Disclaimer:Cellular Respiration is a complex process. Therefore, there are lots of details that can be very confusing, depending on one's previous knowledge. In my write-up, you will find a user-friendly description of cellular respiration.
There are three main metabolic stages in respiration.
- The Krebs cycle
- The electron transport chain and oxidative phosphorylation.
Gylcolysis is subdivided into two stages: energy investment and energy payoff. In the energy investment stage, the cell spends two ATP molecules to allow gylcolysis to occur.
During gylcolysis, glucose is split into two 3-Carbon sugars. These sugars oxidize and the remaining molecules are rearranged to form two molecules of pyruvate. In the next steps of gylcolysis, NADH and a small amount of ATP is produced. Bottom line importance of gylcolysis: It produces pyruvates, NADH, and ATP.
The pyruvates are important in the Kreb Cycle, the next step in cellular respiration. In the presence of oxygen, the pyruvates will move to the mitochondrion. Then, one hydrogen and two electrons are stripped away from each pyruvate. Because of this stripping, one molecule of carbon dioxide is liberated. The remaining two carbons from the pyruvate form an acetyl group. This acetyl group is then joined by co-enzyme A, or CoA.
In the next step, the acetyl-CoA joins with a four-carbon molecule to form citrate. In a series of chemical reactions, electrons and hydrogen are stripped away to make NADH and FADH2. In addition, each acetyl group residue yields a small amount of ATP. Throughout this process, one more carbon dioxide molecule is liberated. End results of the Kreb cycle: 1 ATP, 3 NADH, 1 FADH, 2 CO².
The electron transport chain is a collection of molecules embedded in the inner membrane of the mitochondrion. The previously made NADH and FADH2 are the sources of electrons which will move down the chain. The electrons move down the chain gradually so that there is a slow release of energy and not a sudden explosive release. Oxygen, which has a great affinity for the electrons, is at the bottom of the chain. When the electrons reach the oxygen, there is more energy released and water is given off. In addition to releasing water, the electron transport chain ensures that there is a H+ concentration gradient.
Because of this concentration gradient, hydrogen ions shoot across the membrane in a process called chemiosmosis. The energy of these particles provide the activation energy required for the ADP and free phosphate to join together to form ATP. Finally, it is in this final step that our body generates the most ATP. It creates many ATP molecules for each original sugar molecule that is oxidized.
I used the breakdown of glucose to explain cellular respiration because it is most commonly taught that way. However, it is wise to take note that carbohydrates, fats, and proteins can be processed and used as fuel in the same manner.
Resources: Campbell et al. Biology, 5th edition. Bio lecture, and a night's worth of study for tommorrow's exam.