CHAPTER XVI

THE PHYSICAL CHEMISTRY OF PROTOPLASM

Thus far, we have considered the chemical nature of the various groups of compounds which are found in the tissues of living organisms, laying emphasis upon those which are of plant origin. These compounds constitute the material, or machinery, of the cell, and their various transformations furnish the energy for its operation. We come now to a study of the mode of its operation, or the processes of vital phenomena.

Our knowledge of these matters is not yet far enough advanced to permit a definite statement as to whether there is any difference between the protoplasm of plant tissues and that of animal origin in their modes of action, or in the physical-chemical changes which constitute the vital phenomena in the two groups of living organisms. Thus far, no such differences have been discovered. Hence, in the following discussions, no attempt is made to differentiate between animal and plant protoplasm. Most of the facts and principles which are here presented have been developed as the result of the study of the physiological chemistry of animal life. No similar careful study of plant chemistry has yet been carried out; but preliminary studies seem to indicate that the same general principles apply to all protoplasm, regardless of whether it is of plant or of animal origin. It is possible, of course, that further studies of plant protoplasm will render necessary some modifications of some of these views as applied to the growth of plants; but they are believed to represent the best which is now known of the physical chemistry of the plant-cell activities.

HETEROGENEOUS STRUCTURE OF THE CELL

Examination of cell protoplasm under the microscope reveals that it is not a simple homogeneous mass. In the first place, it has a definite structure, composed of (_a_) a nucleus; (_b_) numerous granular bodies of different sizes and kinds; and (_c_) a clear mass of colloidal material, which (if observed under the ultramicroscope, or photographed by ultra-violet light) is apparently made up of very minute particles of many different types of materials; the whole mass, in the case of plant protoplasm, being generally surrounded by (_d_) a differentiated layer known as the cell-wall. The actual internal structural arrangement of the clear colloidal mass is uncertain; but its properties indicate that it may be considered to be like a mass of foam (resembling a compact mass of soap-bubbles) the compartments of the foam being, of course, very minute and the films themselves almost infinitely thin, the contents of each compartment being probably liquid, and the whole composing a typical colloidal gel of complex composition.

This conception may not be accurate in every detail, but it seems to fit very closely the conditions and reactions of cell protoplasm. Furthermore, it is obvious that the definite structure, or form, of the cell is essential to its life; since, if the structure be destroyed by any kind of mechanical injury (freezing of the cell contents, resulting in the puncturing of the membranes by ice crystals; rupturing of the films, or cell-walls, by grinding with sharp sand, etc.) so as to bring about an intermingling of the parts which are segregated from each other in the organized structure, there results an immediate exhibition of abnormal chemical actions, accompanied by the liberation of carbon dioxide, and the death of the cell.

A proper mental picture of the organization of the cell structure and of the interrelation of all its working parts is suggested by the figure of a well-organized chemical factory, with the different chemical transformations which are involved in the whole process being carried on in different portions, or rooms, of the factory, with the various intermediate and final products regularly and systematically transported from one room to another as they are needed to keep each individual step in the whole process going at the proper rate, and with the different parts of the whole factory working in smooth coordination with each other. Any disturbance of the mechanism in any particular room, or any abnormal condition which breaks down the coordination or results in the mixing of the reagents or processes of adjoining rooms in improper order or proportions, produces instant destruction of the normal process, abnormal reactions take place, and the factory output is interrupted.

No other conception than this one of a definite structure and coordination of the different working parts of a cell can adequately account for the great variety of chemical changes which are constantly going on in any given cell. It is wholly inconceivable that a homogeneous mass of all the varying chemical compounds which are contained in any given quantity of protoplasm could either exist or produce any regular sequence of chemical reactions. Structure, or organization of the cell-contents into separate colloidal compartments, and the segregation of cell-contents into masses having different functions, is essential to any reasonable conception of how the cell performs its various activities.

The best understanding of the structural arrangement is afforded by the conception that protoplasm consists of a colloidal gel, or sometimes a very viscid sol, containing water, salts, carbohydrates, fats, proteins, and enzymes. Evidence in favor of this conception is afforded by the appearance of protoplasm under a high-power microscope, and by the close resemblance of the processes which go on in it, and its responses to external stimuli, to those of an artificial gel of similar chemical composition.

Two different conceptions of the form in which the chemical components exist in this mass have been advanced. One is that they are in true molecular unions, known as "biogens," and that the reactions which take place in the mass may, therefore, be studied from the same basis as are reactions between similar substances when they take place in a beaker or test tube in the laboratory. It would seem, however, that the constantly varying proportions of the materials themselves, and the lack of homogeneity of cell contents, afford insurmountable difficulties to this conception as a basis for the study of cell activities. The other, and seemingly more reasonable, conception is that these bodies exist in the form of colloidal complexes, whose composition may vary within wide limits and whose reactions are responsive to the usual phenomena incident to the colloidal condition of matter.

