36 GENERAL BIOCHEMISTRY 



that the cell wall is not sufficiently strong, the ceil bursts and the 

 semipermeable membranes are broken, allowing unimpeded diffusion 

 of the solutes. On the other hand, a cell placed in a very concentrated 

 solution loses solvent by osmosis and may be partially collapsed as 

 the volume decreases. Usually this latter change does not rupture the 

 semipermeable membrane, so a change in the external concentration 

 will restore the normal shape. 



Membranes differ in their permeability. Some retain small solutes 

 and others allow small ones to pass through, retaining only very 

 large molecules. Cells themselves differ in this regard with their age, 

 temperature, nutritional status, and the presence of a variety of 

 materials in the environment. 



Membranes may actually be chosen to permit a separation of solutes. 

 When one or more solutes are retained by a membrane and others 

 diffuse through, the process is called dialysis. Repeated replacement 

 of solvent permits complete diffusion and removal of all the diffusible 

 solutes. Biological systems depend upon the dialysis of waste products 

 out of, and the dialysis of food materials into, cells. Considerable 

 laboratory use is also made of the phenomenon in isolating and purify- 

 ing all kinds of solutes. 



Dissociation of Water 



Pure water exhibits a slight electrical conductivity, indicating that 

 small numbers of ions are present. These ions must, therefore, arise 

 by dissociation of a few of the water molecules. This process may be 

 represented by 



H2O -> H+ + OH- 



This proton represented by H+ does not appear to enjoy an inde- 

 pendent existence in the presence of water molecules. Moreover, the 

 process is reversible in that the ions recombine to form a molecule. 

 So the overall scheme is represented more accurately as consisting 

 of two steps: 



H2O ±^ H+ + OH- 

 H2O + H+ ±:^ H3O+ 



which are combined to 



2H2O ±^ H3O+ + OH- 



This process is described quantitatively by means of an equilibrium 

 constant which expresses the effect of the concentrations of both 

 reactants and products. For water the constant is 



