Glucose is the fuel needed by cells for energy. During cellular respiration, several steps occur culminating in the production of 32 ATP from a molecule of glucose. At first, glucose undergoes the process of glycolysis and two molecules of pyruvate are produced.
The citric acid cycle is also called the Krebs cycle. In this part of cellular respiration, pyruvate is oxidized to form carbon dioxide. The cycle culminates in the formation of 2 ATP, 6 NADH and 2 FADH2 for two turns of the cycle.
There are 8 steps in the citric acid cycle. Following this cycle, the electron transport chain occurs which powers the synthesis of ATP. NADH and FADH2 are the electron carriers which contain most of the energy extracted from the original energy source which was the glucose. This energy was obtained during the processes of glycolysis and the citric acid cycle.
The electron transport chain in the inner membrane of the mitochondrion is where electrons pass through various proteins eventually to oxygen in small manageable steps. The end product of this stage is the production of ATP.
To summarize, most of the energy in a cell flows from glucose to NADH, to the electron transport chain to a proton-motive force to ATP. The electron carriers are NADH and FADH2.