![]() The carbon that is removed takes two oxygens from pyruvate with it, and exits the body as carbon dioxide. ![]() Between glycolysis and the citric acid cycle, the 3-carbon pyruvate molecule loses a carbon to produce a new, 2-carbon molecule called acetyl-CoA. You’ll recall that in glycolysis, a 6-carbon glucose molecule is split into two 3-carbon molecules called pyruvates. The citric acid cycle produces very little ATP directly and does not directly consume oxygen.īefore it can be used as a substrate in the citric acid cycle, acetyl-CoA must be made. ![]() If this transfer does not occur, the oxidation steps of the citric acid cycle also do not occur. This is considered an aerobic pathway because the NADH and FADH2 produced must transfer their electrons to the next pathway in the system, which will use oxygen. The eight steps of the cycle are a series of redox, dehydration, hydration, and decarboxylation reactions. The electrons transported by electron shuttles represent the high-energy intermediates that are later used to generate ATP. Electron shuttles are small organic molecules, such as NAD+ and FADH, that transport high energy electrons by gaining electrons (through “reduction”) and losing electrons (through “oxidation”). Electron shuttle molecules accept the energy released by these stepwise rearrangements and the subtraction of carbons in the form of electrons. The molecules that enter and move through the citric acid cycle are made mostly of carbon atoms, and these carbon atoms undergo rearrangements throughout the cycle. Unlike glycolysis, the citric acid cycle is a closed loop: the last part of the pathway regenerates the oxaloacetate molecule used in the first step. The trapped energy from the citric acid cycle is then passed on to oxidative phosphorylation, where it is converted to a usable form of cellular energy, adenosine triphosphate (ATP). It captures the energy stored in the chemical bonds of acetyl-CoA from the products of glycolysis in a step-by-step process, trapping it in the form of high-energy intermediate molecules. The citric acid cycle takes place in the matrix of the mitochondria. The citric acid cycle (or the Krebs cycle) is one of the steps in cellular respiration and consists of a series of reactions that produces two carbon dioxide molecules, one GTP/ATP, and reduced forms of NADH and FADH2.
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