The main energy currency of the cell is adenosine triphosphate (ATP). This molecule stores and transports energy required for various biochemical reactions within cells. ATP is generated through cellular respiration, primarily in the mitochondria, using substrates like glucose and fatty acids. When energy is needed, ATP is broken down into adenosine diphosphate (ADP) and inorganic phosphate, releasing energy for cellular processes such as muscle contraction, nerve impulse transmission, and biosynthesis. ATP is critical for sustaining life, as it provides the energy necessary for maintaining cellular function and homeostasis.