

In the evolution of the intelligence, as in that of the body, the development of the individual is but a repetition of the development of the race. Even now children attribute life to everything that moves. For them a little bird still lives in the inside of a watch, and produces the tick-tick of the wheels. In modern times, however, we have learnt that everything in nature moves, so that motion of itself cannot be considered as the characteristic of life.

Heraclitus aptly compares life to a flame. Aristotle says, "Life is nutrition, growth, and decay,—having for its cause a principle which has its end in itself, namely ἐντελέχεια." This principle is itself in need of definition, and Aristotle only substitutes one unknown epithet for another.

Bichat defined life as the ensemble of the functions which resist death. This is to define life in terms of death,—but death is but the end of life, and cannot be defined without first defining life. Claude Bernard rejects all definition of life as insufficient, and incompatible with experimental science.

Some modern physiologists regard sensibility, others irritability, as the characteristic of life, and define life as the faculty of responding, by some sort of change, to an external stimulus. As in the case of movement, we have found by more attentive observation that this faculty also is universal in nature. There is no action without reaction; an elastic body repels the body that strikes it. Every object in nature dilates with heat, contracts with cold, and is modified by the light which it absorbs. Everything in nature responds to exterior action by a change, and hence this faculty cannot be the characteristic of life.

A distinguished professor of physiology was accustomed to teach that the disproportion between action and reaction was the characteristic of life. "Allow a gramme weight to fall on a nerve, and the muscle will raise a weight of ten grammes. This disproportion is the characteristic of life." But there is a much greater disproportion between action and reaction when the friction of a match blows up a powder factory, or the turning of a switch lights the lamps and animates the tramways and the motors of a great city. The disproportion between action and reaction is therefore no characteristic of life.

The essential characteristic of life is often said to be nutrition—the phenomenon by which a living organism absorbs matter from its environment, subjects it to chemical metamorphosis, assimilates it, and finally ejects the destructive products of metamorphosis into the surrounding medium. But this characteristic is also common to a great number of ordinary chemical reactions, so that we cannot call it peculiar to life. Consider, for instance, a fragment of calcium chloride immersed in a solution of sodium carbonate. It absorbs the carbonic ion, incorporates it into a molecule of calcium carbonate, and ejects the chlorine ion into the surrounding medium.

It may be argued that this is merely a chemical process, since the substance which determines the reaction is also modified, the chloride of calcium changing into carbonate of calcium. But every living thing is also changing its chemical constitution during every moment of its existence,—it is this change which constitutes the process of senile involution. The substance of the child is other than that of the ovum, and the substance of the adult is not that of the child. Hence we cannot regard nutrition as the exclusive characteristic of life.

Other authorities regard growth and organization as the essentials of life. But crystals also grow. It was said that the growth of a crystal differed from that of a living thing, in that the former grew by the addition of material from without—the juxtaposition of bricks, as it were—while the latter grew by intussusception, an introduction of fresh material into the substance of the organism. A crystal, moreover, was homogeneous, while the tissues of a living being were differentiated—such differentiation constituting the organization. At the present time, however, we recognize the existence of a great variety of purely physical productions, the so-called "osmotic growths," which increase by a process of intussusception, and develop therefrom a marvellous complexity of organization and of form. Hence growth and organization cannot be considered as the essential characteristics of life.

Since, then, we are totally unable to define the exact boundary which separates life from the physical phenomena of nature, we may fairly conclude that no such separation exists. This is in conformity with the "law of continuity,"—the principle which asserts that all the phenomena of nature are continuous in time and space. Classes, divisions, and separations are all artificial, made not by nature but by man. All the forms and phenomena of nature are united by insensible transition; it is impossible to separate them, and in the distinction between living and non-living things we must content ourselves with relative definitions, which are far from being precise.

Life can only be defined as the sum of all phenomena exhibited by living beings, and its definition thus becomes a mere corollary to the definition of a living being.

The true definition of a living being is that it is a transformer of energy, receiving from its environment the energy which it returns to that environment under another form. All living organisms are transformers of energy.

A living organism is also a transformer of matter. It absorbs matter from its environment, transforms it, and returns it to its environment in a different chemical condition. Living things are chemical transformers of matter.

Living beings are also transformers of form. They commence as a very simple form, which gradually develops and becomes more complicated.

The matter of which a living organism is constituted consists essentially of certain solutions of crystalloids and colloids. To this we may add an osmotic membrane to contain the liquids, and a solid skeleton to support and protect them. Finally, it would seem that a colloid of one of the albuminoid groups is a necessary constituent of every living being.

We may say, then, that a living being is a transformer of energy and of matter, containing certain albuminoid substances, with an evolutionary form, the constitution of which is essentially liquid.

