Article:

Introduction

The electron transport chain (ETC) is the powerhouse of cellular respiration, responsible for generating ATP through a series of redox reactions. Within the ETC, various molecules act as electron carriers, shuttling electrons through a cascade of complexes embedded in the mitochondrial membrane. Among these, flavin stands out as a two-electron carrier, playing a unique and crucial role in bioenergetics.

✅ Correct answer: (c) Flavin

What Is Flavin?

Flavin refers to flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN)—two coenzymes derived from vitamin B2 (riboflavin). These molecules are commonly bound to enzymes such as complex I (NADH dehydrogenase) and succinate dehydrogenase (complex II) in the electron transport chain.

Why Is Flavin a Two-Electron Carrier?

Flavins are capable of accepting and donating either one or two electrons, giving them a biochemical edge:

• One-electron step: Flavin forms a semiquinone intermediate (partially reduced).

• Two-electron step: Flavin is fully reduced (FAD → FADH₂ or FMN → FMNH₂).

This flexibility allows flavins to bridge reactions between molecules that transfer one electron at a time (like iron-sulfur clusters) and those that transfer two (like NADH), acting as a redox mediator.

Other Options Explained

• (a) Cupro proteins:
  Contain copper ions and typically transfer one electron due to their redox state cycling (Cu⁺/Cu²⁺).

• (b) Cytochromes:
  Use heme groups that facilitate one-electron transfers, cycling between Fe²⁺ and Fe³⁺.

• (d) Iron-sulphur proteins (Fe-S clusters):
  Also one-electron carriers, involved in multiple complexes in the ETC.

Only flavin can handle both one- and two-electron chemistry efficiently, making it vital in electron transport and energy conversion.

Role of Flavin in the ETC

1. Complex I (NADH:ubiquinone oxidoreductase):
   FMN accepts two electrons from NADH and passes them to Fe-S clusters.

2. Complex II (succinate dehydrogenase):
   FAD accepts two electrons during the conversion of succinate to fumarate.

Flavins are permanently bound to these enzymes and don’t freely diffuse, unlike NADH or ubiquinone.

Conclusion

Flavin’s ability to accept and donate two electrons sets it apart in the electron transport chain. This makes it indispensable for transferring electrons from molecules like NADH and FADH₂ to the ETC’s redox centers.

✅ Correct Answer: (c) Flavin