Fermentation

Essential Question

How do cells get energy when there is no oxygen?

Sprint as fast as you can, and your muscles run short of oxygen. Bread dough rises, and yogurt thickens, with no oxygen at all. In this lesson, you’ll see how cells keep making ATP without oxygen, what they produce instead, and why it gives them far less energy than aerobic respiration.

What Happens Without Oxygen?

• With O₂: aerobic respiration, about 30 ATP
• Without O₂: glycolysis, then fermentation
• Fermentation makes no more ATP, but lets glycolysis continue
• Total: only 2 ATP per glucose

Without oxygen, the electron transport chain stops, and so does the rest of aerobic respiration (Lesson 8.5). Glycolysis does not need oxygen, so it can keep going and make 2 ATP per glucose. But glycolysis needs a steady supply of NAD⁺. Fermentation is the extra step that recycles NADH back into NAD⁺, so that glycolysis can continue. Fermentation makes no more ATP itself. It happens in the cytoplasm, and it is called anaerobic because it does not use oxygen.

Lactic Acid Fermentation

• Where: cytoplasm
• Input: pyruvate from glycolysis
• Output: lactic acid; no CO₂
• Muscle cells, and bacteria used for yogurt

In lactic acid fermentation, pyruvate is converted to lactic acid. Your muscle cells do this during hard exercise when oxygen cannot reach them fast enough, and the lactic acid builds up and makes the muscles tire. When oxygen is available again, the lactic acid can be converted back into other molecules and used. Some bacteria make lactic acid fermentation their main way of getting energy. People use them to make yogurt, cheese, and sauerkraut. Lactic acid fermentation does not release CO₂.

Alcoholic Fermentation

• Where: cytoplasm
• Input: pyruvate from glycolysis
• Outputs: ethanol and CO₂
• Yeast: bread, beer, and biofuel

Alcoholic fermentation is done by yeast and some bacteria. Each pyruvate loses a CO₂ and becomes ethanol, the alcohol in beer and wine. The CO₂ makes bread dough rise, and the bubbles in beer and sparkling drinks. Ethanol can also be used as a fuel. Alcoholic fermentation makes the same 2 ATP per glucose as lactic acid fermentation, because the ATP comes from glycolysis alone.

How Much ATP Does Fermentation Make?

• Aerobic respiration: about 30 ATP per glucose
• Fermentation: 2 ATP per glucose
• Aerobic gives about 15 times as much
• Fermenting cells use a lot more glucose

Fermentation is much less efficient than aerobic respiration: 2 ATP compared with about 30 per glucose, about 15 times less. Most of the energy of glucose is still in the ethanol or lactic acid, which are not broken down further. A cell that relies on fermentation has to use a lot more glucose to make the same amount of ATP. This is why yeast ferment fast in a sugary solution, and why muscles can sprint using fermentation for only a short time.

Evidence: Yeast and Sugar

• Yeast make CO₂ as they ferment sugar
• More sugar: more CO₂, at first
• The graph levels off at a plateau
• Another factor then limits the rate

You can measure fermentation by collecting the CO₂ that yeast produce. In the sample data, the CO₂ produced in 10 minutes rises from 0 mL with no sugar to 28 mL at 8% sugar. It then levels off at 29 mL, and the same amount is made at 10% and 12% sugar. As in Lesson 8.3, a plateau means that something other than the factor being changed, such as the number of yeast cells or the temperature, has become the limiting factor.

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Worked Examples

Try each problem on your own first, then open the solution to check your work.

Example 1: Comparing ATP Yields

Compare aerobic respiration (about 30 ATP per glucose) with fermentation (2 ATP per glucose). (a) How many ATP are made from 6 glucose by each? (b) How many times as much is that? (c) How many glucose does each need to make 60 ATP?

Show the solution

1. (a) 6 glucose: aerobic: 6 × 30 = 180 ATP. Fermentation: 6 × 2 = 12 ATP.
2. (b) Compare: 180 ÷ 12 = 15 times as much
3. (c) For 60 ATP: fermentation: 60 ÷ 2 = 30 glucose. Aerobic: 60 ÷ 30 = 2 glucose.
4. Meaning: a fermenting cell needs about 15 times as much glucose for the same ATP.
5. Answer: 180 and 12 ATP, 15 times as much. For 60 ATP: 30 glucose by fermentation, 2 by aerobic respiration.

Example 2: Yeast and Sugar Concentration

Use the yeast data. (a) How much more CO₂ is made at 4% sugar than at 2% sugar? (b) What happens above 10% sugar? (c) State a claim about the plateau, with evidence and reasoning.

Show the solution

1. (a) Read the graph: 17 mL at 4% and 9 mL at 2%, so 17 − 9 = 8 mL more.
2. (b) Above 10%: the CO₂ stays at 29 mL at both 10% and 12% sugar.
3. (c) Claim: above about 10% sugar, sugar is no longer the limiting factor.
4. Evidence: raising sugar from 10% to 12% did not change the CO₂ (29 mL both times), while raising it from 2% to 4% added 8 mL.
5. Reasoning: the yeast already have all the sugar they can use, so another factor, such as the number of yeast cells or the temperature, limits the rate.
6. Answer: 8 mL more. Above 10% sugar the rate levels off at 29 mL, so another factor is limiting.

Example 3: Follow the Carbon in Fermentation

Use the model. Glycolysis changes glucose (6 C) to 2 pyruvate (3 C each). In alcoholic fermentation, each pyruvate becomes ethanol (2 C) and CO₂ (1 C). In lactic acid fermentation, each pyruvate becomes lactic acid (3 C). (a) Account for all 6 carbons in each kind of fermentation. (b) Which kind releases CO₂?

Kind of fermentation	Products from 1 glucose	Carbons
Alcoholic	2 ethanol + 2 CO2	?
Lactic acid	2 lactic acid	?

Show the solution

1. (a) Alcoholic: 2 ethanol × 2 C = 4 C, plus 2 CO₂ × 1 C = 2 C. Total: 4 + 2 = 6 C.
2. Lactic acid: 2 lactic acid × 3 C = 6 C. No other product, so total = 6 C.
3. Check: both add up to the 6 carbons in glucose, so carbon is conserved.
4. (b) CO₂: only alcoholic fermentation. This is why yeast make dough rise and muscles do not release CO₂ when they ferment.
5. Answer: Both account for 6 C. Only alcoholic fermentation releases CO₂.

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Practice

Label a model of fermentation, compare ATP made with and without oxygen, and use yeast data to find a limiting factor. Try it on your own first, then check your work with the answer key.

• Fermentation Practice Worksheet (PDF)
• Fermentation Practice Answer Key (PDF)

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Lesson Slides

• 8.6 Fermentation Slides (PDF)

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Big Ideas

• No O₂: glycolysis continues, with fermentation
• Fermentation: lactic acid or ethanol + CO₂
• Fermentation lets glycolysis continue but adds no ATP
• Only 2 ATP per glucose, about 15 times less than aerobic
• Both kinds happen in the cytoplasm

Back to the essential question: when oxygen is missing, the electron transport chain stops, but cells can still make 2 ATP per glucose by glycolysis, with fermentation recycling NADH so that glycolysis can continue. Muscle cells and some bacteria make lactic acid, and yeast make ethanol and CO₂. Fermentation makes about 15 times less ATP than aerobic respiration. Next, you’ll put photosynthesis and respiration together.

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