Which process involves the breakdown of complex molecules to release energy?

The process that involves the breakdown of complex molecules to release energy is catabolism. Catabolism encompasses a series of metabolic pathways that lead to the degradation of larger, more complex molecules into simpler ones. This breakdown releases energy, which is often conserved in the form of ATP (adenosine triphosphate), the primary energy carrier in living cells.

Catabolic pathways are essential for cellular respiration, where organic molecules such as carbohydrates, fats, and proteins are metabolized to produce energy. Examples include glycolysis, where glucose is broken down, and the citric acid cycle, where energy is extracted from acetyl-CoA derivatives.

In contrast, anabolism is the process by which cells synthesize complex molecules from simpler ones, often requiring an input of energy. Metabolism is a broad term that encompasses both catabolism and anabolism, representing the entirety of chemical reactions occurring in the body. Fermentation is a specific type of catabolic process that occurs in anaerobic conditions, where organic compounds are broken down to produce energy without the involvement of oxygen, but it is just one example of catabolic pathways. Thus, catabolism is the correct answer, as it specifically refers to the energy-releasing breakdown of complex molecules.