Oxidative Phosphorylation Quiz
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Questions and Answers

What is the ultimate electron acceptor in the process of cellular respiration?

  • Citric Acid
  • Molecular Oxygen (correct)
  • NADH
  • Ubiquinol

    • Which of the following compounds directly donates electrons to Complex I of the electron transport chain?

  • Fatty Acids
  • FADH2
  • NADH (correct)
  • Ubiquinol

    • What role do the mitochondrial transport systems play?

  • To synthesize ATP
  • To import and export metabolites (correct)
  • To carry out oxidative phosphorylation
  • To enhance the permeability of the inner membrane

    • How do protons move across the mitochondrial membrane during electron transport?

    <p>Via active transport by specific complexes</p> Signup and view all the answers

    What characteristic does the standard reduction potential of +0.815 V indicate about molecular oxygen?

    <p>It is an effective oxidizing agent</p> Signup and view all the answers

    Why might respiratory complexes not be organized in a linear chain?

    <p>They may function independently in parallel pathways</p> Signup and view all the answers

    Which processes primarily generate the NADH used in mitochondrial electron transport?

    <p>Citric acid cycle and fatty acid oxidation</p> Signup and view all the answers

    What is the primary function of the electron transport chain in the mitochondrion?

    <p>To produce proton gradients</p> Signup and view all the answers

    What is the first electron acceptor for the electrons donated by NADH in Complex I?

    <p>Flavin mononucleotide (FMN)</p> Signup and view all the answers

    How many protons are transferred from the matrix to the intermembrane space by Complex I?

    <p>Four protons</p> Signup and view all the answers

    Which half-reaction shows the most positive reduction potential?

    <p>Lipoic acid + 2 H+ + 2 e− ⇌ dihydrolipoic acid</p> Signup and view all the answers

    Which component of the electron transport chain is not a one-electron carrier?

    <p>Ubiquinone</p> Signup and view all the answers

    In mitochondrial electron transport, which molecule is the first electron donor?

    <p>NADH</p> Signup and view all the answers

    What role do iron-sulfur clusters play in the electron transport chain?

    <p>They transfer electrons one at a time</p> Signup and view all the answers

    What happens to ubiquinol during the process of electron transport?

    <p>It is oxidized by Complex III</p> Signup and view all the answers

    What is the outcome of passing electrons to a molecule with a more positive reduction potential?

    <p>The reaction is favorable.</p> Signup and view all the answers

    Which reaction is catalyzed by succinate dehydrogenase?

    <p>Succinate + Q ⇌ fumarate + QH2</p> Signup and view all the answers

    Which equation would most likely be used to calculate the free energy change in redox reactions?

    <p>ΔG = -nℱΔE</p> Signup and view all the answers

    What indicates that the protons taken up from the matrix are not the same ones released into the intermembrane space?

    <p>Presence of proton wires</p> Signup and view all the answers

    Which species would be oxidized in a redox reaction between NADH and pyruvate?

    <p>NADH</p> Signup and view all the answers

    What is the role of O2 in the electron transport chain?

    <p>Final electron acceptor</p> Signup and view all the answers

    What structural change occurs in the Fe-S protein during electron transfer?

    <p>It rotates and moves about 22 Å</p> Signup and view all the answers

    Which statement correctly describes the relationship among oxidants and reductants?

    <p>A reductant can also be an oxidant.</p> Signup and view all the answers

    Which of the following half-reactions is least likely to occur reversibly?

    <p>Acetate− + 3 H+ + 2 e− ⇌ acetaldehyde + H2O</p> Signup and view all the answers

    What is the main outcome of oxidative phosphorylation?

    <p>It synthesizes ATP from ADP and inorganic phosphate.</p> Signup and view all the answers

    Which substance is oxidized during the process of oxidative phosphorylation?

    <p>O2</p> Signup and view all the answers

    How does the protonmotive force contribute to ATP synthesis?

    <p>It creates a concentration gradient of protons that drives ATP synthase.</p> Signup and view all the answers

    What role do the coenzymes NAD+ and ubiquinone play in metabolism?

    <p>They carry electrons from oxidized compounds to the electron transport chain.</p> Signup and view all the answers

    What is a characteristic feature of the oxidation-reduction reaction?

