1. Anaerobic Respiration:
* Yeast cells, like many other organisms, prefer to use aerobic respiration when oxygen is available. This is a highly efficient process that produces a lot of energy (ATP) from glucose.
* However, when oxygen is limited or absent, yeast cells switch to anaerobic respiration, also known as fermentation.
2. Fermentation Process:
* Glycolysis: The first step of both aerobic and anaerobic respiration is glycolysis, which breaks down glucose into pyruvate. This process generates a small amount of ATP.
* Pyruvate Reduction: In fermentation, instead of pyruvate entering the Krebs cycle and electron transport chain (like in aerobic respiration), it is reduced to a different product, depending on the type of fermentation:
* Alcoholic fermentation: Pyruvate is converted into ethanol and carbon dioxide.
* Lactic acid fermentation: Pyruvate is converted into lactic acid.
3. Why Fermentation?
* Energy Production: While fermentation is far less efficient than aerobic respiration, it still generates a small amount of ATP, which is enough to keep the yeast cell alive.
* Regeneration of NAD+: A key enzyme in glycolysis, glyceraldehyde 3-phosphate dehydrogenase (GAPDH), requires NAD+ (nicotinamide adenine dinucleotide) as a cofactor. Fermentation regenerates NAD+ from NADH, which is essential for glycolysis to continue.
In summary:
Yeast cells can perform fermentation because they have the necessary enzymes to break down glucose into products like ethanol or lactic acid in the absence of oxygen. This process provides a limited source of energy and regenerates NAD+, allowing glycolysis to continue and the yeast to survive.
