LIFE AND ENERGY 29 



solution passing through them with very little resistance. This fact 

 may be regarded as further evidence of the presence of a semi- 

 permeable membrane on the surface of the cell. But, on the other 

 hand, it has been objected that the inorganic salts, shown by 

 chemical analysis to be present in the cell, might be combined 

 in a non-dissociable form with the organic constituents, or proteins, 

 of the cell. There are certain methods, which would require more 

 space to describe than can be allowed here, which show that there 

 are free electrolytes inside the cell (p., p. 123). But, apart from this, 

 an indirect proof can be given on the basis of the osmotic pressure 

 of the cell contents, a proof which is instructive in itself. We have 

 seen that the osmotic pressure is that of a 0.3 molar solution. The 

 smallest molecular weight met with amongst proteins is over 3,000 ; 

 haemoglobin has one of 12,000. Assuming that it is 3,000, a 0.3 

 molar solution must contain 90 per cent, of the solute, an impossible 

 amount, since we know that only 20 per cent, at the most, of the 

 cell contents is solid matter. The cell membrane must be 

 impermeable to solutes of small molecular weight. 



Changes in Permeability during Life 



The consideration of the preceding paragraph leads us to a 

 brief statement of what evidence there is with regard to such 

 changes. 



It has been pointed out above that the cell membrane cannot 

 always be semi-permeable as regards food materials sugar, for 

 example when the supply comes to it from the outside, as in the 

 higher animals. We have also seen reason to regard the cell 

 membrane itself as a local concentration of constituents of the cell 

 and of the surrounding medium. Its properties naturally depend 

 on the changes in the cell especially. Hence it is not surprising 

 to find that, in states of activity of the cell, the membrane becomes 

 permeable to substances to which it was previously impermeable. 

 There are not many cases in which, as yet, direct evidence of this 

 has been obtained (P., p. 124). Electrical stimulation of certain 

 contractile cells causes them to lose the pigment which is normally 

 kept within. Again, supposing that the natural electrical resistance 

 of the cells is mainly due to the impermeability of their membranes 

 to the ions of solutions in which they are immersed, it will be clear 

 that this resistance must decrease if the membrane becomes more 

 permeable. Such effects have been detected in muscle in contrac- 

 tion, in the process of fertilisation of egg-cells, and so on (P., p. 141). 

 We shall see later how this change of permeability to ions explains 

 the electrical phenomena which are frequently to be detected when 

 cells enter into activity. 



