Although it is known that many chemical processes taking place within the cell are due to the actions of ferments, and although we can in many cases separate these ferments from living protoplasm and study their action
in vitro
, there still remain considerable discrepancies between the processes as we see them occur
in vivo
and as we study them
in vitro
. One of the most characteristic features of the processes taking place within the living cell is what, for want of a better term, may be called their “adaptability,” that is the delicate sensitiveness with which they respond to very slight changes in the surrounding medium by being retarded, accelerated, or reversed. It is known, of course, that the action of ferments is influenced by changes in temperature or in the alkalinity or acidity of the surrounding medium. But in the case of the living cell these factors remain practically constant, so that their influence can be excluded. We know, too, that many reactions brought about by the action of ferments are reversible, and that the direction in which the ferments act depends upon the concentration of the various substances entering into the reaction. But here again these differences are of an order of magnitude far greater than the variations which exist in the living organism. Its is noteworthy, too, that the equilibrium of a reaction brought about by a ferment separated from living protoplasm lies almost always near the point of complete hydrolysis, and contrasts in that respect markedly with the behaviour of the same ferment when its reaction is studied in the living cell, where the reverse process may be found to occur or where very minute changes in the surrounding medium are sufficient to transfer the point of equilibrium from hydrolysis to synthesis. If we take the liver cell as an example we find that the living cell can, with equal readiness, transform glycogen into glucose and glucose into glycogen. If the liver is removed from the body a marked glucogenolysis occurs, so that a marked amount of sugar is formed, while the power to synthesise glycogen appears to be almost completely inhibited. In other words, the equilibrium point lies now near the point of complete hydrolysis. The same takes places if an extract of the liver is allowed to act on glycogen or on glucose in the concentrations found in the blood. It is known that the formation of glycogen
in vivo
occurs when the percentage of blood-sugar is relatively high, and the reverse process when the blood-sugar concentration falls. But the differences in the concentration of the blood-sugar which accompany these processes are too slight to be an adequate explanation in themselves, especially if we compare them with the large difference of concentration necessary to effect the reversal of a ferment action
in vitro
. Moreover, when the liver is removed from the body the formation of such a large amount of sugar takes place that the hydrolysis of glycogen should be inhibited if the concentration of the sugar was the main factor. Nevertheless the glycogen under these conditions completely disappears. The disappearance of glycogen from the liver in the living animal as the result of “sugar puncture” or under the influence of the thyroid hormone cannot be satisfactorily explained on the ground of changes in concentrations of the reacting substances.
Microglia-Derived Small Extracellular Vesicles Reduce Glioma Growth by Modifying Tumor Cell Metabolism and Enhancing Glutamate Clearance through miR-124