Question

Which of the following is a component of ETS in mitochondria?
A. Phytochrome
B. Plastocyanin
C. Cytochrome oxidase
D. Carotenoids

Answer 1

Hint: The proteins and complexes of molecules that are involved in the mechanism called electron transport are found in the inner membrane. As it is called, the electron transport system (ETS) takes energy from carriers in the matrix and stores it in a form that can be used for the phosphorylation of ADP.

Complete Answer:
- Two energy carriers, nicotine adenine dinucleotide (NAD) and flavin adenine dinucleotide (FAD), are known to donate energy to the ETS. Reduced NAD carries energy to the electron transport chain complex I (NADH-Coenzyme Q Reductase). A bound component of the complex of succinate dehydrogenase (complex II) is FAD. When the substratum succinate binds the complex, it is reduced.
- A key concept is the obligatory forcing of protons into the intermembrane space. Without proton translocation, electrons can not pass through Complex I. If you prevent proton translocation, the transport of electrons is prevented. When you prevent the transport of electrons, you prevent the translocation of protons. The incidents have to occur together or not at all.
- Electron transport carriers are specific in that electrons (and associated free energy) from a specific type of preceding carrier are accepted by each carrier. The electrons pass from complex I to a carrier (Coenzyme Q) which is embedded in the membrane by itself. Electrons are passed from Coenzyme Q to a complex III that is associated with another event of proton translocation.
- Note that the electron path is from Complex I to Complex III to Coenzyme Q. The succinate dehydrogenase complex, Complex II, is a distinct starting point, and is not part of the NADH pathway.
- The pathway from Complex III is to cytochrome c and then to Complex IV (the complex of cytochrome oxidase). Complex IV translocates more protons, and it is at this site that oxygen binds together with protons, and oxygen is reduced to water using the electron pair and the remaining free energy.
- Since molecular oxygen is diatomic, the completion of the reaction sequence for oxygen reduction actually requires two electron pairs and two cytochrome oxidase complexes. This last step in the transport of electrons serves the critical function of removing electrons from the system so that the transport of electrons can function continuously.

The correct Answer is option (C) Cytochrome oxidase.

Note: In the form of a chemiosmotic gradient, the purpose of electron transport is to conserve energy. In turn, the gradient can be exploited for ADP phosphorylation as well as for other purposes. Cell damage is instant and permanent with the cessation of aerobic metabolism.