General Physiology: An Outline of the Science of Life

52), and found that, while they are undergoing digestion, the}r take a perfectly definite path within the cell-body — viz., from the cell-pharynx (Of. Vorticella, p. 146, Fig. 46) to the bottom of the cell and back to the mouth-opening, where the undigested masses are cast out. It is very noteworthy that the food-masses 1 Cf. Verworn ('89, 1). remain for a long time in the concavity which the sausage-shaped nucleus turns toward the interior of the cell, there chiefly to

FIG. 51.— An elongated pseudopodium of Lieberkuhnia in which an ini usorian (Colpidium colpoda) has become caught ; o, b, c, d, e, /, various stages of digestion of the inf usorian. undergo destruction. This indicates that very probably the nucleus takes an important share in the digestion of the food-masses. Just as proteids are transformed by pepsin in an acid and by trypsin in an alkaline solution, so also the insoluble carbohydrates, such as starch, are changed into soluble forms both in intracellular and extracellular digestion by the action of certain enzymes. As has been seen, starch is a polysaccharid, which represents a combination of several sugar molecules in the anhydride form. By the action of the enzyme, e.g., the ptyalin of the saliva and the pancre-

FIG. 52. — Carchesium polypinum, scheme of the path taken by the ingested food in digestion and expulsion of the excreta. The food enters through the pharynx and is transported downward (small circles), where it is stored in the concavity of the sausage-shaped nucleus (the latter is recognised by its containing darker bodies). It remains here for some time at rest (small crosses). Then it passes upward upon the other side (dots) and returns to the middle of the cell, where it undergoes dissohition. The excreta are removed to the outside, through the opening of the cell-mouth. The black line with arrows indicates the direction of the path. (After Greenwood.)

atic juice in animals or the diastase in plants, the polymeric starch molecule is split up through hydrolysis into simple sugar molecules, maltose and dextrose, which are soluble in water. In the intracellular digestion of Infusoria, as M. Meissner ('88) has shown, starch -grains are slowly digested from the outside, so that they appear as if gnawed (Fig. 53), and finally are completely dissolved. Yet from the striking researches of Greenwood ('86, '87)

and Meissner (loc. cit.) it appears that Rhizopoda such as Amoeba, although occasionally taking in starch, are nevertheless unable to digest it. Finally, fats in extracellular digestion are split up, likewise with hydration, by the fat-ferment steapsin into glycerine and fatty acids, the latter uniting with alkalies to form soaps. Glycerine and soaps are soluble and can be resorbed. In the intracellular ingestion of the neutral fat-droplets as such, however, a direct digestion does

not always take place. As FIG. SS.-Starch-grains which have been r. J. . -I * 7 devoured and digested by an infusorian. Meissner has observed, Amoeba (After M. Meissner.) and Infusoria retain ingested fatdroplets within their protoplasm for days unchanged, and Greenwood has found that Amoeba and Actinosphcerium do not digest ingested fat at all. The ferments are physiologically such an extremely interesting group of bodies that it is worth while to examine them somewhat in detail, and especially to become acquainted with their peculiar mode of working. By ferments there is understood a series of highly complex organic bodies belonging to animals and plants, which have the remarkable peculiarity of bringing about certain chemical transformations apparently ivithout undergoing changes themselves.

When two substances act upon each other in an ordinary chemical reaction, both undergo a chemical transformation. With the ferment this appears not to be the case, for, when a large quantity of a chemical compound has been split up by a certain quantity of an enzyme, the original quantity of enzyme is found unchanged in the liquid. Theoretically, an unlimited quantity of material can be decomposed by a small quantity of a ferment. Practically, however, this is usually not possible, because the effectiveness of the ferment gradually becomes diminished by the accumulation of substances resulting from the cleavage.

It is a question, however, whether the ferment, when acting upon other substances, really undergoes no decomposition or is itself destroyed and constantly re-formed, so that in the end the same quantity of ferment is found as at first. In inorganic chemistry there are cases analogous to each possibility. By the terms catalytic action and contact -action in the original sense, chemists understand the property possessed by many substances of decomposing chemical compounds by simple contact. Thus, Sainte-Claire Deville and Debray have found that formic acid can be split up into carbonic acid and hydrogen, not only by certain ferments, but also by finely divided iridium, rhodium and ruth-

