In mitochondrial electron transport, what is the role of Coenzyme Q (ubiquinone)?

Introduction / Context:
Coenzyme Q (ubiquinone, abbreviated Q or CoQ10) is central to the mitochondrial electron transport chain (ETC). Understanding its physical nature and position clarifies how electrons and protons move across the inner mitochondrial membrane to drive ATP synthesis.

Given Data / Assumptions:

• Oxidative phosphorylation in eukaryotic mitochondria.
• Complex I and II feed electrons to Q; Complex III receives electrons from reduced Q (QH2).
• Inner mitochondrial membrane is hydrophobic.

Concept / Approach:
Ubiquinone is a small, isoprenoid-derived, lipid-soluble benzoquinone. Because it dissolves in the inner membrane lipid bilayer, it can freely diffuse laterally to shuttle electrons from membrane-bound dehydrogenases (Complex I/II) to Complex III. It is not the terminal acceptor (that is O2 at Complex IV), nor is it a water-soluble carrier.

Step-by-Step Solution:
Identify CoQ's chemical character: hydrophobic tail → membrane solubility.Trace electron path: NADH/FADH2 → Complex I/II → Q → Complex III.Conclude its role: a lipid-soluble electron carrier within the membrane.

Verification / Alternative check:
Spectral and inhibitor studies (e.g., antimycin A on Complex III) support Q’s mobile carrier role distinct from protein-bound cytochromes.

Why Other Options Are Wrong:

• Direct electron transfer to O2: done by Complex IV, not Q.
• Water-soluble donor: Q is not water-soluble; cytochrome c is water-soluble but peripherally associated.
• Covalent cytochrome cofactor: Q is not covalently bound to cytochromes.

Common Pitfalls:
Confusing Q with cytochrome c; the latter is water-soluble in the intermembrane space, whereas Q is membrane-soluble.

Final Answer:
A lipid-soluble electron carrier