Energy balance of cellular respiration
The glycolysis occurs in the cytosol, two molecules of ATP directly and two molecules of NADH. If oxygen is available, the electrons from NADH enter the electron transport chain of the inner mitochondrial membrane, producing three ATP molecules for every NADH. Therefore, the yield of glycolysis is (6 + 2) 8 ATP.
In some cells, carrying electrons from NADH from glycolysis, the cytosol to the mitochondrial inner membrane lowers the net yield of these two NADH only four ATP instead of 6 ATP, so the performance of glycolysis it will be 6 ATP (adding the two ATP molecules obtained directly).
Then, the passage from pyruvic acid to acetyl-CoA, in the mitochondrial matrix, produces two molecules of NADH for each molecule of glucose, whose electrons in the respiratory chain will produce 6 ATP.
In the Krebs cycle, two molecules of acetyl-CoA are introduced and two of GTP (equal to 2 ATP), six molecules of NADH and two of FADH2 are obtained.
Global reaction of cellular respiration:
Glucose + 6 O 2 + 38 ADP + 38 Pi → 6 CO 2 + 6 H 2 O + 38 ATP