Yeast bioenergetics in the alcohol industry: In industrial brewing and winemaking, Saccharomyces cells convert pyruvic acid (from glycolysis) into carbon dioxide (CO2) and ethyl alcohol (ethanol). What is the name of this metabolic process that proceeds under anaerobic conditions?

Introduction / Context:
Yeasts, especially Saccharomyces cerevisiae, power the alcohol industry by converting sugars into ethanol and carbon dioxide. This question checks whether you can correctly name the metabolic pathway that takes pyruvic acid, produced by glycolysis, and transforms it into the end products that make bread rise and beverages alcoholic.

Given Data / Assumptions:

• Substrate-level breakdown of glucose via glycolysis yields pyruvate (pyruvic acid).
• Conditions are anaerobic (very low or no oxygen).
• End products mentioned are CO2 and ethanol.

Concept / Approach:
Under anaerobic conditions, cells cannot route pyruvate into the full aerobic respiratory chain. Yeasts instead perform alcoholic fermentation: pyruvate is decarboxylated to acetaldehyde (releasing CO2) and then reduced to ethanol. The key purpose is regeneration of NAD+ so that glycolysis can continue to yield ATP by substrate-level phosphorylation when oxidative phosphorylation is unavailable.

Step-by-Step Solution:
Glucose is converted to 2 pyruvate via glycolysis, producing ATP and NADH. In anaerobiosis, pyruvate is decarboxylated to acetaldehyde + CO2 (enzyme: pyruvate decarboxylase). Acetaldehyde is reduced to ethanol using NADH, regenerating NAD+ (enzyme: alcohol dehydrogenase). This closed loop maintains glycolytic flux in the absence of oxygen and yields the observed products: CO2 and ethanol.

Verification / Alternative check:
Observe CO2 bubbles during brewing or dough proofing and measure ethanol accumulation; both confirm fermentation. Lab assays can track NADH/NAD+ cycling or detect alcohol dehydrogenase activity.

Why Other Options Are Wrong:

• Glycolysis: makes pyruvate; it does not convert pyruvate to ethanol.
• Respiration: requires oxygen as the terminal electron acceptor, producing CO2 and water, not ethanol.
• Krebs cycle: an aerobic pathway in mitochondria; pyruvate enters as acetyl-CoA, not producing ethanol.

Common Pitfalls:
Confusing glycolysis (upstream) with fermentation (downstream) or thinking ethanol comes from the Krebs cycle. Remember: fermentation regenerates NAD+ when oxygen is limited.

Final Answer:
fermentation