Which stage of cellular respiration produces the most ATP?

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The total ATP production from the complete oxidation of a glucose molecule to CO2 and H2O under aerobic conditions is shown as below:

  1. Glycolysis provides 2 ATP molecules and 2NADH + 2H+.
  2. Pyruvate oxidation yields 2 NADH + 2H+ only.
  3. Krebs’s cycle gives 2 GTP molecules, 6 NADH + 6H+ and 2FADH2. Generally no distinction is made between ATP and GTP because GTP is changed into ATP in the cytoplasm by an enzyme nucleoside diphosphate  kinase. Therefore, GTP is regarded ATP in the concerned calculations.
  4. ETS produces 32 or 34 ATP molecules, and is the major source of energy for a cell. Its yield is as under
    1. The 2 NADH molecules from glycolysis give 4 ATP molecules if their electrons are introduced into route 2 of ETC by the less efficient shuttle or 6 ATP molecules if their electrons are passed by the more efficient shuttle into route 1 to ETC.
    2. The 2NADH molecules from pyruvate oxidation yield 6 ATP molecules in route 1 of ETC.
    3. The 6 NADH molecules from Krebs’ cycle yield 18 ATP molecules in route 1 of ETC.
    4. The 2 FADH2 molecules from Krebs’ cycle yield 4 ATP molecules in route 2 of ETC.

The 32 or 34 ATP from electron transfers, when added to 4 ATP from glycolysis and Krebs’ cycle, give a grand total of 36 or 38 ATP for each glucose molecule fully oxidized to CO2 and H2O.

The shuttle system seems to vary with the species. Thus, a glucose molecule on complete oxidation produces 36 ATP in most eukaryotic cells, but forms 38 ATP in some species. Glucose + 36/38 Pi + 6O2 → 6CO2 + 42 / 44H2O + 36 / 38 ATP + Heat

The 42/44 H2O molecules shown in the products above include 6H2O molecules produced in the last step of ETS by combination of hydrogen and oxygen, and 36/38 H2O molecules released as by products of the synthesis of ATP. 36 / 38 ADP + 36 / 38 Pi → 36 / 38 H2O

In the prokaryotic cells, oxidation of glucose molecule always provides 38 ATP molecules because NADH is not to enter the mitochondria, which are absent.

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