The term for a reaction that breaks down large molecules into smaller units is called catabolism. Catabolic reactions involve the decomposition of complex macromolecules, such as proteins, lipids, and carbohydrates, into simpler molecules. This process is essential for cellular metabolism, as it releases energy stored in chemical bonds that can be harnessed for cellular activities. Catabolism is a part of the larger metabolic pathway, which includes anabolic reactions that construct larger molecules from smaller subunits. This balance between catabolism and anabolism is crucial for maintaining energy homeostasis in living organisms.