15Feb_12

15Feb_12 - Biological Science I Tuesdays and Thursdays...

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Biological Science I Tuesdays and Thursdays 8:00-9:15, HCB 102 Mondays 5:15-6:15, KIN 1024 15 February 2011 – Lecture 12 Concept 9.2 Glycolysis harvests chemical energy by oxidizing glucose to pyruvate. During glycolysis, glucose, a six-carbon sugar, is split into two three-carbon sugars. These smaller sugars are then oxidized and rearranged to form two molecules of pyruvate, the ionized form of pyruvic acid. Each of the ten steps in glycolysis is catalyzed by a specific enzyme. These steps can be divided into two phases. 1. In the energy investment phase, the cell spends ATP. 2. In the energy payoff phase, this investment is repaid with interest. ATP is produced by substrate-level phosphorylation, and NAD + is reduced to NADH by electrons released by the oxidation of glucose. The net yield from glycolysis is 2 ATP and 2 NADH per glucose. No CO 2 is produced during glycolysis. Glycolysis can occur whether or not O 2 is present. If O 2 is present, the chemical energy stored in pyruvate and NADH can be extracted by the citric acid cycle and oxidative phosphorylation. Concept 9.3 The citric acid cycle completes the energy-yielding oxidation of organic molecules. More than three-quarters of the original energy in glucose is still present in the two molecules of pyruvate. If molecular oxygen is present in eukaryotic cells, pyruvate enters the mitochondrion, where enzymes of the citric acid cycle complete the oxidation of the organic fuel to carbon dioxide. In prokaryotic cells, this process occurs in the cytoplasm. After pyruvate enters the mitochondrion via active transport, it is converted to a compound called acetyl coenzyme A, or acetyl CoA. This step, the junction between glycolysis and the citric acid cycle, is accomplished by a multienzyme complex that catalyzes three reactions. A carboxyl group is removed as CO 2 . The carbon dioxide is fully oxidized and thus has little chemical energy. The remaining two-carbon fragment is oxidized to form acetate. An enzyme transfers the pair of electrons to NAD + to form NADH. Acetate combines with coenzyme A to form the very reactive molecule acetyl CoA. Due to the chemical nature of the CoA group, a sulfur-containing compound derived from a B vitamin, acetyl CoA has a high potential energy. In other words, the reaction of acetyl CoA to yield lower-energy products is highly exergonic. Acetyl CoA is now ready to feed its acetyl group into the citric acid cycle for further oxidation. The citric acid cycle is also called the tricarboxylic acid cycle or the Krebs cycle. 1
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Biological Science I Tuesdays and Thursdays 8:00-9:15, HCB 102 Mondays 5:15-6:15, KIN 1024 15 February 2011 – Lecture 12 The latter name honors Hans Krebs, who was largely responsible for elucidating the cycle’s pathways in the 1930s. The citric acid cycle oxidizes organic fuel derived from pyruvate.
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This note was uploaded on 11/07/2011 for the course BIO bsc2010 taught by Professor Trombley during the Spring '08 term at FSU.

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15Feb_12 - Biological Science I Tuesdays and Thursdays...

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