FOOD MANUFACTURE 37 



out carbon dioxide), the opinion became current that plants 

 and animals differ in their " breathing." As a corollary to 

 this opinion, it was pointed out that animals and plants 

 supplement each other in this process, each taking in what 

 the other gives off, and each living on what the other rejects. 

 Since this impression is still current, its correction must be 

 emphasized. It is clear that photosynthesis has nothing to 

 do with respiration, for respiration is associated with what 

 may be called the act of living, and therefore is carried on by 

 every living thing. If respiration stops, the plant or animal 

 body is dead; in fact, we use respiration as an evidence of 

 life. Therefore plants and animals " breathe " alike, both 

 taking in oxygen and giving out carbon dioxide ; but green 

 plants can carry on the process of photosynthesis also, in 

 connection with which it takes in carbon dioxide and gives 

 out oxygen. The confusion arose from the fact that during 

 the day, when photosynthesis is going on, the amount of the 

 gas exchange involved in the manufacture of carbohydrates is 

 so much greater than the amount involved in respiration 

 that the latter was not noticed ; but if the observation had 

 extended into the night, it would have been discovered that 

 only the gas exchange of respiration was being carried on. 



It may be useful to contrast photosynthesis and respira- 

 tion sharply as follows : photosynthesis occurs only in green 

 cells, requires light, uses carbon dioxide, liberates oxygen, 

 makes organic material, and accumulates energy; while 

 respiration occurs in every living cell, does not require light, 

 uses oxygen, liberates carbon dioxide, uses organic material, 

 and uses energy. 



29. Manufacture of proteins. Carbohydrates are by 

 no means the only foods, and 'therefore photosynthesis is 

 not the only process of food manufacture. Another conspicu- 

 ous group of foods is the proteins, which may be regarded 

 as foods in the most advanced stage, since the living proto- 

 plasm is largely composed of proteins. Carbohydrates, 

 4 



