The Principles of Biology Vol. I/Chapter I.3a
CHAPTER IIIA.
METABOLISM.
§ 23a. In the early forties the French chemist Dumas pointed out the opposed actions of the vegetal and animal kingdoms: the one having for its chief chemical effect the decomposition of carbon-dioxide, with accompanying assimilation of its carbon and liberation of its oxygen, and the other having for its chief chemical effect the oxidation of carbon and production of carbon-dioxide. Omitting those plants which contain no chlorophyll, all others de-oxidize carbon; while all animals, save the few which contain chlorophyll, re-oxidize carbon. This is not, indeed, a complete account of the general relation; since it represents animals as wholly dependent on plants, either directly or indirectly through other animals, while plants are represented as wholly independent of animals; and this last representation though mainly true, since plants can obtain direct from the inorganic world certain other constituents they need, is in some measure not true, since many with greater facility obtain these materials from the decaying bodies of animals or from their excreta. But after noting this qualification the broad antithesis remains as alleged.
How are these transformations brought about? The carbon contained in carbon-dioxide does not at a bound become incorporated in the plant, nor does the substance appropriated by the animal from the plant become at a bound carbon-dioxide. It is through two complex sets of changes that these two ultimate results are brought about. The materials forming the tissues of plants as well as the materials contained in them, are progressively elaborated from the inorganic substances; and the resulting compounds, eaten and some of them assimilated by animals, pass through successive changes which are, on the average, of an opposite character: the two sets being constructive and destructive. To express changes of both these natures the term "metabolism" is used; and such of the metabolic changes as result in building up from simple to compound are distinguished as "anabolic," while those which result in the falling down from compound to simple are distinguished as "katabolic." These antithetical names do not indeed cover all the molecular transformations going on. Many of them, known as isomeric, imply neither building up nor falling down: they imply re-arrangement only. But those which here chiefly concern us are the two opposed kinds described.
A qualification is needful. These antithetic changes must be understood as characterizing plant-life and animal-life in general ways rather than in special ways—as expressing the transformations in their totalities but not in their details. For there are katabolic processes in plants, though they bear but a small ratio to the anabolic ones; and there are anabolic processes in animals, though they bear but a small ratio to the katabolic ones.
From the chemico-physical aspect of these changes we pass to those distinguished as vital; for metabolic changes can be dealt with only as changes effected by that living substance called protoplasm.
§ 23b. On the evolution-hypothesis we are obliged to assume that the earliest living things—probably minute units of protoplasm smaller than any the microscope reveals to us—had the ability to appropriate directly from the inorganic world both the nitrogen and the materials for carbo-hydrates without both of which protoplasm cannot be formed; since in the absence of preceding organic matter there was no other source. The general law of evolution as well as the observed actions of Protozoa and Protophyta, suggest that these primordial types simultaneously displayed animal-life and plant-life. For whereas the developed animal-type cannot form from its inorganic surroundings either nitrogenous compounds or carbo-hydrates; and whereas the developed plant-type, able to form carbo-hydrates from its inorganic surroundings, depends for the formation of its protoplasm mainly, although indirectly, on the nitrogenous compounds derived from preceding organisms, as do also most of the plants devoid of chlorophyll—the fungi; we are obliged to assume that in the beginning, along with the expending activities characterizing the animal-type, there went the accumulating activities characterizing both of the vegetal types—forms of activity by-and-by differentiated.
Though the successive steps in the artificial formation of organic compounds have now gone so far that substances simulating proteids, if not identical with them, have been produced, yet we have no clue to the conditions under which proteids arose; and still less have we a clue to the conditions under which inert proteids became so combined as to form active protoplasm. The essential fact to be recognized is that living matter, originated as we must assume during a long stage of progressive cooling in which the infinitely varied parts of the Earth's surface were slowly passing through appropriate physical conditions, possessed from the outset the power of assimilating to itself the materials from which more living matter was formed; and that since then all living matter has arisen from its self-increasing action. But now, leaving speculation concerning these anabolic changes as they commenced in the remote past, let us contemplate them as they are carried on now—first directing our attention to those presented in the vegetal world.
