Introduction
Oxidative phosphorylation is a critical biochemical process that takes place in the mitochondria, enabling cells to produce ATP, the primary energy currency. This process involves the electron transport chain (ETC) and chemiosmosis, both of which work together to generate large amounts of ATP from the oxidation of nutrients.
Mitochondrial Structure
- Outer Membrane: Permeable to small molecules and ions.
- Inner Membrane: Impermeable to most ions and houses the ETC and ATP synthase.
- Intermembrane Space: Between the inner and outer membranes, where a proton gradient is established.
- Matrix: Contains enzymes for the Krebs cycle and other metabolic processes.
Oxidative Phosphorylation
Electron Transport Chain (ETC)
The ETC is a series of protein complexes and electron carriers located in the inner mitochondrial membrane. It facilitates the transfer of electrons from NADH and FADH₂ to oxygen, the final electron acceptor.
- Complex I (NADH: Ubiquinone Oxidoreductase):
- Transfers electrons from NADH to ubiquinone (coenzyme Q).
- Pumps protons (H⁺) into the intermembrane space.
- Complex II (Succinate Dehydrogenase):
- Transfers electrons from FADH₂ to ubiquinone.
- Does not pump protons.
- Complex III (Cytochrome bc1 Complex):
- Transfers electrons from ubiquinol to cytochrome c.
- Pumps protons into the intermembrane space.
- Complex IV (Cytochrome c Oxidase):
- Transfers electrons to oxygen, reducing it to water.
- Pumps protons into the intermembrane space.
- Complex V (ATP Synthase):
- Uses the proton gradient to synthesize ATP from ADP and inorganic phosphate (Pi).
- Protons flow back into the matrix through ATP synthase, driving ATP production.
- Ubiquinone (Coenzyme Q):
- Lipid-soluble electron carrier that shuttles electrons between Complex I/II and Complex III.
- Cytochrome c:
- A small protein that transfers electrons from Complex III to Complex IV.
Chemiosmosis
Chemiosmosis is the process by which the energy stored in the proton gradient across the inner mitochondrial membrane is used to drive ATP synthesis.
- Proton Gradient: The ETC creates a high concentration of protons in the intermembrane space, generating an electrochemical gradient (proton-motive force).
- ATP Synthase: This enzyme harnesses the energy of the proton gradient as protons flow back into the mitochondrial matrix, catalyzing the formation of ATP from ADP and Pi.
Mechanism
- Electron Transfer: Electrons from NADH and FADH₂ are passed through the ETC, releasing energy at each step.
- Proton Pumping: The energy from electron transfer is used to pump protons across the inner membrane, creating a gradient.
- ATP Production: The flow of protons back into the matrix through ATP synthase drives the phosphorylation of ADP to ATP.
ATP Yield
- NADH: Each molecule contributes to the production of approximately 2.5 ATP.
- FADH₂: Each molecule contributes to the production of approximately 1.5 ATP.
Summary
Oxidative phosphorylation is an essential component of cellular respiration, providing the majority of ATP used by cells for energy-requiring processes. By coupling electron transport with proton pumping and ATP synthesis, the mitochondria efficiently convert biochemical energy into a usable form.
Key Components of the Electron Transport Chain
| Component | Function |
|---|---|
| Complex I | Oxidizes NADH, transfers electrons to ubiquinone, pumps protons. |
| Complex II | Oxidizes FADH₂, transfers electrons to ubiquinone. |
| Complex III | Transfers electrons from ubiquinol to cytochrome c, pumps protons. |
| Complex IV | Transfers electrons to oxygen, forming water, pumps protons. |
| ATP Synthase | Utilizes the proton gradient to synthesize ATP. |
| Ubiquinone | Shuttles electrons between Complex I/II and III. |
| Cytochrome c | Transfers electrons between Complex III and IV. |
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Key Terms
- Oxidative Phosphorylation: The process of ATP production linked to the oxidation of nutrients.
- Electron Transport Chain: A series of protein complexes that transfer electrons and create a proton gradient.
- Chemiosmosis: The movement of protons across a membrane, driving ATP synthesis.
- Proton Gradient: An electrochemical gradient created by the ETC, essential for ATP production.
- ATP Synthase: An enzyme that synthesizes ATP using the energy from the proton gradient.
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References
- Berg, J. M., Tymoczko, J. L., & Stryer, L. (2015). Biochemistry. W. H. Freeman.
- Nelson, D. L., & Cox, M. M. (2017). Lehninger Principles of Biochemistry. W. H. Freeman.
- Voet, D., Voet, J. G., & Pratt, C. W. (2016). Fundamentals of Biochemistry: Life at the Molecular Level. Wiley.