Читать книгу The Organism as a Whole, from a Physicochemical Viewpoint - Jacques Loeb - Страница 7

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1. Each organism is characterized by a definite form and we shall see in the next chapter that this form is determined by definite chemical substances. The same is true for crystals, where substance and form are definitely connected and there are further analogies between organisms and crystals. Crystals can grow in a proper solu­tion, and can regenerate their form in such a solu­tion when broken or injured; it is even possible to prevent or retard the forma­tion of crystals in a supersaturated solu­tion by preventing “germs” in the air from getting into the solu­tion, an observa­tion which was later utilized by Schroeder and Pasteur in their experi­ments on spontaneous genera­tion. However, the analogies between a living organism and a crystal are merely superficial and it is by pointing out the fundamental differences between the behaviour of crystals and that of living organisms that we can best understand the specific difference between non-living and living matter. It is true that a crystal can grow, but it will do so only in a supersaturated solu­tion of its own substance. Just the reverse is true for living organisms. In order to make bacteria or the cells of our body grow, solu­tions of the split products of the substances composing them and not the substances themselves must be available to the cells; second, these solu­tions must not be supersaturated, on the contrary, they must be dilute; and third, growth leads in living organisms to cell division as soon as the mass of the cell reaches a certain limit. This process of cell division cannot be claimed even metaphorically to exist in a crystal. A correct apprecia­tion of these facts will give us an insight into the specific difference between non-living and living matter. The forma­tion of living matter consists in the synthesis of the proteins, nucleins, fats, and carbohydrates of the cells, from the split products. To give an historical example, Pasteur showed that yeast cells and other fungi could be raised on the following sterilized solu­tion: water, 100 gm., crystallized sugar, 10 gm., ammonium tartrate, 0.2 gm. to 0.5 gm., and fused ash from yeast, 0.1 gm.7 He undertook this experi­ment to disprove the idea that protein or organic matter in a state of decomposi­tion was needed for the origin of new organisms as the defenders of the idea of spontaneous genera­tion had maintained.

2. That such a solu­tion can serve for the synthesis of all the compounds of living yeast cells is due to the fact that it contains the sugars. From the sugars organic acids can be formed and these with ammonia (which was offered in the form of ammonium tartrate) may give rise to the forma­tion of amino acids, the “building stones” of the proteins. It is thus obvious that the synthesis of living matter centres around the sugar molecule. The phosphates are required for the forma­tion of the nucleins, and the work of Harden and Young suggests that they play also a rôle in the alcoholic fermenta­tion of sugar.

Chlorophyll, under the influence of the red rays of light, manufactures the sugars from the CO₂ of the air. This makes it appear as though life on our planet should have been preceded by the existence of chlorophyll, a fact difficult to understand since it seems more natural to conceive of chlorophyll as a part or a product of living organisms rather than the reverse. Where then should the sugar come from, which is a constituent of the majority of culture media and which seems a prerequisite for the synthesis of proteins in living organisms?

The investiga­tions of Winogradsky on nitrifying,8 sulphur and perhaps also on iron bacteria have to all appearances pointed a way out of this difficulty. It seemed probable that there were specific micro-organisms which oxidized the ammonia formed in sewage or in the putrefac­tion of living matter, but the attempts to prove this assump­tion by raising such a nitrifying micro-organism on one of the usual culture media, all of which contained organic compounds, failed. Led by the results of his observa­tions on sulphur bacteria it occurred to Winogradsky that the presence of organic compounds stood in the way of raising these bacteria, and this idea proved correct. The bacteria oxidizing ammonia to nitrites were grown on the following medium; 1 gm. ammonium sulphate, 1 gm. potassium phosphate, 1 gm. magnesium carbonate, to 1 litre of water. From this medium, which is free from sugar and contains only constituents which could exist on the planet before the appearance of life, the nitrifying bacteria were able to form sugars, fatty acids, proteins, and the other specific constituents of living matter. Winogradsky proved, by quantitative determina­tion, that with the nitrifica­tion an increase in the amount of carbon compounds takes place. “Since this bound carbon in the cultures can have no other source than the CO₂ and since the process itself can have no other cause than the activity of the nitrifying organism, no other alternative was left but to ascribe to it the power of assimilating CO₂.”9 “Since the oxida­tion of NH₃ is the only source of chemical energy which the nitrifying organism can use it was clear a priori that the yield in assimila­tion must correspond to the quantity of oxidized nitrogen. It turned out that an approximately constant ratio exists between the values of assimilated carbon and those of oxidized nitrogen.” This is illustrated by the results of various experi­ments as shown in Table I.

