1326 INDUCTION PHENOMENA CHAP. 33 



change in ten three-minute "dim" periods, also was about 0.85; this was 

 considered evidence that the "gulp" was due to a complete reversal of 

 photosynthesis — i. e., oxidation of a substrate of the approximate reduction 

 level of a carbohydrate by molecular oxygen. From the initial slope of 

 the "dim light" curve segments, it was calculated that immediately after 

 the cessation of "bright light," the rate of this back reaction (which we may 

 call "antiphotosynthesis," to distinguish it from ordinary respiration), can 

 be ten or more times that of steady respiration of the same cells (measured 

 over extended periods of darkness). 



As reported in more detail in chapter 37D (section 4a), Warburg et al. 

 interpreted the observed "bursts" and "gulps" of O2 and CO2 not as induc- 

 tion phenomena, but as revelations of the hidden mechanism of photosyn- 

 thesis, showing it to consist of a forward, photochemical reaction with a 

 quantum requirement of 1 (one quantum per O2 produced and CO2 con- 

 sumed), and a thermal back-reaction that consumes a large proportion of 

 the products of the forward reaction. This proportion must be large 

 enough for the liberated chemical energy (added, via some "chemosynthetic" 

 mechanism, to the one quantum of light energy used in the forward process) 

 to reduce the net yield of the cycle to a value compatible with the law of 

 conservation of energy (=^2.8 quanta per O2 molecule). 



The postulated partial separation in manometric experiments of the 

 forward photochemical, from the reverse thermal reaction implies that the 

 back reaction has a rate constant as low as a reciprocal minute, so that the 

 light-enhanced consumption of molecular oxygen needs a minute or two to 

 get under way at the beginning of a light period, and is carried over for a 

 minute or two into the following dim period, thus permitting tell-tale 

 "bursts" and "gulps" to be caught by minute-to-minute manometric 

 readings. 



Warburg and co-workers (1951) noted that activity bursts of the above- 

 described type are not observed in carbonate buffers, and suggested that 

 the back reaction is much faster in these media; later (1953, 1954) they 

 found that ChloreUa cultures grown in a different way showed no activ- 

 ity bursts of similar duration also in carbon dioxide solutions. 



Wide variations in the duration of the oxygen bursts and gulps was ob- 

 served also by Damaschke, Todt, Burk and Warburg (1953) with a rapid 

 electrochemical method of oxygen determination. The method was simi- 

 lar to that used by Blinks and Skow, and the results were qualitatively 

 similar to those described above on p. 1319. These measurements, too, 

 showed a rapid oxygen burst at the beginning of illumination, and an oxygen 

 gulp after its termination. (Fig. 33. 6B shows the recordings of gulps after 

 bright periods of 3, 8 and 28 seconds.) 



