NUTRITION 359 



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

 in the position of certain atoms or groups of atoms, which 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, d-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 upCeHi 2 O 6 : 



OH H OH OH 



COH C C C C CH 2 OH = ^-glucose 



I I I I 

 H OH H H 



H OH H H 



I ! I I 



COH C C C C CH 2 OH =/-glucose 



I I I I 

 OH H OH OH 



Further, fruit sugar (d-fructose) is abundant in plants, and its structure is quite 

 different from glucose : 



H OH OH 



I I I 

 CH 2 OH CO C C C CH 2 OH = d-f ructose 



I i I 



OH H H 



Another sugar especially abundant in plants, cane sugar, Ci 2 H 22 Oii, probably has 

 this formula: 



,CH\ CH 2 OH 



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, galactose, and maltose are formed in the 

 course of digestion,- 