TABLE I

	No. 5	No. 6	No. 7	No. 8
	mg.	mg.	mg.	mg.
Oxidized N	722.0	506.1	928.3	815.4
Assimilated C	019.7	015.2	026.4	022.4
Ratio N : C	036.6	033.3	035.2	036.4

It is obvious that 1 part of assimilated carbon corresponds to about 35.4 parts oxidized nitrogen or 96 parts of nitrous acid.

These results of Winogradsky were confirmed in very careful experi­ments by E. Godlewski, Sr.10

The nitrites are further oxidized by another kind of micro-organisms into nitrates and they also can be raised without organic material.

Winogradsky had already previously discovered that the hydrogen sulphide which is formed as a reduc­tion product from CaSO₄ or in putrefac­tion by the activity of certain bacteria can be oxidized by certain groups of bacteria, the sulphur bacteria. Such bacteria, e.g., Beggiatoa, are also commonly found at the outlet of sulphur springs. They utilize the hydrogen sulphide which they oxidize to sulphur and afterwards to sulphates, according to the scheme:

(1) 2H₂S + O₂ = 2H₂O + S₂

(2) S₂ + 3O₂ + 2H₂O = 2H₂SO₄

The sulphuric acid is at once neutralized by carbonates.

Winogradsky assumes that the oxida­tion of H₂S by the sulphur bacteria is the source of energy which plays the same rôle as the oxida­tion of NH₃ plays in the nitrifying bacteria, or the oxida­tion of carbon compounds—sugar and others—in the case of the other lower and higher organisms. Winogradsky has made it very probable that sulphur bacteria do not need any organic compounds and that their nutri­tion may be accomplished with a purely mineral culture medium, like that of the nitrite bacteria. On the basis of this assump­tion they should also be able to form sugars from the CO₂ of the air.

Nathanson11 discovered in the sea water the existence of bacteria which oxidize thiosulphate to sulphuric acid. They will develop if some Na₂S₂O₃, is added to sea water. These bacteria can only develop if CO₂ from the air is admitted or when carbonates are present. For these organisms the CO₂ cannot be replaced by glucose, urea, or other organic substances. Such bacteria must therefore possess the power of producing sugar and starch from CO₂ without the aid of chlorophyll. Similar observa­tions were made by Beijerinck on a species of fresh-water bacteria.12

Finally the case of iron bacteria may briefly be mentioned though Winogradsky’s views are not accepted by Molisch.

We may, therefore, consider it an established fact that there are a number of organisms which could have lived on this planet at a time when only mineral constituents, such as phosphates, K, Mg, SO₄, CO₂, and O₂ besides NH₃, or SH₂, existed. This would lead us to consider it possible that the first organisms on this planet may have belonged to that world of micro-organisms which was discovered by Winogradsky.

If we can conceive of this group of organisms as producing sugar, which in fact they do, they could have served as a basis for the development of other forms which require organic material for their development.

In 1883 the small island of Krakatau was destroyed by the most violent volcanic erup­tion on record. A visit to the islands two months after the erup­tion showed that “the three islands were covered with pumice and layers of ash reaching on an average a thickness of thirty metres and frequently sixty metres.”13 Of course all life on the islands was extinct. When Treub in 1886 first visited the island, he found that blue-green algæ were the first colonists on the pumice and on the exposed blocks of rock in the ravines on the mountain slopes. Investiga­tions made during subsequent expedi­tions demonstrated the associa­tion of diatoms and bacteria. All of these were probably carried by the wind. The algæ referred to were according to Euler of the nostoc type. Nostoc does not require sugar, since it can produce that compound from the CO₂ of the air by the activity of its chlorophyll. This organism possesses also the power of assimilating the free nitrogen of the air. From these observa­tions and because the Nostocaceæ generally appear as the first settlers on sand the conclusion has been drawn that they or the group of Schizophyceæ to which they belong formed the first settlers of our planet.14 This conclusion is not quite safe since in the settlement of Krakatau as well as in the first colonizing of sand areas the nature of the first settler is determined chiefly by the carrying power of wind (or waves and birds).

We may now return from this digression to the real object of our discussion, namely that the nutritive solu­tions of organisms must be very dilute and consist of the split products of the complicated compounds of which the organisms consist. The examples given sufficiently illustrate this statement.

The nutritive medium of our body cells is the blood, and while we take up as food the complicated compounds of plants or animals, these substances undergo a diges­tion, i.e., a splitting up into small constituents before they can diffuse from the intestine into the blood. Thus the proteins are digested down to the amino acids and these diffuse into the blood as demonstrated by Folin and by Van Slyke. From here the cells take them up. The different proteins differ in regard to the different types of amino acids which they contain. While the bacteria and fungi and apparently the higher plants can build up all their different amino acids from ammonia, this power is no longer found in the mammals which can form only certain amino acids in their body and must receive the others through their food. As a consequence it is usually necessary to feed young animals on more than one protein in order to make them grow, since one protein, as a rule, does not contain all the amino acids needed for the manufacture of all the proteins required for the forma­tion of the material of a growing animal.15

3. The essential difference between living and non-living matter consists then in this: the living cell synthetizes its own complicated specific material from indifferent or non-specific simple compounds of the surrounding medium, while the crystal simply adds the molecules found in its supersaturated solu­tion. This synthetic power of trans­forming small “building stones” into the complicated compounds specific for each organism is the “secret of life” or rather one of the secrets of life.

