This chapter explores the core principles of cellular metabolism, emphasizing the enzymatic mechanisms and bioenergetics involved in energy transformation. As heterotrophic organisms, we rely on the metabolic conversion of nutrients such as glucose, amino acids, and fatty acids into energy and materials necessary for growth and cellular functions. Cellular metabolism consists of a complex array of enzyme-driven reactions that transform these nutrients into chemical energy and form cellular macromolecules. The chapter covers the structure of metabolic pathways, the function of enzymes as biological catalysts, and the fundamentals of bioenergetics, which investigate how energy from nutrient metabolism or light is harnessed for cellular activities. It includes discussions on the thermodynamic factors affecting metabolic reactions, ATP production, and the enzymatic processes crucial for capturing energy, converting nutrients, and major degrading pathways to obtain ATPs.