From The Dynamics of Living Protoplasm (1956). Final section, page 275-279.
The study of the colloid chemistry of protoplasm has progressed a long way since I published my book on the subject in 1928. And yet even in 1928 many important truths had begun to emerge and subsequent work has only served to substantiate much of what was known twenty-seven years ago. On the other hand, there is also much new and significant information.
Here is very brief summary is what we know now:
1. In cells generally the major part of the protoplasm is fluid.At any rate, the hyaline part of the protoplasm is fluid. Suspended in the hyaline protoplasm there are granules of one sort or another. If these granules are very numerous, the viscosity of the entire protoplasmic suspension may be rather high. Such dense suspensions may show a dilatancy similar to that of concentrated starch suspensions, that is to say the viscosity may become progressively greater when more andmore shearing force is applied. (In such dilatant suspensions, the viscosity of the dispersion medium is of necessity low).
2. Typically (and perhaps always) the outer part of the protoplasm consists of a rigid cortex. This cortex may constitute a rather delicate membrane containing a single layer of granules, or it may be decidedly thicker than this. The cortex is an extremely important part of the cell.
3. The rigidity of the cortex depends on the fact that it contains calcium bound to protein (and also to lipid). Chemical and physical agents which are able to free the calcium in the cortex from its bound state cause a liquefaction of the cortex.
4. The fact that the cortex can bind calcium ions and can also release them makes it a calcium electrode. This calcium electrode is an important factor in the electrochemical behavior of the cell. But it is not the only factor - in the cell interior (at least of some cells) chloride ions are bound, so that in addition to the calcium electrode the cell also has a chloride electrode. Both of these electrodes contribute to the bioelectric potentials that the cell is capable of producing.
5. The colloidal behavior of protoplasm is not like that of a pure protein or of any ordinary combination of protein and lipid. When a cell is torn or broken, it seals itself by a reaction called the surface precipitation reaction. This reaction is similar to blood clotting, for it does not occur in the absence of calcium, and the calcium ion is necessary because it activates an enzyme system which acts both as a protease and as a clotting enzyme. The surface precipitation reaction, or s.p.r., is the basic colloidal reaction of protoplasm, and many of the effects produced by chemical or physical agents on protoplasm can be interpreted in terms of this reaction. If a cell is broken gently, the s.p.r. results merely in the formation of a new precipitation membrane at the outer boundary of the emerging droplet of protoplasm, but if the cell is broken more harshly, the reaction spreads so as to involve a sizable fraction of the cell or perhaps all of the cell. In an ordinary s.p.r., the contents of the cell become exposed to the calcium of the medium. The same type of reaction can occur if calcium is injected into the cell interior or if calcium is liberated from the cortex and enters the interior. Because of the similarity of the s.p.r. to blood clotting, it is propert to speak of the reaction that occurs in the cell interior as a protoplasmic clotting.
6. Protoplasm is always in a state of equilibrium between the factors that induce clotting and those that tend to prevent clotting. Cells generally contain thrombin or thromboplastin or similar substances; they also contain heparin or heparin-like substances. both the clotting factors (thrombin, thromboplastin, etc.) and the anticlotting factors (heparin) can be liberated from cells and can have an influence on other cells of an organism.
7. When a cell is exposed to stimuli, such as heat, cold, mechanical impact, electric shock, ultraviolet radiation, etc., the cortex is liquefied and calcium is released from the cortex into the cell interior. It is of the very nature of stimuli to produce this effect. Thus, for example, ultraviolet radiation is known to break the carboxyl bond that links calcium to protein, and an electric current would tend to separate calcium ion from calcium proteinate.
8. The freeing of calcium from the cortex would tend to make the outer region of the cell negative to the interior. This would be a factor in the initiation of an action potential. Freeing of calcium from the outer region of the cell would also increase the cell permeability.
9. As the calcium enters the cell interior, it activates an enzyme which is both a protease and a clotting enzyme. This causes the protoplasm or at least some constituent of the protoplasm to undergo a clotting reaction. In a muscle fiber the result of such a clotting reaction is the shortening of the fiber; in a marine egg cell, the clotting reaction results in the formation of the mitotic spindle.
10. The protoplasmic clotting reaction involves a change of SH groups to S-S groups. This oxidative change starts off a chain of oxidative reactions, and accordingly one common result of stimulation and response is an increase in the rate of metabolism.
