The electrons that enter the ETC through Complex II come fromGroup of answer choicesFADH2SuccinateFumarateNADH2 or more of the above

To answer the question step by step:

1. Understand the Electron Transport Chain (ETC): The ETC is a series of protein complexes located in the inner mitochondrial membrane that transfer electrons from electron donors to electron acceptors via redox reactions. This process helps generate a proton gradient that drives ATP synthesis.

2. Identify Complex II: Complex II, also known as succinate dehydrogenase, is one of the protein complexes in the ETC. It plays a dual role in both the citric acid cycle and the ETC.

3. Determine the Electron Donors for Complex II:

   • FADH2: This molecule is produced during the citric acid cycle when succinate is converted to fumarate. FADH2 donates electrons directly to Complex II.
   • Succinate: This is the substrate that is oxidized by Complex II to form fumarate. During this process, electrons are transferred to FAD, forming FADH2, which then donates the electrons to the ETC.
   • Fumarate: This is the product formed after succinate is oxidized. It does not directly donate electrons to the ETC.
   • NADH: This molecule donates electrons to Complex I, not Complex II.

4. Evaluate the Options:

   • FADH2: Correct, as it donates electrons to Complex II.
   • Succinate: Indirectly correct, as it is the substrate that leads to the formation of FADH2, which then donates electrons to Complex II.
   • Fumarate: Incorrect, as it is the product and does not donate electrons.
   • NADH: Incorrect, as it donates electrons to Complex I.

5. Conclusion: The electrons that enter the ETC through Complex II come from FADH2 and indirectly from succinate.

Therefore, the correct answer is: 2 or more of the above.