Final answer:
Complex II is not involved in the flow of electrons from NADH to O2 in electron transport.
Step-by-step explanation:
In electron transport, each of the following respiratory complexes is involved in the flow of electrons from NADH to O2 except complex II. Complex I, III, and IV are all involved in the flow of electrons from NADH. Complex II, also known as succinate dehydrogenase, is unique because it does not pump protons, unlike the other complexes.