40 



OVER-ALL REACTION OF PHOTOSYNTHESIS 



CHAP. 3 



are best brought out by Haworth's prospective formulae: 



Alpha 

 OH OH 



Befa 

 OH H 



l/H H0\ 



OH H 



OH 

 H CHjOH H CHgOH 



Formulae S.II. a- and fi-Glucose 

 (a- and /3-glucopyranoses) 



In ^-glucose, all substituents are in trans-positions, which probably gives the lowest 

 energy and highest stability. 



Fructose is a 2-ketohexose, with the five possible structures: 



A 



CH2OH 



I 



CHOH 



I 

 CHOH 



5 

 6 



CHOH 



CH2OH 



Open-chain 

 ketone 



CHOH 



COH 



I 

 CHOH 



CHOH 



I 

 CHOH 



I 

 CH2OH 



1,2-Enol 

 form 



CH2OH 



COH 



II 

 COH 



I 

 CHOH 



C 



CH2OH 



I 

 COH — 



D 



CH2OH 



I 

 CH 



CHOH 



CH2OH 



2,3-Enol 

 form 



CHOH 



I 

 CHOH O 



I 

 CHOH 



I 

 CHOH- 



2,6-Fructo- 

 pyranose 



CHOH 



I 

 CHOH 



COH — 



O 



CH2OH 



2,5-Fructo- 

 furanose 



Formulae 3. III. Tautomers of Fructose 



Of the two enols, Bi is identical with the enol of glucose; this relation is responsible 

 for the slow interconversion of glucose and fructose in alkaline solutions. The di- 

 phosphates of these two sugars also are identical, and this must be important for their 

 interconversion in living plants. 



In polymerization, glucose usually acts in the pyranoid form, whereas fructose 

 more often enters into polymers in the form of a five-membered furanoid ring. 



All hexoses in plants are optically active and belong to the c?-series. 

 This shows that asymmetric synthesis takes place in the course of the 

 reduction of carbon dioxide, probably through the intervention of an 

 asymmetric enzyme. 



Mention must be made also of inositols, carbocyclic compounds which 

 are isomeric with hexoses, (cf. Formula 3. IV), taste sweet and are included 

 in the general classification of sugars under the name of "cycloses." 

 Inositols are widely distributed in plants (particularly in seeds), and have 

 been identified in leaves, e. g., by H. Miiller (1907), Tanret (1907) and 

 Curtius and Franzen (1916) in quantities of about 0.05% of dry weight. 

 Interesting is the phytin, a calcium-magnesium salt of inositol phosphate, 

 C6H6-[OPO(OH)2]6, which was found in green leaves by Curtius and 

 Franzen. 



