264 INTRINSIC ACTIVITY OF SECRETING CELLS 



cell protoplasm or lipoids may explain such uptake or retention, 

 but neither view can explain more than this. Before passing on 

 to a consideration of the energy changes involved in secretion, and 

 the possible explanation of such changes, it may be well to point 

 out that the view of the formation of unstable chemical com- 

 binations between cell protoplasm and selectively absorbed and 

 retained constituents, fits the observed facts much 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 

 increase in absorption. This sequence of events is precisely what 

 would occur if formation of an unstable or reversible chemical com- 

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

 example, typically in the combination between hemoglobin 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. 