enium, the molecules of the metals undergoing no change. These facts are explained as follows : It is known that according to the mechanical theory of heat the atoms in every molecule are in constant vibratory motion — a phenomenon that is termed intramolecular heat. Upon contact of the molecule of the metals in question with the complex molecule of formic acid this intramolecular vibration of the atoms of the former is transferred to the latter, and combines with the latter's vibration in such a way that another arrangement of atoms results — i.e., a decomposition of the formic acid molecule. According to a different idea, it is the chemical affinity between the atoms of the molecule of the metal and certain atoms of the formic acid molecule that disturbs the intramolecular vibrations of the formic acid atoms in such a way that a rearrangement, i.e., a decomposition, takes place, without, however, the occurrence of a real combination of the atoms of the metal with the corresponding atoms of formic acid. However it be, in every case the intramolecular motion of the atoms in the molecules that are to be broken up becomes disturbed, while the catalytic molecule of the metal remains intact. Such contact-actions are widely known in chemistry. Thus, hydrogen peroxide upon contact with finely divided platinum is changed into water and oxygen without the platinum itself being altered.

In contrast to these pure contact-effects, chemistry recognises other cases in which the effective body remains unchanged only apparently. While bringing about transformations, it is continually altered chemically, but is immediately re-formed again. The end-results in the two cases must be the same, for even in the latter case at the conclusion the body in question is found in its original form. We have already become acquainted elsewhere with such a case. In the manufacture of concentrated sulphuric acid the nitric acid is continually reduced by sulphurous anhydride into nitrous acid, to be re-formed again into nitric acid with the aid of the oxygen of the air.

Which of the two cases does the action of ferments resemble ? Thus far this question has not been decided with certainty. It is very probable, however, that among so-called ferment-actions both cases are present. In the large group of ferments two kinds are distinguished — dissolved unorganised ferments, or enzymes, and solid organised ferments, or ferment-organisms ; the former comprise secretions which are given off to the outside by the living cell and remain constantly effective, the latter consist of the living substance of the cell itself, with the life of which the ferment-action is associated. While in ferment-organisms the ferment-action is extinguished with the life of the cell, the enzymes can be preserved as long as desired as chemical bodies, without losing their power. The cells of yeast (Soccharomycex), which cause the alcoholic fermentation

of beer (Fig. 54), are ferment-organisms, decomposing grape-sugar into alcohol and carbonic acid.1 They produce, however, in addition an enzyme, invcrtin, which is able to convert cane-sugar into grape-sugar. The two actions can be separated from one another. If the yeast-cells be killed by chloroform or ether, it is no longer possible for them to decompose grape-sugar into alcohol and carbonic acid ; but the power of the inverting enzyme continues undiminished, so that the change of cane-sugar into grape-sugar goes on as well as before. In ferment-organisms the living substance exercises the ferment-action only so long as it lives, i.e., its ferment-action is associated with metabolism. This evidently indicates that in ferment-organisms there is realised the second case mentioned above, that which is analogous to the action of nitric acid in the manufacture of sulphuric acid ; while the peculiar fact that the action of the enzymes may be replaced by other FlG> bi.-SaccharomyCeS) yeast-ceiis. (After substances, e.g., metals, suggests Remke.)

also like finely divided metals by pure contact. At present, naturally, this question cannot be decided with absolute certainty. Like the organised ferments the enzymes are highly complex compounds, all of which probably contain nitrogen and are derived from the metabolism of proteids ; they are made ineffective by substances that enter into combination with proteids, as well as by boiling ; within certain limits, however, an increase of temperature is favourable to ferment-action, because thereby the intramolecular vibrations of the atoms are increased.

If the action of ferment-organisms depends actually upon a continual destruction and rebuilding of their own substance, then all living organisms may be regarded as ferment-organisms ; for all living substance transforms food-stuffs in its metabolism while not disappearing itself. Hence the metabolism of living substance can be compared with the metabolism of nitric acid in the above case. The digestion of food-stuffs by the action of ferments is only a preparation for the process of assimilation. Only after the foodstuffs have been brought into the condition in which they can do chemical work, i.e., after they have become dissolved, can their

function in the construction of living substance begin to be exercised. The process of assimilation naturally differs much according to the condition of the ingested food. Differences must be recognised also in assimilation by the two main groups of organisms, plants and animals, corresponding to the differences that have been recognised in their food. It is evident that the processes that lead to the formation of living substance in the plant-cell must constitute a much longer series than in the animal-cell, for the plant must construct the highly complex proteid molecule out of the simplest inorganic compounds, carbonic acid, water, salts and oxygen, while the animal obtains, already formed, the proteid food without which it cannot live, and only needs to use this in its specific manner. We will follow the processes that lead to the assimilation of proteids somewhat in detail in the two series, so far as in general they are known. The lack of our knowledge is realised here as elsewhere.

