Which complex of ETC is not associated with liberation of energy ?
**Core Concept**
The electron transport chain (ETC) is a series of protein complexes located in the mitochondrial inner membrane that generate ATP by harnessing the energy from the transfer of electrons from high-energy molecules to oxygen. The ETC is divided into four complexes: NADH dehydrogenase (Complex I), succinate dehydrogenase (Complex II), cytochrome b-c1 complex (Complex III), and cytochrome oxidase (Complex IV).
**Why the Correct Answer is Right**
Complex II (succinate dehydrogenase) is the only complex in the ETC that is not directly involved in the generation of energy through the transfer of protons across the membrane. Unlike Complexes I, III, and IV, Complex II does not pump protons across the membrane, and therefore, does not contribute to the generation of the proton gradient that drives the synthesis of ATP.
**Why Each Wrong Option is Incorrect**
**Option A:** NADH dehydrogenase (Complex I) is directly involved in the liberation of energy, as it pumps protons across the membrane during the transfer of electrons from NADH to ubiquinone.
**Option B:** Cytochrome b-c1 complex (Complex III) is also directly involved in the liberation of energy, as it pumps protons across the membrane during the transfer of electrons from ubiquinol to cytochrome c.
**Option C:** Cytochrome oxidase (Complex IV) is directly involved in the liberation of energy, as it pumps protons across the membrane during the transfer of electrons from cytochrome c to oxygen.
**Option D:** Although Complex II does not directly contribute to the generation of the proton gradient, it is still an essential component of the ETC and plays a critical role in the transfer of electrons.
**Clinical Pearl / High-Yield Fact**
The electron transport chain is a critical component of cellular respiration, and its dysfunction is associated with various diseases, including mitochondrial disorders and neurodegenerative diseases. Understanding the role of each complex in the ETC is essential for diagnosing and managing these conditions.
**Correct Answer:** D. Complex II is not directly involved in the liberation of energy through the transfer of protons across the membrane.