A living being has but a limited duration. It is born, develops, becomes organized, declines and dies. Through all the metamorphoses of form, of substance, and of energy, informing the whole course of its existence, there is a certain co-ordination, a certain harmony, which is necessary for the conservation of the individual. This harmony we call Life. Discord is disease,—the total cessation of the harmony is Death. When the form is profoundly altered and the substance changed, the transformation of energy no longer follows its regular course, the organism is dead.

After death the colloids which have constituted the form of the living thing pass from their liquid state as "sols" into their coagulated state as "gels." The metamorphoses of form, substance, and energy still continue, but no longer harmoniously for the conservation of the individual, but in dis-harmony for its dissolution. Finally, the form of the individual disappears, the substance and the energy of the living being is resolved and dispersed into other bodies and other phenomena.

The results hitherto obtained from the study of life seem but inconsiderable when compared with the time and labour devoted to the question. Max Verworn exclaims, "Are we on a false track? Do we ask our questions of Nature amiss, or do we not read her answers aright?"

Each branch of science at its commencement employs only the simpler methods of observation. It is purely descriptive. The next step is to separate the different parts of the object studied—to dissect and to analyse. The science has now become analytical. The final stage is to reproduce the substances, the forms, and the phenomena which have been the subject of investigation. The science has at last become synthetical.

Up to the present time, biology has made use only of the first two methods, the descriptive and the analytical. The analytical method is at a grave disadvantage in all biological investigations, since it is impossible to separate and analyse the elementary phenomena of life. The function of an organ ceases when it is isolated from the organism of which it forms a part. This is the chief cause of our lack of progress in the analysis of life.

It is only recently that we have been able to apply the synthetic method to the study of the phenomena of life. Now that we know that a living organism is but the arena for the transformation of energy, we may hope to reproduce the elementary phenomena of life, by calling into play a similar transformation of energy in a suitable medium.

Organic chemistry has already obtained numerous victories in the same direction, and the rapid advance in the production of organic bodies by chemical synthesis may be considered the first-fruits of synthetic biology.

A phenomenon is determined by a number of circumstances which we call its causes, and of which it is the result. Every phenomenon, moreover, contributes to the production of other phenomena which are called its consequences. In order therefore to understand any phenomenon in its entirety, we must determine all its causes both qualitatively and quantitatively.

Phenomena succeed one another in time as consequences one of another, and thus form an uninterrupted chain from the infinite of the past into the infinite of the future. A living being gathers from its entourage a supply of matter and of energy, which it transforms and returns. It is part and parcel of the medium in which it lives, which acts upon it, and upon which it acts. The living being and the medium in which it exists are mutually interdependent. This medium is in its turn dependent on its entourage,—and so on from medium to medium throughout the regions of infinite space.

One of the great laws of the universe is the law of continuity in time and space. We must not lose sight of this law when we attempt to follow the metamorphoses of matter, of energy and of form in living beings. Evolution is but the expression of this law of continuity, this succession of phenomena following one another like the links of a chain, without discontinuity through the vast extent of time and space.

The other great universal law, that of conservation, applies with equal force to living and to inanimate things. This law asserts the uncreateability and the indestructibility of matter and of energy. A given quantity of matter and of energy remains absolutely invariable through all the transformations through which it may pass.

We need not here discuss the question of the possible transformation of matter into ether, or of ether into ponderable matter. Such a transformation, if it exists, would have but little bearing on the phenomena of life. Moreover, it also will probably be found to conform to the law of conservation of energy.

In marked contrast to the permanence of matter and of energy is the ephemeral nature of form, as exhibited by living beings. Function, since it is but the resultant of form, is also ephemeral. All the faculties of life are bound up with its form,—a living being is born, exists, and dies with its form.

The phenomena of life may in certain cases slow down from their normal rapidity and intensity, as in hibernating animals, or be entirely suspended, as in seeds. This state of suspension of life, of latent life as it were, reminds us of a machine that has been stopped, but which retains its form and substance unaltered, and may be started again whenever the obstacle to its progress is removed.

During the whole course of its life a living being is intimately dependent on its entourage. For example, the phenomena of life are circumscribed within very narrow limits of temperature. A living organism, consisting as it does essentially of liquid solutions, can only exist at temperatures at which such solutions remain liquid, i.e. between 0° C. and 100° C. Certain organisms, it is true, may be frozen, but their life remains in a state of suspension so long as their substance remains solid. Since the albuminoid substances which are a necessary component of the living organism become coagulated at 44° C., the manifestations of life diminish rapidly above this temperature. The intensity of life may be said to augment gradually as the temperature rises from 0° to 40°, and then to diminish rapidly as the temperature rises above that point, becoming nearly extinct at 60° C.

