What is the function of the electron transport chain and the specific role played by complex I, II, III, IV, ubiquinone, and cytochrome c?
The correct answer and explanation is:
The electron transport chain (ETC) is a series of protein complexes and molecules embedded in the inner mitochondrial membrane. Its primary function is to transfer electrons from electron donors like NADH and FADH2 to oxygen, the final electron acceptor. This process creates a proton gradient across the membrane, which drives ATP synthesis through oxidative phosphorylation.
Complex I, also called NADH:ubiquinone oxidoreductase, accepts electrons from NADH. It transfers these electrons to ubiquinone (coenzyme Q), while pumping protons from the mitochondrial matrix into the intermembrane space. This proton pumping contributes to the proton gradient.
Complex II, or succinate dehydrogenase, receives electrons from FADH2 generated during the Krebs cycle. Unlike Complex I, Complex II does not pump protons but transfers electrons directly to ubiquinone. This makes it a less significant contributor to the proton gradient.
Ubiquinone is a lipid-soluble electron carrier that shuttles electrons from Complexes I and II to Complex III. It plays a critical role by transferring electrons within the inner membrane and accepting protons from the matrix side.
Complex III, known as cytochrome bc1 complex, transfers electrons from reduced ubiquinone to cytochrome c. During this process, Complex III pumps protons into the intermembrane space, further contributing to the proton gradient necessary for ATP production.
Cytochrome c is a small, soluble protein located in the intermembrane space. It carries electrons one at a time from Complex III to Complex IV.
Complex IV, or cytochrome c oxidase, accepts electrons from cytochrome c and transfers them to molecular oxygen. This step reduces oxygen to water. Complex IV also pumps protons into the intermembrane space, reinforcing the proton gradient.
The proton gradient formed by the actions of Complexes I, III, and IV drives ATP synthase to convert ADP to ATP. This entire process is fundamental for cellular energy production.