Cellular respiration — In the mitochondrial electron transport chain, which molecule serves as the final (terminal) electron acceptor?

Introduction / Context:
The electron transport chain (ETC) in aerobic respiration is a series of redox carriers embedded in the inner mitochondrial membrane. Electrons flow from reduced substrates (such as NADH and FADH2) through complexes and mobile carriers to a terminal electron acceptor. Knowing the identity of this final acceptor is essential to understand how oxidative phosphorylation proceeds and why oxygen is vital for aerobic life.

Given Data / Assumptions:

• Aerobic respiration uses oxygen and produces water.
• ETC components include Complex I–IV and mobile carriers (ubiquinone and cytochromes).
• ATP synthesis is driven by a proton motive force established by proton pumping linked to electron flow.

Concept / Approach:
Electrons donated by NADH and FADH2 pass sequentially to carriers of increasing reduction potential. At Complex IV (cytochrome c oxidase), electrons are finally transferred to molecular oxygen. Oxygen is reduced to water, and this step is tightly coupled to proton pumping that sustains the electrochemical gradient used by ATP synthase.

Step-by-Step Solution:

Verification / Alternative check:
Blocking oxygen availability (e.g., cyanide inhibition at Complex IV or anoxia) halts electron flow, collapses the proton motive force, and stops ATP synthesis—confirming oxygen's role as the terminal electron acceptor.

Why Other Options Are Wrong:

• ADP is the substrate for ATP synthase, not an electron acceptor.
• Cytochrome b is an intermediate carrier within Complex III, not terminal.
• “None of these” is incorrect because oxygen is correct.

Common Pitfalls:
Confusing oxygen with water: oxygen is the acceptor; water is the reduced product. Also, mistaking cytochromes as final recipients; they are intermediates.

Final Answer:
Oxygen