METABOLISM 123 



food substances which we have discussed above exist commonly 

 in forms which are not sohible in water. The advantages of this 

 in the storage of reserve foods is obvious. It is evident, however, 

 that before such foods can be moved or translocated within the 

 plant, and before they can be assimilated into living protoplasm, 

 they must in some way be made soluble; and it is this process of 

 converting an insoluble food into soluble form which is known 

 as digestion. 



Digestion is brought about through the activity of certain 

 highly important but little understood substances known as 

 enzymes or ferinents. Enzymes are concerned not alone in diges- 

 tion but in the production of many other chemical changes in 

 the plant. They occur in great variety and are probably protein 

 in character, although their composition is not definitely known. 

 Enzymes are usually present in exceedingly small quantities but 

 are able to effect profound chemical changes out of all proportion 

 to their bulk. How they do this we do not understand. The 

 enzyme apparently does not enter into the composition of the 

 substance produced, nor does it contribute energy for the process, 

 and it is not consumed or used up. It seems merely to hasten a 

 chemical reaction which might still take place, although very 

 slowly, in its absence. Enzymes have thus been said to "lubri- 

 cate " reactions. Temperature largely controls their rate of activ- 

 ity, each enzyme having an optimum temperature at which it 

 works most rapidly. These remarkable substances may be 

 destroyed by heat and even by certain poisons. Aside from effect- 

 ing digestion, they are concerned with the changes which take 

 place in the various fermentations and in decay; with the process 

 of oxidation in living tissues, and with the synthesis and decom- 

 position of many organic substances. Indeed, most of the 

 metabolic processes of plants and animals are probably depend- 

 ent, in one way or another, upon enzymes. 



It is only the digestive enzymes and their activities with 

 which we are here concerned. Digestion is generally accom- 

 panied by hydrolysis, or the addition of one or more molecules 

 of water to a molecule of the substance to be digested. The 

 sugars are soluble and most of them may be assimilated directly 

 without digestion. Cane sugar, however, is often broken down 

 into glucose and fructose through the agency of the enzyme 

 invertase, thus: 



C12H22O11 -j- H2O = 2C6H12O6 



