The protoplasmic Membrane regarded as a Complex System

As the experiments of Chambers and Reznikoff leave no doubt as to the existence of a protoplasmic membrane, it would be interesting to investigate its structure. Numerous facts suggest that the protoplasmic membrane is not a layer which stands by itself, quite independent of protoplasm. On the contrary, the membrane actually forms part of the protoplasm though this layer, being present on the surface, will possess different properties. The numerous models of protoplasmic membrane developed (47, 60, 70,74, 81, 82) can explain but part of the facts. They concur with one of the permeability theories, the sieve theory, the lipoid theory, etc. Since the permeability of different plant and animal cells for many compounds, investigated previously, cannot be satisfactorily explained from a single theoretic standpoint, we shall have to look for a model comprising all of the permeability theories. To our mind the models developed under the direction of Bungenberg de Jong (14, 15, 30, 31, 100) present a ready answer to this question. Two entirely different parts of these membranes can be distinguished. Firstly, there is an electric part governed by the attraction between the ionized groups (complex relations). In the other, hydrophobe, section the London-Van der Waals forces (symplex relations) play the leading part. When wanting to study the membrane in any given object, we can make use of the fact that various substances may alter the complex and symplex relationship within the membrane. As the membrane has no greater thickness than a few molecules, we cannot immediately measure these changes. We can, however, note the outcome of these changes. When the interval between the particles in the membrane changes, permeability, too, may vary. The protoplasmic membrane being the region where the cell contacts the medium, a change inside the membrane will affect many processes of life. We shall now compare the changes in a biological process, caused by the addition to the medium of various compounds, with the influence exerted by the same substances on membrane models. This gives us some information as to the nature of the membrane (if we discard the eventuality of a compound acting upon an internal system in preference to the membrane). We have examined two different objects (Lathyrus pollen and bakers’ yeast) and more in particular the influence of different compounds on some of their life processes. Germination of Lathyrus pollen is depressed by different substances. As grains choose to burst in distilled water, a constant of 20 per cent, sucrose will have to be used in all our experiments. When measuring the influence of salts, we shall find pollen to be very sensitive to this action. Also the region, where pn exerts no influence, is limited (6—7). During two successive summers we have experimented with Lathyrus; in either year the sequence of the alkali salts presented great variations. This we attribute to a varying amount of electrolyte in the membrane in either year. Even an inappreciable difference in the relative amount of Ca w;ll readily cause important variations in the alkali salts since the antagonism of the ions has an active part in this. We may anticipate a less marked difference in the behaviour of the alkaline earth metals in either summer since antagonism takes a less active part here. This, indeed, is borne out by the facts. Organic substances come up to our expectations of the influence exerted on lipophile coacervates. There will be analogy between a series of organic compounds, placed i n the order of their diminishing activity on oleate coacervates, and a series depressing germination (with the exception of some minor variations). Pollen grains will burst in certain media. Here, too, salts exert an influence even in low concentration (the grains remaining intact). Also alcohols will show this stabilizing action. Since cane sugar hardly permeates into the cells, we may assume the high osmotic value of the surroundings to be responsible for the fact that grains remain intact in high sugar concentration. Comparing them with cane sugar, methyl- and ethyl-alcohol do not stabilize the grains until higher concentration is reached. From this it would appear that methyl- and ethyl-alcohol exert an opening action while propyl-, butyl- and amyl-alcohol are shrinking agents. A similar phenomenon may be observed in oleate coacervates. This also leads us to the conclusion that this protoplasmic membrane must be either nonsensitized at all or but weakly sensitized. If we consider its great sensitivity for electrolytes and variations in pii, the strong antagonism of the ions and the influence of organic compounds, it becomes clear that, in this membrane, symplex relations are more active than complex relations. Hence, in our opinion, the protoplasmic membrane of Lathyrus pollen conforms pre-eminently to the membrane model developed by Bungenberg de Jong and Bonner (a uni-complex of lecithin). To trace the fermentation of glucose by bakers’ yeast in different surroundings, a new method of measuring the rate of fermentation was developed. The many salts, we have experimented with, may be roughly divided into three groups: 1. salts that completely depress fermentation in low concentration (Ag, Cu, Hg, U02 and Th); 2. salts that initially lower fermentation to a given percentage of the blank. They do not exert further influence when the concentration is increased. Ultimately, at abt. i n., these salts will depress fermentation completely (Tl, Be, Zn, Cd, Pb, Mn, Fe' *, Ni, Co, Al, Fe- • •, La and Ce); 3. salts that stimulate fermentation, or those that exert no influence in low concentration. Later, at about 1 n., fermentation will again diminish (Li, Na, K, Rb, Cs, NH4, Mg, Ca, Sr, Ba, luteo and hexol). The first group of salts, it appears, acts upon an internal system. (These salts have great influence also on the volume of the yeast cells.) The influence exerted by the second and third groups of salts can be traced back to one and the same principle. We assume our membrane to be a Ca tri-complex. Now, if we add a salt, Ca will be replaced by another cation. In the one case the tri-complex formed (for example a Ni tri-complex) will be less permeable than the original membrane. In other words, the amount of glucose fermented represents but part of the amount that might be fermented by yeast to which no salt has been added. In the other case (e.g. a K tricomplex) fermentation will be stimulated since the membrane gets more permeable. Decreased fermentation in a concentration of abt. i n. runs parallel with the diminishing volume of the cells; we are faced with an osmotic phenomenon. Upon respiration salts have quite a different influence. This process is retarded in a concentration which is practically identical to that in which volume decreases. Alcohols presumably act upon an internal system, hence we must discard the results of experiments with these compounds when wanting to arrive at conclusions regarding the membrane. In this protoplasmic membrane complex relations play a predominant part. This model will be a tri-complex whose components are lecithin, protein and Ca (Winkler’s model), possibly accompanied by substances which intensify the symplex relations (“sensitizers”). We must look upon the membrane models developed by Bungenberg de Jong and his collaborators as mere examples of an extensive series of potentialities. This series ranges from a lecithin uni-complex (Bonner) to a tri-complex of lecithin, protein and Ca (Winkler). Every one component of these systems may be varied. Evidently, the structure of the protoplasmic membrane cannot be construed from a few single data. It would become necessary to investigate the influence of numerous compounds on those biological processes in which the membrane is expected to be active. These models show the following distinct advantages: they reconcile, with one another, various permeability theories, and, alike, they will allow the differences in the permeability of organisms and organs to be attributed to variations of the membrane components. Our great lack of knowledge about the physico-chemical properties of the membrane components accounts for the fact that we can no more than construe the most important differences existing among protoplasmic membranes of various cells. Minor problems, for example the very remarkable differences occurring in the permeation of isomeric sugars through glomerulus membrane, cannot, at least for the present, be solved. The general importance of complex systems, possessing, amongst other things, lipophile components, is clearly shown by the circumstance that two vastly different objects like Lathyrus pollen and bakers’ yeast have protoplasmic membranes that can be included in the series of potential models without great difficulty.