252 ELEMENTS OF GENERAL SCIENCE 



Precisely the same thing happens in our own bodies when the 

 digested food is dissolved, carried through the body in the 

 blood, and assimilated by the cells. 



Of course protein foods that have never been stored may 

 be assimilated instead of being stored. Carbohydrates may be 

 used for protein-making soon after they are manufactured, or 

 they may be stored and then digested and used as described. 



274. Assimilation and respiration. It will be worth while 

 here to contrast assimilation and respiration. Assimilation is 

 a process by which more protoplasm is constantly made from 

 proteins, and it is therefore a constructive process. By respi- 

 ration the very complex living chemical compound which we 

 call protoplasm is separated into several compounds, which 

 may be as simple as the original proteins or even as simple 

 as carbon dioxide and water. These products of decomposi- 

 tion may be given off as wastes. Respiration is a destructive 

 process by which protoplasm is destroyed and the energy 

 which it yields is made available. 



The whole matter may be represented as like the slope 

 of a hill. We begin at the bottom with carbon dioxide, 

 water, nitrates, etc., and by means of photosynthesis, protein 

 manufacture, and assimilation we ascend to the highest point 

 protoplasm ; the descent on the other side is represented by 

 respiration, which reduces the protoplasm to carbon dioxide, 

 water, nitrogen compounds, and other decomposition products 



