Substances Made from Foods 6i 



ing protoplasm. How non-living materials are transformed into 

 living protoplasm is one of the greatest problems in biology. 



Assimilation. Assimilation may be defined as the process by 

 which protoplasm, cell walls, and other essential constituents of 

 cells are made from foods. Protoplasm must be made in the forma- 

 tion of new cells, and it must be constantly renewed in cells 

 already formed. Of all the foods the proteins most nearly ap- 

 proach protoplasm in composition and are most used in the build- 

 ing of the living matter. However, carbohydrates and fats also 

 enter into the construction of both protoplasm and cell walls. 

 Before being assimilated, the complex foods are broken up into 

 simpler and more active compounds. Assimilation takes place 

 in all living cells, but it is most active in growing parts. Respira- 

 tion is also most active in these parts, and some of the energy 

 liberated by respiration is used in forming other compounds. 



Summary. The many substances of which plants are composed 

 are made from foods. Some of these substances, hke chlorophyll 

 and enzymes, are of vital importance ; others, like tannins, alka- 

 loids, and essential oils, may be merely by-products of the nutri- 

 tive processes. Protoplasm is an organization of many sub- 

 stances possessing various properties. Cell walls are composed 

 primarily of cellulose, which is modified by the addition of other 

 substances. Vitamins, which are formed mostly by plants, are 

 essential additions to the food of animals, and there can be little 

 doubt that the presence of enzymes in the vegetable food of 

 animals aids in digestion. The building of new tissues is known 

 as assimilation, and is considered the culmination of all the chemi- 

 cal processes occurring in cells. Of the sugar made in photo- 

 synthesis by a corn plant, about one fourth is used in respiration, 

 about one half is assimilated in the construction of the plant, and 

 the remaining one fourth is accumulated in various forms of food 

 within the plant. 



REFERENCES 



Thatcher, R. W. The Chemistry of Plant Life. McGraw-Hill Book Co. 

 Haas and Hill. Chemistry of Plant Products. Longmans, Green & Co. 



