This process occurs in the mitochondria.
There are mainly three processes involved in the breakdown of food.
The Krebs Cycle and Glycolysis together produce only 4 ATP molecules and 6 very high energy molecules which breakdown in the
Oxidative Phosphorylation , NADH or Nicotine Amide Adenine Dineucleotide Hydride , and 2 FADH2 or Flavin Adenine Dinucleotide Hydride molecules.
Given below is a picture of the Oxidative phosphorylation reactions.