What process do cells use to produce energy in the absence of oxygen?

Cells produce energy in the absence of oxygen primarily through the process known as anaerobic respiration. During this process, cells convert glucose into usable energy (ATP) using pathways that do not require oxygen. Anaerobic respiration typically yields less energy compared to aerobic respiration but is essential for organisms or conditions where oxygen is not available.

In anaerobic respiration, different organisms have adapted to use various substrates and produce different byproducts. For example, in yeast, anaerobic respiration leads to fermentation, producing ethanol and carbon dioxide. In muscle cells, when oxygen levels are low, the process may also lead to lactic acid fermentation.

Anaerobic respiration is vital for certain environments and conditions, such as in deep water, soil, or even in human muscles during intense exercise. Understanding this process is crucial for grasping how diverse life forms adapt to varying energy needs and environmental conditions.