CHAPTER II.ENERGY IN BIOLOGY.
§ 1. Energy in Living Beings.—§ 2. The First Law of Biological Energetics:—All Vital Phenomena are Energetic Transformations.—§ 3. Second Law:—The Origin of Vital Energy is in Chemical Energy. Functional Activity and Destruction.—§ 4. Third Law:—The Final Form of Energetic Transformation in the Animal is Thermal Energy. Heat is an Excretum.
The theory of energy was thought of and utilized in physiology before it was introduced into physics, in which it has exercised such an extraordinary influence. Robert Mayer was a physicist and a doctor. Helmholtz was equally at home in physiology and in physics. From the outset both had seen in this new idea a powerful instrument of physiological research. The volume in which Robert Mayer expounded, in 1845, his remarkable views on organic movement in relation to nutrition, and Helmholtz’ commentary leave us in no doubt in this respect. The essay on the mechanical equivalent of heat, of a more particularly physical character, is six years later than the earlier work.
The Relations between Energetics and Biology.—The theory of energy is therefore only returning to its cradle; and to that cradle it returns with all the sanction of physical proof, as the most general theoryever proposed in natural philosophy, and the theory least encumbered with hypotheses. It reduces all particular laws to two fundamental principles—that of the conservation of energy, which contains the principles of Galileo and Descartes, of Newton, of Lavoisier, Joule’s law, Hess’s law, and Berthelot’s principle of the initial and final states; and also Carnot’s principle, from which are deduced the laws of physico-chemical and chemical equilibrium. These two principles therefore sum up the whole of natural science. They express the necessary relation of all the phenomena of the universe, their uninterrupted gentic connection, and their continuity.
A priorithere would be little likelihood that a doctrine, so universal and so thoroughly verified in the physical world, could be restricted, and thus be useless to the living world. Such a supposition would be contrary to the scientific method, which always tends to the generalization and the explanation of elementary laws. The human mind has always proceeded thus: it has applied to the unknown order of living phenomena the most general laws of contemporary physics.
This application has been found legitimate, and has been justified by experiment whenever it has been a question of the laws or of the really fundamental or elementary conditions of phenomena. It has, on the other hand, however, been unfortunate when it has stopped short of secondary characteristics. When we now concede the subjection of living beings to these general laws of energetics, we are following a traditional method. There is no doubt that this application is legitimate, and that experiment will justify ita posteriori.
I will therefore grant, as a provisionalpostulate, the consequences of which will have to be ultimately justified, that the living and inanimate world alike show us nothing buttransformations of matterandtransformations of energy. The word phenomenon will have no other signification, whatever be the circumstances under which the phenomenon occurs. The varied manifestations which translate the activity of living beings thus correspond to transformations of energy, to conversions of one form into another, in conformity with the rules of equivalence laid down by the physicists. This conception may be formulated in the following manner:—The phenomena of life have the same claim to be energetic metamorphoses as the other phenomena of nature.
This postulate is the foundation of biological energetics. It may be useful to give some explanation relative to the signification, the origin, and the scope of this statement.
Biological energetics is nothing but general physiology reduced to the principles that are common to all the physical sciences. Robert Mayer and Helmholtz gave the best description of this science, and laid down its limits by defining it as “the study of the phenomena of life regarded from the point of view of energy.”
Our first object will be to define and to enumerate the energies at play in living beings; to determine their more or less easy transformations from one to another,to bring to light the general laws which govern those transformations, and finally to apply them to the detailed study of phenomena. This programme may be divided into four parts.
In the physical world the specific forms of energy are not numerous. When we have mentioned mechanical, chemical, radiant (thermal and photic) energies, electrical energy, with which is blended magnetic energy, we have exhausted the catalogue of natural agents.
But is this list for ever closed? Are vital energies comprised in this list? These are the first questions which we must ask ourselves.
The iatro-mechanical school, ona priorigrounds give an affirmative answer. No doubt there are in the living organism many manifestations which are pure physical manifestations of known energies, mechanical, chemical, thermal, etc. But are all the manifestations of the living being of this order? Are they all, henceforth, reducible to the categories and varieties of energy which are investigated in physics? This is the claim of the mechanical school. But the claim is rash. Our fundamental postulate affirms, in principle, that universal energy is manifested in living beings; but, as a matter of fact, there is no reason for the assertion that it does not assume particular forms, according to the circumstances peculiar to the conditions under which they are produced.
