In a mixture of NAD+, NADH, ubiquinone, and ubiquinol, which compound will be oxidized? Which will be reduced?

The loss of electrons by a molecule, atom, or ion during a process is known as oxidation.On the other hand, the gain of electrons by a molecule, atom, or ion is referred to as reduction.

From the table of standard reduction potentials, the standard reduction potentials of reactions are compared to identify which reactions occur and which do not.

The ubiquinone half-reaction is summarised as,

Ubiquinone $2H^{+}+2e^{-}\square$ubiquinol, ${\xi }^o=0.045V$

The NAD⁺half-reaction is summarized as,

$NA{D}^{+}+H^{+}+2e^{-}\square NADH,{\xi }^o=-0.315V$

The reduction potential of ubiquinone half-reaction is higher than NAD+ half-reaction. Thus, NADH loses electrons and gets oxidized, whereas ubiquinone gains electrons and becomes reduced.

Hence, ubiquinone gets reuced, and NADH is oxidized.