What clew have we in regard to the nature of this synthetic power? We know that the comparatively great velocity of chemical reac­tions in a living organism is due to the presence of enzymes (ferments) or to catalytic agencies in general. Some of these catalytic agencies are specific in the sense that a given catalyzer can accelerate the reac­tion of only one step in a complicated chemical reac­tion. While these enzymes are formed by the action of the body they can be separated from the body without losing their catalytic efficiency. It was a long time before scientists succeeded in isolating the enzyme of the yeast cell which causes the alcoholic fermenta­tion of sugar; and this gave rise to the premature statement that it was not possible to isolate this enzyme since it was bound up with the life of the yeast cell. Such a statement was even made by a man like Pasteur, who was usually a model of restraint in his utterances, and yet the work of Buchner proved him to be wrong.

The general mechanism of the action of the hydrolyzing enzymes is known. The old idea of de la Rive, that a molecule of enzyme combines transitorily with a molecule of substrate; the further idea, which may possibly go back to Engler, that the molecule of substrate is disrupted in the “strain” of the new combina­tion and that the broken fragments fall off or are easily knocked off by collision from the ferment molecule which is now ready to repeat the process, seems to be correct. On the assump­tion that the velocity of enzyme reac­tion is propor­tional to the mass of the enzyme and that de la Rive’s idea was correct, Van Slyke and Cullen were able to calculate the coefficients of the velocity of enzyme reac­tions for the fermenta­tion of urea and other substances, and the agreement between calculated and observed values was remarkable.16

While the hydrolytic action of enzymes is thus clear the synthesis in the cell is still a riddle. An interesting sugges­tion was made by van’t Hoff, who in 1898 expressed the idea that the hydrolytic enzymes should also act in the opposite direc­tion, namely synthetically. Thus it should not only be possible to digest proteins with pepsin but also to synthetize them from the products of diges­tion with the aid of the same enzyme. This expecta­tion was based on the idea that the enzyme did not alter the equilibrium between the hydrolyzed and non-hydrolyzed part of the substrate but only accelerated the rate with which the equilibrium was reached. Van’t Hoff’s idea omitted, however, the possibility that in the transitory combina­tion between enzyme molecule and substrate a change in the molecular configura­tion of the substrate or in the distribu­tion of intramolecular strain may take place. The first apparently complete confirma­tion of van’t Hoff’s sugges­tion appeared in the form of the synthesis of maltose from grape sugar by the enzyme maltase, which decomposes maltose into grape sugar. By adding the enzyme maltase from yeast to a forty per cent. solu­tion of glucose Croft Hill17 obtained a good yield of maltose. It turned out, however, that what he took for maltose was not this compound but an isomer, namely isomaltose, which has a different molecular configura­tion and cannot be hydrolyzed by the enzyme maltase.

Lactose is hydrolyzed from kephyr by an enzyme lactase into galactose and glucose; by adding this enzyme to galactose and glucose a synthesis was obtained not of lactose but of isolactose; the latter, however, is not decomposed by the enzyme lactase.

E. F. Armstrong has worked out a theory which tries to account for this striking phenomenon by assuming “that the enzyme has a specific influence in promoting the forma­tion of the biose which it cannot hydrolyze.”18 The theory is very ingenious and seems supported by fact. This then would lead to the result that certain hydrolytic enzymes may have a synthetic action but not in the manner suggested by van’t Hoff.

The principle enunciated by Armstrong, that in the synthetic action of hydrolytic enzymes not the original compound but an isomer is formed which can not be hydrolyzed by the enzyme, may possibly be of great importance in the understanding of life phenomena. It shows us how the cell can grow in the presence of hydrolytic enzymes and why in hunger the disintegra­tion of the cell material is so slow. It was at first thought that the forma­tion of isomers contradicted the idea of the reversible action of enzymes, but this is not the case; on the contrary, it supports it but makes an addi­tion which may solve the riddle of what Claude Bernard called the creative action of living matter. We shall come back to this problem in the last chapter.

Kastle and Loevenhart demonstrated the synthesis of a trace of ethylbutyrate by lipase if the latter enzyme was added to the products of the hydrolysis of ethylbutyrate, ethyl alcohol, and butyric acid by the same enzyme.19 Taylor20 obtained the synthesis of a slight amount of triolein