Metabolism is all about biochemical reactions that happen within a cell. Energy is stored in adenosine trisphosphate ATP. The two classes of metabolism reactions are: Catabolic pathways- break larger molecules into smaller products. It is exergonic, and energy is released. Anabolic pathways synthesize large molecules from smaller products from catabolism to form larger molecules. It is endergonic, meaning it requires more energy. Oxidation reaction donates electrons while reduction reaction accepts electrons.