Four electron carriers, a, b, c, and d, whose reduced and oxidized forms can be distinguished spectrophotometrically, are required for respiration in a bacterial electron-transport system. In the presence of substrates and oxygen, three different inhibitors block respiration, yielding the patterns of oxidation states shown below. What is the order of the carriers in the chain from substrates to O2, and where do the three inhibitors act?

O and R indicate fully oxidized and fully reduced, respectively.

Carriers Inhibilor 0