[Chap. XI A BIT OF USEFUL CHEMISTRY 99 



oxidation and reduction are seen to be supplementary and simultaneous; 

 this fact is indicated by the term oxicJation-reduction. 



This concept of oxidation and reduction is held to be true for many 

 cases, such as the oxidation of iron and other metals. In the oxidation of 

 methane, sugar, and similar carbon compounds, however, the electrons 

 do not actually pass from one atom to another but are mutually shared 

 by both atoms. Oxidation and reduction are involved in all physiological 

 processes in which energy is chemically bound or liberated. 



Hydrolysis and condensation. Some seeds contain starch but no sugar. 

 When such seeds germinate, sugar appears in them and increases in 

 abundance while the starch gradually disappears. When an animal 

 eats a starchy food, the starch is converted to sugar in its alimentary 

 tract. We are accustomed to say that the starch has been digested to 

 sugar. The chemist refers to this process as hydrolysis because one mole- 

 cule of water is added to the starch molecule for every molecule of sugar 

 separated from it. 



The reverse process also takes place in plant cells, sugar being changed 

 to starch. When several molecules of sugar are combined forming one 

 large molecule of starch, a molecule of water is separated from each 

 molecule of sugar added. Since several molecules are joined into one 

 larger molecule with a loss of H2O this process may be called condensa- 

 tion. Many similar changes in the fat and protein compounds in living 

 cells occur by hydrolysis and condensation. These changes do not in- 

 volve a transfer of electric charges and little or no energy is gained 

 or lost. 



Attention is called to these two contrasting pairs of fundamental chem- 

 ical processes — oxidation versus reduction, and hydrolysis versus con- 

 densation — so that the discussion of food substances, their uses and 

 transformations may be understood more readily. 



In addition to these four chemical processes, there is another in which 

 ions that have the same charge mav replace each other in certain chemi- 

 cal compounds. For example, in a solution Na+ and K+ may replace 

 each other. Likewise OH~ and NH7 may replace each other in certain 

 organic compounds in living cells. This is called chemical substitution. 



The number of atoms in a molecule may be small, as in water ( H-O ) , 

 or it may be very large, as in proteins where each molecule is composed 

 of more than a thousand atoms. Casein, one of the common proteins of 

 milk, has the formula C708Hn3o0224Ni8oS4P4. In nature, outside of living 

 cells, such complicated molecules are rarely formed. One of the reasons 