Thesespecial forms of energymanifested in the conditions suitable to living beings would swell the list drawn up by the physicists. And it would not be the first instance of an extension of this kind. The history of science records many remarkable cases. Scarcely a century has passed since we first heard ofelectrical energy. This discovery in the world of energy, which took place, so to speak, before our very eyes, of an agent which plays so large a part in nature, clearly leaves the door open to other surprises.
We shall therefore concede that there may be other forms of energy at work in living beings than those we already know in the physical world. This reservation would enable us to discover at once the essential characteristics by which vital phenomena are henceforth reduced to universal physics, and the purely formal differences still distinguishing them.
If there are really special energies in living beings, our monistic postulate leads us to assert that these energies are homogeneous with the others, and that they do not differ from them more than they differ among themselves. It is probable that some day they will be discovered external to living bodies, if the material conditions (which it is always possible to imagine) are realized externally to them. And if we must admit that the peculiarity of the medium is such that these forms must remain indefinitely peculiar to living beings, we may assert with every confidence that these special energies do not obey special laws. They are subject to the two fundamental principles of Robert Mayer and Carnot. They are exchanged according to fixed laws with the other physical forms of energies at present known.
To sum up, then, we must establish three categories in the forms of energy which express the phenomena of vitality.
In the first place, most of these energies are thosewhich have already been studied and recognized in general physics. They are the same energies: chemical, thermal, mechanical, with their characteristics of mutability, their lists of equivalents, and their actual and potential stales.
In the second place, it may happen, and it probably will happen, as it happened in the last century in the case of electricity, that some new form of energy will be discovered belonging to the universal order as to the living order. This will be a conquest of general physics as well as of biology.
And finally we may rigorously and provisionally admit a last category ofvital energies properly so called.
It is difficult to give much precision to the idea ofvital energies properly so called.
It will be easier to measure them by means of equivalents than to indicate their nature. Besides, this is the ordinary rule in the case of physical agents. We can measure them, although we know not what they are.
Characteristics of Vital Energies.—We see why we cannot exhibit with precision,a priori, the nature of vital energies. In the first place, they are expressed by what takes place in the tissues in activity, and this cannot at present be identified with the known types of physical, chemical, and mechanical phenomena. This is a first, intrinsic reason for not being able to distinguish them readily, since what takes place is not distinguished by the phenomenal appearances to which we are accustomed.
There is a second, intrinsic reason. These vital phenomena are intermediary, as we shall see, between manifestations of known energies. They lie betweena chemical phenomenon which always precedes them, and a thermal phenomenon which always follows them. They are lost sight of, as it were, between manifestations which strike our attention. Generally speaking, intermediary energies often escape us even in physics. Only the extreme manifestations are clearly seen. In the presence of the organism we are, as it were, in electric lighting works which are run by a fall of water, and at first we only see the mechanical energy of the falling water, of the turbine and dynamo at work, and the photic energy of the lamps which give the light. Electrical energy, an intermediary, which has only a transient existence, does not impose itself on our attention.
And sovital energiesfor this twofold reason, intrinsic and extrinsic, are not readily apparent. To reveal them, the careful analysis of the physiologists is required. They are acts, in most cases silent and invisible, which we should scarcely recognize but by their effects, after they have terminated in familiar, phenomenal forms. This is, for example, what goes on in the muscle in process of shortening, in the nerve carrying the nervous influx, in the secreting gland. And this is what constitutes the different forms of energy which we callvital properties. M. Chauveau and M. Laulanié use the phrasephysiological workto distinguish them.Vital energywould be preferable. It better expresses the analogy of this special form with the other forms of universal energy; it helps us better to understand that we must henceforth consider it as exchangeable by means of equivalents with the energies of the physical world just as they are exchangeable one with another.
It is easy to understand, after these remarks, the significance and the scope of this assertion which contains the first principle of biological energetics—namely, that the phenomena of life have the same claim to be called energetic metamorphoses as the other phenomena of nature.
Irreversibility of Vital Energies.—However, there is one characteristic of vital energies which deserves the closest attention. Their transformations have a direction which is in some measure inevitable. They descend a slope which they never re-ascend. They appear to be irreversible. Ostwald has rightly insisted on this fundamental characteristic, which no doubt is not that of all the phenomena of the living being without exception, but which is certainly that of the most essential phenomena. There are reversible phenomena in organisms; there are energetic transformations which may take place from one form of energy to another, orvice versâ. But the most characteristic phenomena of vitality do not act in this way. We shall presently see that most functional physiological acts begin with chemical and end with thermal action. The series of energetic transformations takes place in an inevitable direction, from chemical to thermal energy. The order of succession of ordinary energies is thus determined in the machine of the organism, and therefore by the conditions of the machine. The order of transformation of vital energies is still more rigorously regulated, and the phenomena of life evolve from childhood to ripened years, and thence to old age, without a possible return.