According to the latter conception, vital activities of cell protoplasm may be due to changes in water content, to electrical disturbances, to the phenomena resulting from the conditions brought about by surface boundaries between the different phases of the gel, to varying osmotic pressure, to changes in chemical reaction, etc., and may be controlled by various stimuli of chemical, physical, or mechanical nature. This conception seems, therefore, to fit most closely the actual conditions under which the protoplasm exists and carries on its vital functions.

With this conception in mind, we may now proceed to a consideration of how the various components of the complex organic colloidal system, and their specific properties, can affect its chemical activities.

The components of the system are, of course, water, salts, and the various organic compounds (fats, proteins, carbohydrates, and enzymes in all cells; and other groups, such as essential oils, tannins, pigments, etc., in cells which have certain special functions to perform) which constitute the solid phase of the colloidal mixture. In addition to the definite chemical properties of each of these component groups, which have been studied in detail in preceding chapters, there are many physical, or physical-chemical, properties of the system as a whole, and of its component parts, which are of the utmost importance in the physiological activities of the protoplasm. These we may now proceed to consider in some detail.

WATER

Water constitutes the largest proportion of the weight of active protoplasm. In living cell contents (except those of such bodies as resting seeds, etc.), water comprises from 70 to 95 per cent of the total weight of the substance; the average proportion being usually between 85 and 90 per cent. The fact that protoplasmic material can exist in turgid form with such high percentages of water as these is due, as has been pointed out, to its existence as a colloidal gel. It is because of this condition that increases in the proportion of water generally increase the turgidity, or turgor, of the protoplasm; instead of, as in all other cases, rendering the mixture less solid and more labile. Losses of water from the protoplasmic gel decrease its "swollen" condition and so render the tissue soft and flabby; while increases in water content swell the gel and make the tissue stiff and turgid. No other condition than that of a colloidal gel could respond in this way to changes in water content.

The formula which is commonly assigned to water is the simplest possible one; namely, H_{2}O. But if the water were really as simple as this, the compound would boil at a very low temperature, would have a very low surface tension, etc.; whereas its actual boiling point, surface tension, etc., are much higher than those of other compounds having a higher molecular weight than is indicated by the formula H_{2}O. Actual measurements of the physical properties of water indicate that at the temperature at which water is a vapor its formula is at least (H_{2}O)_{2}; while at lower temperatures, at which it exists as a liquid, its formula may be (H_{2}O)_{3}, or (H_{2}O)_{4}, or even more complex still. The cause for this association of the compound into multiple molecules undoubtedly lies in the extra valences of the oxygen. In many organic compounds oxygen is undoubtedly tetravalent, and it may be easily conceived that in these complex molecular groupings in the water it exhibits this same property; the possible molecular arrangements being represented by the formulas

H H H H \ / \ / H-O-O-H O=O and | | etc. / \ H-O-O-H H H / \ H H

Such molecules may be conceived to break down very easily, leaving the extra valences of the oxygen available to form linkages with other atoms or molecules. This may constitute one of the ways in which water exerts its remarkable effects both as a solvent and as an accelerator of all kinds of chemical reactions. Other organic compounds which contain tetravalent oxygen are exceedingly active chemically, and there seems to be much to commend this view of the chemical structure of the water molecule.

Probably the most remarkable property of water is its power of solution. No other liquid surpasses water as a solvent. This power, as has been pointed out, is supposed to be due to, or in some way correlated with, the extra valences of the oxygen atoms, which may perhaps unite with similar extra valences of other substances with which the water is brought into contact, and so cause the latter to enter into solution. All kinds of substances dissolve in water, and when in solution, or even when only moistened, are much more active chemically than when dry. This property of water contributes greatly to the possibilities of the chemical reactions which constitute life processes.

Water, likewise, has a higher dielectric constant than any other common liquid. This means that it does not readily conduct electricity, or readily permit electric equilibrium to be established in it; or, in other words, that it is a good insulator. This property permits the existence in it simultaneously of materials having opposite electric charges, or the so-called ionization phenomena; hence, water is the best-known ionizing medium, and ionization favors chemical reactivity.

