Questions: In aerobic respiration O2 is the final electron acceptor, whereas in anaerobic respiration another oxidized ion such as NO3^- is the final electron acceptor. Multiple Choice O2 ; NO3^- NO3 ; O2 O2 ; H2O NO3 ; H2O O2 ; NH3

The answer is the first one: $\mathrm{O}_{2} ; \mathrm{NO}_{3}{ }^{-}$

Explanation for each option:

1. $\mathrm{O}_{2} ; \mathrm{NO}_{3}{ }^{-}$: This is correct. In aerobic respiration, oxygen ($\mathrm{O}_{2}$) is the final electron acceptor. In anaerobic respiration, other oxidized ions such as nitrate ($\mathrm{NO}_{3}{ }^{-}$) can serve as the final electron acceptor.
2. $\mathrm{NO}_{3} ; \mathrm{O}_{2}$: This is incorrect. It reverses the roles of the electron acceptors in aerobic and anaerobic respiration.
3. $\mathrm{O}_{2} ; \mathrm{H}_{2} \mathrm{O}$: This is incorrect. While $\mathrm{O}_{2}$ is the final electron acceptor in aerobic respiration, $\mathrm{H}_{2} \mathrm{O}$ is not an electron acceptor in anaerobic respiration.
4. $\mathrm{NO}_{3} ; \mathrm{H}_{2} \mathrm{O}$: This is incorrect. It incorrectly assigns $\mathrm{NO}_{3}$ as the final electron acceptor in aerobic respiration.
5. $\mathrm{O}_{2} ; \mathrm{NH}_{3}$: This is incorrect. While $\mathrm{O}_{2}$ is the final electron acceptor in aerobic respiration, $\mathrm{NH}_{3}$ is not typically an electron acceptor in anaerobic respiration.

Summary: In aerobic respiration, $\mathrm{O}_{2}$ is the final electron acceptor, whereas in anaerobic respiration another oxidized ion such as $\mathrm{NO}_{3}{ }^{-}$ is the final electron acceptor.