290 PROTOPLASM 



In all probability, the films formed are usually two or more 

 layers thick and so arranged that the oily ends are in contact 

 with each other, and the acid ends (the glyceryl-cholyl-phosphoric 

 acid in the case of lecithin) are in contact with the water or with 

 each other if the film is polymolecular. The films of soap bubbles 

 are probably thus arranged. The iridescence of soap bubbles 

 is due to the interference of light by the lamellae of the poly- 

 molecular film, and the kaleidoscopic change in color is due to 

 the sliding of the soap leaflets one upon the other where they are 

 in contact along their oily surfaces. It is the orientation of such 

 linear and polar molecules that best accounts for the part that 

 fatty substances play in determining the permeability qualities 

 of the plasma membrane. 



The Donnan Equilibrium. — We return again to the Donnan 

 equilibrium (page 203) in explanation of a biological phenomenon, 

 viz., selective permeability in terms of the unequal distribution 

 of ions on the two sides of a membrane. Donnan has himself 

 shown how membrane equilibrium can be applied to permeability 

 problems. Molecules and ions within and without a red blood 

 cell may be distributed as follows : 



If these two solutions are very dilute, it is possible mathematically 

 to determine their distribution (concentration) on the two sides 

 of the membrane (within and without the blood cell). This is 

 true only if there are no other interfering factors, as is so often 

 the case in biological systems when an attempt is made to inter- 

 pret them in terms of relatively simple physicochemical systems. 

 Narcosis. — The first definite hypothesis of narcosis was that 

 of Overton (page 280), who stated that the narcotic effectiveness 

 of a substance is proportional to its lipoid solubility. If the cell 

 surface is coated with lipoids, substances that dissolve them will 

 enter very rapidly. The theory is essentially one of disturbance 

 in cell permeability. It is true that narcotics dissolve lipoids 

 and that they enter the cell very rapidly; a lipoid-permea- 