Again, water has a very high specific heat, a fact which is of the utmost biological importance. It takes more heat to raise the temperature of one gram of water through one degree than is required to produce the same result in any other known substance; or, stated the other way around, a given amount of heat will cause less change in temperature of water than of any other known substance. Further, the latent heat of liquefaction and of vaporization (i.e., the amount of heat required to change the substance from solid to liquid and from liquid to gaseous state, respectively) is greater for water than for any other common substance. These facts are of very great importance in cell-protoplasm. The high specific heat of water provides that the heat liberated by the chemical reactions which take place in the protoplasm can be absorbed by the water of the cell contents, and given off again to other reactions, with very slight effect upon the temperature of the protoplasm itself. Hence, violent changes in temperature, which might be disastrous to the life of the cell, are prevented by the high specific heat of the water which it contains. Similarly, the high latent heat of liquefaction of water, resulting in the giving up of large quantities of heat before it can become solid, or "freeze," tends to prevent freezing and thawing of the cell contents with sudden changes of external temperatures at or near the freezing temperature of water.

As a result of its physical properties, as just briefly described, water accelerates all kinds of chemical reactions in protoplasm, both by solution and by ionization of such substances as undergo electric dissociation; and serves to regulate the temperature of the protoplasmic mass. Furthermore, in organic tissues, most of the important chemical reactions of the protoplasm are reversible hydrolyses; i.e., water actually enters into the reaction or is liberated by it, and the equilibrium point of the reaction is changed by the proportions of water which are present in the reacting mass. Hence, the presence of large proportions of water in the colloidal complex known as protoplasm has a very important influence upon its possibilities of biological reactions.

SALTS

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.

Actual measurements of the osmotic pressure of plant cell have been made. The results are more or less uncertain, because, as has been pointed out, a plant cell is not a definite quantity of uniform protoplasm surrounded by an ideal semi-permeable membrane, but is instead a mass of living matter which is approximately semi-permeable throughout its entire volume and is in a constantly changing condition because of the anabolic and catabolic activities which are going on in it; but values have been obtained which show a normal osmotic pressure as high as fourteen atmospheres in the cells of very turgid plants, such as those of some of the green algæ. Animal cells probably have an osmotic pressure similar to that of the blood which circulates around them, which is approximate that of seven atmospheres.

SURFACE BOUNDARY PHENOMENA

In the preceding chapter, a brief consideration of the phenomena arising at surface boundaries was presented. It was pointed out that when any substance exists in the colloidal, or dispersed, condition, it has relatively enormous surface area and that, consequently, enormous surface boundaries between the dispersed phase and the dispersion medium exist in all colloidal mixtures. Since protoplasm is conceived to exist in the form of a colloidal gel, having a foamlike structure, it is apparent that it has these enormous surface boundaries between the different phases of the system, and that the phenomena arising from this condition are of great importance in its biological activities. The following necessarily brief discussion will serve to give some indication of the physiological importance of the surface boundaries in such a system.

It is easy to see that the molecules which are in the surface layers at the interface, where two phases of a colloidal system are in contact, are under the influence of forces quite different from those which are acting upon the molecules in the interior of either phase. It is apparent that the molecules in the surface layer are exposed on the inner side to the attraction and influence of similar molecules, while on the opposite, or outer, side they are exposed to the influence of molecules of an entirely different kind. This results in a state of tension, known as "surface tension," with the development of resultant forces and energy which profoundly affect the chemical reactivity of the molecules which are present in this surface layer. The so-called "surface energy," which results from this surface tension, produces marked increases in the possibility of chemical reaction between the materials which are present at the surface boundaries. In colloidal gels, this effect is so pronounced, in many cases, as to completely overshadow other types of influences upon reaction velocities. Also, the surface layer of a liquid is compressed by its surface tension, to such an extent that the solubility of substances in this surface layer is greatly increased over that of the same substances in the interior of the liquid, which results in greatly increased concentration of dissolved substances in the surface layer, and so increases the rate of chemical changes which take place there, as contrasted with the rate of the same reactions going on in the interior of the solution. This latter consideration seems to be the factor of largest influence in colloidal catalysis.

But in addition to the increased rate of reaction in the surface layer due to the increased energy available there and to the increased concentration of dissolved substances, there is the possibility that the act of concentration itself bring into play molecular forces which give rise to a resultant increase in chemical potential, or chemical affinity, of the reacting materials, such as has been observed to result in other concentrated solutions. A discussion of the theoretical and mathematical considerations upon which this conception is based would be out of place here, but there is ample experimental evidence to indicate its soundness.

