286 PHOTOSYNTHESIS 



g. The Blackman Reaction. 



A conception of the mechanism of photosynthesis differing somewhat 

 from that of Willstatter and Stoll was developed by Warburg.^^ He con- 

 siders that photosynthesis is not a simple photolysis of carbonic acid. 

 There are rather several steps, of which the first is the "photochemical 

 primary action." In this action oxygen is not split out, but it consists 

 rather in the action of light on the chlorophyll molecule with the forma- 

 tion of the "photochemical primary product." The rate of formation 

 of this product is proportional to the amount of radiant energy absorbed 

 per unit time. The "photochemical primary product" reacts in a secondary 

 reaction with the "acceptor." The "acceptor" is not carbonic acid, but a 

 carbonic acid derivative which is formed in the cell in a chain of chem- 

 ical reactions. There are thus according to this view three reactions : 

 1. the photochemical primary^ reaction, 2. the secondary reaction and 

 3. acceptor formation. The acceptor formation takes place independently 

 of light; it is a "preparatory" (vorbereitende) reaction, the product of 

 which, the acceptor, is consumed in the secondary reaction. 



Warburg considered that in the "primary photochemical action" 1. no 

 oxygen is formed and 2. no substances are formed which automatically 

 split out oxygen. This view is based partially upon the claim of Willstatter 

 and Stoll that chlorophyll adds on carbon dioxide, and partially upon 

 quantum theoretical reasoning which makes a primary splitting of car- 

 bon dioxide by the light which is active in photosynthesis (0.77 [i) highly 

 improbable. 



Warburg's theory of "acceptor" formation was based upon his investi- 

 gations of the influence of carbon dioxide concentration and light in- 

 tensity on the rate of photosynthesis. Above certain high concentrations 

 of carbon dioxide and high illumination intensities the rate of photosyn- 

 thesis does not increase when either of these factors are augmented. Also, 

 under these conditions (high illumination intensity or high carbon dioxide 

 concentration) a rise of 10° in temperature (15 to 25°) causes the rate 

 of photosynthesis to double. This fact, discovered by Blackman, is 

 taken by Warburg to indicate that in the mechanism of carbon dioxide 

 reduction a slowly progressing chemical reaction plays a role and under 

 these conditions determines the rate of photosynthesis. This reaction 

 Warburg designated as the "Blackman reaction." When the light in- 

 tensity is low a temperature coefficient close to unity is obtained ; this 

 is characteristic of photochemical reactions and indicates that under these 

 conditions the light reaction is determining the rate of photosynthesis. 

 When the light intensity is high, a high temperature coefficient is obtained; 

 this is characteristic of an ordinary chemical reaction and indicates that 

 under these conditions a thermal or "dark" reaction is determining the 

 rate of photosynthesis. 



''^ Warburg, Biochcm. Zcit., 100. 230 (1919); 103, 188 (1920); Naturzmss.. 9, 

 354 (1921). Warburg and Negelein, Zcit. physik. Chcm.. 102, 235 (1922); 106, 

 191 (1923). Warburg and Uyesuge, Biochem. Zeit., 146, 486 (1924). 



