352 K. DAMASCHKE, F. TODT, D. BURK, O. WARBURG VOL. 12 (ig53) 



Back reaction of mihanced oxygen consumption 



The initial respiratory curve shown in Fig. 2 had already been proceeding linearly 

 from 4% to 0.445% for a- period of many hours in darkness, without any intervening 

 illumination. After each of the two illuminations shown in Fig. 2, the rate of oxygen 

 consumption, upon cessation of illumination, proceeded at 1.8/0.4 = 4-5 times the 

 normal rate, a marked enhancement, and a very clear demonstration of the light-induced 

 back reaction of the photosynthetic cycle. As shown after the second illumination 

 period, more than ten minutes were required for the back reaction, in this instance, to 

 run its course before re-establishment of the basic respiratory rate. The pressure fall 

 during the back reaction was more nearly first order, as compared with the zero order 

 time course of the normal respiratory fall in oxygen pressure. In Fig. 2, the intensity 

 of white light employed for the two 5 minute illuminations was high enough to result 

 in typical induction periods in the photo phases, following the long initial period of 

 dark adaptation. 



The increase in oxygen consumption induced by the light reaction can also be shown 

 with particular beauty if the Chlorella cell suspension is first brought to compensation 

 of respiration (Figs. 3, 4,5). When additional light is turned on briefly, and then off, 

 any increased rate of back reaction immediately makes itself evident by a fall in oxygen 

 pressure in the system upon cessation of the additional illumination, and the time taken 

 to return to the state of compensation is a measure of the time period of the back 

 reaction of the cycle under the conditions of the experiment. In our experience, this 

 may vary from a few seconds up to a minute or more, depending upon a variety of 

 conditions that need not be detailed here. In general, such a cycle of photo and back 

 reactions is most sharply delineated when the illumination period is made as short as 

 possible with a light intensity high enough to produce a notable amount of oxygen 

 within no more than a few seconds. Then the back reaction, whose rate depends upon 

 the concentration of products built by the light reaction, becomes relatively less im- 

 portant during the illumination period, and relatively more important during the period 

 after cessation of the illumination; that is, the photo and back reactions are thereby 

 separated in time as much as possible. With photo-flash bulbs of extremely high but 

 undetermined intensity per flash, typical cycle curves were obtained qualitatively with 

 single flashes of o.ooi second. 



Fig. 4 is a photograph of one of our original work sheets obtained witli tiio Linien- 

 schreiber recorder of Multiflex Galvanometer No. 3 response. In Curves 2, 3, and 4, which 

 must be read from right to left, the cell suspension was in each instance first brought 

 to compensation at approximately 0.15% O2 with a beam of white light, and then given 

 extra red light (A ^ 644 m/x) of fixed moderate intensity for respectively 28, 8, and 

 3 seconds. The intensity of red light employed with the i)artially light-adapted cultures 

 in Fig. 4 was smaller than that in Fig. 2, in order to avoid the i:)hotosynthetic induction 

 phases seen in the latter, and to permit an electrochemical demonstration (confirming 

 manometric results reported by us many times previously'*'^'"''') of concave downward 

 photo curves whose initial slopes are steepest at first and fall progressively to a constant 

 steady-state value as the rate of the back reaction rises to a constant value. This rise 

 in back reaction rate is indicated not only by the fall in slope as illumination })roceeds, 

 but also by the increasingly greater initial negative slopes observed when the red light 

 is turned off in Curves 4, 3, and 2, respectively. Curve i in I'^ig. 4 is the calibration 



References p. J55. 



