Yeast Uses Alcoholic Fermentation to Produce Ethanol and CO₂
- Yeast regenerates NAD⁺ by converting pyruvate into ethanol and CO₂, not lactate like humans.
- This process is called alcoholic fermentation.
The Two-Step Process
- Decarboxylation of Pyruvate: Pyruvate loses a carbon atom, releasing carbon dioxide and forming ethanal (acetaldehyde).
- Reduction of Ethanal: Ethanal is reduced to ethanol by accepting hydrogen from NADH, regenerating NAD.
Common Mistake
- Don’t confuse anaerobic respiration with fermentation.
- Fermentation refers to the broader process of energy production in the absence of oxygen, while anaerobic respiration specifically describes the metabolic pathway.
Commercial Applications
- Brewing
- Yeast ferments sugars in grape juice (wine) or malted barley (beer) under anaerobic conditions, producing ethanol and CO₂.
- Fermentation stops when ethanol concentration reaches ~15% (toxic to yeast).
- Baking
- CO₂ produced by yeast gets trapped in dough, causing it to rise.
- The ethanol evaporates during baking, leaving no trace in bread.
Note
The ethanol produced during baking evaporates completely, leaving no trace in the finished bread.
Yeast vs. Humans
- Both use glycolysis to break down glucose.
- Yeast: Pyruvate → Ethanol + CO₂
- Humans: Pyruvate → Lactate
Hint
Humans produce lactate, while yeast produces ethanol and CO₂.
Self review
- What are the final products of anaerobic respiration in yeast?
- Why is NAD⁺ regeneration important in anaerobic respiration?
- How is anaerobic respiration used in baking?
- Why is the ATP yield from anaerobic respiration lower than from aerobic respiration?
