Oxidative Phosphorylation Overview

Mitochondrial Structure and Function

• Mitochondria Overview
  • Mitochondria are the cellular powerhouse responsible for ATP production.
  • Essential for cellular respiration and energy conversion.

Key Concepts

• Chemiosmosis

  • A process where energy from the electron transport chain (ETC) is used to pump protons (H+) across the inner mitochondrial membrane, creating a proton gradient that drives ATP synthesis.

• Electron Transport Chain (ETC)

  • A series of protein complexes (complexes I-IV) located in the inner mitochondrial membrane that facilitate electron transfer from NADH and FADH2 to oxygen, leading to ATP synthesis.

Mitochondrial Structure

• Outer Membrane:
  • Permeable to small molecules and ions.
• Inner Membrane:
  • Site of the ETC and ATP synthase; less permeable, containing transport proteins.
• Intermembrane Space:
  • Area between the inner and outer membranes where protons are pumped.
• Matrix:
  • Contains enzymes for the citric acid cycle and the mitochondrial DNA.

Learning Objectives

• Understand the purpose of chemiosmotic coupling during oxidative phosphorylation.
• Outline the flow of electrons through the electron transport chain.
• Analyze the function of ATP synthase in ATP production.

Stages of ATP Production

1. Electron Transport Chain

   • NADH and FADH2 transfer electrons through four complexes:
     • Complex I: NADH dehydrogenase (transfers electrons to ubiquinone - CoQ and pumps protons).
     • Complex II: Succinate dehydrogenase (FADH2 entry point).
     • Complex III: Cytochrome C reductase (exchanges electrons with cytochrome C).
     • Complex IV: Cytochrome C oxidase (final electron transfer to oxygen).

2. Chemiosmotic Coupling

   • Protons are pumped into the intermembrane space creating a proton-motive force (PMF).
   • PMF drives ATP production via ATP synthase.

Role of ATP Synthase

• ATP Synthase Mechanism
  • Functions as a molecular turbine powered by the movement of protons back into the matrix, catalyzing the phosphorylation of ADP to ATP.
  • Overall, it synthesizes 3 ATP molecules per cycle.

Electron Carriers and ATP Yield

• NADH:

  • From citric acid cycle produces 2.5 ATP.
  • Transfers electrons to Complex I.

• FADH2:

  • From glycolysis produces 1.5 ATP due to mitochondrial import.
  • Transfers electrons to Complex II, resulting in fewer protons pumped (only 2 H+) compared to NADH.

Summary of Oxidative Phosphorylation

1. Involves catabolic oxidation of food molecules to synthesize ATP from ADP (phosphorylation).
2. Activated carriers shuttle electrons to the ETC to pump protons:
   • NADH -> Complex I (3 H+ pumped).
   • FADH2 -> Complex II (2 H+ pumped).
3. Electron flow: carrier -> entry complex -> CoQ -> CytC -> O2.
4. Proton pumping results in a steep electrochemical gradient across the inner mitochondrial membrane.
5. The chemiosmotic H+ gradient (PMF) drives ATP synthase for ATP production.