11. The calcium-activated proteolytic enzyme not only causes a clotting reaction, it also tends to digest away the protein that is bound to heparin, or at least to break the bond between protein and heparin. As a result, heparin or some substances like it is liberated, and this heparin inhibits further activity of the enzyme. There is thus a cellular homeostasis, or biostasis, in that the very nature of the response reaction or series of reactions in itself causes an inhibition of excessive reaction (and perhaps also a reversal of the colloidal changes induced by the response).
12. Fat solvent anesthetics liquefy the cortex and free calcium just as stimulating agents do, but they prevent the clotting reaction that would otherwise follow the release of calcium. Under certain conditions, usually in relatively low concentration, the fat solvents liberate calcium but are not able to prevent the clotting of the protoplasm that follows such a liberation. This is the reason that fat solvent anesthetics in low concentration can act to increase rather than decrease excitability.
13. When cells are exposed to an excess of magnesium ion, the calcium of the cortex is to a large extent replaced by magnesium. The result is a cortex in which the Mg/Ca ratio is increased. Hence when the cells are stimulated, it is largely magnesium ion rather than calcium ion that is freed and enters the cell interior. The magnesium ion is far less potent in causing protoplasmic clotting than is the calcium ion. Thus in the sea urchin egg it takes a hundred times as much magnesium to produce a surface precipitation reaction as it does calcium. This is why stimulation of nerve or muscle in the presence of excess magnesium fails to elicit a response.
14. Heparin and heparin-like substances can also have an anesthetic action. They cause a relaxation of smooth and heart muscle and they probably play a part in depressing excitability in hibernating animals and animals in shock.
15. When cells are stimulated or injured, they may give off to the surrounding medium both substances like thromboplastin which promote clotting and substances like heparin which prevent clotting (both of blood and of protoplasm). Thus cells injured severely by burns, by mechanical crushing, by anoxia, by roentgen radiation, etc., can give off substances which have an effect on cells and tissues throughout the organism. On the basis of these facts a theory of shock can be formulated, a theory which would explain why substances like thrombin, trypsin and peptone can cause shock.
The above summary should not be regarded as anything more than the statement of a set of conclusions arrived at on the basis of a considerable amount of data. These data are presented in some detail in the earlier chapters of this book. Moreover, evidence in support of our conclusions is constantly growing.
Our story has been a long and complex one. It has taken us into various aspects of physiology and, indeed, we have invaded the fields of muscle physiology, nerve physiology, cell division, cytochemistry, pathology, and pharmacology, and we have even ventured into the practical field of medical science. Whether the excursions into any or all of these fields have been profitable or not, only the future can tell. Doubtless mistakes in interpretation have been made. In this era of scientific complexity and specialization, an era in which there are so many different branches and subdivisions of science, it is hard for any one person to understand the facts and the points of view of widely divergent disciplines. Concentration in very specialized parts of scientific inquiry is the order of the day; and yet in spite of this, or perhaps because of it, there seems to be a need for a broad outlook, for a theory of theories that would interpret the varied activities of the living substance in terms of a consistent general plan, a plan that is in keeping with and not in contradiction to our knowledge of the physical and colloidal properties of the living material. If, as we believe, these properties are fundamentally similar in almost all types of living cells and organisms, then there should certainly be a fundamental basis for interpreting vital activity, whether it be in a muscle cell, a nerve cell, or in a dividing egg cell.
In attempting to find out what this general plan is, I have tried to view all the facts available to me and to present them as simply and clearly as possible. At the risk of failing to be impressive, I have avoided mathematical formulations and abstruse discussions of physical chemistry. This has been done in the hope that more biologists and more physiologists, and possibly also some of those concerned with clinical problems will become interested in the colloid chemistry of protoplasm. There are so many important facts still to be learned and so few workers in the fied. And at the present time, with new techniques and new apparatus available, it should be possible to make more rapid progress than was possible in the past. But apparatus alone, no matter how complicated or how expensive, is not a substitute for thought and imagination. And the mere gathering of data, no matter how precise they may be, is not necessarily of great value. What is needed is a clear appreciation and understanding of the problems involved. Experiments should be planned, not merely on the basis of apparatus available, but primarily in terms of an attempt to interpret one or another of the mysterious phenomena involved in vital activity.
With properly trained workers, men trained not only in techniques but in ways of thought as well, it should be possible to make rapid progress, so that perhaps before many years pass, what has been presented in this book may constitute but a small fraction of our knowledge concerning the dynamics of living protoplasm.