Section III 



CATABOLISM 



CHAPTER VIII 

 TRANSFERENCE OF FOOD MATERIALS : DIGESTION 



AS the result of synthesis in the plant a reserve of materials 

 is stored in various localities. Seeds, tubers and bulbs 

 are examples of storehouses intended to provide for the 

 future development and growth of the plant. These store- 

 houses are, however, often attacked by parasitic plants and 

 by animals. 



The stored materials are deposited in an insoluble non- 

 diffusible form, but the materials are transferred from the 

 place of formation to the storehouses and from the storehouses 

 to the place where they are used, in the form of soluble 

 diffusible substances. These changes involve the reversible 

 action of enzymes and the conditions which determine the 

 separation of a new phase. These matters and their relation 

 to the Law of Mass Action are discussed in this and the follow- 

 ing chapter. All classes of substances are accumulated as 

 reserves, thus we find carbohydrates, fats and proteins in 

 varying proportions. Foods are classified into carbohydrate, 

 fatty and protein foods depending on the varying proportions 

 of one or other of these classes of substances. 



Carbohydrate is stored largely in the form of starch which is 

 an insoluble substance, fats are insoluble in water and proteins 

 form colloidal solutions which cannot 'diffuse through 

 membranes. In order to transport these substances from one 

 place to another they must be converted into simple soluble 

 diffusible substances. 



The various kinds of substances required for the mainten- 

 ance of the body are best dealt with under the heading of 

 metabolism, but in this chapter we will describe the means by 

 which the insoluble or colloidal materials in the food "are 

 turned into soluble diffusible substances, that is the process of 

 digestion. 



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