Metabolic respiration consists of three main processes. The second of these processes is the Krebs cycle. This process is named for Hans Krebs, the British biochemist who discovered this process. It is also sometimes called the citric acid cycle.
During the first process of metabolic respiration, glycolysis, glucose is broken down into two molecules of ATP and two molecules of pyruvate. The Krebs cycle follows the process of glycolysis, and changes the pyruvate through a series of reactions into ATP. ATP is used to provide energy for the cells.
The Krebs Cycle takes place in the mitochondria of the cell. Unlike glycolysis, which can happen with or without the presence of oxygen, the Krebs cycle is an aerobic process that can only occur when oxygen is present in the cell. We refer to this as a cycle because the product at the beginning is the same product that is left at the end. The result of the process is a loop that can be repeated.
Immediately after glycolysis, the pyruvate is moved into the mitochondria where a series of enzymatic reactions will begin to take place. As the pyruvate, a 3-carbon molecule, enters the mitochondria, oxidation occurs and a carbon is removed in the form of carbon dioxide. The result is the 2-carbon molecule Acetyl CoA. Enzymes then react with the Acetyl CoA to transform it from a 2 carbon molecule to citric acid, a 6-carbon molecule. This is done when the Acetyl CoA combines with a 4 carbon molecule called Oxaloacetic Acid. As the Acetyl CoA is oxidized, and as the carbon dioxide is formed, chemical energy is released. This energy comes in three different forms, NADH, FADH2, and ATP.
When the Krebs cycle is finished, energy will have been created. During Glycolysis, 2 pyruvates are created from each molecule of glucose. There is a net gain of 2 molecules of ATP during Glycolysis. During the Krebs Cycle, those two pyruvates will create 2 molecules of ATP, 10 molecules of NADH and 2 molecules of FADH2.
The energy that is created during the Krebs cycle is only part of what happens. The rest helps explain why we refer to this process as a cycle. The final product of the Krebs Cycle is Oxaloacetic Acid. That is the same product that is used at the beginning of the Krebs cycle to turn the 2 carbon molecule into the 6 carbon molecule that will react with enzymes to form the energy. The cell is now ready to receive more pyruvates from Glycolysis and turn them into energy. It is a circle of life that keeps going on and on if glucose and oxygen are present.
The ATP that is created during the Krebs Cycle is used as energy for the cells. The NADH and the FADH2 are used for the next part of metabolic respiration, the electron transport chain. The electron transport chain is a part of the process called Oxidative Phosphorylation. During this process, more molecules of ATP will be created, but it cannot happen without the Krebs Cycle happening first.