ENERGY CHANGES INVOLVED IN SECRETION 159 



better than the alternative view of solution in cell protoplasm 

 or in cell lipoids. For if the explanation were solution, then the 

 osmotic pressure and amount of substance taken up must be in 

 simple ratio to each other. On doubling the osmotic pressure 

 of any constituent in the lymph, the amount taken up or secreted 

 by the cell ought to be doubled, since for simple solution the 

 coefficient of distribution between cell and lymph must be constant, 

 or, in other words, the relationship between osmotic pressure and 

 amount absorbed by the cell should be a linear one. 



This is not found, however, experimentally to be the case ; the 

 absorption at first rises very rapidly with increasing osmotic 

 pressure, then later the rise in amount absorbed for equal incre- 

 ments of osmotic pressure is much decreased, an almost maximum 

 value is later reached, after which there is hardly any appreciable 

 absorption. This sequence of events is precisely what would 

 occur if formation of an unstable or reversible chemical combina- 

 tion took place at a certain range of pressure, and is seen, for 

 example, typically in the combination between haemoglobin and 

 oxygen. Hence it is most probable that such a type of combina- 

 tion exists in the case of those ions and other cell constituents which 

 are selectively absorbed and retained. 



THE ENERGY CHANGES INVOLVED IN SECRETION 



The work done by the secreting cell in the process of secretion 

 may be considered as divided into two fractions, viz. (1) the work 

 done in increasing the volume energy, or work done against osmotic 

 pressure in increasing the concentration of dissolved substances 

 already present in the lymph, and (2) the work done in increasing 

 chemical energy by the formation in the cell of new substances 

 not present in the lymph from other substances and by means 

 of the chemical energy supplied by other substances present in 

 the lymph. 



It is only for the first of these types of energy production by 

 the cell that accurate quantitative estimations can be made ; 

 because for the second type the chemical energy and amounts 

 of the organic substances formed in the cell, and the chemical 

 energy and amounts of substances used by the cell in their forma- 

 tion, are at present unknown to us. 



