Oxidation of food happens in three easy steps (fig:1.1) :

    1. 1. Primary Metabolism or Digestion
    2. 2. Secondary/ intermediate Metabolism
    3. 3. Tertiary Metabolism



Fig: 1.1


  
Now let's talk about the stages of oxidation of foodstuffs:-

1. First Stage:

Primary Metabolism (fig:1.1):- The process of digestion in the gastrointestinal tract converts all the macromolecules into small units e.g. proteins are digested into amino acids.

2. Second Stage: 

Also called the intermediate stage or secondary metabolism; in this stage, the products of digestion are absorbed then catalyzed to smaller components, and finally oxidized to carbon dioxide. Now the reducing equivalents are mainly generated in the mitochondria by the oxidative pathway and this pathway is nothing else but the citric acid cycle. In this process, NADH and FADH2 both are generated and hence the importance of the second stage and indirectly the importance of the TCA cycle in our system.

3. Third Stage:

The reduced equivalents now enter into the electron transport chain(ETC) or respiratory chain; in this phase, energy is released. This is the tertiary metabolism or internal respiration also called the cellular respiration stage.  The energy production by complete oxidation of one molecule of glucose is almost equivalent to 2850 kJ/mole and in the case of palmitate this energy goes up to 9781kJ/mole.  This energy is then used for final synthetic purposes in the body (1:2)



                                                      Fig.1:2 ATP generation