Energy Yield of Aerobic Respiration:

Total ATP Production from Glucose Oxidation:

Let us now calculate the total ATP production from the complete oxidation of a glucose molecule to CO2 and H2O under aerobic conditions.

1. Glycolysis:

It provides 2 ATP molecules and 2 NADH + 2H+.

2. Pyruvate Oxidation:

It yields 2 NADH + 2H+ only.

3. Krebs Cycle:

It gives 2 GTP molecules, 6 NADH+ 6H+ and 2 FADH2. Generally no distinction is made between ATP and GTP because GTP is changed into as ATP in the cytoplasm by an enzyme nucleoside di-phosphate kinase.

Therefore, GTP is regarded as ATP in the following calculations:

4. ETS:

It produces 32 or 34 ATP molecules, and is the major source of energy for a cell.

Its yield is as under:

(i) The 2 NADH molecules from glycolysis give 4 ATP molecules if their electrons are introduced into route 2 of ETS by the less efficient shuttle, or 6 ATP molecules if their electrons are passed by the more efficient shuttle into route 1 of ETS.

(ii) The 2 NADH molecules from pyruvate oxidation yield 6 ATP molecules in route 1 of ETS.

(iii) The 6 NADH molecules from Krebs cycle yield 18 ATP molecules in route 1 of EFS.

(iv) The 2 FADH2 molecules from Krebs cycle yield 4 ATP molecules in route 2 of ETS.

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 predominating 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.

The net yield of a glucose molecule, on complete oxidation is given below in a simple form, and Figure 7.9 shows aerobic respiration (oxidation) in outline:

 

Advantages of Stepwise Oxidation:

Stepwise oxidation of food in the cells is beneficial in many ways:

1. It enables the cell to store as ATP molecules a relatively high proportion of energy released by oxidation because each energy-yielding step is coupled with ATP synthesis.

2. It loses less energy as heat and this keeps temperature of the cell low to prevent its burning.

3. The rate of energy output can be adjusted according to the cell’s requirement by accelerating or inhibiting the enzyme action at different steps with specific compounds.

4. Intermediates of the respiratory pathway can be used for the synthesis of other biomolecules such as amino acids.