Cellular Respiration (4 of 5) Oxidative Phosphorylation

ch 7, 8 and 9 47 of 51 Cellular Respiration (4 of 5): Oxidative Phosphorylation Oxidative phosphorylation consists of two tightly linked processes - electron transport and ATP synthesis. In electron transport, the NADH and FADH2 produced in the first three stages of cellular respiration are oxidized by O2 (the oxidative part of this stage). These redox reactions also drive the pumping of protons across the inner mitochondrial membrane, creating a proton ( H+) gradient. This H+ gradient is used to power the chemiosmotic synthesis of ATP from ADP and Pi (the phosphorylation part of this stage). As you watch the Oxidative Phosphorylation animation, pay close attention to how electron transport is coupled to the formation of the H+ gradient and ATP synthesis. Part A - The role of O2 in electron transport In mitochondrial electron transport, what is the direct role of O2? Hint 1. The definition of “direct role” A molecule plays a direct role in a chemical reaction if it participates in the reaction as either a reactant or a product, or if it is part of the catalytic machinery (enzymes or cofactors) that allows the reaction to occur. Hint 2. The electron transport chain The electron transport chain (shown in this diagram) consists of a sequence of electron carriers, most of which are components of the four main protein complexes (I - IV) embedded in the inner mitochondrial membrane. Two other substances, Q and Cyt c, are mobile carriers that shuttle electrons between the protein complexes. Note the different locations where electrons from NADH and FADH2 enter the electron transport chain and the role that O2 plays in this process. 6/13/2019, 11:24 AM ch 7, 8 and 9 48 of 51 ANSWER: to provide the driving force for the synthesis of ATP from ADP and Pi to provide the driving force for the production of a proton gradient to function as the final electron acceptor in the electron transport chain to oxidize NADH and FADH2 from glycolysis, acetyl CoA formation, and the citric acid cycle Part B - The effects of anaerobic conditions How would anaerobic conditions (when no O2 is present) affect the rate of electron transport and ATP production during oxidative phosphorylation? (Note that you should not consider the effect on ATP synthesis in glycolysis or the citric acid cycle.) Hint 1. The role of O2 in electron transport O2 is the final electron acceptor in the electron transport chain. Without O2, there is no place for the electrons from NADH and FADH2 (and ultimately from glucose) to go. Hint 2. What is the link between electron transport and ATP synthesis in oxidative phosphorylation? If electron transport stops because there is no O2 to serve as the final electron acceptor, ATP synthesis associated with oxidative phosphorylation also stops. Why? ANSWER: Without electron transport, the inner mitochondrial membrane becomes leaky to protons and the proton gradient rapidly dissipates. ATP synthesis in oxidative phosphorylation is directly driven by electrons moving from NADH and FADH2 to O2. Without electron transport from NADH and FADH2 to O2, the pumping of protons across the inner mitochondrial membrane ceases. The synthesis of ATP from ADP and Pi creates a proton gradient across the inner mitochondrial membrane. This gradient is required for NADH oxidation by O2. ANSWER: Both electron transport and ATP synthesis would stop. Electron transport would stop but ATP synthesis would be unaffected. Electron transport would be unaffected but ATP synthesis would stop. Neither electron transport nor ATP synthesis would be affected. Part C - Comparing the amount of ATP synthesis from NADH and FADH2 NADH and FADH2 are both electron carriers that donate their electrons to the electron transport chain. The electrons ultimately reduce O2 to water in the final step of electron transport. However, the amount of ATP made by electrons from an NADH molecule is greater than the amount made by electrons from an 6/13/2019, 11:24 AM ch 7, 8 and 9 49 of 51 FADH2 molecule. Which statement best explains why more ATP is made per molecule of NADH than per molecule of FADH2? Hint 1. How does the number of protons pumped across the membrane per NADH and FADH2 compare? Once an H+ ion is pumped across the inner mitochondrial membrane and becomes part of the H+ gradient, it has the potential to drive the same amount of ATP synthesis as any