Further, as has been pointed out, colloidal phenomena are essentially due, in large part at least, to the electric charges on the dispersed particles. Electric charges accumulate at the surface of any charged body. Hence, the surface layers in any colloidal system carry its electric charges in highest concentration, and all of the chemical changes which are stimulated by electrical phenomena are most strongly influenced at the surface boundaries between the different phases of the system. This latter consideration affords a satisfactory explanation of the well-known depressing, or stimulating, action of electrolytes, especially acids and bases, upon the enzymic catalysis of protoplasmic reactions.

These few, brief statements are sufficient to indicate how extensively the chemical activities of colloidal protoplasm are influenced by the phenomena arising from the surface boundaries between different materials, which are present in such enormous extent in a colloidal gel. Surface boundary phenomena in a heterogeneous system, such as we have seen protoplasm to be, provide the possibilities for many reactions which would otherwise take place very slowly, if at all. Mere subdivision of the protoplasmic materials into the film, or foam, structure brings into play energies which may predominate over all other types of energy in the system. Here, too, effects may be extraordinarily modified by slight changes in environment, which effects could not be explained by any considerations which govern ordinary chemical reactions. Here, we deal with adsorption and other colloidal phenomena, rather than with ordinary stoichiometric combinations.

Indeed, it is not too much to say that the differences between the chemical phenomena which are called "vital" and those which take place in ordinary laboratory reactions are due to the fact that the former are manifestations of the interchanges of energy between the different phases of a heterogeneous colloidal system, while the latter are governed by the laws of ordinary stoichiometric combinations.

ELECTRICAL PHENOMENA OF PROTOPLASM

The investigations of this phase of the physical chemistry of protoplasm have dealt almost exclusively with animal tissues and reactions, and have included the study of such phenomena as nerve impulses, muscular contractions, heart-beats, glandular secretions, etc. Tissues which respond to nerve, or brain, control are, of course, not found in plants. But there is plenty of experimental evidence to show that plant protoplasm carries electrical charges and exhibits electrical phenomena which are similar in character to those of animal tissues. In fact, it has been shown that the contraction of the lobes of the Venus' fly trap, when they close over an imprisoned insect, are accompanied by electrical phenomena in the leaf tissues which are precisely similar to those which take place in an animal muscle when it contracts. It seems probable that many of the observations and conclusions which have been derived from the study of the electrical disturbances in animal tissues may later be found to have definite applications to the vital phenomena of plant cells. Hence, it seems proper to give some brief consideration to these matters here.

The statement has been made that "every active living cell is essentially an electric battery," and it is believed that every activity of living matter, such as the rhythmic contraction of the heart, the passage of a nerve impulse, etc., is accompanied by an electric disturbance in the protoplasm of the tissues in question. Experimental proof of this electrical disturbance has been repeatedly obtained, by connecting a delicate galvanometer in a circuit through the living tissue which is undergoing different activities and obtaining widely varying readings of the instrument as the different phenomena are in progress, or by connecting the instrument with muscular tissue and observing its fluctuations with either the irregular contractions of a voluntary muscle or with the rhythmic contractions of a heart muscle.

By means of such investigations as those just mentioned, it has been found that the part of the protoplasm which is most active is always electro-negative to the part which is less so; that is, the electric current flows from the more active to the less active portion of the protoplasm.

Many different explanations of the origin of the electric current which develops when the protoplasm is stimulated into activity have been suggested; but none of them have, as yet, any experimental confirmation. The most that can be said is that whenever any stimulus excites the protoplasm into activity, there is instantly developed in it an electrical disturbance, which continues as long as the action is in progress. Recent investigations, which have shown that there is a direct relation between many of the vital processes of protoplasm and the ratio of the electrolytes which it contains, particularly the ratio of sodium and potassium to calcium, would seem to indicate that the development of the electrical disturbance is a direct result of variations in the proportions of the salts of these metals, either brought about by, or themselves causing, changes in the permeability of the protoplasm, following the stimulus which determines the nature of the activity which it is to undergo. But there is as yet no indication concerning the mechanism by which this stimulation, with its resultant electrical phenomena, is transmitted to the protoplasm and accomplishes its characteristic effects.

ACIDITY OR ALKALINITY OF PROTOPLASM

The preceding sections of this chapter have dealt almost exclusively with the physical properties of protoplasm; including the phenomena of solution, ionization, surface boundary effects, and electrical disturbances, and their probable effects upon the chemical reactions which constitute its biological activities. It is necessary now to consider another phase of the physical chemistry of protoplasm, namely, its chemical reaction, whether acid, alkaline, or neutral, the effects of variation of this condition upon the activity of the protoplasm, and the mechanism by which it tends to preserve its own proper reaction in this respect.

