THE LEAF AND ITS FUNCTIONS 175 



As a concrete example of enzyme activity in plants, we may consider 

 the process by which starch is split up into monosaccharide molecules. 

 Starch being so nearly insoluble, its conversion into soluble form may be 

 regarded as a kind of intracellular digestion comparable to the digestive 

 process in animals. The splitting is accomplished by two specific enzymes. 

 The first of these, diastase, changes starch back to the sugar, maltose, by 

 the addition of w/2 molecules of water to each starch molecule n(C6Hi O 5 ), 

 as shown by the following equation, in which we may take n as equal to 

 200: 



Starch + Water (yields) Maltose + Energy 



molecule molecules , molecules 



200(C 6 HioO B ) + 100H 2 O -* IOOC12H22OH + Energy 



(in the presence 

 of diastase) 



As soon as this reaction has taken place, the maltose is generally converted 

 into glucose by the action of the second enzyme, maltase, as shown by the 

 equation 



This particular kind of molecule splitting is called hydrolysis — the 

 union of a complex compound with water, accompanied by its breaking 

 up into less complex compounds. The process outlined above is just the 

 reverse of that by which the glucose was synthesized into starch. 



The enzyme diastase is similar to and perhaps identical with the ptyalin 

 of human saliva. Maltase of plants is the same enzyme as that found in 

 the small intestine of man, where it breaks down maltose sugar just as 

 it does in the plant cell. Some of the other enzymes present in plants are 

 invertase, splitting cane sugar into glucose and fructose, and found also 

 in the small intestine of man; cellulase, changing cellulose to glucose, and 

 not present in vertebrates; lipase, mentioned above as an example of 

 enzymes with reversible action, changing fats to fatty acids and glycerin 

 or vice versa, and found also in the human stomach. Various proteinases 

 that convert proteins to amino acids by a series of steps, much as pepsin 

 and trypsin work together in man, are also present. 



We closed our account of the human body, considered as an illustration 

 of the structure and functioning of the individual animal, without con- 

 sideration of the reproductive organs. So we shall also do with the plant. 

 Flowers and fruits and all the processes associated with their formation 