§ 23c. The decomposition of carbon-dioxide ( § 13)—the separation of its carbon from the combined oxygen so that it may enter into one or other form of carbo-hydrate,—is not now ordinarily effected, as we must assume it once was, by the undifferentiated protoplasm; but is effected by a specialized substance, chlorophyll, imbedded in the protoplasm and operating by its instrumentality. The chlorophyll-grain is not simply immersed in protoplasm but is permeated throughout its substance by a protoplasmic network or sponge-work apparently continuous with the protoplasm around; or, according to Sachs, consists of protoplasm holding chlorophyll-particles in suspension: the mechanical arrangement facilitating the chemical function. The resulting abstraction of carbon from carbon-dioxide, by the aid of certain ethereal undulations, appears to be the first step in the building up of organic compounds—the first step in the primary anabolic process. We are not here concerned with details. Two subsequent sets of changes only need here to be noted—the genesis of the passive materials out of which plant-structure is built up, and the genesis of the active materials by which these are produced and the building up effected.
The hydrated carbon which protoplasm, having the chlorophyll-grain as its implement, produces from carbonic acid and water, appears not to be of one kind only. The possible carbo-hydrates are almost infinite in number. Multitudes of them have been artificially made, and numerous kinds are made naturally by plants. Though perhaps the first step in the reduction of the carbon from its dioxide may be always the same, yet it is held probable that in different types of plants different types of carbo-hydrates forthwith arise, and give differential characters to the compounds subsequently formed by such types: sundry of the changes being katabolic rather than anabolic. Of leading members in the group may be named dextrin, starch, and the various sugars characteristic of various plants, as well as the cellulose elaborated by further anabolism. Considered as the kind of carbo-hydrate in which the products of activity are first stored up, to be subsequently modified for divers purposes, starch is the most important of these; and the process of storage is suggested by the structure of the starch-grain. This consists of superposed layers, implying intermittent deposits: the probability being that the variations of light and heat accompanying day and night are associated now with arrest of the deposit and now with recommencement of it. Like in composition as this stored-up starch is with sugar of one or other kind, and capable of being deposited from sugar and again assuming the sugar form, this substance passes, by further metabolism, here into the cellulose which envelopes each of the multitudinous units of protoplasm, there into the spiral fibres, annuli, or fenestrated tubes which, in early stages of tissue-growth, form channels for the sap, and elsewhere into other components of the general structure. The many changes implied are effected in various ways: now by that simple re-arrangement of components known as isomeric change; now by that taking from a compound one of its elements and inserting one of another kind, which is known as substitution; and now by oxidation, as when the oxy-cellulose which constitutes wood-fibre, is produced.
Besides elaborating building materials, the protoplasm elaborates itself—that is, elaborates more of itself. It is chemically distinguished from the building materials by the presence of nitrogen. Derived from atmospheric ammonia, or from decaying or excreted organic matter, or from the products of certain fungi and microbes at its roots, the nitrogen in one or other combination is brought into a plant by the upward current; and by some unknown process (not dependent on light, since it goes on equally well if not better in darkness) the protoplasm dissociates and appropriates this combined nitrogen and unites it with a carbo-hydrate to form one or other proteid—albumen, gluten, or some isomer; appropriating at the same time from certain of the earth-salts the requisite amount of sulphur and in some cases phosphorus. The ultimate step, as we must suppose, is the formation of living protoplasm out of these non-living proteids. A cardinal fact is that proteids admit of multitudinous transformations; and it seems not improbable that in protoplasm various isomeric proteids are mingled. If so, we must conclude that protoplasm admits of almost infinite variations in nature. Of course pari passu with this dual process—augmentation of protoplasm and accompanying production of carbo-hydrates—there goes extension of plant-structure and plant-life.
To these essential metabolic processes have to be added certain ancillary and non-essential ones, ending in the formation of colouring matters, odours, essential oils, acrid secretions, bitter compounds and poisons: some serving to attract animals and others to repel them. Sundry of these appear to be excretions—useless matters cast out, and are doubtless katabolic.
The relation of these facts here sketched in rude outline to the doctrine of Evolution at large should be observed. Already we have seen how ( § 8a), in the course of terrestrial evolution, there has been an increasingly heterogeneous assemblage of increasing heterogeneous compounds, preparing the way for organic life. And here we may see that during the development of plant-life from its lowest algoid and fungoid forms up to those forms which constitute the chief vegetal world, there has been an increasing number of complex organic compounds formed; displayed at once in the diversity of them contained in the same plant and in the still greater diversity displayed in the vast aggregate of species, genera, orders, and classes of plants.
