Question

Which electron transport chain complex does not pump protons across the mitochondrial inner membrane?

• A. Complex I
• B. Complex II
• C. Complex III
• D. Complex IV

Hint:

Remember: \[ \boxed{ \begin{aligned} \text{Proton pumps} &: \text{Complex I, III, IV} \text{No proton pumping} &: \text{Complex II} \end{aligned} } \]

Answer 1

The Correct Option is B

Step 1: Recall the proton-pumping complexes. In the mitochondrial electron transport chain, \[ \begin{aligned} \text{Complex I} &\rightarrow \text{Pumps protons}, \text{Complex II} &\rightarrow \text{Does not pump protons}, \text{Complex III} &\rightarrow \text{Pumps protons}, \text{Complex IV} &\rightarrow \text{Pumps protons}. \end{aligned} \]

Step 2: Identify the correct complex. Complex II (Succinate dehydrogenase) transfers electrons from succinate to ubiquinone but does not contribute to the proton gradient. Hence, \[ \boxed{\text{Complex II}} \] is the correct answer. Therefore, \[ \boxed{(B)} \] is the correct answer.