MCB 150 Lecture 11: Cellular Respiration

Announcements

• No lecture or student hours on Friday of this week.

  • Pre- and post-class questions will be held as usual.

  • A lecture recording from Fall 2024 covers the content.

• Review sessions for exam questions in 200 Burrill Hall: - Today: 3:30-5:00 PM

  • Tuesday: 10:30-Noon

  • Thursday: 9:30-11:00 AM

Phase 2: Pyruvate Oxidation and Krebs Cycle

• The lecture transitions to discussing the later stages of cellular respiration, specifically Pyruvate Oxidation and the Krebs Cycle (also known as the Citric Acid Cycle).

Net Results of Glycolysis and Krebs Cycle

• The lecture will likely cover the net results of Glycolysis (Phase 1) and the Krebs Cycle combined (Phase 2), detailing the outputs in terms of ATP, NADH, and FADH2 for every glucose molecule processed.

Problems at the End of Krebs Cycle

• The lecture brings up the issues remaining at the conclusion of the Krebs Cycle:

  1. NAD+ Regeneration: The cycle hasn't yet replaced the NAD+ consumed; instead, more NADH has been produced.

     • NAD+ is essential for glycolysis and the Krebs cycle to continue, as it acts as an oxidizing agent, accepting electrons during several key reactions.

     • The regeneration of NAD+ ensures the continuation of these energy-producing pathways.

  2. FADH2 Re-oxidation: The generated FADH2 needs to be re-oxidized.

     • FADH2, like NADH, is a crucial electron carrier produced during the Krebs Cycle. Its re-oxidation is necessary for the electron transport chain to function.

     • This process helps in transferring electrons to the electron transport chain, contributing to the proton gradient formation.

  3. Energy Transfer to ATP: The energy held by NADH and FADH2 hasn't been effectively transferred to ATP.

     • The primary goal of cellular respiration is to convert the energy stored in glucose into ATP, the cell's energy currency.

     • NADH and FADH2 store high-energy electrons, which must be used to create ATP through oxidative phosphorylation.

• Oxygen Dependence: It addresses that aerobic respiration requires oxygen, even though the Krebs Cycle itself doesn't directly use oxygen.

  • Oxygen serves as the final electron acceptor in the electron transport chain. Without oxygen, the electron transport chain would halt, and ATP production would drastically decrease.

• Coupling to the Electron Transport Chain: The necessity of oxygen is due to the Krebs Cycle's linkage with the Electron Transport Chain (ETC), which is an oxygen-dependent process.

  • The electron transport chain is where most of the ATP is generated. It relies on oxygen to efficiently move electrons and pump protons, creating the electrochemical gradient needed for ATP synthase.

Oxidative Phosphorylation

• The lecture introduces oxidative phosphorylation, which includes the electron transport chain and chemiosmosis.

The Electron Transport Chain (ETC)

• NADH Oxidation: NADH donates its electrons to the first carrier in the ETC, regenerating NAD+.

  • NADH carries high-energy electrons from glycolysis and the Krebs cycle to the ETC.

  • By donating these electrons, NADH becomes NAD+, which can then return to glycolysis and the Krebs cycle to pick up more electrons.

  • NAD+ is now free to participate in further redox reactions.

• Electron Transfer and Energy Release: Electrons are passed sequentially between carriers at progressively lower energy levels.

  • As electrons move from one carrier to another, they release energy.

  • The released energy is used to pump protons across the membrane, creating a proton gradient (electrochemical gradient).

• Final Electron Acceptor: The last electron carrier transfers electrons to oxygen, which then combines with protons to form water (H_2O).

  • Oxygen's high electronegativity makes it an ideal final electron acceptor, pulling electrons through the ETC.

  • This step accounts for the production of water in cellular respiration.

• FADH2 Contribution: FADH2 also contributes electrons to the ETC, but at a later point (bypassing Complex I).

  • FADH2 delivers electrons to Complex II, bypassing Complex I.

  • This results in fewer protons being pumped across the membrane, reducing its contribution to the electrochemical gradient.

Energy Levels of Electron Carriers

• The lecture describes the successive drop in energy levels as electrons move through the electron transport chain.

Result of Electron Transport Chain

• The primary result of the ETC is the regeneration of cofactors (NAD+ and FAD) and the establishment of a proton gradient, but no ATP is directly produced at this stage.

  • The proton gradient is crucial for chemiosmosis, the process that directly drives ATP synthesis.

Proton Gradient

• The synthesized proton gradient is unstable, driving protons to flow back into the mitochondrial matrix.

  • The high concentration of protons in the intermembrane space creates a strong electrochemical gradient.

  • Protons flow back into the matrix through ATP synthase, an enzyme that uses the energy from this flow to synthesize ATP.

ATP Production

• All the energy derived from glucose breakdown (minus any energy lost as heat) is ultimately used to synthesize ATP:

  Glucose + 6O2 \rightarrow 6CO2 + 6H_2O

• Theoretical ATP Yield: Calculation of theoretical ATP production from full oxidation of one glucose molecule: - Glycolysis:

  • 2 ATP - 2 \text{ ATP}

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    • 2 NADH (x 2 ATP each) - 4 \text{ ATP}

  • Krebs Cycle: - 2 GTP (≡ ATP) - 2 \text{ ATP}

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    • 8 NADH (x 3 ATP each) - 24 \text{ ATP}

    • 2 FADH2 (x 2 ATP each) - 4 \text{ ATP}

  • Total (theoretical): - 36 \text{ ATP per Glucose molecule}