    <p>One reactant undergoes oxidation while the other is reduced.</p> Signup and view all the answers

    What type of bond is hydrolyzed during ATP synthesis?

    <p>Phosphoanhydride bond</p> Signup and view all the answers

    What is the primary function of Complex IV in the electron transport chain?

    <p>To translocate protons to the intermembrane space</p> Signup and view all the answers

    How many protons are translocated to the intermembrane space for every two electrons donated by cytochrome c?

    <p>Two</p> Signup and view all the answers

    Why is oxidative phosphorylation considered an indirect process of energy transformation?

    <p>It links electron transport to ATP synthesis rather than coupling them directly.</p> Signup and view all the answers

    Which of the following pathways is mainly responsible for generating reduced cofactors like NADH?

    <p>Glycolysis and the citric acid cycle</p> Signup and view all the answers

    What is the ΔG°' value for the overall reaction of NADH to O2?

    <p>–218.2 kJ mol–1</p> Signup and view all the answers

    Which statement accurately describes the coupling of electron transport and ATP synthesis?

    <p>Energy from the proton gradient is used to drive ATP synthase</p> Signup and view all the answers

    Which of the following correctly describes the role of oxygen in the electron transport chain?

    <p>It is the final electron acceptor</p> Signup and view all the answers

    Which complex is responsible for the reduction of ubiquinone (Q) to ubiquinol (QH2)?

    <p>Complex I</p> Signup and view all the answers

    How is the pH difference across the mitochondrial membrane related to energy?

    <p>It contributes to the proton motive force</p> Signup and view all the answers

    What are the redox groups in the respiratory electron transport chain categorized by?

    <p>Number of electrons they can accept or donate</p> Signup and view all the answers

    What role does the carboxylate side chain of the c subunit play in ATP synthase?

    <p>It acts as a proton binding site.</p> Signup and view all the answers

    What happens to the c subunit after it binds a proton?

    <p>It rotates, allowing another c subunit to bind a proton.</p> Signup and view all the answers

    How do conformational changes in the β subunits influence ATP production?

    <p>They alter the affinity for adenine nucleotides in different states.</p> Signup and view all the answers

    What regulates ATP synthase in eukaryotes, particularly under varying pH levels?

    <p>Inhibitory factor 1 (IF1).</p> Signup and view all the answers

    What structural change does IF1 undergo when the matrix pH drops?

    <p>It dimerizes and forms extended α helices.</p> Signup and view all the answers

    Which statement about the αβ hexamer of ATP synthase is true?

    <p>It remains stationary while the c ring and γ subunit rotate.</p> Signup and view all the answers

    What is the primary function of ATP synthase?

    <p>To synthesis ATP from ADP and inorganic phosphate.</p> Signup and view all the answers

    Which of the following statements about the process involving ATP synthase is false?

    <p>All α and β subunits contribute equally to catalysis.</p> Signup and view all the answers

    Study Notes

    Oxidative Phosphorylation

    • Living organisms obey the laws of thermodynamics.

    • Transporters obey the laws of thermodynamics, allowing solutes to move down concentration gradients or using ATP to move against the gradients.

    • Coenzymes like NAD+ and ubiquinone collect electrons from oxidized compounds.

    • A reaction that hydrolyzes a phosphoanhydride bond in ATP results in a large negative free energy change.

    Learning Objectives

    • Summarize the thermodynamics of oxidation-reduction reactions.

    • Map the path of electrons through the redox groups of the electron transport pathway.

    • Explain how the protonmotive force links electron transport and ATP synthesis.

    • Describe the structure and operation of ATP synthase.

    The Thermodynamics of Oxidation-Reduction Reactions

    • Outline the thermodynamics of oxidation-reduction reactions.

    • Predict the direction of electron transfer in a mixture of two substances.

    Harvesting of Free Energy

    • Metabolic fuels like glucose, fatty acids, and amino acids, as well as the oxidation of acetyl carbons to CO2, yield the reduced cofactors NADH and ubiquinol (QH2).

    • These reduced cofactors, reoxidized, release free energy.

    • That free energy is used to synthesize ATP in a process called oxidative phosphorylation.

