128 THE CELL 



3. In both kingdoms a great many identical, or at any rate 

 very similar, processes occur, by means of which complex chemical 

 combinations are produced. These, however, differ essentially 

 from the synthetical methods employed by chemists for the pro- 

 duction of different organic compounds. In the chemistry of 

 the cell, whether plant or animal, ferments play an important 

 part (diastase, pepsin, trypsin, etc.). By the term ferment is 

 understood an organic substance, produced by the living cell, 

 of which an exceedingly minute quantity is sufficient to bring 

 about a considerable chemical effect, and which, without being 

 itself, to any appreciable extent, consumed, is able to produce 

 characteristic chemical changes both in carbo-hydrates and 

 albuminous bodies. 



" Le chimisme du laboratoire est execute a l'aide d'ag-ents et 

 d'appareils que le chimistre a crees, et le chimisme de l'etre 

 vivant est execute a l'aide d'agents et d'appareils que l'organisme 

 a crees " (Claude Bernard IV. la). 



In the following pages we will consider the individual phenomena 

 of metabolism, chiefly from a morphological point of view, with- 

 out entering more fully into the chemical processes, which for the 

 most part are very complicated, and as yet to a great extent 

 obscure. During the course of metabolism three stages may be 

 recognised : the absorption of new material, the consequent trans- 

 formation effected in the interior of the protoplasm, and the 

 excretion of waste products. We will first consider together 

 the first and third of these stages, and later on the second by 

 itself. 



I. Absorption and Excretion. All cells absorb gases, and 

 also substances in a fluid or dissolved, and hence diffusible, con- 

 dition ; finally many cells can make use of solid substances as food. 

 These three series of phenomena must be considered apart. 



1. The Absorption and Excretion of Gaseous Material. Proto- 

 plasm can absorb the most various kinds of substances in a 

 gaseous condition (oxygen, nitrogen, hydrogen, carbon dioxide, 

 carbon monoxide, nitrous oxide, ammonia, chloroform, ether, and 

 a large number of similar substances). 



Amongst these substances, oxygen and carbon dioxide are the 

 only ones of general importance in metabolism, and of these 

 oxygen is the more important. 



Without the absorption of oxygen, that is to say without 

 respiration, life cannot continue. With very few exceptions 



