ELECTRON TRANSPORT CHAIN - CHAPATER 7

Overview of Cellular Respiration

  • Cellular Respiration: A biochemical process that converts food into energy (ATP) in cells, utilizing oxygen and producing carbon dioxide.

Citric Acid Cycle (Krebs Cycle)

  • Products Generated:

    • Produces 3 NADH and 1 FADH2 per cycle.

    • High energy electrons are carried by NADH and FADH2.

Electron Transport Chain (ETC)

  • Function:

    • Transfers electrons from NADH and FADH2 through a series of protein complexes in the inner mitochondrial membrane.

Complexes in the ETC

  • Complex I, II, III, IV:

    • Complex I: Accepts electrons from NADH.

    • Complex II: Receives electrons from FADH2.

    • Complex III and IV: Continue the electron transfer process.

  • Coenzyme Q (CoQ):

    • Organic molecule that carries electrons between Complex I/II and Complex III.

  • Cytochrome C:

    • Non-protein electron carrier in the membrane.

Electron Transfer Process

  • NADH delivers electrons to Complex I, which passes them through various carriers:

    • Pathway: NADH -> Complex I -> CoQ -> Complex III -> Cytochrome C -> Complex IV -> Oxygen (O2) to make water.

Oxygen's Role

  • Final Electron Acceptors:

    • Oxygen receives electrons from Complex IV, combining with protons to form water (H2O).

    • Essential for completing the electron transport, preventing backup in the chain.

Proton Motive Force (PMF)

  • Mechanism:

    • The movement of electrons through the ETC also pumps protons (H+) into the intermembrane space, creating a concentration gradient.

    • This gradient stores energy known as proton motive force.

  • Analogy:

    • Similar to smells diffusing in a room; protons want to diffuse back across the membrane to reach equilibrium.

ATP Synthesis via ATP Synthase

  • ATP Synthase:

    • Consists of two parts (F0 and F1) and is driven by PMF.

    • Protons flow back through ATP synthase, catalyzing the formation of ATP from ADP and inorganic phosphate.

Chemiosmosis

  • Definition:

    • The process of using a proton gradient to drive ATP synthesis is termed chemiosmosis, part of oxidative phosphorylation.

Contributions from NADH and FADH2

  • NADH:

    • Conducts electrons through 3 proton pumping sites, leading to synthesis of approximately 3 ATP per NADH.

  • FADH2:

    • Directs electrons through 2 pumping sites, producing about 2 ATP per FADH2.

Summary of Overall Yield from Glucose Metabolism

  • Total Products per Glucose:

    • Glycolysis:

      • 2 ATP (net)

      • 2 NADH

      • 0 FADH2

      • 0 CO2

    • From Pyruvate to Acetyl CoA:

      • 2 NADH (1 per pyruvate)

      • 2 CO2 (1 per pyruvate)

    • Citric Acid Cycle (for 2 cycles):

      • 2 ATP (GTP)

      • 6 NADH

      • 2 FADH2

      • 4 CO2

  • Total Yield:

    • Combining outputs leads to a theoretical maximum of 38 ATP from one glucose molecule.

Mitochondrial Structure and Function

  • Inner Membrane:

    • Highly folded to increase surface area, allowing for more electron complexes and ATP synthase.

  • Outer Membrane:

    • More permeable, allowing easier passage of small molecules.

Implications of Oxygen Availability

  • Aerobic vs Anaerobic Respiration:

    • Organisms can adapt to survive without oxygen, using fermentation pathways to regenerate NAD from NADH to allow glycolysis to continue.

    • Example: Lactic Acid Fermentation:

      • Occurs in muscle cells during intense exercise, converting pyruvate to lactic acid to regenerate NAD.

Reactive Oxygen Species (ROS)

  • Danger of ROS:

    • Can damage proteins, lipids, and DNA if excess free radicals are produced during mitochondrial processes, especially at Complex IV.

Treatment and Damage Control

  • Antioxidants:

    • Dietary antioxidants (e.g., Vitamin E) are believed to mitigate damage from ROS, although the body can handle some ROS through its own stabilization mechanisms.

Final Thoughts

  • Understanding the intricacies of cellular respiration highlights how energy is utilized and conserved within biological systems.