The Citric Acid Cycle Overview
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Questions and Answers

Which complex in the electron transport chain directly passes electrons from NADH to CoQ?

  • Complex III
  • Complex I (correct)
  • Complex IV
  • Complex II

Complex II is involved in the transfer of electrons from FADH2 to CoQ.

True (A)

What is the role of coenzyme Q (CoQ) in the electron transport chain?

To collect and transfer electrons from Complex I and II to Complex III.

The enzyme __________ catalyzes the conversion of succinate into fumarate in the citric acid cycle.

<p>succinate dehydrogenase</p> Signup and view all the answers

What is a characteristic of the standard reduction potential of electrons transferred from succinate to CoQ?

<p>Insufficient to drive ATP synthesis (C)</p> Signup and view all the answers

Cytochrome C is an integral part of Complex I.

<p>False (B)</p> Signup and view all the answers

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

<p>To transfer electrons from coenzyme Q to cytochrome c.</p> Signup and view all the answers

Match the following complexes with their respective electron donors:

<p>Complex I = NADH Complex II = FADH2 Complex III = CoQ Complex IV = Cytochrome c</p> Signup and view all the answers

Which molecules transfer electrons between respiratory complexes?

<p>Coenzyme Q and cytochrome c (D)</p> Signup and view all the answers

NADH can participate in one-electron transfer.

<p>False (B)</p> Signup and view all the answers

What is the final electron acceptor in the electron transport chain?

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

The oxidation states of FMN and CoQ allow them to accept and donate ________ electrons.

<p>one or two</p> Signup and view all the answers

Match the following proton carriers with their characteristics:

<p>NADH = Two-electron donor FMN = One or two-electron acceptor CoQ = Electron conduit between two-electron donor and one-electron acceptors Cytochrome C = One-electron acceptor</p> Signup and view all the answers

Which statement about cytochromes is correct?

<p>They alternate between Fe(II) and Fe(III) oxidation states during electron transport. (A)</p> Signup and view all the answers

Iron-sulfur clusters differ by more than one formal charge based on the number of Fe atoms.

<p>False (B)</p> Signup and view all the answers

What influences the sequence of electron transport in the chain?

<p>Relative affinity for electrons</p> Signup and view all the answers

Cytochrome c is a membrane-bound protein that cannot move freely.

<p>False (B)</p> Signup and view all the answers

What hypothesis explains how electron transfer leads to ATP synthesis?

<p>The chemiosmotic hypothesis.</p> Signup and view all the answers

The process by which Complex III produces two molecules of reduced cytochrome c is called the ________ cycle.

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

Match the following components with their functions in the electron transport chain:

<p>Complex I = Transfers electrons from NADH to CoQ Complex II = Transfers electrons from succinate to CoQ Complex III = Transports electrons to cytochrome c Complex IV = Reduces oxygen to water</p> Signup and view all the answers

Which of the following statements about the proton motive force is true?

<p>It is a combination of membrane potential and H+ concentration difference. (D)</p> Signup and view all the answers

Complex IV acts as a monomer and contains six subunits.

<p>False (B)</p> Signup and view all the answers

The primary role of electron transport chain proteins is to transfer ________ and facilitate ________ transport.

<p>electrons, proton</p> Signup and view all the answers

Flashcards

Electron Transport Chain

A series of protein complexes that transfer electrons from electron donors to electron acceptors, releasing energy for ATP synthesis.

Electron Donors (NADH + H+ and FADH2)

Molecules that provide electrons to the electron transport chain.

Final Electron Acceptor

The molecule that accepts the electrons at the end of the electron transport chain.

Iron-Sulfur Clusters

Protein components in the electron transport chain that facilitate electron transfer.

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Cytochromes

Proteins in the electron transport chain containing heme groups that carry electrons by changing Fe oxidation states.

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FMN and CoQ

Electron carriers capable of one or two electron transfer, connecting NADH to cytochromes.

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Reduction Potential

Measure of a molecule's affinity for electrons, determining electron flow order.

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Ubiquinone (UQ)

A small molecule that moves electrons between respiratory complexes in the electron transport chain.

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Complex III

A protein complex in the electron transport chain that accepts electrons from CoQ and transfers them to cytochrome c.

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Q Cycle

A two-step process in Complex III where two electrons from CoQ are transferred to cytochrome c, using two separate pathways.

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Complex IV (Cytochrome c Oxidase)

The final protein complex in the electron transport chain that accepts electrons from cytochrome c and reduces molecular oxygen to water.

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Chemiosmotic Hypothesis

Theory explaining how ATP is generated from the energy of electron transport by creating a proton gradient across the inner mitochondrial membrane.

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Proton Motive Force

The energy stored as a combination of the chemical gradient of protons and the electrical potential across the mitochondrial membrane.

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How is ATP Synthesized?

The proton motive force drives ATP synthesis by allowing protons to flow back across the inner mitochondrial membrane through ATP synthase, which converts this energy into ATP.

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Electron flow direction

Electrons flow downhill through the electron transport chain, releasing energy used to pump protons uphill, creating the proton motive force.

