PHOTOSYNTHESIS 461 



reaction requires the participation of a number of co-factors, 

 the most important of which is ascorbic acid. 



It is most likely that in the formation of ATP by these 

 reactions, ' photochemical phosphorylation ' follows a similar 

 course to oxidative phosphorylation. The initial hydrogen 

 donor (reduced substance) is photochemically produced 

 ' active hydrogen ' in, for example, the shape of reduced forms 

 of pyridine nucleotides, while the oxidising agent is oh 

 formed by the photolysis of water, hydrogen peroxide, or 

 even molecular oxygen. In this reaction of photochemical 

 oxidative phosphorylation there probably take part many 

 of the ordinary respiratory mechanisms (e.g. cytochromes and 

 flavines) and oxidative cycles (e.g. the tricarboxylic cycle of 

 Krebs) with which we shall become acquainted in more 

 detail in our exposition of the mechanism of respiration. 



Calculations show that the fundamental reaction of 

 * raising ' CO2 to the level of carbohydrates requires the par- 

 ticipation of four electrons and three molecules of ATP, one 

 of which is expended on the phosphorylation of ribulose 

 monophosphate before its carboxylation by CO2. 



As a synopsis we give here a gi'eatly simplified scheme of 

 the interactions of the separate aggregates in the general 

 process of photosynthesis (Fig. 39). 



A detailed knowledge of the photosynthetic apparatus of 

 green plants shows that hardly any of their catalytic mechan- 

 isms or even of their whole aggregate of mechanisms show 

 anything which is new in principle. In most cases we find 

 the very same or analogous mechanisms in various colourless 

 organisms or in photosynthetic bacteria. 



Thus, even before the appearance of green plants, before 

 the development of the present-day forms of photosynthesis, 

 these chemical mechanisms existed, but they were scattered 

 rather than being integrated into a single complex system. 

 This unification of previously existing mechanisms took 

 place during the development of the photosynthetic appara- 

 tus. It could only have been formed during the process of 

 evolution of organisms on the basis of pre-existing systems 

 and aggregates. 



Continuing our analogy with the motor-car engine, we 

 may say that, as the history of technology shows, such an 



