402 PHYSIOLOGY 



as assimilation. To give it a name is about all that can be done at pres- 

 ent, for until very much more is known of the chemistry of proteins, of 

 which protoplasm chiefly consists, practically nothing can be known of 

 the details of assimilation. 



Metabolism. The important steps in nutrition are these : (i) the 

 making of carbohydrates in green parts properly lighted out of H 2 COjj; 

 (2) varied modification of these and incorporation of nitrogen (often also 

 sulfur and phosphorus) from mineral salts to form amides and finally 

 proteins; (3) the assimilation of proteins into protoplasm. On the whole 

 these steps are upward; the material becomes, though with many 

 fluctuations, gradually more and more complex, until it enters upon its 

 final, most complex, least stable, living condition. It is maintained for 

 a time at the high level as living stuff, or it becomes a part of some more 

 permanent portion of the body, like the cell wall; or it is broken up and 

 reduced gradually to simpler compounds, some perhaps to be rebuilt 

 into living matter again, some to break into simpler and simpler com- 

 pounds and to leave the body (e.g. as CO 2 , H 2 , etc.). 



Metabolism is an old general name for all the chemical changes in 

 a living organism. The constructive phases of nutrition are often 

 summed up in the term anabolism or constructive metabolism; the de- 

 structive phases as catabolism or destructive metabolism. In the former 

 the processes tend to be synthetic; in the latter analytic. Having con- 

 sidered the synthetic processes, the analytic ones demand attention in the 

 next chapter. 



