20 PRINCIPLES OF GENERAL PHYSIOLOGY 



more than likely that chemical facts will sooner or later find their description 

 in terms of molecular physics. The enormous molecules and aggregates of 

 molecules which play so large a part in vital phenomena differ from simple 

 small molecules in that they already begin to show the properties of matter 

 in mass, especially those connected with the development of surface. This fact 

 will l)e found to account for many otherwise puzzling phenomena, and cannot 

 be ignored with impunity. Instances will be found in later pages of this book. 



A few words are advisable with respect to the separation by chemical methods 

 of various cell constituents. The view is held by Kanitz (1910, p. 234) that it 

 is impossible to obtain any such substance in the form in which it existed in 

 the living cell. He calls attention to the fact that, in the living cell, reactions 

 must be supposed to be in continual progress, never actually arriving at equilibrium. 

 A system in equilibrium is, in fact, dead, as will be seen better in the next chapter. 

 But when a cell is acted upon by the reagents necessary to extract its con- 

 stituents, the various reactions are supposed by Kanitz to be brought at once into 

 equilibrium. The researches of Fletcher and Hopkins on lactic acid formation in 

 muscle, already quoted, show that this is not necessarily the case. When muscle 

 is heated to 40 C. so that it passes into heat rigor, the maximum amount of 

 lactic acid to be obtained is formed (p. 266), about 0'52 per cent, as zinc lactate. 

 When, on the contrary, the resting muscle is crushed under ice-cold alcohol, only 

 0'02 per cent, is obtained (p. 260). This is sufficient to show that the reaction 

 producing lactic acid is stopped, practically at once, by destruction of the muscle 

 structure at a low temperature. The manipulation requisite to extract it does 

 not cause the reaction to proceed to completion. We may also call to mind that 

 a reaction may be stopped at once by the addition of a chemical agent ; as, for 

 example, the hydrolysis of cane-sugar by the enzyme invertase, when a mercury 

 salt is added, by which the enzyme is destroyed (see Chapter X.). 



The following considerations, due to Hopkins (1912, p. 218), will show that 

 the amount of a particular substance extracted from a cell is no index to its 

 importance in the series of reactions going on in the living cell. The metabolism 

 of the cell undoubtedly takes place in such a series of reactions that the products 

 of one form the starting point of the next following. The various component 

 reactions of this chain will almost certainly not progress at the same rate. 

 Suppose, then, that the first component is kept constant in concentration by 

 continuous supply, as will usually be the case. Then the amount of the products 

 of each reaction present at any given moment will be in inverse ratio to the rate 

 at which they change into the next member of the chain. It is clear that, in such 

 a state of " dynamic equilibrium" the actual amount of chemical change taking 

 place in each reaction must be the same; so that, if the rate at which any 

 particular step is decomposed into the succeeding one is less than that at which 

 it is produced from the preceding one, there will be a heaping up until the larger 

 quantity reacting will compensate for the lesser rate of change. In symbolic 

 form : 



K,[A] = K 2 [B] = K 3 [C] = K 4 [D] = K 6 [E] = etc. 



where Kj, K 2 , K, K 4 , etc., are the respective velocity constants of the reactions, 

 and [A], [B], [C], [D], etc., are the corresponding concentrations, iir accordance 

 with the law of mass action. It is plain that, if Kj is small and K., large, [A] 

 must be large and [B] small, and so on. One important result of this fact is 

 that, when a cell is killed, the amount of any particular body present may be 

 very small, although all the members of the chain of reactions may have passed 

 through this stage. 



The movements of naked protoplasm have already been referred to incidentally. 

 For more detailed description, memoirs such as those of Jensen (1902) Kiihne 

 (1884), or Ewart (1903), may be consulted. 



