Question

Mitochondria known as the "powerhouse" of the cell, generates ATP via oxidative phosphorylation complexes. Which among the following is known as complex I?

• A. Succinate dehydrogenase
• B. Ubiquinol-cytochrome c oxidoreductase
• C. Cytochrome c oxidase
• D. NADH: ubiquinone oxidoreductase

Hint:

• Mitochondrial Electron Transport Chain Complexes:
  • Complex I: NADH dehydrogenase (NADH:ubiquinone oxidoreductase)
  • Complex II: Succinate dehydrogenase
  • Complex III: Cytochrome bc\(_1\) complex (Ubiquinol-cytochrome c oxidoreductase)
  • Complex IV: Cytochrome c oxidase
  • Complex V: ATP synthase
• Complexes I, III, and IV are proton pumps.

Answer 1

The Correct Option is D

The electron transport chain (ETC) in mitochondria consists of a series of protein complexes (I, II, III, IV) and mobile electron carriers (Ubiquinone/Coenzyme Q, Cytochrome c) that transfer electrons from NADH and FADH\(_2\) to oxygen, ultimately producing water and generating a proton gradient used by ATP synthase (Complex V) to produce ATP. The complexes are:

• Complex I: NADH:ubiquinone oxidoreductase (also known as NADH dehydrogenase complex). It accepts electrons from NADH and transfers them to ubiquinone (CoQ). It also pumps protons from the mitochondrial matrix to the intermembrane space.
• Complex II: Succinate dehydrogenase (or succinate-CoQ reductase). It accepts electrons from succinate (via FADH\(_2\) which is part of the complex) and transfers them to ubiquinone. It does NOT pump protons. (Option a)
• Complex III: Ubiquinol-cytochrome c oxidoreductase (or cytochrome bc\(_1\) complex). It accepts electrons from ubiquinol (reduced CoQ) and transfers them to cytochrome c. It pumps protons. (Option b)
• Complex IV: Cytochrome c oxidase. It accepts electrons from cytochrome c and transfers them to molecular oxygen (O\(_2\)), reducing it to water (H\(_2\)O). It pumps protons. (Option c)
• Complex V: ATP synthase. Uses the proton gradient generated by Complexes I, III, and IV to synthesize ATP from ADP and Pi.

Therefore, Complex I is NADH:ubiquinone oxidoreductase. \[ \boxed{\text{NADH: ubiquinone oxidoreductase}} \]