To consider first the plants, a simple experiment shows the first step which the plant takes in the series of processes that lead to assimilation. In a cylindrical tube, provided with a bulb closed above (Fig. 55) and graduated, a green leaf is placed by means of a wire, and a certain measured quantity of carbonic acid is allowed to flow in. The lower end of the tube is closed by means of mercury, and the whole is allowed to stand for some hours in the sunlight. If then the contents of the tube be tested gasometrically, it is found that the carbonic acid has disappeared, and in place of it an equal volume of oxygen is in the tube. Since the volume of the carbonic acid is equal to the volume of the oxygen contained in it, the experiment proves not only that the plant has taken up carbonic acid and given off oxygen, but also that it has given off as much oxygen as was contained in the carbonic acid. The first stage toward assimilation . in the plant is, therefore, a

FIG. 55.— Apparatus for the investigation of the cleavage of carbonic acid in the green parts of plants. (After Detmer.) cleavage of carbonic acid ; this takes place in the green plant-cell under the influence of sunlight. The plant gives off oxygen to the outside. As to the fate of the retained ' carbon, microscopic observation gives us information. It shows, namely, that in proportion to the destruction of the carbonic acid starch is formed in the chlorophyll-grains themselves, and is laid down in the form of small, highly refractive granules (Fig. 23, p. 81, and Fig. 56). Moreover, by a series of experiments Sachs has shown that as soon as the breaking-up of the carbonic acid ceases in darkness the formation of starch also ceases, immediately to begin again in the light along with the destruction of carbonic acid. Since starch contains, in addition to carbon, only hydrogen and oxygen in the same relative proportion as in water, it can be derived only by synthesis from the carbon that is set free arid the water that is received through the roots. Starch is, therefore, the first assimilation-product to appear.

" If," says Sachs, ('82)," starch is the first and sole visible product of assimilation, it follows directly that all other organic compounds of the plant must originate by chemical metamorphosis from it." It will be remembered that no carbon was present in the artificial nutrient solution in which plants were allowed to grow.1 If, therefore, laterthe plant manufactures other carbohydrates, fats, and finally proteids, all of which contain carbon, it can employ only starch as the starting-point. Of course almost nothing is known concerning the special chemical transformations which starch undergoes further. But an idea can be formed, at least in gross outline, of the further processes of assimilation. The fact that from the starch soluble varieties of sugar can be derived very easily by cleavage with hydration, is at once understood when it is borne in mind that starch is a polymeric molecule of the anhydride of sugar. Hence it can pass into the condition of the soluble carbohydrates, and this is necessary in order to make possible further chemical syntheses. The formation of fatty oils out of starch can also be directly observed. If unripe seeds of certain plants, e.g., Pceonia, which contain carbohydrates and no fats, be allowed to lie in moist air, it is found after some time that all starch has disappeared, but fatty oil has appeared in its place. But much more complicated is the origin of proteid from carbohydrates. Since in addition to the atoms of carbohydrate proteid contains nitrogen and sulphur,

FIG. 56. —Starch appearing as transparent scales in chlorophyll-bodies. A, Chlorophyllbodies lying in the cell. B, Chlorophyll-bodies undergoing division. (After Sachs.) which the plant receives through its roots from nitrates and sulphates only, complicated transformations of these salts and then syntheses with the carbohydrate atoms must take place, the details of which are thus far wholly unknown. As to how, finally, the proteid molecule, synthetically formed, is employed further in the living substance for purposes of construction, at present, on account of our extremely scanty knowledge of the chemical constitution of proteids, we can say absolutely nothing. Here an enormous field is offered for future physiological investigation.

In animals, the path from the ingested food to the living proteid molecule is of course essentially shorter, for all animals without exception need for their nutrition proteids already prepared. But what happens further to the proteids that have been peptonised by digestion is not fully known. After the investigations of Salvioli ('80), Hofmeister ('82), Neumeister ('90), and others, no doubt can be entertained that the peptones as such disappear in the cells of the wall of the intestine, in other words, they are transformed in the cell itself. If pieces of the intestinal mucous membrane of a rabbit be placed in a liquid that contains peptone, in which the cells of the intestinal wall exist during life, after some time it is found that all peptone has disappeared. If, moreover, a solution of peptone be injected into the blood of an animal, in a short time the whole quantity of peptone is excreted unchanged in the urine ; and in normal life the blood is always free from peptones. These two experiments prove undoubtedly that the peptones become changed on their way through the cells of the intestinal wall. But little is thus far known as to the kind of change within the cells. Perhaps some of the peptones are broken down immediately into simpler substances by a retrogressive proteid metamorphosis. It is certain that many are changed back into proteid and pass into the juices of the body along with the proteid resorbed directly without peptonisation. This dissolved proteid circulates throughout the body with the bloodcurrent, bathes the cells of all tissues, and is withdrawn by the cells from the blood, to be broken down within them. Hence it happens that in a remarkably short time all the proteid taken into the body, beyond a certain quantity, appears as urea, uric acid, etc., in the urine.