    Recap of Oxidation

    • One reactant is oxidized while the other is reduced.

    • Loss of electrons is oxidation.

    • Gain of electrons is reduction.

    Oxidative Phosphorylation in Context

    • ATP synthesis isn't directly coupled to a single chemical reaction. It is an indirect process of energy transformation.

    • Electrons flow from reduced compounds like NADH and QH2 to an oxidized compound like O2.

    • The reduced cofactors NADH and QH2 are generated in the oxidative catabolism of amino acids, monosaccharides, and fatty acids.

    • These cofactors are reoxidized using molecular oxygen, and the energy powers ATP synthesis.

    Recap of Oxidation-Reduction

    • In reactions involving FADH2, electrons are transferred as H atoms.

    • When NAD+ is involved, the electron pair takes the form of a hydride ion (H−).

    • Electrons usually travel in pairs in biological systems.

    Reduction Potential

    • A substance's tendency to accept electrons (become reduced) is described by its standard reduction potential (E′) at 1 M, 1 atm, pH 7, and 25°C.

    • It's customary to consider one substance at a time (a half-reaction).

    Standard Reduction Potentials of Some Biological Substances

    • Includes a table of half-reactions with their standard reduction potentials (E′).

    The Actual Reduction Potential

    • The actual reduction potential depends on the actual concentrations of the oxidized and reduced species.

    • The Nernst Equation is used to calculate the actual reduction potential.

    Free Energy Change

    • The free energy change can be calculated from the change in reduction potential.

    Overview of Mitochondrial Electron Transport

    • Passing electrons to a more positively charged molecule on a scale is favorable (ΔG < 0).

    • NADH is the first electron donor, and O2 is the final electron acceptor, reduced to H2O.

    • The reduction potentials of electron carriers are indicated. Oxidation-reduction reactions release free energy, mediated by complexes I, III, and IV.

    Mitochondrial Electron Transport

    • The redox potential energy of NADH and FADH2 is released stepwise via the electron transport chain.

    Review

    • Explain why an oxidation-reduction reaction must include both an oxidant and a reductant.

    • When two reactants are mixed together, predict which one will become reduced and which will become oxidized.

    • Select two half-reactions from the provided table that would likely form a freely reversible redox reaction.

    Section 15.2 Mitochondrial Electron Transport

    • Map the path of electrons through the redox groups of the electron transport pathway.

    • Explain why the mitochondrion includes a variety of transport systems.

    • Identify the sources of electrons for complexes I, III, and IV.

    • Describe the mechanisms for transporting protons across the mitochondrial membrane.

    • Explain why respiratory complexes may not actually form a chain.

    Cellular Respiration

    • In aerobic organisms, NADH and ubiquinol are reoxidized by molecular oxygen in a process called cellular respiration.

    • O2 is a more effective oxidizing agent.

    • Electrons are shuttled from NADH to O2 in a multistep process, the respiratory electron transport chain (ETC).

    Electron Transport Takes Place in the Mitochondrion

    • The inner mitochondrial membrane is impermeable to ions.

    • The intermembrane space has an ionic composition similar to the cytosol.

    Experimental Imaging Helps Us Know What Mitochondria Look Like

    • Includes images of electron micrographs, 3D reconstructions, and fluorescence micrographs of mitochondria.

    Cofactors Transfer Electrons to the ETC

    • Much NADH/QH2 is generated by the citric acid cycle (inside mitochondria).

    • Fatty acid oxidation yields NADH/QH2.

    • Reduced cofactors transfer electrons to the respiratory electron transport chain.

    The Malate-Aspartate Shuttle System

    • A system for transferring "reducing equivalents" (electrons) to the matrix.

    Mitochondrial Transport System for ATP, ADP and Pi

    • ATP translocase: Binds ATP/ADP and changes conformation to release nucleotide on opposite sides of the inner mitochondrial membrane.

    Summary of Mitochondrial Electron Transport

    • The electron transport chain is associated with the mitochondrial inner membrane.

    Complex I Transfers Electrons from NADH to Ubiquinone

    • Complex I is the largest electron transport protein in the mitochondrial respiratory chain.

    • Electron transport occurs in the peripheral arm of Complex I, via prosthetic groups/redox centers.

