Life at the Cell and Below-Cell Level. The Hidden History of a Fundamental Revolution in Biology
"Dr. Ling is one of the most inventive biochemist I have ever met."
How the Membrane Theory Began
Moritz Traube (1826-1894), a Berlin tradesman and amateur research scientist, made an elementary but history-making discovery.17 When a drop of copper sulfate solution is brought into contact with a drop of potassium ferrocyanide solution, a thin layer of reddish-brown copper-ferrocyanide precipitate forms at, and blankets, the entire boundary. After that, no further formation of precipitate occurs. Thus the thin layer of precipitated material formed between the two drops of solutions has halted further passage to the other side of the copper ion as well as the ferrocyanide ion.
Traube published his finding in 1867.17 Its significance was promptly recognized by Wilhelm Pfeffer. By allowing the copper ferrocyanide
precipitate to form within the wall of a porous porcelain cylinder, Pfeffer transformed Traube's
fragile layer of copper-ferrocyanide precipitate into
a practical experimental model strong enough to withstand not only routine
handling but even unilateral application of mechanical pressure. Placing
sucrose solutions of different strengths on either side of such a fortified
copper-ferrocyanide membrane, he saw water moving
from the dilute to the concentrated side18—reminiscent of what Abbe Nollet witnessed across his
dead animal membrane and what Dutrochet observed in
and out of living mature plant cells.
Pfeffer also found that this osmotic water movement could be brought to a stop by applying to the side containing the more concentrated sucrose solution a pressure of just enough strength (to be referred to as osmotic pressure). Now if one side contains sucrose solutions of different concentrations (represented as C) and the other side contains plain water, or if the temperature was varied while Ñ is held constant, then the osmotic pressure, ÿ, was shown to be proportional to C, and to the absolute temperature, T, respectively.18
Dutch botanist, Hugo de Vries (1848-1935), who introduced the important mutation theory to genetics,363 p 34 brought Pfeffer's exciting findings to the attention of physico-chemist, J. H. van't Hoff—whose name was mentioned above. Not long after, van't Hoff discovered that an equation resembling the perfect gas law (i.e., PV = RT, where P is the pressure applied to a (perfect) gas occupying a volume, V, at the absolute temperature, T and R is the gas constant) could correctly predict both sets of relationships Pfeffer's meticulous work had brought to light:
π V = R' T (1)
where V is the volume of solution containing one mole of sucrose and thus equal to 1/C. n, T have the meanings given above. R' is a constant. Substituting Pfeffer's experimentally determined values of 71, V (= 1/C) and T into Equation 1, van't Hoff obtained the constant R', which in numerical value, is close to that of the gas constant, R (1.987 cal. deg.-1 mole-1).
For the origin of the osmotic pressure, van't Hoff introduced his bombardment theory. Pointing out the analogy of the perfect gas law and the van't Hoff equation shown above (Equation 1), he wrote: "In the former case, the pressure is due to the impacts of gaseous molecules on the walls of the containing vessel, and in the latter to the impacts of the molecules of dissolved substance on the semipermeable membrane."13 p 664; 14 (For unexpected later developments, see [11.3 (7)].)
Thus Pfeffer's accurate study of osmotic pressure paved the way for what is often referred to as van't Hoffs solution theory. Continued investigations on both model systems and living (mature) plant cells led Pfeffer to ideas on the living plant cells, which were later referred to as Pfeffer's membrane theory (see below).
The membrane theory has been widely attributed to Pfeffer, even though the term, membrane theory, was not cited in Pfeffer's "Osmotische Untersuchungen" published in 1877 nor in the second edition reissued unaltered in 1921, long after Pfeffer had "moved away" from the study of osmosis.18 pp õõii-xxiii
The interweaving theories summarized above have also conjointly made the membrane theory the first coherent general theory of cell physiology. This theory can explain on the basis of the simple postulation of living cells as membrane-enclosed dilute solutions, four major subjects of cell physiology: (i) cell volume control, (ii) selective solute distribution, (iii) selective solute permeability and (iv) cellular electrical potentials.
In Chapter 4 immediately following, I shall examine the major supportive evidence for the membrane theory—focused primarily on the existence of a semipermeable cell membrane—as well as what has become of them eventually. Supportive evidence for free cell water and free cell potassium ion (K+) came later and will be presented in Chapter 5.