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Electron Transport Chain (ETC)

A series of protein complexes embedded in the inner mitochondrial membrane that transfer electrons from NADH and FADH2 to oxygen, generating a proton gradient for ATP synthesis.

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CoQ (Ubiquinone)

A small, mobile molecule in the inner mitochondrial membrane that acts as an electron carrier, accepting them from Complex I and II and passing them to Complex III.

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Standard Reduction Potential

A measure of a molecule's tendency to gain electrons, measured under standard conditions.

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ATP Synthesis Coupling

The energy released from electron transfer in the ETC is used to pump protons across the membrane, creating a gradient that drives ATP synthesis.

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Proton Gradient

The difference in proton concentration across the inner mitochondrial membrane, created by the ETC and used to generate ATP.

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Study Notes

The Citric Acid Cycle (CAC)

  • Also known as the Krebs cycle or TCA cycle
  • A series of 8 chemical reactions
  • Releases stored energy through the oxidation of acetyl coenzyme A (acetyl-CoA) derived from carbohydrates, fats, and proteins
  • Catabolic process, involving degradation
  • Major free energy conservation system in most organisms
  • Amphibolic (both catabolic and anabolic), as some pathways use intermediates for biosynthesis
  • Ancient component of metabolism
  • Enzymes localized in the mitochondrial matrix (except succinate dehydrogenase, which is in the inner mitochondrial membrane)

Citric Acid Cycle Functions

  • Accounts for the major portion of carbohydrate, fatty acid, and amino acid oxidation
  • Generates precursors for the biosynthesis of fatty acids, steroids, porphyrins, purine and pyrimidine nucleotides, and certain amino acids

Reactions of the Cycle

  • Citrate Synthase: Condensation of acetyl-CoA and oxaloacetate to yield citrate
  • Aconitase: Rearranges citrate to isocitrate
  • Isocitrate Dehydrogenase: Oxidative decarboxylation of isocitrate to α-ketoglutarate, producing the cycle's first COâ‚‚ and NADH
  • α-Ketoglutarate Dehydrogenase Complex: Oxidative decarboxylation of α-ketoglutarate to succinyl-CoA, producing a second COâ‚‚ and NADH.
  • Succinyl-CoA Synthetase: Conversion of succinyl-CoA to succinate; a substrate-level phosphorylation reaction forming GTP from GDP and Pi
  • Succinate Dehydrogenase: Oxidation of succinate to fumarate, reducing FAD to FADHâ‚‚. This enzyme is embedded in the inner mitochondrial membrane
  • Fumarase: Hydration of fumarate to malate
  • Malate Dehydrogenase: Oxidation of malate to oxaloacetate, producing NADH

Net Result of Cycle

  • Per Acetyl-CoA molecule: 2 CO2, 3 NADH, 1 FADH2, 1 GTP (converted to ATP)

Regulation of the Citric Acid Cycle

  • Complex regulation; rate-controlling enzymes include citrate synthase, isocitrate dehydrogenase, and α-ketoglutarate dehydrogenase

Electron Transport and Oxidative Phosphorylation

  • Catabolism of carbohydrates, lipids, and proteins feeds into the CAC
  • CAC is the major source of reduced electron acceptors NADH and FADHâ‚‚.
  • Final step under aerobic conditions is oxidative phosphorylation
  • During oxidative phosphorylation, electrons from NADH and FADHâ‚‚ are passed through a series of electron carriers in the inner mitochondrial membrane, ultimately reducing oxygen to water.

Electron Transport Chain (ETC) Components

  • Four respiratory complexes embedded in the inner mitochondrial membrane

  • Associated with various redox-active prosthetic groups (e.g., iron-sulfur clusters, heme groups) with increasing reduction potential.

  • Three complexes (I, III, IV) actively pump protons across the inner mitochondrial membrane from the matrix to the intermembrane space

  • Coenzyme Q (Ubiquinone) and cytochrome c relay electrons between the complexes

  • Electrons flow from NADH/FADH2 to oxygen

  • Two electrons from NADH or FADH2 yield 2.5 or 1.5 ATP.

Chemiosmotic Hypothesis

  • Electron flow through the ETC creates a proton gradient (proton motive force) across the inner mitochondrial membrane.
  • The electrochemical potential (proton-motive force) provides the energy necessary for ATP synthesis during oxidative phosphorylation
  • Protons flow back into the matrix through ATP synthase, driving ATP synthesis.

ATP Synthase

  • Enzyme embedded in the inner mitochondrial membrane
  • Fo subunit forms a proton channel, and F1 subunit is the enzymatic core
  • Rotation of Fo subunit causes conformational changes in F1 subunit leading to ATP synthesis from ADP and Pi

Regulation of Oxidative Phosphorylation

  • Regulation is linked to the cell's need for ATP, typically regulated by ADP/ATP ratios
  • High ADP stimulates pathway activity; low ADP slows down the process. This is often referred to as acceptor control

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Citric Acid Cycle & Carbohydrate Metabolism PDF

Description

This quiz covers the essential aspects of the Citric Acid Cycle, also known as the Krebs cycle or TCA cycle. You'll explore the series of reactions, their functions in energy conservation, and the role of various enzymes involved. Test your understanding of this crucial metabolic pathway.

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