RELATION OF AIR TO FOOD MANUFACTURE 85 



94. Comparison with a factory. It may be interesting to 

 sum up the process of photosynthesis by comparing it with 

 a factory. The comparison may be made as follows : 



The factory Green part of the plant 



Machinery Chloroplasts 



Energy Light 



Raw material Water and carbon dioxide 



Finished product .... Carbohydrates 



Waste product Oxygen 



95. Importance of plant synthesis. The food supply of all 

 living things depends upon the work of green plants. Car- 

 bohydrates, fats, and proteins make up a very large part of 

 our own food and of the food of many animals. Animals 

 are unable to synthesize these foods from simple compounds. 

 This is just as true when we eat meat as when we eat vege- 

 tables. Cattle eat grass and grain and make them over into 

 their own flesh, but they are just as unable as we are to 

 use the very simple substances carbon dioxide, water, and 

 nitrate which are used by the plants. The green plants 

 and the work which they do are therefore of the very 

 greatest importance. 



96. Manufacture of fat. Although the carbohydrates form 

 a large part of all plants, they are by no means the only 

 substances manufactured by plant cells. We are all familiar 

 with such fats as olive oil, cottonseed oil, and corn oil. Most 

 plants produce such substances and store them in their seeds. 

 Like the carbohydrates the fats contain only carbon, hydro- 

 gen, and oxygen and are undoubtedly produced from the 

 carbon dioxide of the air and water. We do not know as 

 yet all the details of how the plant does this. 



97. Manufacture of protein. A third group of substances 

 of great importance which are synthesized by plants are the 

 proteins. In addition to carbon, hydrogen, and oxygen, all 

 proteins contain nitrogen and also a small amount of sulphur. 

 Most of the carbon, hydrogen, and oxygen are obtained 



