CHAPTER XXII 

 DIGESTION 



Digestion is the great secret of life. 



— S. Smith. 



Anabolism and Catabolism. — In the preceding chapters, the 

 processes which have been considered were the constructive phases 

 of metabolism. These building up processes may all be included 

 in the term anabolism, by which is meant those physical and chemi- 

 cal changes within the organism which result in the construction 

 of larger molecules of material from smaller ones. The formation 

 of the carbohydrates, proteins, fats, etc., are all anabolic, but this 

 formation of materials is, in large part, only preparatory to their 

 later use for the release of energy, one of the two main purposes 

 of food. Sugars, fats, and the other foods, which are burned within 

 the plant are not of use until they are so oxidized; hence, after 

 its formation in the leaves or in storage organs, the food must be 

 transported to the place where it is to be burned. The distance 

 which the food must be transported may be very short, but owing 

 to the impermeability of the plasma membrane, large molecules 

 cannot be transported as such. They must first be broken down 

 into smaller particles and put into a soluble form. This putting 

 into solution, which is accompanied by the addition of water, 

 resulting in a product more like water and hence more soluble, 

 is called digestion, which is thus seen to be a form of hydrolysis. 

 Digestion is one of the processes which result in the formation of 

 smaller molecules from larger; it is therefore a phase of catabo- 

 lism or destructive metabolism. 



Extra- and Intra-Cellular Digestion. — Digestion is brought 

 about by enzymes which are either produced inside the cell, 

 where they do their work, or else are excreted and do their work 

 outside the cell. The former type of digestion is intracellular and 

 the latter, extracellular. In plants which make their own food and 

 store it up within their own cells, digestion is largely intracellular, 

 while with animals and with plants such as the parasites and 

 saprophytes, which get their food ready-made, digestion is mostly 



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