§ 23d. On passing to the metabolism characterizing animal life, which, as already indicated, is in the main a process of decomposition undoing the process of composition characterizing vegetal life, we may fitly note at the outset that it must have wide limits of variation, alike in different classes of animals and even in the same animal.
If we take, on the one hand, a carnivore living on muscular tissue (for wild carnivores preying upon herbivores which can rarely become fat obtain scarcely any carbo-hydrates) and observe that its food is almost exclusively nitrogenous; and if, on the other hand, we take a graminivorous animal the food of which (save when it eats seeds) contains comparatively little nitrogenous matter; we seem obliged to suppose that the parts played in the organic processes by the proteids and the carbo-hydrates can in considerable measures replace one another. It is true that the quantity of food and the required alimentary system in the last case, are very much greater than in the first case. But this difference is mainly due to the circumstance that the food of the graminivorous animal consists chiefly of waste-matter—ligneous fibre, cellulose, chlorophyll—and that could the starch, sugar, and protoplasm be obtained without the waste-matter, the required bulks of the two kinds of food would be by no means so strongly contrasted. This becomes manifest on comparing flesh-eating and grain-eating birds—say a hawk and a pigeon. In powers of flight these do not greatly differ, nor is the size of the alimentary system conspicuously greater in the last than in the first; though probably the amount of food consumed is greater. Still it seems clear that the supply of energy obtained by a pigeon from carbo-hydrates with a moderate proportion of proteids is not widely unlike that obtained by a hawk from proteids alone. Even from the traits of men differently fed a like inference may be drawn. On the one hand we have the Masai who, during their warrior-days, eat flesh exclusively; and on the other hand we have the Hindus, feeding almost wholly on vegetable food. Doubtless the quantities required in these cases differ much; but the difference between the rations of the flesh-eater and the grain-eater is not so immense as it would be were there no substitution in the physiological uses of the materials.
Concerning the special aspects of animal-metabolism, we have first to note those various minor transformations that are auxiliary to the general transformation by which force is obtained from food. For many of the vital activities merely subserve the elaboration of materials for activity at large, and the getting rid of waste products. From blood passing through the salivary glands is prepared in large quantity a secretion containing among other matters a nitrogenous ferment, ptyaline, which, mixed with food during mastication, furthers the change of its starch into sugar. Then in the stomach come the more or less varying secretions known in combination as gastric juice. Besides certain salts and hydrochloric acid, this contains another nitrogenous ferment, pepsin, which is instrumental in dissolving the proteids swallowed. To these two metabolic products aiding solution of the various ingested solids, is presently added that product of metabolism in the pancreas which, added to the chyme, effects certain other molecular changes—notably that of such amylaceous matters as are yet unaltered, into saccharine matters to be presently absorbed. And let us note the significant fact that the preparation of food-materials in the alimentary canal, again shows us that unstable nitrogenous compounds are the agents which, while themselves changing, set up changes in the carbo-hydrates and proteids around: the nitrogen plays the same part here as elsewhere. It does the like in yet another viscus. Blood which passes through the spleen on its way to the liver, is exposed to the action of "a special proteid of the nature of alkali-albumin, holding iron in some way peculiarly associated with it." Lastly we come to that all-important organ the liver, at once a factory and a storehouse. Here several metabolisms are simultaneously carried on. There is that which until recent years was supposed to be the sole hepatic process—the formation of bile. In some liver-cells are masses of oil-globules, which seem to imply a carbo-hydrate metamorphosis. And then, of leading importance, comes the extensive production of that animal-starch known as glycogen—a substance which, in each of the cells generating it, is contained in a plexus of protoplasmic threads: again a nitrogenous body diffused through a mass which is now formed out of sugar and is now dissolved again into sugar. For it appears that this soluble form of carbo-hydrate, taken into the liver from the intestine, is there, when not immediately needed, stored up in the form of glycogen, ready to be re-dissolved and carried into the system either for immediate use or for re-deposit as glycogen at the places where it is presently to be consumed: the great deposit in the liver and the minor deposits in the muscles being, to use the simile of Prof. Michael Foster, analogous in their functions to a central bank and branch banks.