    • Electrons travel to a next redox center with an increasing reduction potential.

    Flavin Mononucleotide (FMN)

    • The first electron acceptor for NADH in Complex I is FMN.

    • This transfers electrons to a type of redox center, an iron-sulfur (Fe-S) cluster.

    Iron-Sulfur Clusters

    • These are one-electron carriers.

    • Electrons travel between Fe-S clusters before reaching ubiquinone.

    Complex I Function

    • As electrons are transferred from NADH to ubiquinone, Complex I transfers four protons from the matrix to the intermembrane space via a proton wire.

    Reactions That Contribute to the Ubiquinol Pool

    • The reduced quinone product of reactions joins a pool of quinones.

    • The pool of reduced quinones is augmented by other oxidation-reduction reactions.

    • One such reaction is catalyzed by succinate dehydrogenase (Complex II).

    Complex III Transfers Electrons from Ubiquinol to Cytochrome c

    • Ubiquinol is reoxidized by complex III (ubiquinol:cytochrome c oxidoreductase or cytochrome bc1).

    • Cytochromes are proteins with heme prosthetic groups.

    • The flow of electrons through complex III is complicated. Two electrons must split up, going through a series of one-electron carriers.

    The Heme Group of a b Cytochrome

    • Heme groups of cytochromes undergo reversible one-electron reduction/oxidation. The central Fe atom cycles between Fe3+ (oxidized) and Fe2+ (reduced) states.

    Cytochromes

    • Cytochromes are proteins with heme prosthetic groups. Heme in cytochrome c undergoes reversible one-electron transfers.

    Structure of Mammalian Complex III

    • The complex has eight transmembrane helices in each monomer.
    • Includes iron-sulfur protein and cytochrome c1.
    • Two heme groups from cytochrome b and the heme group of cytochrome c1 form a pathway for electron transfers from ubiquinol to cytochrome c.

    The Q Cycle

    • QH2 donates one electron to the iron-sulfur protein and the second electron to cytochrome b, releases 2 protons.
    • The first electron travels to cytochrome c1 and then cytochrome c.
    • The oxidized ubiquinone accepts an electron to form half-reduced semiquinone (Q−).
    • The second cycle happens, and 2 more protons are released in the intermembrane space. Two electrons from QH2 reduced two cytochrome c molecules.

    Complex III Function

    • QH2 transfers electrons to ubiquinone, resulting in reoxidation of ubiquinone and the release of four protons into the intermembrane space.

    Cytochrome c

    • Cytochrome c transfers electrons one at a time between Complexes III and IV.
    • Four electrons are consumed to reduce molecular oxygen to water, and four protons are relayed from the matrix.

    Structure of Complex IV (Cytochrome c Oxidase)

    • The complex comprises 13 subunits. Includes heme groups and copper ions.
    • Shown in space-filling form.

    Complex IV Function

    • For every two electrons donated by cytochrome c, two protons are translocated in the intermembrane space, with O2 reduced to H2O.

    Review

    • Describe the components of a mitochondrion.
    • List transport proteins in the inner mitochondrial membrane.
    • Draw a diagram of electron-transport complexes and mobile carriers.
    • List different redox group types and identify as one- or two-electron carriers.
    • Explain why O2 is the final electron acceptor in the chain.
    • Describe proton wire operation.
    • Write equations to describe overall redox reactions carried out by each mitochondrial complex.
    • Compare the arrangement of an ETC and a supercomplex.

    Section 15.3 Chemiosmosis

    • Explain how the protonmotive force links electron transport and ATP synthesis.
    • Describe the formation of the proton gradient.
    • Relate the pH difference of the proton gradient to free energy change.

    How Much Energy is Available from Electron Transport?

    • The standard reduction potentials are calculated for Complexes I, III, and IV. Each complex releases a significant quantity of energy. The total amounts to −218.2 kJ mol-1 for the process NADH → O2.

    Generation of a Proton Gradient

    • The proton-translocating activity of ETC complexes generates a proton gradient across the membrane.
    • The proton gradient's energy drives ATP synthase.
    • Protons cannot diffuse back into the matrix because the membrane is impermeable to ions. This creates the protonmotive force.

