What Is the Krebs Cycle?
The Krebs cycle, also known as the citric acid cycle or the tricarboxylic acid (TCA) cycle, is a series of enzyme-catalyzed chemical reactions that form a central part of aerobic respiration in cells. Its primary function is to harvest high-energy electrons from carbon-based molecules, transferring them to electron carrier molecules like NADH and FADH2.
Section 2: Location and Key Process
In eukaryotic cells, the Krebs cycle takes place in the matrix of the mitochondria. The cycle begins when a two-carbon molecule, acetyl-CoA (derived from the breakdown of glucose), combines with a four-carbon molecule, oxaloacetate, to form a six-carbon molecule called citrate. Through a series of eight steps, this citrate molecule is systematically broken down, releasing carbon dioxide and regenerating the original oxaloacetate molecule to continue the cycle.
Section 3: A Practical Example: Products Per Turn
For each single molecule of acetyl-CoA that enters the cycle, one complete turn produces: 2 molecules of carbon dioxide (CO2), 3 molecules of NADH, 1 molecule of FADH2, and 1 molecule of ATP (or an equivalent, GTP). The CO2 is released as a waste product, while the energy-rich NADH and FADH2 molecules proceed to the final stage of cellular respiration, the electron transport chain.
Section 4: Importance in Metabolism
The Krebs cycle is the central hub for the metabolism of carbohydrates, fats, and proteins. It is the primary source of the electron carriers that fuel the electron transport chain, where the majority of a cell's ATP is produced. Beyond energy production, intermediates from the cycle also serve as building blocks for synthesizing amino acids, fatty acids, and other essential organic molecules.