LIFE AND ENERGY 15 



and mechanism of an atom when it enters into chemical combina- 

 tion with another atom, and that it is in this way that energy is 

 given off when, for example, carbon combines with oxygen, and 

 that energy must be supplied when a chemical system of low 

 potential is to be raised to one of higher potential, as when carbon 

 dioxide is changed to sugar under the influence of the sun's rays. 



There is one more point in connection with these two factors 

 of energy. The quantity of heat energy in bodies of the same 

 chemical composition at the same temperature is proportional to 

 their mass. Thus, a litre of water at 100 has twice the heat 

 energy of half a litre at the same temperature. So that if we 

 mix the two together we shall have three times as much heat 

 energy as we have in the half litre, but there is no change in the 

 temperature. The capacity factors, therefore, add together, while 

 the intensity factors do not. 



The Cell Membrane 



We have seen that if we reduce the tension at a boundary 

 surface, we reduce the free energy present. Now, nearly all 

 substances when added to water have this property, and the degree 

 to which the tension is reduced is in proportion to the amount of 

 the active material present, up to a certain value. Therefore, 

 suppose that there are things present in the protoplasm of an 

 amoeba that lower the surface tension of water, the more of these 

 that concentrate themselves at the contact surface of the organism 

 with the water, the greater is the decrease of free energy. The 

 second law of energetics tells us that this will happen. The name 

 "adsorption" has been given to the process. Such an accumula- 

 tion of a substance at the interface between two phases may go 

 so far as to exceed the limit of solubility of the substance, so 

 that it is deposited out of solution, and forms a more or less 

 coherent or rigid membrane. The fact can be well seen by blow- 

 ing a bubble with a solution of the vegetable product called 

 saponin, which is not very soluble in water, but has a powerful 

 effect in lowering surface energy (E ., p. 168). Certain substances 

 which we know to be present in protoplasm have properties like 

 that under discussion. We should expect, therefore, to find them 

 taking a chief part in the production of the cell membrane. 

 These are especially those which have a fatty nature, and also the 

 proteins, whose nature we shall learn in the next chapter. Fats 

 have a particularly marked effect in lowering the surface tension 

 of water. That at the contact surface between water and air is 

 notably depressed by merely stirring the water with the finger. 



The cell membrane is then to be regarded as a part of the 