The earlier methods of investigation of the chemical reaction of protoplasm were all based upon its color reactions to various staining agents. These sometimes led to erroneous conclusions, because of the effects of the staining agent itself upon the tissue; some stains are poisonous and result in the death of the protoplasm, others do not easily penetrate the semi-permeable colloidal mass, others are themselves changed by the oxidizing or reducing action of the protoplasm, etc. Again, colloidal adsorption effects often lead to the so-called "capillary segregation" of added staining materials. So that this method of study must be used with great care, or wholly erroneous conclusions will be reached, and many of the earlier reports have subsequently been found to be incorrect.

The recent improvements in the apparatus and methods for the determination of hydrogen-ion concentration have afforded a much more trustworthy method of determining the actual acidity or alkalinity of such materials than is obtained by color reactions, and this method is now being extensively used in the study of the reaction of active protoplasm.

It must be kept in mind that protoplasm is an heterogeneous mass and not an homogeneous solution, so that it is not always possible to determine the actual conditions as to neutrality of different parts of the protoplasm of a single cell, for example. Hence, one of the best methods of determining the reaction which is favorable to the life and activity of any given type of protoplasm is to investigate the reaction of a liquid medium in which the cells live and grow; this plan being based upon the assumption that a cell is not likely to have a reaction different from that of the medium which is favorable to its growth.

The results of all of the many investigations which have dealt with this problem point to the conclusion that the normal reaction for living protoplasm is either neutral or very faintly alkaline; but that it becomes acid when the cell is working in the absence of sufficient oxygen, and after the death of the cell.

The first effect of a change in the reaction toward acidity of the protoplasm is a decrease in the rate of respiration of the tissue, while increased alkalinity stimulates respiratory activity. Whet carried to the point of actual acidity, the respiratory coefficient becomes negative, and the cell actually gives off carbon dioxide because of the stoppage of the synthetic processes.

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.

The reactions which take place in cell protoplasm, as a result of the action of either physical or chemical stimuli, are accompanied by electrical disturbances, which may be either caused by, or the result of, changes in the electrical charges of the mineral salts which are present in the gel. Such changes, like the chemical reactions which they accompany, may be regarded as reversible and mutually self-regulatory; so that the protoplasm has not only the possibilities of enormous chemical reactivity, but also the mechanism for self-regulation of its actions, the products or results from any given series of changes generally tending to reverse the process by which they are proceeding and so to restore the condition of normal equilibrium.

Finally, the most characteristic difference between the reactions which go to make up the vital activities of a living cell and those of the same chemical substances when in inanimate form in the laboratory lies in the presence in the colloidal mass of the accelerating catalysts known as enzymes, which are produced by the protoplasm itself in some way which is as yet wholly unknown; and which not only add to the possibilities of rapid chemical change which are afforded by the colloidal nature of the material, but also, because of their extreme sensitiveness to minute changes in environmental conditions, serve to govern both the rate and the direction of the individual chemical reactions which constitute the vital activities of the protoplasmic mass. These enzymes are not distributed uniformly through any given cell, or organism, but are localized in different parts of the cell or tissue and so give to its different parts the ability to perform their various different functions.

References

ATKINS, W. R. G.--"Some Recent Researches in Plant Physiology," 328 pages, 28 figs., London, 1916.

CZAPEK, F.--"Chemical Phenomena of Life," 152 pages, New York, 1911.

CZAPEK, F.--"Ueber eine Methode zur direkten Bestimmung der Oberflächenspannung der Plasmahaut von Pflanzen," 86 pages, 3 figs., Jena, 1912.

HÖBER, M. R.--"Physikalische Chemie der Zelle und der Gewebe," 671 pages, 55 figs., Leipzig, 1911.

LIVINGSTON, B. E.--"The Rôle of Diffusion and Osmotic Pressure in Plants," 149 pages, Chicago, 1903.

MCCLENDON, J. F.--"Physical Chemistry of Vital Phenomena," 248 pages, Princeton University Press, 1917.

MACDOUGAL, D. T.--"Hydration and Growth," Publication No. 297, Carnegie Institution of Washington, 176 pages, 52 figs., Washington, D. C., 1920.

SPEIGEL, L., trans. by LUEDEKING, C. and BOYLSTON, A. C.--"Chemical Constitution and Physiological Action," 155 pages, New York, 1915.

THOMPSON, D'A. W.--"On Growth and Form," 793 pages, 408 figs., Cambridge, 1917.

WILLOWS, R. S. and HATSCHEK, E.--"Surface Tension and Surface Energy and their Influence on Chemical Phenomena," 116 pages, 21 figs., New York, 1919, (2d ed.).