38 OVER-ALL REACTION OF PHOTOSYNTHESIS CHAP. 3 



B. The Products of Photosynthesis* 



1. The Carbohydrates 



Experiments described in the preceding section indicate that the direct 

 products of photosynthesis belong to the class of carbohydrates. How- 

 ever, by the time when the quantity of the photosynthate becomes suf- 

 ficient for chemical analysis, the carbohydrate fraction is found to contain 

 a variety of compounds of different degree of polymerization, and it is 

 unlikely that they all are the primary products of photosynthesis. 

 Which of them, if any, is the primary product, is a moot question. 

 Before presenting the arguments advanced on behalf of different con- 

 tenders for this distinction, it seems advisable to give a short review of 

 the structure and properties of the most common plant carbohydrates — 

 pentoses, hexoses and their various polymers. For a detailed presenta- 

 tion of sugar chemistry, the reader is referred to the monographs by 

 Pringsheim (1932), Bernhauer (1933), Armstrong and Armstrong (1934) 

 and Micheel (1939). 



(a) Pentoses 



These Cs sugars occur abundantly in many plants, usually not in the 

 free soluble form, but as the so-called pentosans, i. e., anhydrous com- 

 pounds of the composition (C6H804)i. The pentosans are mostly found 

 in the supporting structures — cell walls, wood fibers, etc., and are thus 

 not directly associated with photosynthesis. However, they are more 

 easily hydrolyzed than cellulose, and sometimes serve as reserve ma- 

 terials, thus coming into closer relation with nutritional processes. 



Some authors, Nef (1910), for example, thought that the synthesis of 

 pentoses must be independent of that of hexoses; others, as Lob and 

 Pulvermacher (1910), suggested that pentoses are intermediates in the 

 formation of hexoses. The production of starch from externally supplied 

 pentoses (c/. page 260) indicates that plants contain enzymes capable of 

 bringing about the conversion of pentoses into hexoses. On the other 

 hand, it is known that pentoses can be produced by degradation of 

 hexoses (by the intermediary of hexuronic acids) . According to Spoehr, 

 Smith, Strain and Milner (1940), albino maize plants provided with su- 

 crose as the only source of carbon, produce uronic acids and pentosans 

 from this food. This supports the opinion of those authors, including 

 Ravenna (1911) and Tollens (1914), who believed that pentoses in plants 

 are secondary products of transformation of the hexoses. The pentosans 

 deposited in the cell walls and wood fibers must be produced there from 

 products transported by the sap, which usually contains only glucose, 

 fructose and sucrose (and no free pentoses). 



* Bibliography, page 57. 



