Question

Which of the following is/are TRUE about the electron carrier, ubiquinone (coenzyme Q)?

• A. Its ability to accept two electrons, one at a time, enables ubiquinone to function at the junction between a 2-electron donor and a 1-electron acceptor
• B. Being small and hydrophobic, ubiquinone readily shuttles between protein-based electron transfer complexes within the membrane
• C. Its hydrophilic nature and high affinity for protons enable ubiquinone to transport protons readily within the intermembrane space of mitochondria
• D. Its ability to interact with Heme C of cytochromes enables electron transport in the mitochondrial membrane

Hint:

Ubiquinone = Coenzyme Q = Lipid-soluble electron carrier between Complex I/II and III. Accepts 2e$^-$ sequentially → forms semiquinone intermediate.

Answer 1

The Correct Option is A, B

Step 1: Recall ubiquinone’s properties.
Ubiquinone (Coenzyme Q) is a small, lipid-soluble electron carrier in the mitochondrial inner membrane.
It shuttles electrons between complexes I/II and III.
Step 2: Analyze statements.
(A) True — It can accept two electrons one by one, functioning as a bridge between 2e$^-$ donors (NADH) and 1e$^-$ acceptors (cytochromes).
(B) True — Its hydrophobic nature allows diffusion within the lipid bilayer.
(C) False — Ubiquinone is hydrophobic, not hydrophilic.
(D) Partially true but incomplete. Direct electron transfer to cytochromes occurs via cytochrome b and c$_1$, not directly by ubiquinone.
Step 3: Conclusion.
Hence, statements (A) and (B) are correct.