What is the main source of ATP synthesis in cells?

The main source of ATP synthesis in cells is glucose. Most ATP synthesis happens during cellular respiration within the mitochondrial matrix. This generates approximately 32 ATP molecules per molecule of glucose that is oxidized. The glucose is catabolized into three successive processes to produce ATP: glycolysis, TCA or Krebs cycle, and oxidative phosphorylation. During glycolysis, glucose becomes converted into pyruvate and the amount of ATP generated is low. Then, the pyruvate is converted to acetyl coenzyme A (Acetyl-CoA) which enters the Krebs cycle to produce NADH molecules. Then, the NADH is combined with oxygen to generate a proton gradient across the inner mitochondrial membrane. The energy released from these oxidation/reduction reactions is utilized for production of large quantities of ATP by mitochondrial ATP synthase. The proton gradient across the inner mitochondrial membrane is necessary in order for ATP molecules to be produced.