THE CARBOHYDRATES 



39 



(6) Hezoses (Monosaccharides) 



Among the compounds of this group, glucose and fructose are most 

 widely distributed in plants. They are found in the sap of practically 

 all leaves (as first proved by Brown and Morris in 1893) in quantities 

 which depend on species as well as on the previous treatment of the plant. 

 Starvation may reduce their concentration to zero, while intense photo- 

 synthesis may raise it to 10 or 15% of the dry weight of the leaf. Free 

 fructose is sometimes more abundant than free glucose; Brown and 

 Morris (1893), for example, found in Tropaeolum majus leaves four times 

 more fructose than glucose, and Gast (1917) found up to eight times 

 more in leaves of five different species. Equal quantities of glucose and 

 fructose are contained in cane sugar (sucrose), which is present in all 

 green leaves, while the most common highly polymeric carbohydrates, 

 starch and cellulose, are built entirely of glucose units, thus making the 

 latter the most abundant single organic compound on earth. 



It is often forgotten that the photosynthesis by higher plants is far 

 inferior, in its yield, to the photosynthesis by the microscopic organisms 

 of the plankton. The tendency to extend to the whole plant world con- 

 clusions reached in the study of the highly developed land plants, is not 

 without its danger. It is therefore important to note that hexoses have 

 been found also in many algae, although no systematic information about 

 their distribution in those organisms is as yet available. 



Hexoses other than glucose and fructose are rare in green plants. 

 Clements (1932) was unable to find mannose in leaves of 42 species. 

 Galactose is only encountered in the esterified form, as galactosides, or 

 in condensation products with other sugars; while sorbose was definitely 

 identified only in fruit juices. 



Glucose (and other aldohexoses) act chemically as mixtures of three or four different 

 tautomeric forms. 



A B C D 



1 CHO CHOH CHOH-n CHOH — 



2 

 3 



4 

 5 

 6 



CHOH 



I 

 CHOH 



I 



CHOH 



I 

 CHOH 



CH2OH 



Open-chain 



aldehyde 



(Baeyer 1870) 



COH 



CHOH 



i 

 CHOH 



I 

 CHOH 



I 

 CH2OH 



Enol form 

 (Fischer 1895) 



CHOH 



CHOH 



I 

 CHOH 



I 

 CH 



I 

 CH2OH 



1,5-Glucopyranose 



(a- and j3-glucose) 



(Haworth 1926) 



CHOH 



I 

 CHOH 



O 



CH 



CHOH 



CH2OH 



1 ,4-Glucof uranose 



(7-glucose) 



(ToUens 1883) 



Formulae 3.1. Tautomers of Glucose 



The most stable structure is the six-membered ring C. These formulae describe 

 all aldohexoses; the particular spacial arrangements characterizing the a- and ^-glucose 



