H. GAFFRON 



etc., and hence could not proceed very rapidly. The picture changed 

 radically, however, the moment some cells happened to combine the 

 photochemically accelerated utilization of water as hydrogen donor 

 with a reaction allowing for the elimination of oxidation products by 

 liberating free oxygen. 



As long as there was an abundant supply of carbon dioxide, 

 of water, and of light energy, the accumulation of organic matter could 

 continue unchecked. Oxygen appeared in quantities as free gas in 

 the atmosphere. This was followed by the enormous multiplication 

 of organisms capable of using the oxidation of organic compounds for 

 synthetic reactions.* Carbon dioxide released by respiration was 

 again utilized in the photosynthetic reaction. Some million years 

 later, the cycle of carbon arrived at a steady state. The concentration 

 of carbon dioxide in the air is now 0.03% and barely sufficient to 

 support maximum photosynthesis in full sunlight. Most plants do 

 better when carbon dioxide is added artificially. 



This picture of photosynthesis as a process that developed 

 gradually from less complicated reactions is supported by the following 

 observations. In the plant, the catalytic systems bringing about the 

 assimilation of carbon dioxide can be distinguished by the differentiat- 

 ing effects of metabolic poisons. The whole process is specific for 

 carbon dioxide, just as other metabolic reactions are specific for their 

 substrates. The various catalysts seem to consist of proteins combined 

 with special reaction groups. Chlorophyll itself is bound to protein 

 in the cell. The reduction of carbon dioxide is not an exclusive 

 privilege of the green plants. Besides normal photosynthesis we dis- 

 tinguish at present the following types of carbon dioxide reduction: 



(a) In the dark (10,23) coupled with 



(7) bacterial fermentations (methane bacteria, propionic 

 acid bacteria), 



(2) bacterial and plant oxidations (sulfur bacteria, 

 "Knallgas" bacteria, unicellular algae), 



(3) metabolic reactions in animal tissues. 



(b) In the light (6,7,19), with the simultaneous consumption 



* Van Niel has demonstrated a gradual adaptation to aerobic conditions 

 with some strains of purple bacteria, which at first grew only anaerobically in the 

 light (20). 



34 



