How is energy from oxidation reactions transported to the cristae of mitochondria by reduced NAD and FAD?
How is energy from oxidation reactions transported to the cristae of mitochondria by reduced NAD and FAD?
Answer
The process of transporting energy from oxidation reactions to the cristae of mitochondria via reduced NAD (NADH) and FAD (FADH₂) is a critical component of cellular respiration, particularly during oxidative phosphorylation. Here’s how this process works:
1. Production of Reduced NAD and FAD
During metabolic processes such as glycolysis, the link reaction, and the Krebs cycle, NAD+ and FAD are reduced to NADH and FADH₂, respectively. This occurs when they accept electrons (and protons) released during the oxidation of substrates:
- Glycolysis: Produces 2 NADH from glucose.
- Krebs Cycle: Produces 6 NADH and 2 FADH₂ from one glucose molecule (accounting for two turns of the cycle).
These reduced coenzymes carry high-energy electrons that are crucial for ATP production.
2. Transport to the Mitochondria
NADH produced in the cytoplasm must be transported into the mitochondria for further processing:
- Shuttle Systems: The electrons from cytoplasmic NADH can be transferred into the mitochondria via shuttle systems such as the malate-aspartate shuttle or glycerol phosphate shuttle. These systems effectively transport electrons while regenerating NAD+ in the cytoplasm.
- Once inside the mitochondria, NADH can directly enter the electron transport chain (ETC).
3. Role in the Electron Transport Chain
Electron Transfer
- Entry into the ETC:
- NADH donates its electrons to Complex I (NADH-Q oxidoreductase), where it is oxidized back to NAD+. This releases high-energy electrons that are passed through a series of protein complexes in the inner mitochondrial membrane.
- FADH₂, produced in the Krebs cycle, donates its electrons to Complex II (succinate-Q reductase). This complex does not pump protons but connects to the same electron transport chain.
- Redox Reactions:
- As electrons move through Complexes I-IV of the ETC, they undergo a series of redox reactions, releasing energy at each step. This energy is harnessed to pump protons (H⁺ ions) from the mitochondrial matrix into the intermembrane space, creating an electrochemical gradient.
Creation of a Proton Gradient
- The pumping of protons creates a proton gradient across the inner mitochondrial membrane, with a higher concentration of H⁺ ions in the intermembrane space compared to the matrix. This gradient represents stored potential energy known as the proton-motive force.
4. ATP Synthesis
- Protons flow back into the mitochondrial matrix through ATP synthase, a protein complex that synthesizes ATP from ADP and inorganic phosphate (Pi) using the energy released from this proton movement.
- The final step in electron transport involves oxygen acting as the terminal electron acceptor at Complex IV, where it combines with electrons and protons to form water (H₂O).
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