Electron Transport Chain and Cellular Respiration

Electron Transport Chain (ETC) Overview

• Most components of the electron transport chain are proteins, with some soluble components involved.
• Key soluble component: Ubiquinone (Coenzyme Q)
• A lipid-soluble compound that acts as an electron carrier.
• Transfers electrons between protein complexes in a taxi-like manner.

Redox Potential and Electron Acceptors

• Different electron acceptors have varying redox potentials, which influence their ability to accept electrons.
• Electrons flow from carriers with lower affinity to those with higher affinity in the ETC, promoting spontaneous electron transfer.

Organization of the Electron Transport Chain

• The ETC is organized into four main protein complexes (I to IV):

1. Complex I: NADH dehydrogenase
   • Accepts electrons from NADH, oxidizing it to regenerate NAD⁺.
   • Pumps protons from the mitochondrial matrix into the intermembrane space, creating a proton gradient.
2. Complex II: Succinate dehydrogenase
   • Interacts with FADH₂ from the citric acid cycle, transferring electrons to ubiquinone.
   • Does not pump protons across the membrane.
3. Complex III: Cytochrome c reductase
   • Accepts electrons from ubiquinone and passes them to cytochrome c.
   • Pumps four protons into the intermembrane space for every two electrons transferred.
4. Complex IV: Cytochrome c oxidase
   • Accepts electrons from cytochrome c and reduces oxygen to form water.
   • Also pumps protons into the intermembrane space.

Free Energy Changes

• As electrons move through the complexes, their free energy decreases:
• Electrons start at a higher energy level in NADH and end at a much lower energy level when accepted by oxygen.
• The energy released during these transfers is used to pump protons across the membrane, facilitating ATP production.

The Proton Gradient

• The proton gradient established in the intermembrane space creates an electrochemical gradient.
• Protons flow back into the matrix through ATP synthase, driving ATP production via oxidative phosphorylation.

ATP Synthase Mechanism

• ATP Synthase functions as a molecular machine:
• Protons flow through ATP synthase, inducing a rotational motion that helps attach a phosphate group to ADP, generating ATP.
• The rotational movement is driven by the transmembrane proton gradient.

Summary of Inputs and Outputs

• Inputs of the ETC:

• Electrons from NADH and FADH₂.

• Oxygen as the final electron acceptor.

• Outputs of the ETC:

• Water (as a byproduct).

• A proton gradient across the inner mitochondrial membrane.

• Regeneration of NAD⁺ and FAD.

Importance of Aerobic vs Anaerobic Respiration

• Aerobic Respiration:

• More efficient due to the use of oxygen as the final electron acceptor.

• Produces about 29-38 ATP per glucose molecule.

• Anaerobic Respiration:

• Uses alternative electron acceptors, resulting in less ATP production.

• Fermentation:

• Allows for regeneration of NAD⁺ in the absence of oxygen, enabling glycolysis to continue.

• Efficient for short-term energy but yields only 2 ATP per glucose.

Types of Fermentation

1. Lactic Acid Fermentation:

• Muscles perform this under low oxygen conditions, producing lactate and NAD⁺.

1. Alcoholic Fermentation:

• Yeast and some bacteria convert pyruvate to ethanol and CO₂.

Conclusion

• Oxidative phosphorylation is critical for ATP production in cellular respiration, primarily through the establishment and utilization of a proton gradient. Aerobic conditions are optimal for efficiency, while anaerobic processes present alternative survival strategies for organisms in oxygen-poor environments.