CHAPTER XXVII 



DIGESTION 



Digestion. — The conversion of complex, usually insoluble foods into 

 simple, usually soluble forms, in or through the agency of living organ- 

 isms is known as digestiori. The most familiar digestive reaction in plants 

 is the conversion of starch to glucose. The summary equation for this 

 process is: 



(C6Hio05)„ + « H2O -^ n CoHisOe 



Starch Glucose 



Actually, as already discussed in Chap. XXII, this reaction takes place 

 in several stages. 



Digestion of starch is a process of hydrolytic decomposition. This is true 

 of all other digestive processes, as will be evident from other equations given 

 later in the chapter. Furthermore the reaction representing the digestion of 

 any substance is an exact reverse of the condensation reaction by which it 

 was originally produced. If the direction of the arrow is reversed in the 

 above equation, for example, it will then represent the process of starch 

 synthesis. 



Digestion may occur in any living plant cell, but obviously the quan- 

 tities of food hydrolyzed are likely to be greatest in those cells in which foods 

 have accumulated in abundance. The leaf cells in which starch manufacture 

 occurs are also a seat of much digestive activity as practically all of the starch 

 accumulated in such cells is sooner or later digested into glucose and trans- 

 located in the form of sugars to other tissues of the plant. 



The basic chemical mechanism of digestion is, as in a number of other 

 fundamental physiological processes, similar in plants and in animals, although 

 in its mechanical aspects digestive processes in the higher animals are much 

 more complex than in plants. This is due to the presence in all of the 

 more complex forms of animal life of a more or less highly organized system 

 of digestive organs. In the higher plants digestion is usually an intracellular 

 phenomenon. In all living organisms, however, the same fundamental types 

 of foodstuffs — carbohydrates, fats, and proteins — are hydrolyzed in the process 

 of digestion. 



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