46 OVER-ALL REACTION OF PHOTOSYNTHESIS CHAP. 3 



There does not seem to be any basis for arguing whether fructose and glucose are 

 independent products, or whether one is a "primary" and the other a "secondary" 

 sugar. Some leaves contain more free glucose, others more free fructose; while glucose 

 usually predominates in the polymeric carbohydrates. Endo (1936) found only glucose 

 in some green algae {C odium latum) , and only fructose in others {Cladophora Wrightiana) . 

 In vitro, glucose, fructose and mannose are interconvertible in alkahne solutions (the so- 

 called Lobry de Bruin-van Eckenstein reaction). This conversion, which is supposed 

 to proceed through the intermediary of the enols Bi and B2 (Formula 3.III), inevitably 

 produces a certain proportion of mannose. The leaf cells are not alkahne, but neutral 

 or acid; and the leaves apparently contain no mannose (page 39). These facts have 

 been used as arguments against the glucose-fructose interconversion in the leaves. 

 However, Spoehr and Strain (1929) showed that in presence of sodium phosphate the 

 interconversion can be achieved, in vitro, also in neutral or even shghtly acid solution. 

 Fructose, kept at 37° C. in shghtly acid Na2HP04 solution (pH 6.7) was found to 

 contain, after 165 hours, 8.5% mannose and 28% glucose. 



An enzyme (isomerase) is known which converts glucose monophosphate into 

 fructose monophosphate; while the diposphates of these two sugars are identical. Thus, 

 the interconversion in Uving plants probably occurs by a combination of phosphatiza- 

 tion with the action of specific enzymes. 



In addition to glucose and fructose, the distinction of being the first 

 Ce products of photosynthesis was claimed also for the inositols. Crato 

 (1892) and Kogel (1919) suggested that these cyclic compounds, con- 

 sisting entirely of HCOH groups, are the parent substances of all other 

 sugars in plants. Gardner (1943) suggested that the first carbohydrate 

 formed by photosynthesis may be the triose, glycer aldehyde; but the only 

 basis for this hypothesis was that trioses are the last carbohydrates which 

 occur in respiration (c/. Chapter 9, page 223). 



If it is implausible that disaccharides could precede monosaccharides 

 in the synthesis of carbohydrates, it is, a fortiori, even less probable that 

 starch could be formed directly by photosynthesis (as has occasionally 

 been suggested, e. g., by Baly). True, one could conceive of a mecha- 

 nism of photosynthesis in which new — CHOH links would be added to 

 a chain, growing from some "carrier" molecule, and hexoses and other 

 simple sugars would be produced by an enzymatic breakdown of these 

 chains only after they have grown to a considerable length. It is, how- 

 ever, unlikely that this hypothetical intermediate long-chain product 

 should be identical with leaf starch. The prevailing opinion is that the 

 latter is only a temporary storage product, deposited in the chloroplasts 

 during intense photosynthesis, when more sugar is formed than can be re- 

 moved by translocation. Sapozhnikov (1889, 1890, 1891, 1893) found 

 that detached leaves of Vitis vinifera and V. labrusca can accumulate up 

 to 25-50% dry weight in starch before becoming "choked" with this 

 product. According to Winkler (1898) the formation of starch grains 

 sets in when the concentration of glucose exceeds 0.2%, and reaches a 

 maximum at 10% glucose in the leaf sap. 



