54 CHEMISTRY OF PLANT LIFE 



TETRASACCHARIDES 



A complex saccharide, known as stachyose, which is found in 

 the tubers of Stachys tuberifera, is said by some investigators 

 to be a tetrasaccharide and by others to have the formula 

 CseH^Osi 7H2O (i.e., a hexasaccharide) . It is a crystalline solid, 

 with a faintly sweetish taste, and a specific rotatory power of 

 + 148. When hydrolyzed it yields glucose, fructose, and two 

 (or more) molecules of galactose. 



As will be pointed out later (see Chapter XIV), all chemical 

 reactions which are involved in vital phenomena, including those 

 of plant growth and metabolism, are controlled by enzymes. The 

 biochemical reactions which the soluble carbohydrates undergo 

 afforcl such excellent illustrations of the relation of the molecular 

 configuration of an organic compound to the possibility of the 

 action of an enzyme upon it, that it seems desirable to discuss this 

 relationship at this point, rather than to postpone it until after 

 the nature of enzyme action has been considered. Undoubtedly, 

 the student, after he has studied the nature of enzymes and their 

 mode of action, as presented in Chapter XIV, will find it profitable 

 to return to this section and review the facts here presented, as 

 illustrating the principles and mechanism of enzyme action. 

 But a consideration, at this time, of the relation of the molecular 

 configuration of the sugars to their biochemical reactions cannot 

 fail to add- interest to the study of these matters from the chemical 

 and biological standpoints. 



It has been known for a long time that the dextro- and levo- 

 isomers of a compound which contains one or more asymmetric 

 carbon atoms are affected differently by biological agents, such as 

 yeasts, moulds, bacteria, etc. Pasteur, as early as 1850, showed 

 that the green mould, Penidllium glaucum, when growing in solu- 

 tions of racemic acid (a mixture of equal molecules of d- and 

 Z-tartaric acids) uses up only the rf-acid, leaving the I- form abso- 

 lutely untouched. Later, it was found that the same green mould 

 attacks Z-mandelic acid in preference to the d- form; whereas the 



