TRANSIENTS FOR CARBON DIOXIDE 



431 



medium, the amounts are too small to register under our conditions. 

 Presence of carbon dioxide either in water or in diluted neutral phos- 

 phate media, or in pure solutions of mixed sodium and potassium 

 carbonates, on the other hand, leads to conspicuous changes in the 

 recorded pH upon illuminating or on darkening. 



Thus we proceed under the assumption that our apparatus responds 

 mainly to those pH changes which are due to a metabolic carbon 

 dioxide exchange. A release of CO2 by respiration or fermentation 

 lowers pH and the recorded curves in our figures slope up. Photo- 

 synthesis increases pH and the curves slope downward. A horizontal 



Fig. 1. Absence of acidity changes not attributable to carbon dioxide when light 

 is turned on or off. Curves a and b: Scenedesmus in 0.002 M phosphate buffers 

 nearly free of CO2. Curve c control in 0.002 M bicarbonate. 



trace means that carbon dioxide is neither released nor absorbed, or 

 that both processes balance exactly. 



What happens behind the cell wall within the cell is another matter. 

 Obviously the method cannot distinguish between the decomposition 

 of intracellular carbonates and true decarboxylations. 



]\Iany years ago I came to the conclusion that gross transients 

 lasting for a minute or more are due to the interference of general 

 metabolic reactions or their products with the normal course of photo- 

 synthesis, and that such phenomena do not reflect simply the se- 

 quence of incipient reactions building up the conditions in the steady 

 state, for their cause evidently does not reside in the photosynthetic 

 mechanism proper (G). In the main this has been borne out by the de- 

 velopment in the intervening years. For instance, after a short ana- 

 erobic incubation, when respiratory and fermentative reactions are at 



