Cellular Respiration (3 of 5) Acetyl CoA Formation and the Citric Acid Cycle

ch 7, 8 and 9 41 of 51 Cellular Respiration (3 of 5): Acetyl CoA Formation and the Citric Acid Cycle Before beginning this tutorial, watch the animation showing the second and third stages of cellular respiration – acetyl CoA formation and the citric acid cycle (also known as the Krebs cycle). In these two stages, the pyruvate produced in glycolysis is completely oxidized to CO2, with the production of NADH, FADH 2, and some ATP. Consider carefully the unique cyclic sequence of reactions of the citric acid cycle and the many redox reactions that occur within it. Acetyl CoA Formation. 6/13/2019, 11:24 AM ch 7, 8 and 9 42 of 51 Cytric Acid Cycle. Part A - Carbon atoms in acetyl CoA formation and the citric acid cycle During acetyl CoA formation and the citric acid cycle, all of the carbon atoms that enter cellular respiration in the glucose molecule are released in the form of CO2. Use this diagram to track the carbon-containing compounds that play a role in these two stages. Drag the labels from the left (which represent numbers of carbon atoms) onto the diagram to identify the number of carbon atoms in each intermediate in acetyl CoA formation and the citric acid cycle. Labels may be used more than once. Hint 1. How many carbon atoms are in pyruvate and acetyl CoA? In glycolysis, glucose (a six-carbon molecule) is broken down to form two molecules of pyruvate. Then, during acetyl CoA formation, pyruvate is converted to acetyl CoA through a series of reactions. Which statement is true about the number of carbon atoms in the compounds involved in acetyl CoA formation? ANSWER: Both pyruvate and acetyl CoA are three-carbon molecules. Pyruvate is a six-carbon molecule, while acetyl CoA is a five-carbon molecule. One molecule of CO2 is released. Pyruvate is a three-carbon molecule, while acetyl CoA is a two-carbon molecule. One molecule of CO2 is released. Both pyruvate and acetyl CoA are six-carbon molecules. Both pyruvate and acetyl CoA are two-carbon molecules. Hint 2. Carbon atoms in citrate, the first intermediate in the citric acid cycle Citrate (citric acid), the first intermediate in the citric acid cycle, contains six carbon atoms. ANSWER: 6/13/2019, 11:24 AM ch 7, 8 and 9 43 of 51 Reset Help Part B - Net redox reaction in acetyl CoA formation and the citric acid cycle In the sequential reactions of acetyl CoA formation and the citric acid cycle, pyruvate (the output from glycolysis) is completely oxidized, and the electrons produced from this oxidation are passed on to two types of electron acceptors. Drag the labels on the left to show the net redox reaction in acetyl CoA formation and the citric acid cycle. Note that two types of electron carriers are involved. Hint 1. Redox reactions 6/13/2019, 11:24 AM ch 7, 8 and 9 44 of 51 Under normal circumstances, redox reactions always occur in pairs. In the first of the two reactions, the electron donor is oxidized (it loses electrons). In the second reaction, the electron acceptor is reduced (it gains the electrons lost by the first compound). A generic redox reaction involving the transfer of two electrons is illustrated here. Note that compounds A and B each exist in two forms: One form is reduced (it carries the extra electrons) while the other form is oxidized (it does not carry the extra electrons). In the reaction shown here, the electron donor is the reduced form of compound A and the electron acceptor is the oxidized form of compound B. Hint 2. What are the electron carriers in acetyl CoA formation and the citric acid cycle? In acetyl CoA formation and the citric acid cycle, as in glycolysis, electron carriers shuttle the electrons removed from carbon-containing intermediates to a later stage in cellular respiration. The most common pair of electron carriers in cellular respiration is NAD+ (the oxidized form) and NADH (the reduced form): In the citric acid cycle, however, another pair of electron carriers shuttles electrons in addition to NAD+/NADH. What is the second pair of electron carriers in the citric acid cycle? ANSWER: ADP (the oxidized form) and ATP (the reduced form) H2O (the oxidized form) and O2 (the reduced form) FAD (the oxidized form) and FADH2 (the reduced form) O2 (the oxidized form) and H2O (the reduced form) FADH2 (the oxidized form) and FAD (the reduced form) ATP (the oxidized form) and ADP (the reduced form) Hint 3. Can you identify the electron donor and acceptor in a reaction? One of the reactions in the citric acid cycle is the oxidation of the six-carbon molecule isocitrate to the five-carbon molecule alpha-ketoglutarate, with the release of one molecule of CO2. isocitrate + NAD+ alpha-ketoglutarate + NADH + CO2 In this reaction, the electron donor is __________ and the electron acceptor is __________. ANSWER: isocitrate/NAD+ alpha-ketoglutarate/NADH NADH/NAD+ isocitrate/alpha-ketoglutarate NAD+/NADH 6/13/2019, 11:24 AM ch 7, 8 and 9 45 of 51 ANSWER: Reset Help Part C - Why is the citric acid cycle a cyclic pathway rather than a linear pathway? In the oxidation of pyruvate to acetyl CoA, one carbon atom is released as CO2. However, the oxidation of the remaining two carbon atoms—in acetate—to CO2 requires a complex, eight-step pathway—the citric acid cycle. Consider four possible explanations for why the last two carbons in acetate are converted to CO2 in a complex cyclic pathway rather than through a simple, linear reaction. Use your knowledge of the first three stages of cellular respiration to determine which explanation is correct. Hint 1. What is the fate of carbon atoms entering the citric acid cycle? With each turn of the citric acid cycle, two carbon atoms from the acetyl group of acetyl CoA join with the four-carbon oxaloacetate (the last intermediate in the citric acid cycle) to form citrate, a six-carbon acid. In the remaining steps of the citric acid cycle, the six-carbon citrate is converted back to the four-carbon oxaloacetate. What happens to the other two carbon atoms? ANSWER: 6/13/2019, 11:24 AM ch 7, 8 and 9 46 of 51 They are used to convert ADP and Pi to ATP. They are used to convert NAD+ to NADH. They are released as two CO2 molecules. They are converted into pyruvate. They increase the number of carbon atoms stored in the intermediates of the citric acid cycle. Hint 2. How many carbon atoms are in the molecules from which CO2 is released in the citric acid cycle? Two molecules of CO2 are produced for each molecule of acetyl CoA that enters the citric acid cycle. These two molecules of CO2 are removed from specific carbon intermediates in the cycle. How many carbon atoms are present in the intermediates from which CO2 is removed? How many carbon atoms are present in the intermediates from which CO2 is removed? Hint 1. Reactions that release CO2 in the citric acid cycle This diagram shows the carbon skeletons of the intermediates in the citric acid cycle. Locate the intermediates involved in reactions in which CO2 is released to determine how many carbon atoms they contain. ANSWER: 2 3 4 5 6 2 or 6 4 or 5 5 or 6 6/13/2019, 11:24 AM ch 7, 8 and 9 47 of 51 ANSWER: More ATP is produced per CO2 released in cyclic processes than in linear processes. It is easier to remove electrons and produce CO2 from compounds with three or more carbon atoms than from a two-carbon compound such as acetyl CoA. Redox reactions that simultaneously produce CO2 and NADH occur only in cyclic processes. Cyclic processes, such as the citric acid cycle, require a different mechanism of ATP synthesis than linear processes, such as glycolysis. 6/13/2019, 11:24 AM