Voit ('81) thought that this proteid that is broken down ought to be distinguished as " circulating proteid " from the " tissue proteid/' which is employed for the formation of tissues, since he assumed that the destruction of the circulating proteid took place in the blood, in the liquids, of the body. But the reason for such a distinction has disappeared, since Pfliiger ('93) and Schondorff ('93) have shown recently by very careful investigations that the breaking-down of the proteid dissolved in the blood does not take place in the, blood itself, but in the tissue-

cells. Under certain circumstances, however, the cells also retain a small part of the proteid dissolved in the blood, either employing it for the increase of its living substance, as in growth, or storing it up in the protoplasm, as in fattening, in the form of reserve, i.e., passive, proteid which is not ordinarily consumed in metabolism. Under certain conditions, as in fasting or during the development of eggs, such passive, indifferent, reserve-proteid can again be drawn into the metabolism. The vitellin in egg-cells is such a substance.

Regarding the fate of the ingested fats and carbohydrates as few details are known as regarding the finer transformations of proteids. The fat which is taken as such into the cells frequently remains for a long time as reserve-material. Likewise the fat that is split up into glycerine and fatty acids and resorbed can be changed back into neutral fat in the cell ; this is proved by the striking experiments of J. Munk ('84), who, by feeding fat-free soaps or free fatty acids, was able to cause a storing-up of tissue-fat in dogs that had fasted and become extremely lean. In a similar manner the grape-sugar that is split off from the carbohydrates can be transformed synthetically into glycogen in the tissue-cells, especially in the cells of the liver and the muscles, and can be stored up as such. Regarding the further fate of this stored fat and glycogen, however, it is known only that they can be consumed during fasting and during excessive muscle-work, that, therefore, they represent a reserve-material which acts in cases of need as " compensation-food " in Pfltiger's sense.

Our knowledge of the processes of dissimilation of living substances is much more meagre than that of assimilation. We really know only that living substance is continually undergoing decomposition, for this is apparent from the output of decomposition-products. But as to the path from the complex proteid compounds to the end-products, as to the special chemical transformations that take place, our knowledge is very incomplete, since as yet the composition of proteids is known very slightly.

But one fact at least is certain, namely, that the most of all those substances that result from the decomposition of the proteid molecule are not groups of atoms that were preformed as such in the molecule and are now simply split off, but they are derived from certain cleavage-products by successive syntheses ; this takes place either at the moment of decomposition by rearrangement of the atoms in the proteid molecule itself, as in the case of carbonic acid, or later outside of the proteid molecule by combination with other cleavage-products and a simultaneous rearrangement of atoms, as is the case, e.g., in the formation of uric acid. Thus far it is not known

that any product of proteid-decomposition originates by the simple cleavage of preformed groups of atoms. It is important to become acquainted with the most essential derivatives of the disintegrating proteid molecule. As has been found by the investigation of the substances that are contained in living substance,1 there can be distinguished among these products of proteid transformation two groups — those containing nitrogen, and those not containing nitrogen. Representatives of each group appear in every cell, but their special composition differs in individual cases according to the characteristic metabolism of the cell. Among the substances that contain nitrogen the most wide-spread are urea, uric acid, hippuric acid, creatin, and the nuclein bases — xanthin, hypoxanthin or sarkin, guanin, and adenin. Regarding the majority of these substances, thus far it is not known how they originate from the decomposition of proteids, but for some at least hypotheses concerning their immediate forerunners have been formed. Thus, from the fact, which Schroder discovered, that ammonium carbonate introduced into the fresh, excised, still-living liver of a dog leaves the liver as urea, it has been supposed that ammonium carbonate is the forerunner of urea, that from it the livercells prepare urea by a transformation of the atoms and the giving-off of two molecules of water : —

