Final answer:
In the absence of oxygen, organisms can generate ATP through fermentation or anaerobic respiration, which involve glycolysis and the use of alternative electron acceptors instead of oxygen.
Step-by-step explanation:
In the absence of oxygen, organisms have developed various alternative pathways to generate energy, which are critical for survival in anoxic conditions. Fermentation and anaerobic respiration are two principal anaerobic processes that allow for ATP production without oxygen.
Fermentation
An important way of making ATP without oxygen is called fermentation. This process involves glycolysis, but not the other stages of aerobic respiration. Lactic acid fermentation is one form, where pyruvate is converted to lactic acid and NADH is regenerated into NAD+, keeping glycolysis running. This is evident in human muscle cells during intense exercise when oxygen is scarce. Moreover, alcoholic fermentation converts pyruvate into ethyl alcohol and CO2, also regenerating NAD+.
Anaerobic Respiration
In anaerobic conditions, organisms can also perform anaerobic respiration, where inorganic molecules other than oxygen act as the final electron acceptor. For instance, methanogens use carbon dioxide to oxidize NADH to methane. Other bacteria can perform this type of respiration using substances like sulfate as electron acceptors.
These alternative pathways are vital not only for individual organisms' survival but also play an important role in global cycles like the carbon and nitrogen cycles.