Cellular respiration — In the overall reaction C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + energy, which reactant is being reduced during the process?

Introduction / Context:
Redox bookkeeping is essential for understanding oxidative metabolism. In aerobic respiration, electrons from glucose ultimately reduce oxygen to water. Recognizing which species are oxidized and reduced clarifies the roles of electron carriers and the electron transport chain.

Given Data / Assumptions:

• Overall reaction: glucose + oxygen → carbon dioxide + water + energy.
• Oxidation = loss of electrons; reduction = gain of electrons.
• We identify the species that gains electrons.

Concept / Approach:
Carbon atoms in glucose are oxidized to CO2 (higher oxidation state), meaning glucose loses electrons. Molecular oxygen (O2) accepts those electrons and protons to form water (H2O), so oxygen is reduced. CO2 is an oxidation product, not reduced; “energy” is not a chemical species and cannot be reduced. Water is the reduced product of oxygen, not itself the electron acceptor in this equation.

Step-by-Step Solution:

Verification / Alternative check:
Standard redox potentials show O2/H2O as a strong oxidant couple; its positive potential drives electron flow downhill through the respiratory chain.

Why Other Options Are Wrong:

• CO2: end product of oxidation of carbon; not reduced.
• H2O: product after reduction of O2; not the electron acceptor.
• Energy: not a chemical species; cannot be reduced.

Common Pitfalls:
Focusing on oxygen’s name rather than its role; despite the term “oxidation,” oxygen here is reduced to water.

Final Answer:
O2