4 02 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.,C0 3 ; 

 (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 C0 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. 



