Chapter 9 Cellular Respiration and Fermentation

O the interior of the organelle is filled with layers

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o The interior of the organelle is filled with layers of sac-like structures called cristae. o The region inside the inner membrane but outside the cristae is the mitochondrial matrix. Pyruvate moves across the mitochondrion's outer membrane through small pores. Enry into the matrix occurs through a membrane protein called the pyruvate carrier, located in the inner membrane. Transport into the matrix requires ATP. Inside the mitochondrion, pyruvate reacts with a coupound called coenzyme A (CoA). CoA acts as a coenzyme by accepting and transferring an acetyle group to a substrate. Pyruvate reactes with CoA to produce acetyl CoA. The reaction sequence occurs inside an enzyme complex called pyruvate dehydrogenase. In eukaryotes, pyruvate dehydrogenase is located in the mitochondrial matrix. In archaea and bacteria, pyrucate dehydrogenase is located in the cytosol. As pyruvate is being processed, NAD + is reduced to NADH and one of the carbons in the pyruvate is oxidized to CO 2 . The remaining two-carbon acetyl unit is transferred to CoA. Acetyl CoA is the final product final product of the pyruvate processing step in glucose oxidation. o Pyruvate, NAD + , and CoA go in; CO 2 , NADH, and acetyl CoA come out. When supplies of ATP are abundant, however, the process shuts down. The rate of phosphorylation increases when other products (CoA and NADH) are at high concentration. These regulator changes are examples of feedback inhibition.
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Carboxylic acids have the form R-COOH. The redox reactions that produce carboxylic acids also sometimes produce carbon dioxide. The citric acid cycle starts with citrate, which becomes citric acid when protonated. The citric acid cycle is also known as the Krebs cycle and tricarboxylic acid (TCA) cycle, because it involves acids with three carboxyl groups. The energy released by the oxidation of one molecule of acetyl CoA is used to produce three molecules of NADH, on of FADH 2 , and one of guanosine triphosphate (GTP) or ATP through substrate-level phophorylation. Whether GTP or ATP is produced depends on the type of cell being considered. For example, GTP appears to be produced in the liver cells of mammals, while ATP is produced in muscle cells. In bacteria and archaea, the enzymes responsible for the citric acid cycle are located in the cytosol. In eukaryotes, most of the enzymes responsible for the citric acid cycle are located in the mitochondrial matrix. Because glycolysis produces two molecules of pyruvate, the cycle turns twice for each molecule of glucose processed in cellular respiration. How is the Citric Acid Cycle Regulated? Reaction rates are high when ATP is scarce; reaction rates are low when ATP is abundant. The enzyme that converts acetyl CoA to citrate is shut down when ATP binds to it. (Feedback Inhibition) Feedback inhibition also occurs at two points later in the cycle: 1.
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