Cells need a steady supply of ATP to power movement, active transport, biosynthesis, and many other life processes. Glucose is a common fuel, but cells can extract its energy in different ways depending on whether oxygen is available. Cellular respiration uses oxygen and releases much more ATP from each glucose molecule.
Fermentation does not require oxygen, but it produces far less ATP and mainly keeps glycolysis running.
Key Facts
- Overall aerobic respiration: C6H12O6 + 6 O2 -> 6 CO2 + 6 H2O + about 30 to 32 ATP
- Glycolysis: glucose -> 2 pyruvate + 2 ATP net + 2 NADH
- Fermentation occurs after glycolysis when O2 is unavailable and regenerates NAD+ from NADH.
- Lactic acid fermentation: pyruvate + NADH -> lactate + NAD+
- Alcoholic fermentation: pyruvate -> ethanol + CO2, while NADH is oxidized to NAD+
- ATP yield per glucose: fermentation = 2 ATP net, aerobic respiration = about 30 to 32 ATP
Vocabulary
- ATP
- ATP is the main energy-carrying molecule cells use to power chemical work.
- Glycolysis
- Glycolysis is the cytoplasmic pathway that splits glucose into two pyruvate molecules and produces a net gain of 2 ATP.
- Aerobic respiration
- Aerobic respiration is the oxygen-requiring process that breaks down glucose to produce large amounts of ATP.
- Fermentation
- Fermentation is an anaerobic process that regenerates NAD+ so glycolysis can continue making a small amount of ATP.
- NAD+
- NAD+ is an electron carrier that accepts electrons during glycolysis and must be regenerated for glycolysis to continue.
Common Mistakes to Avoid
- Saying fermentation makes no ATP is wrong because glycolysis still produces a net gain of 2 ATP before fermentation regenerates NAD+.
- Thinking oxygen is used in fermentation is wrong because fermentation is anaerobic and occurs when oxygen is absent or limited.
- Forgetting that glycolysis happens before both pathways is wrong because both aerobic respiration and fermentation begin with glucose being split into pyruvate.
- Claiming lactic acid fermentation and alcoholic fermentation have the same products is wrong because lactic acid fermentation produces lactate, while alcoholic fermentation produces ethanol and carbon dioxide.
Practice Questions
- 1 A muscle cell breaks down 12 glucose molecules by lactic acid fermentation. How many net ATP molecules are produced?
- 2 A yeast cell breaks down 5 glucose molecules by alcoholic fermentation. How many pyruvate molecules are produced during glycolysis, and how many net ATP molecules are made?
- 3 A runner sprints so intensely that oxygen delivery to muscle cells cannot keep up with demand. Explain why fermentation helps the cells keep working briefly even though it produces much less ATP than aerobic respiration.