Nicotinamide nucleotide transhydrogenase (NNT) in the inner mitochondrial membrane catalyzes the reaction NADH + NADP⁺ → NAD⁺ + NADPH to generate the NADPH needed for certain reactions that help destroy reactive oxygen species.

(a) Explain why the NNT reaction cannot be driven primarily by the difference in substrate reduction potentials (Δℰ).

(b) In fact, the NNT reaction is driven by proton translocation across the membrane (from outside to inside). How does operation of the transhydrogenase affect the efficiency of oxidative phosphorylation?