In the course of Glycolysis, the Krebs Cycle, and the Electron Transport Chain, 40 molecules of ATP are actually formed. Why is the net yield for the cells only 38 molecules? What other factors can reduce the yield of ATP?

In cellular respiration, the glucose molecule is thoroughly broken down and a great amount of energy is used to from ATP molecules. During this process, a series of metabolic pathways occur in an organism’s cell, converting biochemical energy from nutrients to ATP.

The first metabolic stage in cellular respiration is Glycolysis. Glycolysis is a metabolic pathway that is found in the cytoplasm of cells in all living organisms and is anaerobic, or doesn’t require oxygen. In the Glycolysis Preparatory Phase (energy-investment phase), 2 molecules of ATP are used to phosphorylate glucose and fructose 6-phosphate. However, these 2 ATP molecules are later paid off, with dividend, in the Glycolysis Pay-Off Phase. In this phase, 4 molecules of ATP are produced via substrate-level phosphorylation (yielding a net total of 2 molecules of ATP). In addition to these 4 molecules of ATP, the Glycolysis stage also produces 2 H2O molecules, 2 NADH molecules and 2 H+ ions. Also, the 2 three-carbon sugars (formed by the splitting of the glucose molecule) are oxidized and converted into two molecules of pyruvate.

Next, if molecular oxygen is present, the 2 pyruvate molecules produced in the Glycolysis stage enter the mitochondrion, where the enzymes of the citric acid cycle complete the oxidation of the organic fuel. Before entering the Krebs Cycle, however, the pyruvate’s carboxyl group is lost, and the remaining two-carbon fragments become oxidized, releasing 2 molecules of NADH. After becoming oxidized, each of the two-carbon fragments (acetates) becomes attached to the coenzyme A, becoming acetyl CoA.

Then, the 2 CoA molecules enter the Krebs Cycle. Via eight individual steps, each catalyzed by a specific enzyme, the Krebs Cycle oxidizes these molecules to produce 2 molecules of ATP, 6 molecules of NADH, and 2 molecules of FADH2. Also, two waste products – H2O and CO2 are produced during this stage.

Lastly comes the Electron Transport Chain. In this stage, the NADH and FADH2 molecules produced in the previous two stages (including the brief junction step between the Glycolysis and Krebs Cycle) are made to produce ATP through the proton-motive force. In this stage, every molecule of NADH produces about 3 ATP molecules and every FADH2 molecule produces 2 ATP molecules. (The ratio of the number of NADH/FADH2 molecules and ATP is not a whole number. However, they are rounded off to 3 and 2, respectively). As a result, by totaling the number of NADH and FADH2 molecules from the previous two stages, we get a total of 10 molecules of NADH and 2 molecules of FADH2. These molecules, altogether, produce 34 molecules of ATP (via the oxidative phophorylation). Adding these 34 molecules of ATP to the 6 molecules of ATP, produced via the substrate-level phosphorylation in the first two stages, yields a total of 40 molecules of ATP. However, since 2 ATP molecules are used up in the Phosphorylation of glucose and fructose 6-phosphate in the Glycolysis Preparatory Phase, the NET YIELD of the ATP is only 38 molecules (2+2+34).

There are 3 main reasons that can influence/reduce the yield of ATP:

• First, the phosphorylation and the redox reactions are not directly coupled to each other, so the ratio of number of NADH/FADH2 molecules to the number of ATP molecules is not a whole number. One NADH molecule can produce enough proton-motive force for synthesis of 2.5 to 3.3 ATP and one molecule of FADH2 can generate enough force to synthesize anywhere from 1.5 to 2.0 ATP.
• Second, the ATP yields varies slightly depending on the type of shuttle used to transport electrons from the cytosol into the mitochondrion. If the electrons are passed to NAD+, then 3 molecules of ATP can be produced per each NADH. However, if the electrons are passed to FAD, then the ATP yield can decrease to 2 molecules of ATP per each FADH.
• Lastly, another reason that can reduce the ATP yield is the use of proton-motive force generated by the redox reactions of respiration to drive other kinds of work. If all the proton-motive force generated by the electron transport chain were used to drive ATP synthesis, one glucose can produce 36 (34 if FAD shuttle is used) molecules of ATP via the oxidative phophorylation. However, if lesser proton-motive force is used, then the ATP yield via oxidative phophorylation would be lesser.