CHEMICAL COMPOSITION OF PLANTS 39 



in the latter group are found glucose, fructose, galactose, man- 

 nose, and others. Until recently, the structure of monoses was 

 supposed to be represented by rows of carbon atoms with 

 hydrogen atoms and hydroxyl groups attached to them. Accord- 

 ing to this assumption, the structure of arabinose, glucose, and 

 fructose was represented by the following formulas : 



O 



C 



H 

 OH— C— H H— C— OH 



O— C— OH " OH— C— H 



C^ CH2OH 



H C=0 



OH— C— H 

 H— C— OH 



H— C— OH H— C— OH 



I I 



CH2OH H— C— OH 



H— C— OH 

 CH2OH 



CH2OH 



Z-Arabinose d-Glucose d-Fructose 



Recently, on the basis of their physical and chemical proper- 

 ties, which are too complex to describe here, it was suggested 

 (Haworth) that pentoses and hexoses possess the structure of a 

 six-membered ring consisting of 5 carbon atoms and 1 atom of 

 oxygen (oxygen bridge). When only atoms of one kind are con- 

 tained in a ring structure, the compound is said to be homo- 

 cychc; when different kinds of atoms are present, they are called 

 heterocychc compounds. This structure may be illustrated by 

 the formulas shown on p. 40. 



When the hydrogen and hydroxyl ot other groups are arranged 

 about the carbon atom in a symmetrical pattern, the compound 

 is called symmetric. When they are arranged asymmetrically, 

 the compound itself is asymmetric. Asymmetric arrangement 

 of the atoms or atomic groups around the carbon atom ^ gives 

 the compound characteristic optical properties that can be used 

 to identify it or to analyze its solutions. 



Owing to the presence of asymmetric carbon atoms, all the 

 natural monosaccharides are optically active and are capable of 

 rotating the plane of polarization of light either to the right or 

 to the left. On this capacity of rotating the plane of polarized 

 light is based a simple quantitative method of determining sugars 