An instructive parallelism may be noted between these processes carried on in the animal organism and those carried on in the vegetal organism. For the carbo-hydrates named, easily made to assume the soluble or the insoluble form by the addition or subtraction of a molecule of water, and thus fitted sometimes for distribution and sometimes for accumulation, are similarly dealt with in the two cases. As the animal-starch, glycogen, is now stored up in the liver or elsewhere and now changed into glucose to be transferred, perhaps for consumption and perhaps for re-deposit; so the vegetal starch, made to alternate between soluble and insoluble states, is now carried to growing parts where by metabolic change it becomes cellulose or other component of tissue and now carried to some place where, changed back into starch, it is laid aside for future use; as it is in the turgid inside leaves of a cabbage, the root of a turnip, or the swollen underground stem we know as a potato: the matter which in the animal is used up in generating movement and heat, being in the plant used up in generating structures. Nor is the parallelism even now exhausted; for, as by a plant starch is stored up in each seed for the subsequent use of the embryo, so in an embryo-animal glycogen is stored up in the developing muscles for subsequent use in the completion of their structures.
§ 23e. We come now to the supreme and all-pervading metabolism which has for its effects the conspicuous manifestations of life—the nervous and muscular activities. Here comes up afresh a question discussed in the edition of 1864—a question to be reconsidered in the light of recent knowledge—the question what particular metabolic changes are they by which in muscle the energy existing under the form of molecular motion is transformed into the energy manifested as molar motion?
There are two views respecting the nature of this transformation. One is that the carbo-hydrate present in muscle must, by further metabolism, be raised into the form of a nitrogenous compound or compounds before it can be made to undergo that sudden decomposition which initiates muscular contraction. The other is the view set forth in § 15, and there reinforced by further illustrations which have occurred to me while preparing this revised edition—the view that the carbo-hydrate in muscle, everywhere in contact with unstable nitrogenous substance, is, by the shock of a small molecular change in this, made to undergo an extensive molecular change, resulting in the oxidation of its carbon and consequent liberation of much molecular motion. Both of these are at present only hypotheses, in support of which respectively the probabilities have to be weighed. Let us compare them and observe on which side the evidence preponderates.
We are obliged to conclude that in carnivorous animals the katabolic process is congruous with the first of these views, in so far that the evolution of energy must in some way result solely from the fall of complex nitrogenous compounds into those simpler matters which make their appearance as waste; for, practically, the carnivorous animal has no carbo-hydrates out of which otherwise to evolve force. To this admission, however, it should be added that possibly out of the exclusively nitrogenous food, glycogen or sugar has to be obtained by partial decomposition before muscular action can take place. But when we pass to animals having food consisting mainly of carbo-hydrates, several difficulties stand in the way of the hypothesis that, by further compounding, proteids must be formed from the carbo-hydrates before muscular energy can be evolved. In the first place the anabolic change through which, by the addition of nitrogen, &c., a proteid is formed from a carbo-hydrate, must absorb an energy equal to a moiety of that which is given out in the subsequent katabolic change. There can be no dynamic profit on such part of the transaction as effects the composition and subsequent decomposition of the proteid, but only on such part of the transaction as effects the decomposition of the carbo-hydrate. In the second place there arises the question—whence comes the nitrogen required for the compounding of the carbo-hydrates into proteids? There is none save that contained in the serum-albumen or other proteid which the blood brings; and there can be no gain in robbing this proteid of nitrogen for the purpose of forming another proteid. Hence the nitrogenizing of the surplus carbo-hydrates is not accounted for. One more difficulty remains. If the energy given out by a muscle results from the katabolic consumption of its proteids, then the quantity of nitrogenous waste matters formed should be proportionate to the quantity of work done. But experiments have proved that this is not the case. Long ago it was shown that the amount of urea excreted does not increase in anything like proportion to the amount of muscular energy expended; and recently this has been again shown.