other H+ ion, regardless of where it was pumped across the membrane. Thus, an H+ ion derived from the oxidation of NADH is equivalent to an H+ ion derived from the oxidation of FADH2. Because more ATP is made from an NADH molecule than from an FADH2 molecule, what must be true about the number of H+ ions that these two molecules contribute to the H+ gradient? ANSWER: NADH contributes fewer H+ ions than FADH2. NADH contributes more H+ ions than FADH2. Both NADH and FADH2 contribute the same number of H+ ions . Hint 2. NADH and FADH2 donate electrons at different positions along the electron transport chain Although NADH and FADH2 have very similar functions in cellular respiration (they are both reduced electron carriers), they donate their electrons at different points along the electron transport chain. ANSWER: Fewer protons are pumped across the inner mitochondrial membrane when FADH2 is the electron donor than when NADH is the electron donor. It takes more energy to make ATP from ADP and Pi using FADH2 than using NADH. There is more NADH than FADH2 made for every glucose that enters cellular respiration. The H+ gradient made from electron transport using NADH is located in a different part of the mitochondrion than the H+ gradient made using FADH2. FADH2 is made only in the citric acid cycle while NADH is made in glycolysis, acetyl CoA formation, and the citric acid cycle. 6/13/2019, 11:24 AM ch 7, 8 and 9 50 of 51 Part D - The effect of gramicidin on oxidative phosphorylation When the protein gramicidin is integrated into a membrane, an H+ channel forms and the membrane becomes very permeable to protons (H+ ions). If gramicidin is added to an actively respiring muscle cell, how would it affect the rates of electron transport, proton pumping, and ATP synthesis in oxidative phosphorylation? (Assume that gramicidin does not affect the production of NADH and FADH 2 during the early stages of cellular respiration.) Sort the labels into the correct bin according to the effect that gramicidin would have on each process. Hint 1. Review the Oxidative Phosphorylation animation In oxidative phosphorylation, electron transport is coupled to the synthesis of ATP via a proton gradient that forms across the inner mitochondrial membrane. In the Oxidative Phosphorylation animation, watch how the proton gradient is created by electron transport and how it is used in the synthesis of ATP. Hint 2. How to approach the problem You know that membranes treated with gramicidin become very leaky to protons. Consider these four questions (in this order) to help you evaluate how gramicidin alters oxidative phosphorylation. 1. 3. Is a proton gradient across the inner mitochondrial membrane required for ATP synthesis during oxidative phosphorylation? 2. What effect does a membrane that is very leaky to protons have on the ability of the mitochondrion to maintain a proton gradient across that membrane? What effect does the ability of the mitochondrion to maintain a proton gradient have on the rate of proton pumping? 4. How is the rate of electron transport related to the rate of proton pumping, and are these rates affected by the membrane being leaky to protons? Hint 3. How would gramicidin affect electron transport and proton pumping? Think about how gramicidin would affect proton pumping driven by electron transport. 6/13/2019, 11:24 AM ch 7, 8 and 9 51 of 51 Which of the following statements is correct? ANSWER: Electron transport would continue to pump protons across the inner mitochondrial membrane, but gramicidin would allow the protons to leak back across the membrane. Gramicidin would prevent electron transport from pumping protons across the inner mitochondrial membrane. Hint 4. How would gramicidin affect the proton gradient across the inner mitochondrial membrane? If gramicidin creates proton channels, think about how this would affect the proton gradient across the inner mitochondrial membrane. What would happen to the proton gradient in the presence of gramicidin? ANSWER: Protons would be pumped through the channel against their electrochemical gradient and the proton gradient would increase. Protons would diffuse back through the channel down their electrochemical gradient and the proton gradient would dissipate. Nothing. ANSWER: Reset Help Score Summary: Your score on this assignment is 95.3%. You received 14.77 out of a possible total of 15.5 points. 6/13/2019, 11:24 AM