    Protonmotive Force for Driving Phosphorylation of ADP

    • The free energy from proton passage back into the matrix is insufficient to drive ADP phosphorylation. However, the combined total of 10 protons translocated released 218.2 kJ mol-1 from NADH to O2 is enough.

    Review

    • Describe the importance of mitochondrial structure for generating the protonmotive force.
    • Identify proton sources for the transmembrane gradient.
    • Explain why the proton gradient has both a chemical and an electrical component.

    Section 15.4 ATP Synthase

    • Describe the structure and operation of ATP synthase;
    • Recognize the structural components of ATP synthase.
    • Identify the energy transformations that occur in ATP synthase;
    • Describe the binding change mechanism.
    • Explain why P:O ratios are non-integral.
    • Explain why oxidative phosphorylation is coupled to electron transport.

    Oxidative Phosphorylation

    • Protons move back into the matrix, using the energy stored in the electrochemical gradient to make ATP. The chemiosmotic hypothesis.

    Components of ATP Synthase

    • The protein that uses the electrochemical proton gradient is F-ATP synthase (or Complex V). The Fo part functions as a transmembrane channel, allowing protons to flow back into the matrix.
    • The F₁ component catalyzes the reaction ADP+P = ATP+H₂O

    Structure of ATP Synthase

    • Proton transport requires rotation of the c ring past the stationary a subunit.
    • The carboxylate side chain of conserved aspartate or glutamate residues acts as a proton binding site on each c subunit.

    Mechanism of Proton Transport by ATP Synthase

    • The c subunit picks up a proton from the intermembrane space and the binding neutralizes the carboxylate group, freeing it from the positively charged arginine residue.
    • The protonated c subunit moves away, and a slight rotation of the c ring positions another c subunit for proton uptake.

    Structure of the F1 Component of ATP Synthase

    • The three αβ pairs change their conformations as the y subunit rotates.
    • As each proton moves across the membrane, the c ring and Y subunit rotate.
    • The αβ hexamer doesn't rotate.

    Production of ATP

    • ATP synthase uses mechanical energy (rotation) to attach a phosphoryl group to ADP.
    • Rotation-driven conformational changes alter the affinity of the catalytic β subunits for adenine nucleotides.

    Regulation of ATP Synthase in Eukaryotes

    • In eukaryotes, ATP synthase is regulated by inhibitory factor 1 (IF1).
    • Different forms of IF1 are intrinsically disordered at high matrix pH values when the electron transport chain is pumping protons into the intermembrane space.

    ATP Synthase Dimers

    • When the pH drops, IF1 dimerizes, forming extended alpha helices that insert between the a and b subunits of F1 and contact the y shaft. Binding prevents ATP synthase from carrying out the binding change mechanism.

    Powering Human Muscles

    • Cells cannot stockpile ATP.
    • Phosphocreatine plays a role. Different activity levels require different energy systems.

    Babies Can't Shiver

    • Brown fat deposits burn energy to create heat and maintain body temperature.
    • Cells are packed with iron-rich mitochondria.
    • Localized in the neck, shoulders, upper arms, spine, and tummy.
    • Overlaps blood vessels.

    Where Else Does Uncoupling Happen?

    • Hibernating mammals have enhanced respiration rates via the various complexes of the ETC during hibernation.

    2,4-Dinitrophenol (DNP)

    • Initially used for weight loss (due to uncoupling).
    • But caused severe health problems and was banned in the US. DNP disrupts the proton gradient, preventing ATP production.

    Review

    • Draw a diagram of ATP synthase, indicating stationary and rotating parts.
    • Explain how ATP synthase dissipates the proton gradient, the three conformational states of the ß subunits and their role in ATP synthesis, and how it could operate in reverse to hydrolyze ATP.
    • Explain varied proton translocation numbers per ATP synthesized across species.
    • Discuss the availability of reduced substrates as a primary mechanism for regulating oxidative phosphorylation.

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    Description

    Test your understanding of oxidative phosphorylation and its thermodynamic principles. This quiz covers key concepts like electron transport pathways, the operation of ATP synthase, and the relationship between the protonmotive force and ATP synthesis. Prepare to explore the intricacies of energy harvesting in living organisms.

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