The process by which living organisms produce energy without oxygen is called anaerobic respiration. This metabolic pathway allows cells to generate energy in the absence of oxygen, using alternative electron acceptors. In many organisms, such as yeast, this leads to fermentation, producing byproducts like ethanol and carbon dioxide. In animal cells, anaerobic respiration results in the production of lactic acid. Anaerobic respiration is crucial for organisms living in environments devoid of oxygen and during intense physical activity when oxygen is limited, enabling them to sustain energy production even under challenging conditions.