GENERAL PRINCIPLES OF BIOCHEMICAL ENERGETICS 135 



D. Sources of Free Energy 



The organisms in Nature obtain their vital energy from two main free 

 energy sources. One is situated in the biosphere itself, it is the covalent 

 energy of the organism's food and depends upon the properties of the 

 external electronic orbits of the constituent atoms. 



The other is outside the biosphere : it is nuclear energy coming into it 

 in the form of light. 



Ordinary hydrogen has a nucleus made up of a single proton around 

 which a single electron revolves. 



The atom of ordinary helium has two protons and two neutrons in the 

 nucleus and two external electrons. If we take the exact masses of a proton 

 and a neutron, add them together and multiply by two, we obtain a figure 

 of 4-03304 units. But, if we measure the mass of the helium nucleus, we 

 shall obtain 4'00279. Between the calculated and the measured value there 

 is a difference of 0-03025 units, equivalent to an energy value of 28-2 MeV 

 (millions of electron volts). This is the energy of binding the mass lost in 

 order to keep together such particles as the protons which, because of their 

 extremely small size, develop considerable forces of repulsion. 



Since this binding energy is in the hydrogen atom and is equal to 

 28-2 MeV in the helium atom, the transformation of a hydrogen atom into a 

 helium atom will liberate 28'2 MeV. This is what happens in the centre of 

 the sun which, because of its considerable diameter (1,392,000 km or 109 

 times the diameter of the earth), has a resulting gravitational field strong 

 enough to retain its hydrogen which forms 99% of its weight. The centre 

 of the sun has a temperature of 20,000,000° C and is under a pressure of 

 several tens of thousands of atmospheres, so that the kinetic energy of the 

 hydrogen atoms is sufficient for collisions between them to bring about 

 nuclear reactions. The sun converts hydrogen atoms into helium atoms by 

 means of a cyclic process in which carbon acts as a catalyst. It is this 

 nuclear energy which reaches the biosphere in the form of heat and various 

 forms of radiation, in particular those which are utilized by organisms 

 containing chlorophyll. 



However, a great many organisms use sources of non-nuclear energy. 

 This energy is derived from the change in energy level of electrons during 

 the atomic rearrangements which accompany the changes in structure of 

 nutrient molecules, and those which accompany oxido-reduction reactions. 

 These changes in potential are expressed in electron-volts (1 electron-volt 

 = the kinetic energy acquired by a particle carrying the charge on an 

 electron when accelerated by a potential of one volt). The commonly used 

 unit is a million times greater (MeV). The energy corresponding to an 

 electron-voh is equal to 1-60207 ± 0.00007 X lO-^^ gj-gs. The energy 

 in calories per mole corresponding to one electron-volt per molecule 

 = 23-05285 ±3-2 cal/mole). 