The laws of biological energetics are three in number. First of all, there is the fundamental principle which we have just developed, and which is, so to speak, laid downa priori; and there are two other principles, those established by experiment and summing-up, as it were, the multitude of known physiological effects. Of these two experimental laws, one refers to theoriginand the other to thetermination of the energies developed in living beings.
The Origin of Vital Energy.—Vital energies have their origin in one of theexternal or common energies—not in any one we choose, as might be supposed, but in one only: chemical energy. The third principle will show us that they terminate in another energy or a few others, also completely fixed.
It follows that the phenomena of life must appear to us to be a circulation of energy which, starting from one fixed point in the physical world, returns to that world by a few points, also fixed, after a transient passage through the animal organism.
Or more precisely, it is a transposition from the realm of matter into the world of energy, of the idea of thevital vortexof Cuvier and the biologists. They defined life by its most constant property—nutrition. Nutrition was exactly this current of matter which the organism obtains from without by alimentation, and which it throws out again by excretion; and the even momentary interruption of which, if complete, would be the signal of death. The cycle of energy is the exact counterpart of this cycle of matter.
The second truth taught us by general physiology, a truth which physiology learned from experiment, is enunciated as follows:—The maintenance of life consumes none of its energy. It borrows from the external world all the energy which it expends, and borrows it in the form of potential chemical energy.This is a translation into the language of energetics of the results acquired in animal physiology during the last fifty years. No comment is needed to exhibit the importance of such a truth. It reveals the origin of animal activity. It reveals the source from which proceeds that energy which at some moment of its transformations in the animal organism will be avital energy.
Theprimum movensof vital activity is, therefore, according to this law, the chemical energy stored up in the immediate principles of the organism.
Let us try to follow, for a moment, this energy through the organism and to specify the circumstances of its transformations.
Organic Functional Activity, and the Destruction of Reserve-stuff.—Let us suppose then, for this purpose, that our attention is directed to a given limited part of this organism, to a certain tissue. Let us seize it, so to speak, by observation at a given moment, and let us make an examination of the functional activity starting from this conventional moment. This functional activity, like all other vital phenomena, will be the result, as we have just explained, of a transformation of the potential chemical energy contained in the materials held in reserve in the tissue. This is our first perceptible fact. This energy, when disengaged, will furnish to the vital action the means by which it may be prolonged.
There is, then, afunctional destruction. There is, atthe beginning of the functional process, and by a necessary effect of that very process, a liberation of chemical energy; and that can only take place by a decomposition of the immediate principles of the tissue, or, as we may say, by a destruction of organic material. Claude Bernard insisted on this consideration, that the vital function is accompanied by a destruction of organic material. “When a movement is produced, when a muscle is contracted, when volition and sensibility are manifested, when thought is exercised, when a gland secretes, then the substance of the muscles, of the nerves, of the brain, of the glandular tissue, is disorganized, is destroyed, and is consumed.” Energetics enables us to grasp the deeply-seated reason of this coincidence between chemical destruction and the functional activity, the existence of which Claude Bernard intuitively suspected. A portion of organic material is decomposed, is chemically simplified, becomes less complex, and loses in this kind of descent the chemical energy which it contained in its potential state. It is this energy which becomes the very texture of the vital phenomenon.
It is clear that the reserve of energy thus expended must be replaced, because the organism remains in equilibrium. Alimentation provides for this.
How does it provide for it? This is a question which deserves detailed examination. We cannot incidentally treat it in full; we can only indicate its main features.
How the supply of Reserve Stuff is kept up.—We know that food does not directly replace the reserve of energy consumed by the functional activity. It is not its potential chemical energy which replaces,purely and simply, the energy brought into play, consumed, or, better still, transformed in the active organ, or tissue. Food as it is introduced, inert food, does not, in fact, take up its placeas it is, without undergoing changes in that organ and that tissue, in order to restore thestatus quo ante.
Before building up the tissue it will have undergone various modifications in the digestive apparatus. It will have also undergone changes in the circulatory apparatus, in the liver, and in the very organ we are considering. It is after all these changes that assimilation takes place. It will find its place and will have then passed into the state ofreserve.
The food digested, modified, and finally incorporated as an integral part in the tissue in which it will be expended, is therefore in a new state, differing more or less from its state when it was ingested. It is a part of the living tissue in the state of constitutive reserve. Its potential chemical energy is not the same as that of the food introduced. It may differ from it very remarkably in consequence of sudden alterations.
