Plants and fungi such as yeast can also respire without oxygen, for example yeast in a sealed fermenter or plant roots in waterlogged soil. As in mammals, the problem is regenerating NAD so that glycolysis can continue, but the route is different. It is called alcoholic fermentation.It happens in two steps in the cytoplasm:
1. Pyruvate (3C) is decarboxylated to ethanal (2C), releasing carbon dioxide. The enzyme is pyruvate decarboxylase.
2. Ethanal is reduced to ethanol by reduced NAD, catalysed by alcohol dehydrogenase. This regenerates NAD.pyruvate → ethanal + CO2 ethanal + reduced NAD → ethanol + NAD Overall:C6H12O6 → 2C2H5OH + 2CO2 The net yield is still only 2 ATP per glucose, from glycolysis.Differences from lactate fermentation:
• carbon dioxide is produced;
• there are two steps, not one;
• it cannot be reversed: ethanol cannot be converted back to pyruvate, so the energy in it is lost to the cell.Ethanol is toxic. Yeast cells die when the ethanol concentration reaches about 15%, which limits the strength of fermented drinks. Plants that survive flooding, such as rice, tolerate ethanol better.Humans use this process in brewing (ethanol) and baking (carbon dioxide makes bread rise).
1. Pyruvate (3C) is decarboxylated to ethanal (2C), releasing carbon dioxide. The enzyme is pyruvate decarboxylase.
2. Ethanal is reduced to ethanol by reduced NAD, catalysed by alcohol dehydrogenase. This regenerates NAD.
• carbon dioxide is produced;
• there are two steps, not one;
• it cannot be reversed: ethanol cannot be converted back to pyruvate, so the energy in it is lost to the cell.Ethanol is toxic. Yeast cells die when the ethanol concentration reaches about 15%, which limits the strength of fermented drinks. Plants that survive flooding, such as rice, tolerate ethanol better.Humans use this process in brewing (ethanol) and baking (carbon dioxide makes bread rise).