Which process is primarily responsible for generating energy in cells when oxygen is not available?

The process that generates energy in cells when oxygen is not available is anaerobic respiration. During anaerobic respiration, cells convert glucose into energy through pathways that do not require oxygen. This process is crucial for many organisms, especially those that live in environments devoid of oxygen or during conditions when oxygen cannot be supplied quickly enough to meet energy demands.

In contrast, aerobic respiration depends on oxygen for the complete breakdown of glucose, producing carbon dioxide and water alongside a large amount of energy. Photosynthesis, while an essential biological process, is primarily a way for plants to capture solar energy and convert it into chemical energy in the form of glucose; it does not directly generate energy in the absence of oxygen. Cellular respiration encompasses both aerobic and anaerobic processes but does not specifically address the scenario where oxygen is unavailable. Thus, anaerobic respiration is the correct choice since it specifically refers to energy generation under low or absent oxygen conditions.