Chapter 3 



THE OVER-ALL REACTION AND THE PRODUCTS 

 OF PHOTOSYNTHESIS 



In the first chapter, photosynthesis was characterized as the reversal 

 of combustion, a process by which carbon dioxide and water are com- 

 bined to form organic matter, Avhile oxygen escapes into the medium. 

 This definition describes what may be called "normal photosynthesis" of 

 the higher plants, mosses and algae. In recent years, some significant 

 variations of this process have been discovered, which occur regularly in 

 pigmented bacteria, and can be induced artificially in certain algae. The 

 present chapter deals with the over-all reaction of normal photosynthesis, 

 while chapters 5 and 6 will be devoted to its modifications in bacteria 

 and algae. 



A. The Quantitative Balance of Photosynthesis * 



1. The Photosynthetic Quotient 



The relative amounts of oxygen and carbon dioxide exchanged in 

 photosynthesis were first determined by de Saussure in 1804. He found 

 the volume of oxygen evolved, AO2, to be smaller (by as much as 30-40%) 

 than the volume of carbon dioxide consumed by the plants, — ACO2. 

 According to his analyses, the "missing" oxygen was converted into 

 nitrogen. We cannot blame de Saussure for this error, for it was riot 

 until sixty years later that the methods of quantitative plant physiology 

 were sufficiently improved to allow a better determination of the "photo- 

 synthetic quotient," Qpi 



(3.1) Qp = AO2/- ACO2 



(The term "photosynthetic qutotient" has been used by many authors, 

 for example, by Willstatter and Stoll, to designate the inverse ratio, 

 — ACO2/AO2; this difference calls for care in the quotation of numer- 

 ical results.) 



When Boussingault redetermined the photosynthetic quotient in 

 1864, he found Qp values varying between 0.8 and 1.2, with an average 

 close to unity. In his measurements, the net production of oxygen was 

 compared with the net consumption of carbon dioxide. However, even 



* Bibliography, page 56. 



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