We do not know for certain at the expense of what category of foods this or that given organ builds up its reserve stuff. There is a belief, for instance, according to M. Chauveau, that the muscle does its work at the expense of the reserve of glycogen which it contains. The potential chemical energy of this substance would be a source of muscular mechanical energy. But we do not know exactly at the expense of what foods, albumenoids, fats, or carbohydrates the muscle builds up the reserve ofglycogenexpended during its contraction. It is probable that it builds it up at the expense of each of the three categories after the various more or less simple alterations undergoneby the materials in the digestive tube, the blood, the liver, or other organs.
This building up of reserve stuff, the complement and counterpart offunctional destruction, is not chemical synthesis. It is, on the contrary, generally, and on the whole, a simplification of the food that has been introduced. This is true, at least as far as the muscle is concerned. However, to this operation, Claude Bernard has given the name oforganizing synthesis, but the phrase is not a happy one. But in no case was the eminent physiologist deceived as to the character of the operation. “The organizing synthesis,” says he, “remains internal, silent, hidden in its phenomenal expression, gathering together noiselessly the materials which will be expended.”
These considerations enable us to understand the existence of the two great categories into which the eminent physiologist divides the phenomena of animal life: the phenomena of thedestruction of reserve-stuffcorresponding tofunctional facts—that is to say expenditures of energy; and theplastic phenomenaof thebuilding-up of reservesof organic regeneration, corresponding tofunctional repose—i.e., to the supply of food to the tissues.
Distinction between Active Protoplasm and Reserve-stuff.—If it is not exactly in these terms that Claude Bernard formulated this fruitful idea, it is at any rate in this way that it is to be interpreted. This can be done by giving it a little more precision. We apply more rigorously than that great physiologist the distinction drawn by himself betweenreally active and living protoplasmand thereserve-stuffwhich it prepares. To the latter is restricted thedestruction by the functional activity and the building up by repose.
The classification of Claude Bernard is strictly true for reserve-stuff. It is easy to criticize the wavering and, as it were, dimly groping expressions in which the celebrated physiologist has shrouded his ideas. The old adage will excuse him:Obscuritate rerum verba obscurantur. In the depths of his ignorance he had a flash of genius; perhaps he did not find the definitive and, as it were, clearly-cut formula defining what was in his mind. But, in this respect, he has left his successors an easy task.
The Law of Functional Assimilation.—The progress of physiological knowledge compels us therefore to distinguish in the constitution of anatomical elements two parts—the materials ofreserve-stuffand thereally activeandliving protoplasm. We have just seen how the reserve-stuff behaves, alternately destroyed by functional activity, and built up afterwards by the ingestion of food, followed by the operations of digestion, elaboration, and assimilation. It remains to ask how this really living and protoplasmic matter behaves. Does it follow the same law? Is it destroyed during the functional activity, and is it afterwards replaced? As to this we can express no opinion. M. le Dantec fills a gap in our knowledge, in this respect, by an hypothesis. He assumes that this essentially active matter grows during functional activity, and is destroyed during repose. This is what he calls thelaw of functional assimilation. The protoplasm would therefore behave in an exactly contrary manner to the reserve-stuff. It will be its counterpart. But this is only an hypothesis which, in the present state of our knowledge, cannot beverified by experiment We are at liberty to assert either that the protoplasm increases by functional activity or that it is destroyed. Neither the arguments nor the objections pro or con have any decisive value. The facts alleged on either side are capable of too many interpretations.[10]
The only favourable argument (not demonstrative) is furnished by energetics. It is this. There-building of the protoplasmis not like theorganisation of reserve-stuff, a slightly complicated or even simplified phenomenon, as happens in the case of the reserve of muscular glycogen. The glycogen, in fact, is built up at the expense of foods chemically more complex. It is, on the contrary, a clearly synthetic phenomenon, certainly of chemical complexity, since it ends in building up the active protoplasm which is, in some measure, of the highest scale of complexity. Its formation at the expense of the simplest alimentary materials requires, therefore, an appreciable quantity of energy.
The assimilation which organizes the active protoplasm therefore requires energy for its realization. Now, at the moment of functional activity, and by a necessary consequence thereof, the chemical destruction or simplification of the substance of reserve takes place. Here is something that meets the case, and we may note the coincidence. It does not mean that the disposable energy is really used to increase the protoplasm, nor that the protoplasm itself is thereby increased. It merely signifies that the wherewithal exists to provide for that increase if it takes place.
