When yeast cells are O₂ starved, fermentation serves as the source of energy. This results in the production of:

Key Points

• In cellular respiration, glucose is converted into pyruvate in the cytoplasm by the process of glycolysis.
• The fate of pyruvate then depends on the presence or absence of oxygen.
• Fermentation is the process by which molecules such as glucose are broken down anaerobically.
• There are two main types of fermentation:

Alcohol Fermentation - 

• Alcoholic fermentation is the process by which ethanol and carbon dioxide are produced from sugars like fructose, glucose, and sucrose by yeasts during their cellular respiration.
• It is an anaerobic process. i.e. it occurs in the absence of oxygen.
• Alcoholic fermentation has many uses such as for the production of alcoholic beverages, ethanol fuel, and bread cooking.

Lactic Acid Fermentation - 

• This type of fermentation occurs in animal muscle cells.
• Sugar or glucose is converted into lactic acid and CO₂.
• During exercise, the occurrence of muscle cramps is due to lactic acid formation.
• In this case, the energy expenditure is faster than the oxygen supplied to the muscle cells, which creates an anaerobic condition.

Hence, the correct answer is option 1.

Additional Information

• In aerobic conditions, the pyruvate is converted to acetyl-coA, which then enters the TCA cycle for ATP synthesis.
• This process takes place in the mitochondria of a cell.