ift 



PRINCIPLES OF GENERAL PHYSIOLOGY 



" lipoids," are essential, in addition to water and inorganic salts. Carbohydrate 

 is probably equally important. 



A certain theory, that of " bioyen molecules." has attracted many investigators 

 (Verworn, 1903). According to this view, living matter consists of large mole- 

 cules, with permanent central nucleus, and a great number of " side-chains," in the 

 chemical sense. These side-chains are supposed capable of oxidation, reduction, 

 methylation, and so forth. Under certain conditions, parts of the biogen molecules 

 may be split off, but the essential phenomena of life are associated with changes in 

 which these giant molecules take part as components of chemical reactions, taking 

 place according to the ordinary laws of mass action, equivalent combining 

 proportion, etc. In the thoughtful address of Prof. Hopkins to the British 

 Association (1912, p. 220), which will be read with much profit, we find the 

 following criticism, which seems to me to be entirely justified : 



" This view conceives of the unit 

 of living matter as a definite, if very 

 large and very labile molecule, and 

 conceives of a mass of living matter as 

 consisting of a congregation of such 

 molecules in that definite sense in 

 which a mass of, say, sugar is a con- 

 gregation of molecules, all like to one 

 another. In my opinion, such a view 

 is as inhibitory to productive thought 

 as it is lacking in basis. It matters 

 little whether in this connection we 

 speak of a ' molecule,' or, in order to 

 avoid the fairly obvious misuse of a 

 word, we use the term 'biogen,' or 

 any similar expression with the same 

 connotation. Especially, I believe, is 

 such a view unfortunate when, as 

 sometimes, it is made to carry the 

 corollary that simple molecules, such 

 as those provided by food-stuffs, only 

 suffer change after they have be- 

 come in a vague sense a part of 

 such a giant molecule or biogen. Such 

 assumptions became unnecessary as 

 soon as we learnt that a stable sub- 

 stance may exhibit instability after it 

 enters the living cell, not because it 

 loses its chemical identity, and the 

 chemical properties inherent in its 

 own molecular structure, by being 



built into an unstable complex, but because in the cell it meets with agents (the 

 intracellular enzymes) which catalyse certain reactions of which its molecule is 

 normally capable." 



If carbohydrate utilised by the cell becomes part of the protoplasmic molecule 

 before oxidation, it is difficult to suppose that the nitrogenous part of the 

 molecule would escape breakdown. That this is not so is shown by some 

 experiments of Kosmski (1902), who grew Aspergillus in water, and on sugar 

 solution. In the case of growth on pure water, the carbon dioxide given off falls 

 at once, but to a value which is not zero, but about one quarter of that when 

 grown on sugar. This production may possibly be that of the protoplasmic 

 substance itself. When transferred to solutions containing sugar, the carbon 

 dioxide rises at owe, indicating direct utilisation without previous combination 

 -k "biogen" (see Fig. 18). 



5 - 



I 2345678 



FIG. 18. CHANGE IN RESPIRATORY META- 

 BOLISM OF ASPERGILLCS NIGER, ACCORD- 

 ING TO PRESENCE OF GLUCOSE. 



Ordinates milligrams of carbon dioxide per hour. 



Abscissa; time in hours. 



At the second hour, the nutrient solution, containing 

 glucose, was changed for tap water. The combus- 

 tion processes immediately decreased to a low level, 

 but were as suddenly restored, at the seventh hour, 

 by addition of normal glucose nutrient solution. 



There is apparently no store of food material. The 

 steady MM. ill respiratory exchange in the absence of 

 food is probably due to consumption of the organised 

 structure of the cells. 



(From experiments by Kosi'nski, 1902.) 



with 



The use by bacteria of energy obtained from oxidation of inorganic sulphur, etc., 

 is further evidence of non-oxidation of protoplasm itself. In many of these cases, 



