Fermentation

No oxygen, what then?

Glycolysis makes 2 NADH from each glucose. The cell has only a small stock of NAD$^+$. If all of it is turned into NADH, glycolysis stops, because the oxidation of PGAL needs NAD$^+$.

So the cell must change NADH back into NAD$^+$. If oxygen is present, the mitochondria do this. If there is no oxygen, fermentation does it. In fermentation, pyruvic acid takes the hydrogen from NADH. That is why this stage is a part of respiration and not a waste.

Alcoholic fermentation

Yeast carries out alcoholic fermentation. It takes two steps.

1. The enzyme pyruvic acid decarboxylase removes a carbon dioxide from each pyruvic acid. The product is acetaldehyde. Two pyruvic acid molecules give 2 carbon dioxide molecules.
2. The enzyme alcohol dehydrogenase passes hydrogen from NADH + H$^+$ to acetaldehyde. The product is ethanol, and NAD$^+$ is made again.

For one glucose: 2 ethanol, 2 carbon dioxide and 2 net ATP.

Lactic acid fermentation

Some bacteria make lactic acid from pyruvic acid. In the cells of our muscles, the same thing happens when a hard exercise uses oxygen faster than the blood can bring it.

There is only one step. The enzyme lactate dehydrogenase reduces pyruvic acid to lactic acid. The reducing agent is NADH + H$^+$, which is oxidised back to NAD$^+$. No carbon dioxide is released.

For one glucose: 2 lactic acid and 2 net ATP.

The two side by side

• Alcoholic: two enzymes, pyruvic acid decarboxylase and then alcohol dehydrogenase.
• Lactic acid: one enzyme, lactate dehydrogenase.

How good is fermentation?

Fermentation is poor value.

• Less than 7 per cent of the energy of glucose is released, and only a part of that is trapped in ATP.
• The net gain is only 2 ATP for each glucose.
• The product is harmful in the end, either an acid or an alcohol. Yeast poisons itself when the alcohol reaches about 13 per cent.

For a cell to get all the energy of the glucose, it must oxidise the pyruvic acid completely. That needs oxygen, and the next course follows that story.

You should find that the ATP and the NAD$^+$ do not change with the kind, but the carbon dioxide appears only in the alcoholic kind.

Try it in Python

The program compares the products of the two fermentations of some glucose.

Practise with new numbers

Common mistakes

• Saying that lactic acid fermentation releases carbon dioxide. It does not.
• Saying that fermentation needs oxygen. It is the way to go on without it.
• Forgetting that NADH is turned back into NAD$^+$. That is the purpose of fermentation. Glycolysis cannot go on without it.
• Thinking that fermentation makes a lot of ATP. The net gain is only 2 ATP for each glucose.
• Giving one enzyme for the whole alcoholic fermentation. There are two: pyruvic acid decarboxylase, then alcohol dehydrogenase.

Remember these

• Fermentation takes place without oxygen and makes NAD$^+$ again.
• Alcoholic: pyruvic acid decarboxylase gives acetaldehyde and carbon dioxide, and alcohol dehydrogenase gives ethanol. Yeast.
• Lactic acid: lactate dehydrogenase. Some bacteria and muscle cells. No carbon dioxide.
• Net gain of 2 ATP, and less than 7 per cent of the energy of glucose released. Yeast dies at about 13 per cent alcohol.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Why must NADH be turned back into NAD$^+$ when there is no oxygen?

2. Name the two enzymes of alcoholic fermentation, in order.

3. How does the lactic acid fermentation of muscle differ from alcoholic fermentation in its products?

4. Why is fermentation called a poor source of energy?

5. What happens to yeast when the alcohol in its medium reaches about 13 per cent?