22 INTRODUCTION TO GENERAL PHYSIOLOGY 



in the ordinary animal cells. In certain plant cells it is higher 

 still, and may amount to more than eleven atmospheres. It may 

 be asked, why do such cells escape being burst? In the plant 

 cell there is a rigid case around the protoplasm, so that the 

 osmotic pressure makes the cell very stiff (turgor\ thus preserving 

 the form and uprightness of even the fragile stalks of plants. If 

 the osmotic properties of the cell are destroyed (E., p. 171), the 

 rigidity of the structure disappears and the stalk collapses. In 

 the case of the animal cell, which is devoid of such a protection, 

 the liquid in which it lies has the same osmotic pressure as itself, 

 is " isotonic" so that the pressure on both sides of the membrane 

 is the same. When the outer fluid has a lower osmotic pressure 

 (hypotonic), the cells swell or burst. If the plant cell is surrounded 

 by a liquid of the same osmotic pressure as itself, its internal 

 pressure is compensated and the turgor disappears. 



We may next take a further step. We have seen that the 

 osmotic pressure is proportional to the number of molecules in 

 a given volume and, as the student is well aware, so is the pressure 

 of a gas. What we have called a molar solution contains one 

 gram molecule in a litre. A gas at atmospheric pressure contains 

 one gram molecule in 22.4 litres. Therefore, if we want to have 

 one gram molecule in one litre of a gas, we must compress it, so 

 that 22.4 litres become one litre. This requires, by Boyle's law, 

 a pressure of 22.4 atmospheres. If we want only 0.3 gm. molecule 

 in a litre we require a pressure of only 22.4x0.3 = 6.7 atmospheres, 

 identical with the osmotic pressure of a solution of the same 

 molar strength. As mentioned above, however, when accurate 

 measurements of osmotic pressures of solutions are made, it is 

 found that, as would be expected with liquids, we have to make 

 allowance for the space occupied by the molecules in a more 

 important degree than in gases, although it has to be done in this 

 case also, as the reader is probably aware, from his study of the 

 Van der Waals' "equation of state" a necessary modification of 

 the simple Boyle's law of simple, direct relation between pressure 

 and volume. We see, nevertheless, that the pressure of a gas 

 and the osmotic pressure of a solution are fundamentally the 

 same, and depend on the molecular concentration. 



The dissolving of a substance in a solvent, as is well known, 

 raises the boiling point and lowers the freezing point of this 

 solvent. The efifefct is again found to be proportional to the 

 molecular concentration, and can therefore be used to measure 

 the latter. The boiling point is only of limited application in 

 physiology, since changes occur in the solutions with which we 

 have to deal when the temperature is raised much above that of 

 warm-blooded animals. The depression of the freezing point of a 



