OSMOSIS 189 



to possess a cell content which has about one fifth the sodium concen- 

 tration and fifty times the potassium concentration of sea water in which 

 it normally exists. 



In a certain sense, the human skin may be considered as functioning 

 like a semi-permeable membrane. The work of Whitehouse, Hancock, 

 and Haldane [1932] has shown that a large proportion of the moisture 

 which is given off from the skin during rest under ordinary conditions of 

 temperature passes through the skin by osmosis or diffusion. The 

 osmotic loss was demonstrated through different effects of baths in pure 

 water and strong salt solutions. They foimd that osmotic loss increased 

 very rapidly as the temperature of the skin rose, but that, with a suffi- 

 cient rise, a point was reached where the presence of liquid sweat over the 

 whole skin interrupted the process completely. 



Since the introduction of the cell theory in 1838 the general conclusion 

 that living matter must be enclosed in a protective membrane in order 

 to survive in its environment has become definitely established. A 

 fundamental property of a living cell is the semi-permeability of its 

 limiting membrane. Upon the death of the cell the semi-permeability 

 of its membrane is usually entirely lost. The limiting membrane of 

 protoplasm is not a dead partition, but an intricate dynamic structure 

 maintaining the control between its internal and external environment. 

 The external environment is usually a dilute solution containing a 

 changing amount of nutrient material. 



Osmosis 



Osmosis may be defined as the passage of a fluid or fluids through a 

 membrane which separates two liquid phases consisting of a common 

 solvent and one or more solutes with different concentrations in the 

 two phases. 



The unfertilized eggs of the sea urchin are typical natural osmometers. 

 They are normally spherical in shape, which readily permits their change 

 in volume to be determined by measuring their diameters. The eggs 

 can be swollen by introducing them into diluted sea water. Shrinkage 

 will take place when they are transferred from diluted to concentrated 

 sea water. McCutcheon and Lucke [1927] discovered that the rate at 

 which water passed through the membrane in exosmosis as well as 

 endosmosis in the unfertilized sea-urchin egg followed the same law as 

 that expressing the velocity of a monomolecular chemical reaction, 

 namely, that 



dx it ^ 



- = k(o - x) 



