A second effect of change in reaction of protoplasm is to alter the enzymic activity of the cell. As has been pointed out, enzymes are extraordinarily sensitive to minute changes in the reaction of the medium in which they are working. A change toward acidity in protoplasm immediately results in the stimulating of carbohydrate-splitting enzymes, which increases the supply of easily oxidizable simple carbohydrates, thereby tending to compensate for the decrease in respiratory activity. Further, increase in acidity increases proteolysis, thereby liberating alkaline ammonia-derivatives which tend to neutralize the rising acidity and so to restore normal neutrality or alkalinity. Thus it will be seen that in the very great sensitivity of its enzyme catalysts to slight changes in the reaction of the medium, the protoplasm possesses a very efficient mechanism for regulating changes and restoring equilibrium, if the latter be disturbed by any abnormal conditions. It should also be noted, at this point, that the almost universal presence in protoplasm of salts of carbonic and phosphoric acids acts as an additional "buffer" against pronounced changes in reaction of the material; the bicarbonates acting by means of their ready release or absorption of carbon dioxide, and the phosphates by their easy change from mono-sodium phosphate to di-sodium phosphate, and vice versa, the former being slightly acid and the latter slightly alkaline in reaction.
A third effect of increasing acidity is that it induces increased imbibition of water by the colloidal gel and causes swelling of the tissue. After death, when the reaction of the protoplasm becomes pronouncedly acid, this swelling often proceeds to the point of rupturing of the cell-wall, or internal membranes of the protoplasm, thus permitting the entrance of the putrefactive bacteria and hastening the decay of the tissue.
Finally, comparatively slight variations in the reaction of the protoplasm produce enormous changes in its colloidal condition, affecting in a very marked degree its permeability, its power of adsorption, etc.
It is clear, therefore, that variations in the chemical reaction of protoplasm profoundly affect its colloidal condition, its enzymic activity, and its respiratory processes. This necessarily brief survey is sufficient to indicate how important to the activity of the protoplasm is the chemical reaction of the material, and the mechanism with which it is provided for maintaining the favorable condition of neutrality or slight alkalinity.
SUMMARY
It is evident that, within the limits of a single chapter, it has been possible to give only a very brief and incomplete discussion of some of the most important applications of the principles of physical chemistry to the properties and activities of protoplasm. Therefore, it may be profitable to summarize briefly these into a series of definite statements which may serve as a review of the principles which have been discussed in the preceding chapters, as applied to the activities of protoplasm.
Protoplasm is a complex hydrogel, composed of an heterogeneous mixture of proteins, fats, and carbohydrates, arranged in a foamlike structure, the compartments of the gel being filled with an aqueous solution of the soluble organic products of synthesis and of varying proportions of mineral salts which are of the same general nature as those of sea-water.
The gel is not uniform throughout the volume of any given cell, but is differentiated in different parts into what are known as the nucleus, the chloroplasts, the plasma of the cell, etc.
The vital activities of the cell consist in chemical reactions which are controlled by comparatively slight changes in the electrolyte distribution, or other environmental changes which affect the colloidal condition of the mass and, generally speaking, result in changes of the water content of the plasma, most such chemical changes being essentially reversible hydrolytic reactions.
The components of active protoplasm are in a condition most favorable to chemical reactions by reason of the enormous surface area of the colloidal material, resulting in abundance of available energy, intimate contact of the reacting materials, and the nearest possible approach to the condition of true solution which can be obtained without the loss of stable form and structure.