Fermentation stoichiometry — oxygen requirement: In the classic fermentation of one molecule of glucose to ethanol and carbon dioxide (as in yeast), how many molecular oxygen (O2) molecules are required as reactants?

Introduction / Context:
Fermentation is an anaerobic energy-yielding process in which an organic molecule acts as both electron donor and electron acceptor. The textbook example is alcoholic fermentation, where glucose is converted to ethanol and carbon dioxide by yeast under oxygen-limited conditions.

Given Data / Assumptions:

• Alcoholic fermentation pathway: glucose → pyruvate → acetaldehyde → ethanol + CO2.
• Electron acceptor is an organic intermediate (acetaldehyde), not O2.
• Goal is NADH reoxidation to NAD+ to sustain glycolysis.

Concept / Approach:
Because no external terminal electron acceptor is used, molecular oxygen is not required as a reactant in fermentation. The pathway operates with internal redox balance: NADH produced in glycolysis is consumed when acetaldehyde is reduced to ethanol, regenerating NAD+.

Step-by-Step Solution:
Glycolysis: glucose → 2 pyruvate + ATP + NADH. Decarboxylation: pyruvate → acetaldehyde + CO2. Reduction: acetaldehyde + NADH → ethanol + NAD+. Count O2 reactants: none are used; answer is zero.

Verification / Alternative check:
Balanced equations show conservation of electrons without O2. In the presence of oxygen, yeast preferentially respire (Crabtree effect in high glucose aside), but the question specifies fermentation, which by definition excludes O2 as electron acceptor.

Why Other Options Are Wrong:

• 1, 2, 36, >10: These imply aerobic respiration or oxidative processes, not fermentation.

Common Pitfalls:
Confusing the need for small amounts of oxygen for sterol or unsaturated fatty acid synthesis in yeasts with the stoichiometric requirement of O2 in the fermentative pathway itself; the latter is zero.

Final Answer:
0 molecular oxygen molecules are required for alcoholic fermentation of glucose.