Question
Explain why FADH2 produces fewer ATP molecules than NADH when it passes its electrons down the Electron transport chain.
Answers
Answered by
GPT-4o mini
FADH2 produces fewer ATP molecules than NADH during cellular respiration primarily due to its point of entry into the electron transport chain (ETC) and the subsequent processes of oxidative phosphorylation.
Here's a breakdown of the reasons:
1. **Entry Point into the Electron Transport Chain**:
- NADH donates its electrons to Complex I (NADH dehydrogenase) of the electron transport chain. This complex contributes to the pumping of protons (H⁺ ions) across the inner mitochondrial membrane into the intermembrane space, creating a proton gradient. This process generates enough energy to synthesize approximately 2.5 ATP per NADH molecule.
- FADH2, on the other hand, donates its electrons to Complex II (succinate dehydrogenase). This complex does not pump protons into the intermembrane space. As a result, FADH2 contributes less to the proton gradient than NADH, leading to fewer ATP molecules being produced.
2. **Proton Gradient and ATP Synthesis**:
- The electron transport chain creates a proton gradient by actively transporting protons across the mitochondrial membrane when electrons pass through Complexes I, III, and IV. This gradient is then used by ATP synthase to produce ATP through a process known as chemiosmosis.
- Because FADH2 bypasses Complex I, it does not contribute to the proton influx associated with that complex. Instead, it links to the chain at Complex II, where fewer protons are pumped, resulting in less energy available for ATP generation compared to NADH.
3. **ATP Yield**:
- On average, the oxidation of one NADH molecule leads to the synthesis of about 2.5 ATP.
- The oxidation of one FADH2 molecule results in the synthesis of approximately 1.5 ATP.
- This difference is primarily due to the variations in the proton pumping activity of the different complexes in the electron transport chain associated with these two electron carriers.
In summary, the key reason FADH2 results in fewer ATP molecules than NADH is that FADH2 enters the electron transport chain at Complex II, which pumps fewer protons compared to the entry of NADH at Complex I. This leads to a smaller proton gradient and, consequently, less ATP production during oxidative phosphorylation.
Here's a breakdown of the reasons:
1. **Entry Point into the Electron Transport Chain**:
- NADH donates its electrons to Complex I (NADH dehydrogenase) of the electron transport chain. This complex contributes to the pumping of protons (H⁺ ions) across the inner mitochondrial membrane into the intermembrane space, creating a proton gradient. This process generates enough energy to synthesize approximately 2.5 ATP per NADH molecule.
- FADH2, on the other hand, donates its electrons to Complex II (succinate dehydrogenase). This complex does not pump protons into the intermembrane space. As a result, FADH2 contributes less to the proton gradient than NADH, leading to fewer ATP molecules being produced.
2. **Proton Gradient and ATP Synthesis**:
- The electron transport chain creates a proton gradient by actively transporting protons across the mitochondrial membrane when electrons pass through Complexes I, III, and IV. This gradient is then used by ATP synthase to produce ATP through a process known as chemiosmosis.
- Because FADH2 bypasses Complex I, it does not contribute to the proton influx associated with that complex. Instead, it links to the chain at Complex II, where fewer protons are pumped, resulting in less energy available for ATP generation compared to NADH.
3. **ATP Yield**:
- On average, the oxidation of one NADH molecule leads to the synthesis of about 2.5 ATP.
- The oxidation of one FADH2 molecule results in the synthesis of approximately 1.5 ATP.
- This difference is primarily due to the variations in the proton pumping activity of the different complexes in the electron transport chain associated with these two electron carriers.
In summary, the key reason FADH2 results in fewer ATP molecules than NADH is that FADH2 enters the electron transport chain at Complex II, which pumps fewer protons compared to the entry of NADH at Complex I. This leads to a smaller proton gradient and, consequently, less ATP production during oxidative phosphorylation.