NUTRITION 



359 



complexity of even the simplest carbohydrates, nor of the fact that a mere dilference 

 in the position of certain atoms or groups of atoms, wliich does not affect the per- 

 centage composition at all, gives wholly different chemical and physical characters 

 to the substance. 



Thus, grape sugar (glucose) exists in two forms, one of which rotates a beam of 

 polarized light to the right and the other to the left; the one, rf-glucose, is abundant 

 in plants ; the other, /-glucose, does not occur in nature but has been made arti- 

 ficially. The difference is shown partly in the three following structural formulas, 

 which all sum upC6Hi206: 



OH H OH OH 



I I. I I 

 COH C C C C CH2OH 



I I i I 

 H OH H H 



= if-glucose 



H OH H H 



I ! I I 



COH C C C C CH2OH 



I I i I 

 OH H OH OH 



=/-glucose 



Further, fruit sugar {d-iructose) is abundant in plants, and its structure is quite 



different from glucose : 



H OH OH 



I I I 

 CH2OH CO C C C CH2OH = (/-f ructose 



I i I 

 OH H H 



Another sugar especially abundant in plants, cane sugar, Ci2H220ii, probably has 



this formula: 



/CU:^ CH2OH 



I 

 -O tC 



CHOH 



q I 



CHOH 



I 

 CH2OH 



the modified 

 glucose unit 



CHOH 



\l 

 \CH 



I 

 CH2OH 



the modified 

 fructose unit 



saccharose 



and when it breaks at the — O — bond, it takes up H -OH and resolves itself into a 

 molecule of glucose and a molecule of fructose. These two hexose sugars, glucose 

 and fructose, and the disaccharide, cane sugar, are the only sugars which occur in 

 abundance in plants ; though mannose, g^-lactose, and maltose are formed in the 

 course of digestion. 



