8. NADH donates its electron to electron transport chain through primary acceptor
(1) CoQ          (2) FMN
(3) FAD          (4) Cytochrome oxidase

NADH is a crucial electron carrier in cellular respiration, donating high-energy electrons to the electron transport chain (ETC) in mitochondria. Understanding the initial step of electron donation by NADH helps reveal how energy conversion occurs during oxidative phosphorylation.

NADH and the Electron Transport Chain

The electron transport chain is a series of protein complexes embedded in the inner mitochondrial membrane. It facilitates electron transfer from electron donors like NADH and FADH2 to molecular oxygen, coupled with proton pumping to generate ATP.

In eukaryotes, the electron transport chain pathway for NADH is:

NADH → Complex I (NADH dehydrogenase) → Ubiquinone (CoQ) → Complex III → Cytochrome c → Complex IV → O2

Primary Electron Acceptor for NADH: FMN

• Complex I (NADH dehydrogenase) is the first protein complex in the chain and accepts electrons directly from NADH.

• Within Complex I, the primary electron acceptor is flavin mononucleotide (FMN), a prosthetic group derived from vitamin B2 (riboflavin).

• FMN accepts two electrons from NADH, becoming reduced to FMNH2.

• These electrons are then passed through a series of iron-sulfur (Fe-S) clusters within Complex I before being transferred to ubiquinone (CoQ).

Why FMN Is the Correct Primary Acceptor

• FMN is tightly bound to Complex I and is the first molecule to receive electrons from NADH.

• It acts as a redox-active prosthetic group, facilitating electron transfer within the complex.

• This initial transfer is essential for the subsequent proton pumping and energy conversion steps.

Clarifying Other Options

Summary

• NADH donates electrons to the electron transport chain at Complex I.

• The primary electron acceptor within Complex I is FMN (flavin mononucleotide).

• FMN accepts electrons from NADH and passes them through iron-sulfur clusters to ubiquinone.

• This step initiates the process of proton pumping and ATP synthesis.

Final Answer:

(2) FMN