How do yeast cells perform anaerobic respiration?

Yeast cells perform anaerobic respiration, also known as fermentation, by converting glucose into ethanol and carbon dioxide.

In the absence of oxygen, yeast cells undergo a process called anaerobic respiration or fermentation. This is a metabolic pathway that allows them to continue producing energy, albeit less efficiently than aerobic respiration. The process begins in the same way as aerobic respiration, with the breakdown of glucose in the cytoplasm to produce pyruvate in a process known as glycolysis. This process also produces a small amount of ATP, the cell's energy currency, and NADH, a carrier molecule for electrons and hydrogen ions.

However, in the absence of oxygen, the pyruvate cannot be fully oxidised in the mitochondria as it would be in aerobic respiration. Instead, it undergoes fermentation. In yeast cells, this involves the conversion of pyruvate into ethanol and carbon dioxide in a two-step process. First, pyruvate is decarboxylated to form ethanal, releasing a molecule of carbon dioxide. This reaction is catalysed by the enzyme pyruvate decarboxylase and requires a coenzyme derived from vitamin B1, thiamine pyrophosphate.

The ethanal is then reduced to ethanol by the enzyme alcohol dehydrogenase, using the hydrogen ions and electrons carried by NADH. This regenerates the NAD+ needed for glycolysis to continue, allowing the cell to keep producing ATP even in the absence of oxygen. This process is crucial for yeast cells, as it allows them to survive in environments where oxygen is scarce or absent, such as deep within a lump of dough. However, it is less efficient than aerobic respiration, producing only 2 ATP molecules per glucose molecule compared to the 38 ATP molecules produced by aerobic respiration.