On this statement a criticism has been made to the following effect:—Considering that muscle will contract when deprived of oxygen and blood and must therefore contain matter from which the energy is derived; and considering that since carbonic acid is given out the required carbon and oxygen must be derived from some component of muscle; it results that the energy must be obtained by decomposition of a nitrogenous body. To this reasoning it may be objected, in the first place, that the conditions specified are abnormal, and that it is dangerous to assume that what takes place under abnormal conditions takes place also under normal ones. In presence of blood and oxygen the process may possibly, or even probably, be unlike that which arises in their absence: the muscular substance may begin consuming itself when it has not the usual materials to consume. Then, in the second place, and chiefly, it may be replied that the difficulty raised in the foregoing argument is not escaped but merely obscured. If, as is alleged, the carbon and oxygen from which carbonic acid is produced, form, under the conditions stated, parts of a complex nitrogenous substance contained in muscle, then the abstraction of the carbon and oxygen must cause decomposition of this nitrogenous substance; and in that case the excretion of nitrogenous waste must be proportionate to the amount of work done, which it is not. This difficulty is evaded by supposing that the "stored complex explosive substance must be, in living muscle, of such nature" that after explosion it leaves a "nitrogenous residue available for re-combination with fresh portions of carbon and oxygen derived from the blood and thereby the re-constitution of the explosive substance." This implies that a molecule of the explosive substance consists of a complex nitrogenous molecule united with a molecule of carbo-hydrate, and that time after time it suddenly decomposes this carbo-hydrate molecule and thereupon takes up another such from the blood. That the carbon is abstracted from the carbo-hydrate molecule can scarcely be said, since the feebler affinities of the nitrogenous molecule can hardly be supposed to overcome the stronger affinities of the carbo-hydrate molecule. The carbo-hydrate molecule must therefore be incorporated bodily. What is the implication? The carbo-hydrate part of the compound is relatively stable, while the nitrogenous part is relatively unstable. Hence the hypothesis implies that, time after time, the unstable nitrogenous part overthrows the stable carbo-hydrate part, without being itself overthrown. This conclusion, to say the least of it, does not appear very probable.
The alternative hypothesis, indirectly supported as we saw by proofs that outside the body small amounts of change in nitrogenous compounds initiate large amounts of change in carbonaceous compounds, may in the first place be here supported by some further indirect evidences of kindred natures. A haystack prematurely put together supplies one. Enough water having been left in the hay to permit chemical action, the decomposing proteids forming the dead protoplasm in each cell, set up decomposition of the carbo-hydrates with accompanying oxidation of the carbon and genesis of heat; even to the extent of producing fire. Again, as shown above, this relation between these two classes of compounds is exemplified in the alimentary canal; where, alike in the saliva and in the pancreatic secretion, minute quantities of unstable nitrogenous bodies transform great quantities of stable carbo-hydrates. Thus we find indirect reinforcements of the belief that the katabolic change generating muscular energy is one in which a large decomposition of a carbo-hydrate is set up by a small decomposition of a proteid.[1]
§ 23f. A certain general trait of animal organization may fitly be named because its relevance, though still more indirect, is very significant. Under one of its aspects an animal is an apparatus for the multiplication of energies—a set of appliances by means of which a minute amount of motion initiates a larger amount of motion, and this again a still larger amount. There are structures which do this mechanically and others which do it chemically.
Associated with the peripheral ends of the nerves of touch are certain small bodies—corpuscula tactus—each of which, when disturbed by something in contact with the skin, presses on the adjacent fibre more strongly than soft tissue would do, and thus multiplies the force producing sensation. While serving the further purpose of touching at a distance, the vibrissæ or whiskers of a feline animal achieve a like end in a more effectual way. The external portion of each bristle acts as the long arm of a lever, and the internal portion as the short arm. The result is that a slight touch at the outer end of the bristle produces a considerable pressure of the inner end on the nerve-terminal: so intensifying the impression. In the hearing organs of various inferior types of animals, the otolites in contact with the auditory nerves, when they are struck by sound-waves, give to the nerves much stronger impressions than these would have were they simply immersed in loose tissue; and in the ears of developed creatures there exist more elaborate appliances for augmenting the effects of aerial vibrations. From this multiplication of molar actions let us pass to the multiplication of molecular actions. The retina is made up of minute rods and cones, so packed together side by side that they can be separately affected by the separate parts of the images of objects. As each of them is but 1⁄10,000th of an inch in diameter, the ethereal undulations falling upon it can produce an amount of change almost infinitesimal—an amount probably incapable of exciting a nerve-centre, or indeed of overcoming the molecular inertia of the nerve leading to it. But in close proximity are layers of granules into which the rods and cones send fibres, and beyond these, about 1⁄100th of an inch from the retinal layer, lie ganglion-cells, in each of which a minute disturbance may readily evolve a larger disturbance; so that by multiplication, single or perhaps double, there is produced a force sufficient to excite the fibre connected with the centre of vision. Such, at least, judging from the requirement and the structure, seems to me the probable interpretation of the visual process; though whether it is the accepted one I do not know.
