Final answer:
In the absence of oxygen, cells must perform fermentation to produce ATP and recycle NADH into NAD+, enabling glycolysis to continue. There are two main types of fermentation, alcoholic and lactic acid fermentation, both of which play crucial roles in energy production under anaerobic conditions.
Step-by-step explanation:
When oxygen is not available, cells must perform fermentation to function. Fermentation allows for the production of ATP from glucose by anaerobic respiration. It is a vital metabolic process that enables cells to regenerate NAD+ from NADH, thus allowing glycolysis to continue and produce energy for the cell.
There are two important types of fermentation: i) alcoholic fermentation ii) lactic acid fermentation.
Alcoholic fermentation is used by yeast and some types of bacteria to convert carbohydrates into alcohol and carbon dioxide, while lactic acid fermentation occurs in human muscles during intense exercise when oxygen is scarce, converting pyruvate into lactic acid.
It is important to understand that fermentation does not involve an electron transport chain and directly produces only a small amount of ATP. The main purpose of fermentation is to oxidize NADH to NAD+ so that glycolysis can continue, indirectly supporting ATP production in the absence of oxygen.