Final answer:
Cells must be in an oxygen-deprived environment to engage in fermentation, which results in lactic acid fermentation in animal cells and alcohol fermentation in yeast. This process regenerates NAD+, allowing glycolysis to continue and ATP to be produced even without oxygen.
Step-by-step explanation:
The condition that must exist for a cell to engage in fermentation is the absence of oxygen. Fermentation is a metabolic process that converts sugar to acids, gases, or alcohol in the absence of oxygen (an anaerobic environment). In animal cells, this often results in lactic acid fermentation, where pyruvate, the end product of glycolysis, is reduced to form lactate. This type of fermentation allows for the regeneration of NAD+, which is essential for the continuation of glycolysis, even when oxygen is not available to the cell.
When oxygen is scarce, such as during intense exercise when muscle cells may not receive enough oxygen, they switch to lactic acid fermentation to ensure energy production. In microorganisms like yeast, fermentation leads to alcohol production, which humans have harnessed for making alcoholic beverages and causing bread to rise. Furthermore, various bacteria can switch between aerobic respiration and fermentation depending on whether oxygen is present (facultatively anaerobic) or may solely rely on fermentation in the absence of oxygen (obligate anaerobes).
The balanced chemical equation for the fermentation of glucose by yeast is:
C6H12O6 → 2 C2H5OH + 2 CO2
This reaction depicts the conversion of glucose to ethanol and carbon dioxide.