The Chemistry of Plant Life

by Roscoe Wilfred Thatcher

Active protoplasm contains mineral salts in solution. These are of the same general nature as those found in sea-water, which is the original habitat of the earlier evolutionary forms of living matter. Or, it might be said that both plants and sea-water derive their mineral salts from the same source, namely the soluble salts of the soil. Recent investigations have shown that the proportions of sodium ions to calcium ions in sea-water are precisely those which maintain fats, proteins, etc., in a true colloidal emulsion; and that comparatively small variations in the ratio of these two cations produce very marked effects upon the colloidal conditions of these substances in an artificial colloidal preparation, which resemble very closely the changes which apparently take place in cell protoplasm under the influence of narcotics, or nerve stimulants, in blood-coagulation, in the parthogenetic development of germ cells, in cancerous growth of tissues, etc. In other words, in so far as it has been studied in this respect, cell plasma exhibits exactly the same responses to variations in the proportions of salts (electrolytes) in solution, that artificial emulsions of oils (fats) in water do; and the normal, or critical, equilibrium proportion of these electrolytes for all colloidal complexes is that in which they occur in sea-water. It must be admitted that there is as yet no definite evidence that the observations which have been made upon the protoplasm of animal tissues will apply equally well to plant cell protoplasm. But many of the phenomena which have been studied in animal tissues have what are apparently similar, if not identical, effects in plant tissues, and it seems reasonable to suppose that these conclusions apply generally to protoplasm of either animal or plant origin.

The effects which salts produce in protoplasm are undoubtedly due to the fact that, when in solution, they readily ionize and conduct the electric current. A discussion of the nature and importance of the theory of dissociation of electrolytes in solution, or the so-called "ionization theory," which has done so much to clear up otherwise unexplainable properties of solutions, would be out of place here. But it may be noted that the ionized condition of salts in solution accounts for the avidity, or "strength," of acids and bases; for the increased osmotic pressure of such solutions; for the conduction of the electric current through solutions; and for the effects of these dissolved electrolytes upon the colloidal condition of many substances, since this is due to the electric charge on the dispersed particles.

Hence, the presence of salts in solution in the water of the protoplasm has a tremendous influence upon the osmotic pressure (which governs the movement of dissolved materials into and out of the cell protoplasm); upon the colloidal condition of the cell contents (which controls all the effects due to the surface boundary phenomena which are discussed below and which are responsible for a large part of the remarkable chemical activity of the protoplasm); upon the electrical phenomena (which constitute many of the stimulations which the protoplasm receives); and upon the acidity or alkalinity of the cell contents (which determine the nature of the respiratory, or oxidation, reactions of the protoplasm and, indirectly, its life or death).

The general nature of these physical-chemical properties of the protoplasm and of the relation of electrolytes in solution to them may now be considered in some detail.

OSMOTIC PRESSURE

Osmotic pressure is one of the chief factors in controlling the amount of water in the protoplasm. As is well known, the phenomenon known as "osmosis" is the passage of solvents, or of dissolved substances, into or out of any tissue, or substance, through the membrane which surrounds it. In the case of a cell, the membrane in question may be either the cell-wall or the internal colloidal films which are distributed throughout the entire mass of the cell contents.

From the standpoint of their relation to osmosis, membranes may be either impermeable, in which case neither solvent nor dissolved materials can pass through them; semi-permeable, which permit the passage of the solvent, but not that of dissolved crystalloidal substances; or permeable, which permit the free passage through them of both solvents and solutes. The first and last of these types of membranes have no effect upon osmotic pressure; but osmotic pressure is at once set up whenever a semi-permeable membrane is interposed between solutions of different concentrations. It is due to the molecular motion of both the liquid and the dissolved solids, as a result of which a greater number of molecules are "bombarding," or pressing upon the membrane from the side of the more concentrated solution. This sets up an unequal pressure upon the two sides of the membrane, and if the latter be semi-permeable there will result a passage of the liquid through the membrane toward the denser solution so as to equalize the pressure. The resultant tendency is for the solutions on the two sides of the membranes to become equal in concentration by movement of the liquid from the less dense to the more dense portion, instead of by movement of the dissolved materials toward the less dense part of the solution as in the case of diffusion when solutions of different concentrations are brought in contact with no membrane to interfere with free diffusion.

Osmotic pressure tends, therefore, to force the movement of solvents through semi-permeable membranes from more dilute toward more concentrated solutions. Protoplasm acts in general as an approximately semi-permeable membrane or material. For example, if the concentration of sugar in any given mass of protoplasm becomes greater, by reason of the photosynthetic activity, osmotic pressure is set up and water enters the mass, thus preventing loss of turgidity due to increased concentration. Similarly, any other increase in concentration of synthetic products is compensated for by entrance of water because of increased osmotic pressure, unless the products are insoluble and, therefore, incapable of effecting the osmotic pressure.

Hence, osmotic pressure provides for the movement of water into and out of protoplasm and so tends to keep the proportion of water uniform throughout the entire tissue. It will at once occur to the reader, however, that if the statements in the preceding paragraph were unqualifiedly true, and if the protoplasmic mass were absolutely semi-permeable in character, there would be no possibility of the passage of dissolved solids into or out of the cell; i.e., if the protoplasm acted as an ideally semi-permeable membrane, only water could pass into or out of it. But we know that mineral salts from the soil must pass into any cell before the synthesis of proteins, etc., can proceed, and that the fats, carbohydrates, proteins, etc., which are synthetized in vegetative cells pass from these to other organs of the plant for use or storage. The obvious explanation for this condition of things in the plant is that protoplasm (and, indeed, this is equally true for practically all known membranes) is not absolutely impermeable to dissolved crystalloids; or, in other words, semi-permeability generally means only that the solvent passes through the membrane more readily and more rapidly than do the dissolved materials in it. Even colloidal materials will diffuse through most common membranes, although at so slow a rate that the process is scarcely observable by ordinary methods of study. Hence, the actual permeability of the protoplasm permits the movement of both water and dissolved solids from one part of the organism to another; but its approximation of semi-permeability produces osmotic pressure and induces freer movement of water than of dissolved substances, and so provides for turgidity of the cells and for equalization of the water content of different portions of the protoplasmic mass.

It is clear, therefore, that osmotic pressure plays an important part in the physical mechanism of cell activities and in the regulation of the proportion of water contained in the protoplasm, with its consequent effects upon the chemical reactions which may go on in the cell.