156 FINE-STRUCTURE OF PROTOPLASM II 



that dehydrogenation plays an important part in the chemistry of 

 fermentation. It is, therefore, likely that to a certain extent this also 

 applies to the formation of methylene bridges between neighbouring 

 polypeptide chains. It is known that in asphyxia the cytoplasm often 

 liquefies; this may be due partly to hydrogenation processes, resulting 

 from increased partial pressure of hydrogen. 



The hydrogen pressure in protoplasm is characterized by its negative 

 logarithm in much the same way as the hydrogen ion concentration. The 

 Ph is derived from the product of the ionic concentrations (cH+) • (cOR-) = 

 1 0-1*. Similarly, the product of the Hg and Og partial pressures in water 

 is constant. It amounts to (tH2)'^-t02 = iQ-^^ in which the pressures are 

 expressed in atmospheres. Thus the Hg and O2 pressures are mutually 

 dependent in the same way as the H+ and OH" concentrations. The hydro- 

 gen and oxygen pressure or the so-called redox potential of a solution in 

 water can therefore be characterized by a single number. For this purpose 

 we choose the negative logarithm of the hydrogen pressure, which is 

 designated as r^. 



If hydrogen is made to flow through a system under atmospheric pressure, 

 the hydrogen pressure amounts to i atm., or, written in exponential form : 

 10" atm., which means that r^ = o. On the other hand, if oxygen flows 

 through the system, tOg = i, and accordingly (tHg)" = iQ-^^ or ry = 41. 

 Obviously the rjj of a system can vary between o and 41. Small values of r^ 

 indicate lack of oxygen, larger ones on the contrary are indicative of fav- 

 ourable aerobic conditions, r^ (like p^) can be measured directly with the 

 aid of a potentiometer (Fig. 103) or indirectly with the help of suitable dyes 

 (MiCHAELis, 1933) which lose colour at a certain r^ as a result of hydrogena- 

 tion (for instance, methylene blue and indigo). The analogies between p^ 

 and Th are listed in Table XVIII. The characteristic values of the rjj scale 

 are apparent from the following list : 



% 



I at. Og (oxygen electrode) 41 



air (1/5 at. oxygen) 4°^1 



hydrogen and oxygen pressure in equilibrium .... 27.3 



H2 pressure =2-02 pressure (middle of redox scale) . 20.5 



, 1 ^ ( aerobic life o 



border or ; 1 ■ i-r ° 



( anaerobic lire 



I at. H2 (hydrogen electrode) o 



Table XIX gives a few r^ measurements in living cytoplasm (Needham, 

 1925, RiES, 1938). The values are not strictly comparable, since according 

 to the equation Hg — 2 el~ ^ 2H+ the value of rjj is a function of p^- This 

 dependence is apparent from Fig. 103 (according to Bladergroen, 1945). 

 If the electric redox potential E (with respect to the platinum hydrogen 



