Cellular Respiration - Electron Transport Chain and Chemiosmosis Notes

Overview of Electron Transport Chain and Chemiosmosis

Summary of Previous Stages of Cellular Respiration

  • Oxidation of Glucose

    • Energy extracted from glucose, a carbon-based molecule.

    • Carbons released as carbon dioxide (CO₂) when fully oxidized.

    • Two of six carbons lost during pyruvate oxidation; four lost in Krebs cycle (TCA cycle).

  • Energy Carriers Generated

    • Formation of ATP; not the primary energy currency extracted (only a little produced).

    • High-energy electron carriers formed: NADH and FADH₂.

    • These carriers transport high-energy electrons to the electron transport chain (ETC) in mitochondria.

Electron Transport Chain (ETC) and Chemiosmosis

  • Location

    • Process occurs across the inner mitochondrial membrane.

    • Protein complexes embedded in the membrane essential for ETC and chemiosmosis.

  • Function of the ETC

    • Components

      • Four major protein complexes: Complex I, Complex II, Complex III, and Complex IV.

    • Process

      • High-energy electrons are supplied by NADH (from Complex I) and FADH₂ (from Complex II).

      • Complex I oxidizes NADH to regenerate NAD⁺, crucial for continued cellular respiration.

        • Electrons from NADH are transferred to Complex I, producing low-energy form NAD⁺ and releasing low-energy hydrogen ions (protons).

      • Electrons pass from Complex I to Complex II then to Complex III.

      • Complex II handles FADH₂ only, does not pump protons.

        • Complex III and Complex IV pump protons into the intermembrane space, creating a proton gradient.

        • High proton concentration in the intermembrane space, low concentration in the mitochondrial matrix.

    • Oxygen as Final Electron Acceptor

      • Low-energy electrons are ultimately delivered to oxygen at Complex IV.

      • Oxygen combines with protons to form water (H₂O), the final waste product of the ETC.

      • Emphasize: No carbon-containing molecules are processed in the ETC; focus is solely on electrons from NADH and FADH₂.

Importance of the Proton Gradient

  • Proton Gradient

    • Created by pumping protons from the mitochondrial matrix to the intermembrane space.

    • High concentration of protons represents a form of potential energy.

Chemiosmosis

  • ATP Synthase Functionality

    • ATP synthase catalyzes the production of ATP using the proton gradient created by the ETC.

      • F0 Domain: Embedded in the inner mitochondrial membrane; allows protons to flow through, causing rotation.

      • F1 Domain: Projects into the mitochondrial matrix; facilitates ATP synthesis from ADP and inorganic phosphate (Pi).

  • Process

    • Proton movement drives the rotation of F0, which spins the central stalk of ATP synthase.

    • Mechanical energy from the spinning converts ADP and inorganic phosphate to ATP.

  • Analogy

    • Similar to a water wheel in a dam: water flows down, turning the wheel; energy released is harnessed to perform work (ATP synthesis).

Chemical Processes in ATP Synthase

  • Mechanism of ATP Production

    • Proton flow down the gradient alters conformational states of the F1 domain.

    • Changes occur in the binding sites to facilitate the formation of ATP from ADP + Pi.

  • Cycle

    • Protons released back into the mitochondrial matrix post ATP synthesis, allowing the process to continue.

Summary of ATP Production

  • Net ATP Yield per Glucose

    • Glycolysis: 2 ATP

    • Pyruvate Oxidation: 2 NADH

    • Citric Acid Cycle: 6 NADH and 2 FADH₂

    • Electron Transport and Chemiosmosis: Theoretical yield of 30-36 ATP, with real-world results closer to 30 ATP due to inefficiencies and energy costs of transport (e.g., mitochondrial import).

Alternate Pathways under Anaerobic Conditions

  • Consequences of Oxygen Deficiency

    • In the absence of oxygen, ETC halts; electrons get stuck, NADH and FADH₂ build up, and NAD⁺ becomes scarce.

    • This leads to shutdown of glycolysis, citric acid cycle, and pyruvate oxidation due to lack of NAD⁺.

    • Results in excessive pyruvate which may be converted to lactate or undergo fermentation to regenerate NAD⁺ for glycolysis.

      • Lactic acid fermentation (done in muscle cells) vs. alcoholic fermentation (done by yeast).

Overall Implication

  • Feedback Mechanisms

    • Regulation of metabolic pathways via feedback inhibition (e.g., accumulation of ATP can inhibit key enzymes to slow down cellular respiration).

    • Ensures homeostasis and a balance of energy use and cellular needs.

Final Notes

  • The process of metabolic pathways (anabolism vs. catabolism).

  • Importance of understanding how different substrates feed into these systems (e.g., fatty acids into acetyl CoA).

  • Efficiencies and Energetics

    • Only ~36% efficiency in converting glucose energy to ATP; energy loss primarily as heat.

    • Discuss scenarios of loss, e.g. heart attack situations affecting cellular respiration due to lack of oxygen delivery.

  • Conclusion

    • Importance of cellular respiration in life-sustaining processes and the relevance of pathways for biochemical regulation and energy production.