322 PHOTOSYNTHESIS 



formed into the other. Starch may then be built up from glucose as described 

 later in this chapter, and sucrose synthesized from glucose and fructose, prob- 

 ably under the influence of the enzyme sucrase (Chap. XXVH), according to 

 the following equation: 



CeHiaOe + CeHiaOe > C12H22O11 + H2O 



Glucose Fructose Sucrose 



Both Starch and sucrose are temporary storage products in the mesophyll 

 cells. The proportions of these two compounds built up from the simple 

 sugars depend upon the species and the intracellular metabolic conditions. A 

 number of species do not synthesize starch in the leaves but sucrose seems to 

 be of universal occurrence in green plants. 



The Photosynthetic Ratio. — For many years it has been known that the 

 ratio of the volume of oxygen released to the volume of carbon dioxide ab- 

 sorbed during photosynthesis is approximately one. This ratio is called the 



photosynthetic ratio \r^\-\ Precise determinations of the photosynthetic 



ratio are difficult to make, principally because of the simultaneous occurrence 

 of respiration in all living cells. The gaseous exchanges accompanying respira- 

 tion — consumption of oxygen and release of carbon dioxide — may complicate 

 attempts to measure photosynthetic ratios, particularly since the value of the 

 respiratory ratio is not necessarily constant, even for the same tissue. Never- 

 theless it is of considerable theoretical importance that exact data regarding 

 the photosynthetic ratio be obtained. Knowledge of the magnitude of this 

 ratio permits certain deductions to be made regarding the nature of the 

 products of photosynthesis. If the primary product of photosynthesis is a fat, 

 a protein, or some other non-carbohydrate, the photosynthetic ratio would not 

 be one. Experimental verification of the theoretical value of one for the 

 photosynthetic ratio will therefore help to validate the generally accepted 

 equation for photosynthesis. 



Of the numerous attempts to determine the photosynthetic ratio of plants 

 with precision the best is probably that of Willstatter and StoU (19 18). 

 They made determinations of the photosynthetic ratio under an experimental 

 arrangement such that the rate of photosynthesis was 20 to 30 times as great 

 as the rate of respiration. Under these conditions any slight errors intro- 

 duced by the gaseous exchanges accompanying respiration are relatively small. 



CO') 

 2 This ratio is sometimes written -77^ but since this form is universally used 



for the respiratory ratio (Ch^p. XXIX) it seems better to adopt the form ^ 

 for the photosynthetic ratio. 



