In glycolysis there is a direct yield of two ATP molecules by degraded glucose molecules. They also form two NADH molecules2 which, in the respiratory chain, provide energy for the synthesis of of six ATP molecules.
During the Krebs cycle, the two Acetyl-CoA molecules lead to direct production of two ATP molecules. They also form also six NADH molecules2 and two FADH molecules2 which, in the respiratory chain, provide energy for the synthesis of eighteen ATP molecules (for NAD) and four ATP molecules (to the ADF).
The complete energy accounting of aerobic respiration is therefore: 2 + 6 + 6 + 2 + 18 + 4 = 38 ATP. The summary of all steps results in the following general equation:
1 C6H12O6 + 6 O2 + 38 ADP + 38 P 6 CO2 + 6H2O + 38 ATP
The Metabolic Importance Of The Krebs Cycle
By studying aerobic respiration, we start from glucose molecules. Other substances, however, such as protein and fat, can also serve as an energy fuel. Once properly transformed, these substances produce acetyl molecules, the basic fuel of the Krebs cycle.
The Krebs cycle is the breathing stage in which acetyl-CoA from food molecules is "disassembled" in CO2 and H2O, and the energy produced is used in the synthesis of ATP.
However, the Krebs cycle does not only participate in energy metabolism: as the various substances in the cycle form, part of them can be “diverted” and will serve as the raw material for the synthesis of organic substances (anabolism).
For example, some of the substances used by cells to make amino acids, nucleotides and fats come from the Krebs cycle.
See the picture below for the steps of cellular respiration and their location: