PLANT STRUCTURE AND COMPOSITION 203 



Rubber 



Rubber antl the related gutta-percha are representatives of the 

 polyterpene class. Rubber has been detected in greater or lesser quan- 

 tities in several thousand species of plants, although only a few of 

 them contain sufficient quantities to be considered significant. The 

 ability to produce rubber is scattered through a large number of 

 plant families, but is not found in monocotyledonous plants, gym- 

 nosperms, or any of the lower plants. All the rubber producers are 

 among tropical or temperate plants. Gutta-percha is produced by a 

 few tropical species, especially by Palaquiiun gutta and Mimusops 

 halata. Achras sapota, a tree from Central America, is the source of 

 chicle, the base for chewing gum. Chicle is a mixture of gutta and 

 triterpenols. 



Structurally, rubber and gutta are isomers. The hydrocarbons of 

 rubber possess the cis configuration, whereas the trans configuration 

 predominates in gutta-percha. The latter also has a lower molecular 

 weight than rubber. Molecular weights of 30,000 to 100,000 have been 

 reported for gutta, whereas the molecular weight of rubber is 300,000 

 to 500,000. 



CH 



3 



1 H 



CH2 C— C CH2 CHo 



CH.2 CH2 G C GH2 



1 H 



central unit in rubber 

 CIS configuration about double bonds 



GH3 GH3 



I I 



GH2 C GH2 C CH, 



GH2 C CHo G GH2 



H H 



central unit in gutta-percha 

 trans configuration about double bonds 



Rubber occurs in the plant in the form of microscopic particles 

 suspended in the serum of specialized latex cells or vessels. Synthesis 

 takes place in the cytoplasm of the cell. Rubber is suspended in both 

 the cytoplasm as minute particles and the vacuolar fluid as larger par- 

 ticles. Recent reports have indicated that mevalonic acid (page 202) 

 is the precursor in rubber synthesis, just as in the triterpene, squalene. 