But now, carrying with us the conception made clear by the first cases and suggested by the last, we shall appreciate the extent to which this general physiological method, as we may call it, is employed. The convulsive action caused by tickling shows it conspicuously. An extremely small amount of molecular change in the nerve-endings produces an immense amount of molecular change, and resulting molar motion, in the muscles. Especially is this seen in one whose spinal cord has been so injured that it no longer conveys sensations from the lower limbs to the brain; and in whom, nevertheless, tickling of the feet produces convulsive actions of the legs more violent even than result when sensation exists: clearly proving that since the minute molecular change produced by the tickling in the nerve-terminals cannot be equivalent in quantity to the amount implied by the muscular contraction, there must be a multiplication of it in those parts of the spinal cord whence issue the reflex stimuli to the muscles.
Returning now to the question of metabolism, we may see that the processes of multiplication above supposed to take place in muscle, are analogous in their general nature to various other physiological processes. Carrying somewhat further the simile used in § 15 and going back to the days when detonators, though used for small arms, were not used for artillery, we may compare the metabolic process in muscle to that which would take place if a pistol were fired against the touch-hole of a loaded cannon: the cap exploding the pistol and the pistol the cannon. For in the case of the muscle, the implication is that a nervous discharge works in certain unstable proteids through which the nerve-endings are distributed, a small amount of molecular change; that the shock of this causes a much larger amount of molecular change in the inter-diffused carbo-hydrate, with accompanying oxidation of its carbon; and that the heat liberated sets up a transformation, probably isomeric, in the contractile substance of the muscular fibre: an interpretation supported by cases in which small rises and falls of temperature cause alternating isomeric changes; as instance Mensel's salt.
Ending here this exposition, somewhat too speculative and running into details inappropriate to a work of this kind, it suffices to note the most general facts concerning metabolism. Regarded as a whole it includes, in the first place, those anabolic or building-up processes specially characterizing plants, during which the impacts of ethereal undulations are stored up in compound molecules of unstable kinds; and it includes, in the second place, those katabolic or tumbling-down changes specially characterizing animals, during which this accumulated molecular motion (contained in the food directly or indirectly supplied by plants), is in large measure changed into those molar motions constituting animal activities. There are multitudinous metabolic changes of minor kinds which are ancillary to these—many katabolic changes in plants and many anabolic changes in animals—but these are the essential ones.[2]
- ↑ This interpretation is said to be disproved by the fact that the carbo-hydrate contained in muscle amounts to only about 1.5 of the total solids. I do not see how this statement is to be reconciled with the statement cited three pages back from Professor Michael Foster, that the deposits of glycogen contained in the liver and in the muscles may be compared to the deposits in a central bank and branch banks.
- ↑ Before leaving the topic let me remark that the doctrine of metabolism is at present in its inchoate stage, and that the prevailing conclusions should be held tentatively. As showing this need an anomalous fact may be named. It was long held that gelatine is of small value as food, and though it is now recognized as valuable because serving the same purposes as fats and carbo-hydrates, it is still held to be valueless for structural purposes (save for some inactive tissue); and this estimate agrees with the fact that it is a relatively stable nitrogenous compound, and therefore unfit for those functions performed by unstable nitrogenous compounds in the muscular and other tissues. But if this is true, it seems a necessary implication that such substances as hair, wool, feathers, and all dermal growths chemically akin to gelatine, and even more stable, ought to be equally innutritive or more innutritive. In that case, however, what are we to say of the larva of the clothes-moth, which subsists exclusively on one or other of these substances, and out of it forms all those unstable nitrogenous compounds needful for carrying on its life and developing its tissues? Or again, how are we to understand the nutrition of the book-worm, which, in the time-stained leaves through which it burrows, finds no proteid save that contained in the dried-up size, which is a form of gelatine; or, once more, in what form is the requisite amount of nitrogenous substance obtained by the coleopterous larva which eats holes in wood a century old?