Fermentation is a set of pathways that lets cells keep making a small amount of ATP when oxygen is unavailable or limited. It begins after glycolysis, the process that breaks glucose into pyruvate and captures energy in ATP and NADH. Fermentation matters because it keeps glycolysis running by recycling NADH back into NAD+.
This is why some microbes, muscle cells, and food production processes can function without aerobic respiration.
Key Facts
- Glycolysis: glucose + 2 NAD+ + 2 ADP + 2 Pi → 2 pyruvate + 2 NADH + 2 ATP + 2 H2O
- Fermentation does not make ATP directly, but it allows glycolysis to continue making ATP.
- The main purpose of fermentation is to regenerate NAD+ from NADH.
- Lactic acid fermentation: pyruvate + NADH → lactate + NAD+
- Alcoholic fermentation: pyruvate → acetaldehyde + CO2, then acetaldehyde + NADH → ethanol + NAD+
- Net ATP yield from fermentation with glycolysis is 2 ATP per glucose molecule.
Vocabulary
- Fermentation
- Fermentation is an anaerobic process that regenerates NAD+ so glycolysis can continue producing ATP.
- Glycolysis
- Glycolysis is the pathway that splits one glucose molecule into two pyruvate molecules and produces a net gain of 2 ATP.
- NAD+
- NAD+ is an electron carrier that accepts electrons during glycolysis and is needed for the pathway to keep running.
- Lactic Acid Fermentation
- Lactic acid fermentation converts pyruvate into lactate while oxidizing NADH back to NAD+.
- Alcoholic Fermentation
- Alcoholic fermentation converts pyruvate into ethanol and carbon dioxide while regenerating NAD+.
Common Mistakes to Avoid
- Saying fermentation produces lots of ATP is wrong because the ATP comes only from glycolysis, giving a net yield of 2 ATP per glucose.
- Forgetting NAD+ regeneration is wrong because without NAD+, glycolysis stops and the cell loses its quick anaerobic ATP source.
- Mixing up lactic acid and alcoholic fermentation is wrong because lactic acid fermentation makes lactate, while alcoholic fermentation makes ethanol and CO2.
- Thinking fermentation requires oxygen is wrong because fermentation is used when oxygen is absent or not being used as the final electron acceptor.
Practice Questions
- 1 A yeast cell ferments 12 glucose molecules. How many net ATP molecules are produced by glycolysis during this fermentation?
- 2 If one glucose molecule produces 2 NADH during glycolysis, how many NAD+ molecules must be regenerated to fully ferment 8 glucose molecules?
- 3 During intense exercise, muscle cells may switch more strongly to lactic acid fermentation. Explain why regenerating NAD+ helps the muscle cell continue producing ATP even when oxygen delivery is limited.