How many functional protein complexes are present in the Electron Transport Chain (ETC)?

The Electron Transport Chain (ETC), which is a crucial component of cellular respiration occurring in the inner mitochondrial membrane, consists of four main functional protein complexes. These complexes are integral to the process of oxidative phosphorylation, where they facilitate the transfer of electrons derived from NADH and FADH2 to molecular oxygen.

The four functional protein complexes are:

1. Complex I (NADH dehydrogenase)

2. Complex II (succinate dehydrogenase)

3. Complex III (cytochrome bc1 complex)

4. Complex IV (cytochrome c oxidase)

Each of these complexes serves a distinct role in transferring electrons and pumping protons (H+) across the membrane, contributing to the proton gradient that drives ATP synthesis through ATP synthase.

Additionally, there is Complex V, often referred to as ATP synthase, but it is not considered a part of the electron transport pathway. Its primary role is to synthesize ATP using the proton gradient established by the electron transport complexes. This distinction is important in determining the count of functional complexes dedicated to electron transport versus ATP synthesis.

Thus, the answer identifying four functional protein complexes accurately reflects the structure and function of the ETC.