Fermentation (SQA National 5 Biology): Revision Note

Fermentation is also known as anaerobic respiration

• It refers to respiration in the absence of oxygen

The fermentation reactions take place in the cytoplasm of cells, rather than in the mitochondria

Fermentation starts in the same way as aerobic respiration

• A molecule of glucose is broken down to two molecules of pyruvate, releasing enough energy to yield two molecules of ATP

In the absence of oxygen, the two pyruvate molecules are converted into lactate (lactic acid) instead of carbon dioxide and water

From one glucose molecule, four molecules of ATP are produced, but two are used to start the process, therefore giving a net gain of only two ATP molecules overall

• This is much less than aerobic respiration

• This is why anaerobic respiration is less efficient

In plant and yeast cells, the process of fermentation is different from that of animal cells

Glucose is again broken down into two molecules of pyruvate, which releases a net gain of two molecules of ATP

• Remember that four molecules of ATP are produced, but two are used to start the process

The two pyruvate molecules are converted to carbon dioxide and ethanol (alcohol)

• No extra ATP is made during the conversion to carbon dioxide and ethanol

The products of fermentation in plant and yeast cells are used commercially to make bread rise and to brew alcoholic drinks