The main energy source for cellular activities is adenosine triphosphate (ATP). ATP serves as the primary energy currency within cells, providing the energy necessary for various biochemical reactions, such as muscle contraction, nerve impulse propagation, and biosynthesis. ATP is generated through cellular respiration, primarily in the mitochondria, where glucose and other substrates are metabolized. Additionally, ATP can be produced via photophosphorylation during photosynthesis in plants. Understanding the role of ATP is crucial in biology, as it underlies energy transfer and utilization across all living organisms.