{"id":666,"date":"2017-04-16T02:54:37","date_gmt":"2017-04-16T02:54:37","guid":{"rendered":"https:\/\/pressbooks.hcfl.edu\/bio1\/chapter\/7-6-aerobic-respiration-the-citric-acid-cycle\/"},"modified":"2025-08-29T18:15:09","modified_gmt":"2025-08-29T18:15:09","slug":"7-6-aerobic-respiration-the-citric-acid-cycle","status":"publish","type":"chapter","link":"https:\/\/pressbooks.hcfl.edu\/bio1\/chapter\/7-6-aerobic-respiration-the-citric-acid-cycle\/","title":{"raw":"Aerobic Respiration, Part 2: Oxidation of Pyruvate and The Citric Acid Cycle","rendered":"Aerobic Respiration, Part 2: Oxidation of Pyruvate and The Citric Acid Cycle"},"content":{"raw":"If oxygen is available, aerobic respiration will go forward. In eukaryotic cells, the pyruvate molecules produced at the end of glycolysis are transported into mitochondria (Figure 1<\/strong>), which are the sites of cellular respiration. In order for pyruvate, the product of glycolysis, to enter the next pathway, it must undergo several changes. The conversion is a three-step process.\n\n[caption id=\"attachment_665\" align=\"alignnone\" width=\"622\"]\"Mitochondria Figure 1<\/strong> Diagram of a human mitochondrion. Recall that mitochondria have two membranes: an inner and an outer membrane. Between the two membranes is a region known as the intermembrane space. The mitochondrial matrix is located inside the inner membrane. Photo credit PsChemp,<\/a>\u00a0Wikimedia.<\/a>[\/caption]\n

Oxidation of Pyruvate<\/h2>\nIn eukaryotic cells, the pyruvate molecules produced at the end of glycolysis are transported into the mitochondrial matrix<\/strong> (the middle region of the mitochondria) (Figure 1<\/strong>). In the mitochondrial matrix, pyruvate will be transformed into a two-carbon acetyl group by removing a molecule of carbon dioxide. This also produces NADH. The acetyl group is\u00a0picked up by a carrier compound called coenzyme A (CoA), which is made from vitamin B5. The resulting compound is called acetyl CoA<\/strong>\u00a0(Figure 2<\/strong>). Acetyl CoA can be used in a variety of ways by the cell, but its major function is to deliver the acetyl group derived from pyruvate to the next pathway in glucose catabolism.\n\n[caption id=\"attachment_665\" align=\"alignnone\" width=\"300\"]\"oxidation Figure 2<\/strong>\u00a0Upon entering the mitochondrial matrix, a multi-enzyme complex converts pyruvate into acetyl CoA. In the process, carbon dioxide is released and one molecule of NADH is formed.[\/caption]\n

\u00a0Acetyl CoA to CO2<\/sub><\/h2>\nIn the presence of oxygen, acetyl CoA delivers its acetyl group to a four-carbon molecule, oxaloacetate, to form citrate, a six-carbon molecule with three carboxyl groups; this pathway will harvest the remainder of the extractable energy from what began as a glucose molecule. This single pathway is called by different names: the citric acid cycle<\/strong> (for the first intermediate formed\u2014citric acid, or citrate\u2014when acetate joins to the oxaloacetate), the TCA cycle<\/strong> (since citric acid or citrate and isocitrate are tricarboxylic acids), and the Krebs cycle<\/strong>, after Hans Krebs, who first identified the steps in the pathway in the 1930s in pigeon flight muscles.\n\nLike the conversion of pyruvate to acetyl CoA, the citric acid cycle <\/strong>in eukaryotic cells also takes place in the matrix of the mitochondria (Figure 1<\/strong>). Unlike glycolysis, the citric acid cycle is a closed loop: the last part of the pathway regenerates the compound used in the first step. The eight steps of the cycle are a series of chemical reactions that produces the following from each<\/em>\u00a0of the two molecules of pyruvate produced per molecule of glucose that originally went into glycolysis (Figure 3<\/strong>):\n