In the electron transport chain, four electrons are required to fully reduce one O₂ molecule to two H₂O molecules. Given this information, how many NADH molecules are required?

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11ecba2d_0ec5_a0fc_a3bf_f39b7d498c82_MD0008_00 NADH is a two-electron carrier. Turning O₂ into water requires four electrons, so two NADH molecules would need to pass their electrons through the ETC to fully reduce O₂. NADH, FADH₂, and UQH₂ can each carry up to two electrons. NADH passes them both together; FADH₂ and UQH₂ can pass them one at a time. Cytochrome C can carry only one electron, so four cyt-C_red molecules are required to fully reduce oxygen. (Choice A) If NADH could carry four electrons instead of two, only one NADH molecule would be required to fully reduce O₂ to water. (Choice C) NADH carries two electrons, so only two NADH molecules are required to fully reduce O₂. (Choice D) Four single-electron carriers, such as cytochrome C, would be required for full reduction of O₂. NADH carries two electrons. Educational objective: Electron donors and acceptors in the ETC have different carrying capacities. Full reduction of oxygen to water requires four electrons, so two copies of a two-electron carrier such as NADH would be required to fully reduce oxygen. Four copies of a one-electron carrier, such as reduced cytochrome C, are required.

In the Electron Transport Chain, Four Electrons Are Required to Fully