ANAEROBIC RESPIRATION AND FERMENTATION 313 



The glucose unites with the phosphate forming a hexose-phosphate 

 which in turn is split up, with the addition of water, into glucose 

 and the phosphate. If the phosphates and the zymase are sepa- 

 rated by filtration, fermentation is stopped ; but when the two are 

 added, the decomposition of the sugar proceeds as before. The 

 phosphate is hence a true coenzyme. 



Role of Fermentation. — In an elementary text there is not 

 space to go further into the complex matter of fermentation, 

 especially since it concerns the lower plants rather than the seed 

 plants. The precise survival role of fermentation is difficult to 

 understand. It seems to be an inefficient substitute for respira- 

 tion and consequently, under anaerobic conditions, has distinct 

 survival value; in the struggle for existence, plants which can 

 live under anaerobic conditions are placed at an advantage. It 

 is hard, however, to see what can be gained by obligate anaerobes 

 which must live in the absence of oxygen; and, in those cases in 

 which the organism is poisoned by its own products, fermentation 

 would seem to be a decided disadvantage. 



Since, as we shall see in our study of the chemistry of the process, 

 all respiration is to an extent anaerobic, it is possible that anaerobic 

 plants were the first organisms to develop on the surface of the 

 earth, their development occurring at a time when there was. 

 little or no free oxygen in the earth's atmosphere. Anaerobic 

 respiration should perhaps, therefore, be considered not in any 

 sense as an adaptation but rather as a stage of evolutionary de- 

 velopment. 



Chemistry of Respiration.— Before attempting to discuss this 

 very complex problem, it is necessary to state that we have at the 

 present time no single theory that will explain all the known facts 

 of respiration. Some workers have attempted to form a theory 

 from the phenomena as seen in only plants or animals, without 

 taking into account all phases of life; and the tendency has been 

 (as often happens) to form hypotheses in order to explain a par- 

 ticular set of laboratory findings. In the end, many of the difficul- 

 ties will probably be found to be due to the fact that many of the 

 hypotheses have been formed upon phenomena seen in test tubes, 

 while the organism is a much more complicated piece of apparatus. 

 Thus the study of respiration at present is a jig-saw puzzle. The 

 pieces are here, but they have not yet been put together; and the 

 truth is, we are not absolutely sure that we have all the pieces. 



