Question

Ubiquinone is one of two mobile electron carriers in the electron-transport chain. Where

does the additional pair of electrons reside in the reduced ubiquinone molecule?
(a) The electrons are added directly to the aromatic ring.
(b) The electrons are added to each of two ketone oxygens on the aromatic ring.
(c) The electrons are added to the hydrocarbon tail, which hides them inside the
membrane bilayer.
(d) Both electrons, and one proton, are added to a single ketone oxygen bound to the
aromatic ring.

Answer 1

In the reduced ubiquinone molecule, the additional pair of electrons is added to each of two ketone oxygens on the aromatic ring, converting ubiquinone into ubiquinol. This takes place within the electron transport chain, where ubiquinone acts as a crucial mobile electron and proton carrier.

Step-by-step explanation:

The student has asked where the additional pair of electrons resides in the reduced ubiquinone molecule. The correct answer to this question is that the electrons are added to each of two ketone oxygens on the aromatic ring, which corresponds to option (b). In the process of ubiquinone reduction within the electron transport chain, a lipid-soluble molecule ubiquinone (Q) accepts two electrons and two protons to become ubiquinol (QH2).

The ubiquinone molecule plays a crucial role by acting as a mobile electron carrier. It transports electrons derived from NADH and FADH2 from complexes I and II to complex III within the inner mitochondrial membrane. During this process, it gains two electrons and two protons, thus becoming reduced.

In summary, the two electrons that are added to ubiquinone reside on each of the two ketone oxygens of the aromatic ring structure, converting ubiquinone into its reduced form, ubiquinol.