It is thereforepossiblethat the active protoplasm follows the law of functional assimilation; but it iscertainthat the reserve-stuff follows the law laid down by Claude Bernard.
All these considerations definitely result in the confirmation of this second law of general physiology, according to which all vital energies are borrowed from the potential chemical energy of the reserve-stuff of alimentary origin.
The third law of biological energetics is also drawn from experiment. It relates no longer to the point of departure of the cycle of animal energy, but to its final position.The energetic transformations of the animal end in thermal energy.
This is the most novel part of the theory, and, if we may say so, that least understood by physiologists themselves. The energy resulting from the chemical potential of food, having passed through the organism (or simply through the organ which we are considering in action), and having given rise to phenomenal appearances more or less diversified, more or less dim or clear, obscure or obvious, which are the characteristic or still irreducible manifestations of vitality, finallyreturns to the physical world. This return takes place (with certain exceptions which will be presently indicated) under the ultimate form of thermal energy. This we are taught by experiment. The phenomena of functional activity are exothermal.
Real vital phenomena thus lie between the chemical energy which gives rise to them, and the thermal phenomena to which they in their turn give rise. The place of the vital fact in the cycle of universal energy is therefore completely determined. This conclusion is of the utmost importance to biology. It may be expressed in a concise formula which sums up in a few words all that natural philosophy can teach as to energetics applied to living beings. “Vital energy is a transformation of chemical energy into thermal energy.”
Exceptions.—There are some exceptions to the rigour of this statement, but they are not many in number. We must first of all remark that it applies toanimal lifealone.
In the case of vegetables, looked at as a whole, the law must be modified. Their vital energy has another origin, and another final form. Instead of being the destroyers of chemical potential energy, they are its creators. They build up by means of the inert and simple materials afforded them by the atmosphere and the soil, the immediate principles by which their cells are filled. Their vital functional activity forms by synthesis of the reserves, carbo-hydrates (sugars and starches), fats, albuminoid nitrogenous materials—that is to say, the same three principal categories of foods as those used by animals.
And to return to the latter, it should be observed that thermal energy is not the only final form of vitalenergy, as this dogmatic statement would have it supposed. It is only the principle of the final forms. The cycle of energy occasionally terminates in mechanical energy (phenomena of motion) and in a less degree in other energies; such as, for example, the electrical energy produced by the functional activity of the nerves and muscles in all animals, or in the functional activity of special organs in rays, torpedo-fish, and the malapterurus electricus, or finally, in the photic energy of phosphorescent animals. But these are secondary facts.
Heat is an Excretum.-The third principle of biological energetics may be therefore thus enunciated:—Vital energy in its final form becomes thermal energy.This principle teaches us that if chemical energy is the primitive generating form of vital energies, thermal energy is the form of waste, of emunctory, the degraded form as the physicists would say. Heat is in the dynamical order an excretion of animal life, as urea, carbonic acid and water, are excreta in the substantial order. By a false interpretation of the principle of the mechanical equivalence of heat, or through ignorance of Carnot’s principle, certain physiologists have fallen into error when they still speak of the transformation of heat into motion or into into electricity in the animal organism. Heat is transformed into nothing in the animal organism. It is dissipated. Its utility arises not from its energetic value, but from the part it plays as a primer in the chemical reactions, as has been explained with reference to the general characteristics of chemical energy.
The Effect of Energetics on our Knowledge of the Relations of the Universe.—The consequences of these principles of energetic physiology, which give us somuch and which are so clear, are of the greatest importance from the practical as well as from the theoretical point of view.
In the first place, they show us the position and the rank of the phenomena of life in the universe as a whole. They throw fresh light on the noble harmony of the animal and vegetable kingdoms which Priestley, Ingenhousz, Senebier, and the chemical school of the beginning of the nineteenth century discovered, and which was expounded by Dumas with incomparable lucidity and brilliance. Energetics is expressed in a line. “The animal world expends the energy accumulated by the vegetable world.” It extends these views beyond the living kingdoms. It shows how the vegetable world itself draws its activity from the energy radiated by the sun, and how animals restore it again, in dissipated heat, to the cosmic medium. It extends the harmony of the two kingdoms to the whole of nature. The new science makes of the whole universe one connected system.
From a more limited point of view, and so that we may not restrict ourselves to a consideration of the domain of animal physiology, the laws of energetics sum up and explain a multitude of facts and of experimental laws—for example, the law of the intermittence of physiological activity, the facts of fatigue, the rôle and the general principles of alimentation, and the conditions of muscular contraction.