CHAPTER XIX 

 USES OF FOOD IN PLANTS 



III. SUBSTANCES MADE FROM FOOD 



All plants contain a variety of substances made from foods. Some of 

 them are important constituents of cell structures; others, such as 

 chlorophyll and enzymes, are essential in certain plant processes. Several 

 others seem to be of no essential survival value or detriment to the plant. 

 Foods are the source of building materials as well as the source of 

 chemically bound energy. All the organic compounds of which cells are 

 composed are constructed directly or indirectly from foods. 



The manner in which cellulose may be formed from sugar by chemical 

 condensation was described in Chapter XIV. It is briefly indicated as 

 follows: 



Glucose > Cellulose + Water 



n CeHioOe > (CeHioOa)^ H2O + (n - 1)H20 



This is merely a condensation process in which n represents an un- 

 known number. The value of n may vary in the same cell and also in 

 diflFerent plants. It probably represents a number greater than 50, and 

 perhaps much greater. After condensation all the chemically bound 

 energy that was in the sugar is in the cellulose. 



Cutin and suberin are fat-like substances that occur in the walls of cer- 

 tain cells. They are formed from sugar in the same manner in which fats 

 and oils are formed: oxidation-reduction processes precede condensation: 



Sugar > Glycerin ^^ 



\ > Water + Suberin or cutin 



Sugar > Fatty acids 



(oxidation-reduction) (condensation) 



The fatty acids in suberin and cutin are unlike those in ordinary fats 

 and oils. Wax is foimed in a similar manner, but both the fatty acids and 

 the alcohol ( glycerin ) are different. 



These two examples show the ways in which constituents of plant 



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