i8 ELEMENTS OF BIOLOGY 



is required to build up a sugar molecule, then when that molecule 

 is broken down in protoplasm, energy is released. Energy is con- 

 verted from one form to another without decrease; when a quan- 

 tity disappears at any place, either in the organism or in the non- 

 living world, an exactly equal quantity appears at another place in 

 some form or forms. Energy is not created anew; it merely changes 

 form. An animal does not create energy in order to move; it 

 merely transforms energy from some potential supply. 



All chemical reactions involve changes in the distribution of en- 

 ergy. A common example is the effect produced when oxygen and 

 carbon unite in the presence of a sufficient supply of oxygen. A 

 single atom of carbon unites with two atoms of oxygen, forming 

 the gas CO2, with the release of energy. This energy makes itself 

 evident in the form of heat. The amount of energy released by this 

 reaction is the same, whether the reaction takes place slowly or 

 rapidly, or whether it takes place as the burning of fuel in a stove 

 or of sugar in the body of an animal. 



Energy and Life. Some chemical reactions require energy; 

 in other words, they cannot occur in the absence of heat or energy 

 in some form. Both sorts of reactions, those which release, and 

 those which take up energy, occur in the living organism. Energy- 

 releasing reactions liberate energy that makes the reactions which 

 require energy possible. Furthermore, living objects are constantly 

 dissipating energy as they live, as heat, or as mechanical work. 

 Some compounds in the organism must therefore contain potential 

 chemical energy. The ultimate source of this energy must be from 

 outside the organism; otherwise a living object would be a per- 

 petual motion machine. We know this is not true. So protoplasm, 

 the physical basis of life, is composed of compounds arranged as a 

 system in which energy is being constantly transformed from po- 

 tential to kinetic and from one form to another, requiring for its 

 maintenance a constant inflow of potential energy and constantly 

 exhibiting evidence that work is being done. 



