Biochemical and Biophysical Phenomena 755 



or by the slow process of passage from cell to cell. The phenomenon of 

 conduction is very essential to ensure efficient distribution of materials 

 to those regions where they are required or from those regions where 

 they are not desired. Without molecular movements in the diffusion of 

 substances from one part of a cell to another, the protoplasm would 

 soon become lifeless. 



Surface Tension 



Surface tension may be defined as the greater tension or attraction 

 between molecules on the surface of a liquid than between those beneath. 

 All molecules of a substance exert an enormous attraction for each other. 

 This property is called cohesion. In the deeper portions of a volume of 

 liquid each molecule is attracted by adjacent molecules with equal force 

 in all directions. However, on the surface of the liquid, the liquid 

 molecules are attracted downward by the lower molecules of the liquid 

 and attracted upward by the molecules of the gases of the air. The 

 attraction of the liquid molecules for each other is greater than the at- 

 traction of the gas molecules for the liquid molecules. Hence, the 

 attraction forces on the surface molecules of the liquid are unequal. 

 Equilibrium is attained only when the surface is made as small as pos-. 

 sible by reducing the number of liquid molecules on the surface. This 

 produces a tendency for the surface to occupy the least amount of space. 

 When a droplet of oil is immersed in water, the former will assume a 

 spherical shape, and the boundary, known as the interface, between the 

 oil and water is in a state of tension and therefore represents an equi- 

 librium between forces. This tendency for surfaces to contract because 

 of tension is known as surface tension. Naturally, surface tension differs 

 widely among various materials. 



Any substance which reduces surface tension has a tendency to ac- 

 cumulate at the surface. When ether is added to water, the ether 

 molecules accumulate in greater numbers at the surface of the water 

 than elsewhere in the water. The amount of potential energy at the 

 surface of an ether-water mixture is much less than at the surface of 

 pure water. If the area of the surface of a substance is reduced, there is a 

 release of energy. Surface tension in living protoplasm is constantly being 

 reduced by the presence of fats. In protoplasm the energy relation of the 

 interfaces (boundaries) between the colloidal particles and their suspend- 

 ing medium is constantly changing. In the living process new compounds 

 are constantly formed, and different sorts of molecules appear and dis- 