Another condition indispensable to life is the presence of oxygen. Life, compared by Heraclitus to a flame, is a combustion, an oxydation, for which the presence of oxygen at a certain pressure is indispensable. There are, it is true, certain anærobic micro-organisms which apparently exist without oxygen, but these in reality obtain their oxygen from the medium in which they grow.

Life is also influenced by light, by mechanical pressure, by the chemical composition of its entourage, and by other conditions which we do not as yet understand. In each case the conditions which are favourable or noxious vary with the nature of the organism, some living in air, some in fresh water, and others in the sea.

Formerly it was supposed that the substance of a living being was essentially different from that of the mineral world, so much so that two distinct chemistries were in existence—organic chemistry, the study of substances derived from bodies which had once possessed life, and inorganic chemistry, dealing with minerals, metalloids, and metals. We now know that a living organism is composed of exactly the same elements as those which constitute the mineral world. These are carbon, oxygen, hydrogen, nitrogen, phosphorus, calcium, iron, sulphur, chlorine, sodium, potassium, and one or two other elements in smaller quantity. It was formerly supposed that the organic combinations of these elements were found only in living organisms and could be fashioned only by vital forces. In more recent times, however, an ever increasing number of organic substances have been produced in the laboratory.

Organic bodies may be divided into four principal groups. (1) Carbohydrates, including the sugars and the starches, all of which may be considered as formed of carbon and water. (2) Fats, which may be considered chemically as the ethers of glycerine, combinations of one molecule of glycerine and three molecules of a fatty acid, with elimination of water. (3) Albuminoids, substances whose molecules are complex, containing nitrogen and sulphur in addition to carbon, oxygen, and hydrogen. The albuminoid of the cell nucleus also contains phosphorus, and the hæmoglobin of the blood contains iron. (4) Minerals or inorganic elements, such as chloride of sodium, phosphate of calcium, and carbonic acid. This group also includes water, which is the most important constituent, since it forms more than a moiety of the substance of all living creatures.

Wöhler in 1828 accomplished the first synthesis of an organic substance, urea, one of the products of the decomposition of albumin. Since then a large number of organic substances have been prepared by the synthesis of their inorganic elements. The most recent advance in this direction is that of Emile Fischer, who has produced polypeptides having the same reactions as the peptones, by combining a number of molecules of the amides of the fatty acids.

In the further synthesis of organic compounds the problems we have before us are of the same order as those already solved. There is no essential difference between organic and inorganic chemistry; living organisms are formed of the same elements as the mineral world, and the organic combinations of these elements may be realized in our laboratories, just as in the laboratory of the living organism.

Not only so, but a living being only borrows for a short time those mineral elements which, after having passed through the living organism, are returned once again to the mineral kingdom from which they came.

All matter has life in itself—or, at any rate, all matter susceptible of incorporation in a living cell. This life is potential while the element is in the mineral state, and actual while the element is passing through a living organism.

Mineral matter is changed into organic matter in its passage through a vegetable organism. The carbonic acid produced by combustion and respiration is absorbed by the chlorophyll of the leaves under the stimulus of light—the oxygen of the carbonic acid being returned to the air, while the carbon is utilized by the plant for the formation of sugar, starch, cellulose, and fats.

Thus plants are fed in great part by their leaves, taking an important part of their nourishment from the air, while by their roots they draw from the earth the water, the phosphates, the mineral salts, and the nitrates required for the formation of their albuminoid constituents. A vegetable is a laboratory in which is carried out the process of organic synthesis by which mineral materials are changed into organic matter. The first synthetic reaction is the formation of a molecule of formic aldehyde, CH₂O, by the combination of a molecule of water with an atom of carbon.

From this formic aldehyde, or formol, we may obtain all the various carbohydrates by simple polymerization, i.e. by the association of several molecules, with or without elimination of water. Thus two molecules of formol form one molecule of acetic acid, 2CH₂O = C₂H₄O₂. Three molecules of formol form a molecule of lactic acid, 3CH₂O = C₃H₆O₃. Six molecules of formol represent glucose and levulose, 6CH₂O = C₆H₁₂O₆. Twelve molecules of formol minus one molecule of water form saccharose, lactose, cane sugar, and sugar of milk, 12CH₂O = C₁₂H₂₂O₁₁ + H₂O; n times six molecules of formol minus one molecule of water, n(C₆H₁₀O₅), form starch and cellulose.

Animals derive their nourishment from vegetables either directly, or indirectly through the flesh of herbivorous animals. The mineral matter, rendered organic in its passage through a vegetable growth, is finally returned by the agency of animal organisms to the mineral world again, in the form of carbonic acid, water, urea, and nitrates. Thus vegetables may be regarded as synthetic agents, and animals and microbes as agents of decomposition. Here also the difference is only relative, for in certain cases vegetables produce carbonic acid, while some animal organisms effect synthetic combinations. Moreover, there are intermediary forms, such as fungi, which possessing no chlorophyll are nourished like animals by organic matter, and yet like vegetables are able to manufacture organic matter from mineral salts.

