The primary form of energy used by living organisms is adenosine triphosphate (ATP). ATP serves as the energy currency within cells, powering various biological processes, including metabolism, muscle contraction, and nerve impulse transmission. It is produced during cellular respiration and photosynthesis, allowing organisms to convert stored energy from food into a usable form. ATP's ability to release energy quickly makes it essential for cellular activities that require immediate energy, highlighting its critical role in sustaining life and enabling organisms to respond to environmental changes.