ENERGY TRANSFERS AND BIOLOGICAL OXIDATIONS 167 



2. Loss ol hydrogen atoms by a molecule or ion as in 



CoU.OH + DPN+ + H2O '"^ > CH.CHO + DPNH + H,0+ 



alcohol 

 dehydrogenase 



J. Gain of oxygen atoms by a molecule or ion as in 



CH3CHO + IO2 -> CH3COOH 



Jn every case the oxidation is accompanied by reduction, which for 

 simplicity may be regarded as the opposite of oxidation, namely, a 

 gain in electrons, a gain in hydrogen atoms, or a loss of oxygen atoms. 

 The second example deserves some special mention since DPN + 

 gains one electron and one hydrogen atom. Usually hydrogen atoms 

 are transferred in pairs in biological reactions. In this case ethanol 

 loses two hydrogen atoms, one going to the DPN + , while the other 

 splits into its electron and proton. The electron is acquired by the 

 DPN+ while the proton is set free into the medium and combines 

 with a water molecule. The structural details are shown on page 140. 

 Not all biological reactions are oxidation-reduction reactions. But 

 those that are may be identified by the application of these three 

 criteria. 



Oxidation of Foods and Substrates 



Some of the substances assimilated from the environment may be 

 incorporated directly into the structural and functional machinery 

 of the cell. Others are modified first and then used for these same 

 purposes. Still other foods are oxidized to provide all necessary 

 energy. This last function of foodstuffs forms the subject of present 

 concern. 



In order to classify the information, the overall change in the sub- 

 strate or food is arbitrarily subdivided as follows: 



1. The food is assimilated or digested. Macromolecules are split, 

 usually by hydrolysis, into molecules small enough to pass through 

 cell membranes. These digestive processes are given special considera- 

 tion later. 



2. These smaller molecules undergo a series of reactions leading to 

 still smaller molecules. Many carbohydrates go by a common path- 

 way to pyruvate. This widespread series of reactions has come to be 

 called glycolysis. 



3. Pyruvate (ionized pyruvic acid) and several other compounds 

 then undergo additional reactions. In plants and animals these reac- 

 tions form a cyclic series known as the citric acid cycle, the Krebs 



h 



