88 INTRODUCTION TO EVOLUTION 



We should note particularly that this reaction occurs only under very 

 special conditions. If we mix carbon dioxide and water in a test tube they 

 do not combine to form glucose. Why not? The fundamental reason is 

 that the combining of carbon dioxide and water to form a carbohydrate 

 requires appropriate application of energy. The forming of carbohydrate 

 is an "uphill reaction," and energy is required to drive anything uphill, as 

 everyone knows. Where can the needed energy be obtained and how can 

 it be applied? Green plants have the ability to harness energy from the 

 sun's radiations for this particular synthesis. Actually, then, a molecule 

 of glucose contains not merely carbon, hydrogen, and oxygen; it also con- 

 tains a certain amount of energy which was not present in the carbon 

 dioxide and water from which it was formed. In carbon dioxide and wa- 

 ter molecules the atoms are bound together by electrical forces of rela- 

 tively little energy. In glucose, however, the atoms are bound together in 

 a more complex manner. The accompanying diagram shows the arrange- 

 ment of atoms in a molecule of one form of glucose. The five carbon 



CHoOH 



HO 



OH 



atoms and one oxygen atom are joined together in a hexagonal ring. The 

 connecting lines of the ring represent bonds holding the atoms together. 

 These are called covalent bonds. They result from the pairing of an 

 electron from one atom with an electron from its neighbor in the ring. In 

 a sense the atoms "hold hands" by means of their electrons. Energy is 

 expended in building this chemical structure, and this energy must be ob- 

 tained from a source, the sun, outside the plant itself. The "locked-up" 

 energy contained in such molecules is of paramount importance for 

 animals. 



As we have seen, animals are ultimately dependent upon plants for 

 food. Animals eat plants, and the carbohydrates manufactured by the lat- 

 ter are digested and transported to the cells of the body. In this way ani- 

 mal cells are supplied with fuel. In the animal cells the chemical reac- 

 tion given above is reversed: 



CeHioOe + 6 Oo ^ 6 CO2 + 6 H2O 