But this is not conclusive, it is only a provisional hypothesis, for the possibility is not to be excluded summarily that within the organism itself still other substances are employed for the synthesis of urea. With somewhat more certainty we know the forerunner of uric acid, which is that substance in which, in reptiles and birds, the greater part of the nitrogen that is derived from the decomposition of proteids leaves the body. Its forerunner is ammonium lactate. From experiments which Gaglio ('86) carried out upon dogs it follows that the lactic acid of the blood is derived from the decomposition of proteid, for the quantity of lactic acid in the blood increases and decreases together with the quantity of proteid food, and is wholly independent of the quantity of ingested carbohydrate. While lactic acid is always found in the blood, under normal conditions no trace of it occurs in the urine ; it must, therefore, undergo transformation before it is excreted. Minkowski('86) made these relations clear by an experiment, in which he showed that geese after the extirpation of the liver excrete very small quantities of uric acid but large quantities of lactic acid and ammonia, both of the latter in the quantitative relations of ammonium lactate. From this important fact Minkowski rightly concluded that ammonium lactate is a preliminary stage in the formation of uric acid, from which uric acid arises by rearrangement. We can also

conjecture with great probability as to the synthesis of hippuric acid, which arises from the decomposition of proteids, especially in the metabolism of herbivora. By boiling with mineral acids or alkalies hippuric acid is split into benzoic acid and glycocoll by hydrolysis, and by heating under a high pressure these two substances can again be united into hippuric acid with the loss of water. It is, therefore, supposed that in the body of the herbivore, where the possibility exists of the derivation of benzoic acid from proteid or the aromatic compounds of the food, and of glycocoll from gelatine-yielding substances derived from proteid, hippuric acid is formed synthetically from these two substances. As a matter of fact, not only in the body of the herbivore, but even in the carnivore, the formation of hippuric acid may be brought about artificially by introducing benzoic acid into the stomach, this acid then uniting with glycocoll into hippuric acid in an unknown manner in the tissues. In contrast with this, nothing whatever is known concerning the origin of creatin. Creatin, together with creatinin, which is derived from it with loss of water, is the substance in which muscle-cells give off chiefly the nitrogen that comes from the decomposition of their proteid. Just as little is known concerning its fate as concerning its origin ; for, although it is found in muscles in considerable quantity, only small quantities of it appear in the urine ; hence it appears to undergo in some manner transformations in the body itself. Finally, regarding the nuclein bases, it is known only that they are derived from the decomposition of nucleins and their derivatives ; the details of the process are unknown.

Among the non-nitrogenous transformation-products of proteids, the most important are fats, carbohydrates, lactic acid, and carbonic acid. These also are derived from the proteid molecule, not by a simple cleavage but by rearrangement and synthetic processes. The theory th&kfat can arise from proteid by transformation has been much disputed. The pathological process of the so-called fatmetamorphosis of cells, in which fat appears in the place of proteid, so that at the end of the process the cells are dead and filled with fat, necessarily led to the idea that here proteid is transformed into fat. But the objection was possible that in the course of the disease the proteid of the cell is forced out by the fat coming in from the outside. Notwithstanding this possibility, this important question has been decided experimentally in favour of the former view. Leo ('85) made experimental use of the fact that phosphorus poisoning causes an extremely rapid fat-metamorphosis, especially of the liver-cells. From a number of frogs he selected six individuals of equal size and weight, killed them and determined their fat-contents. He then took six other individuals, poisoned them with phosphorus and killed them after three days. The determination of fat revealed a considerably greater fat-

contents in the latter than in the former. This experiment proves that fat must actually have arisen in phosphorus poisoning. Franz Hofmann ("72), however, performed an experiment which showed directly the origin of fat from proteid. He took a quantity of eggs from the bluebottle fly (Musca vomit oria) and divided them by weight into two equal portions. One of these portions he employed for the determination of the fat-contents, the other he laid upon blood, the small quantity of fat contained in which was likewise determined. The larvae of the flies creeping out of the eggs fed upon the blood and grew. After they were grown, Hofmann determined the quantity of fat in them, and found that they contained ten times as much fat as the eggs and the blood together. On account of its minute quantity, the blood-sugar need not be considered in the fat-formation. Hence the fat could have come only from the proteid of the blood. After these experiments it is no longer doubtful that fat can originate from proteid. Neither can doubt exist concerning the origin of carbohydrates (grape-sugar and glycogen) from proteid. It has been known for a long time that in severe forms of diabetes mellitus, even with complete lack of carbohydrates in the food, the quantity of grapesugar excreted in the urine is considerably increased by the consumption of an increased quantity of proteid. Likewise, Claude Bernard has observed that in dogs in which the glycogen had been used up by fasting, glycogen is stored in greater quantity when they are fed abundantly upon pure proteid food ; and in a dog that had been fed for four days with pure fibrin after fasting twenty-one days, Mering ("77) found more than sixteen grains of glycogen in the liver. Numerous similar observations have been made, and the origin of carbohydrates from proteids is now assured.