The work of combustion begun by the animal organism is finished by the action of micro-organisms, who complete the oxydation—the re-mineralization of the chemical substances drawn originally from the inorganic world by the agency of plant life.

To sum up. Vegetables obtain their nourishment from mineral substances, which they reduce, de-oxydize, and charge with solar energy. Animal organisms on the contrary oxydize, and micro-organisms complete the oxydation of these substances, returning them to the mineral world as water, carbonates, nitrates, and sulphates.

Thus matter circulates eternally from the mineral to the vegetable, from the vegetable to the animal world, and back again. The matter which forms our structure, which is to-day part and parcel of ourselves, has formed the structure of an infinite number of living beings, and will continue to pursue its endless reincarnation after our decease.

This endless cycle of life is also an endless cycle of energy. The combination of carbon with water carried out by the agency of chlorophyll can only take place with absorption of energy. This energy comes directly from the sun, the red and orange light radiations being absorbed by the chlorophyll. The arrest of vegetation during the winter months is due not so much to the lowering of temperature as to the diminution of the radiant energy received from the sun. In the same way shade is harmful to vegetation, since the radiant energy required for growth is prevented from reaching the plant.

The energy radiated by the sun is accumulated and stored in the plant tissues. Later on, animals feed on the plants and utilize this energy, excreting the products of decomposition, i.e. the constituents of their food minus the energy contained in it. Thus the whole of the energy which animates living beings, the whole of the energy which constitutes life, comes from the sun. To the sun also we owe all artificial heat, the energy stored up in wood and coal. We are all of us children of the sun.

The radiant energy of the sun is transformed by plants into chemical energy. It is this chemical energy which feeds the vital activity of animals, who return it to the external world under the form of heat, mechanical work, and muscular contraction, light in the glow-worm, electricity in the electric eel.

There is a marked difference between the forms affected by organic and inorganic substances. The forms of the mineral world are those of crystals—geometrical forms, bounded by straight lines, planes, and regular angles. Living organisms, on the contrary, affect forms which are less regular—curved surfaces and rounded angles. The physical reason for this difference in form lies in a difference of consistency, crystals being solid, whereas living organisms are liquids or semi-liquids. The liquids of nature, streams and clouds and dewdrops, affect the same rounded forms as those of living organisms.

Living beings for the most part present a remarkable degree of symmetry. Some, like radiolarians and star-fish, have a stellate form. In plants the various organs often radiate from an axis, in such a manner that on turning the plant about this axis the various forms are superposed thrice, four, or more often five times in one complete revolution. It is remarkable how often this number five recurs in the divisions and parts of a living organism. In other cases the similar parts are disposed symmetrically on either side of a median line or plane, giving a series of homologous parts which are not superposable.

The most important characteristic of a living being is its form. This is implicitly admitted by naturalists, who classify animals and plants in genera and species according to the differences and analogies of their form.

All living beings are composed of elementary organizations called cells. In its complete state, a cell consists of a membrane or envelope containing a mass of protoplasm, in the centre of which is a nucleus of differentiated protoplasm. This nucleus may in its turn contain a nucleolus. In some cases the cell is merely a protoplasmic mass without a visible envelope, so that a cell may be defined as essentially a mass of protoplasm provided with a nucleus.

A living organism may consist merely of a single cell, which is able alone to accomplish all the functions of life. Most living beings, however, consist of a collection of innumerable cells forming a cellular association or community. When a number of cells are thus united to constitute a single living being, the various functions of life are divided among different cellular groups. Certain cells become specialized for the accomplishment of a single function, and to each function corresponds a different form of cell. It is thus easy to recognize by their form the nerve cells, the muscle cells which perform the function of movement, and the glandular cells which perform the function of secretion. The cells of a living being are microscopic in size, and it is remarkable that they never attain to any considerable dimensions.

In order that life may be maintained in a living organism, it is necessary that a continual supply of aliment should be brought to it, and that certain other substances, the waste-products of combustion, should be eliminated. In order to be absorbed and assimilated, the alimentary substances must be presented to the living organism in a liquid or gaseous state. Thus the essential condition necessary for the maintenance of life is the contact of a living cell with a current of liquid. The elementary physical phenomenon of life is the contact of two different liquids. This is the necessary condition which renders possible the chemical exchanges and the transformations of energy which constitute life. It is in the study of the phenomena of liquid contact and diffusion that we may best hope to pierce the secrets of life. The physics of vital action are the physics of the phenomena which occur in liquids, and the study of the physics of a liquid must be the preface and the basis of all inquiry into the nature and origin of life.