Final answer:
Electrons from Complex II are transferred to ubiquinone (Q), which then delivers its electrons to Complex III. The ultimate acceptor of electrons at the end of the electron transport chain is oxygen.
Step-by-step explanation:
The student asked: What accepts the electron after it passes through the second electron transport complex? In the electron transport chain within the mitochondria, electrons from NADH and FADH2 are transported through a series of complexes. After passing Complex II, the electrons are transferred to ubiquinone (Q), also known as coenzyme Q. Once reduced to ubiquinol (QH2), ubiquinone delivers its electrons to Complex III. Ubiquinone acts as a mobile carrier between Complex II and Complex III in this process. However, the final electron acceptor after the entire electron transport chain, at Complex IV, is oxygen (O2), which gets reduced to water (H2O).