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ADP, or adenosine diphosphate, derives from the purine base adenine bound to a ribose sugar, forming the nucleoside adenosine. When a phosphate group attaches, the molecule becomes a nucleotide: adenosine monophosphate (AMP). Adding a second phosphate yields ADP, and a third creates the high‑energy adenosine triphosphate (ATP). AMP, together with other monophosphate nucleotides, constitutes the building blocks of DNA.
ATP stores the energy that drives virtually every biochemical reaction. Converting ADP back into ATP requires an input of energy—plants harness sunlight in photosynthesis, while animals metabolize glucose. Once formed, ATP releases energy when it hydrolyzes to ADP, allowing cells to perform work. Cells recycle their ATP/ADP pool roughly every minute; without this cycle, an organism would need to consume its own body mass in ATP each day to survive.
ATP powers muscle contraction by enabling actin–myosin cross‑bridge cycling. A myosin head binds an actin filament, hydrolyzes ATP to ADP, releases the filament, then rebinds to begin another cycle. This process underpins all muscular movement, from heartbeats to reflexes.
Beyond energy transfer, ADP and ATP orchestrate numerous physiological functions. They facilitate ion transport that generates neuronal signals, and ADP released by platelets recruits more platelets to seal vascular injuries. Additionally, ADP influences DNA repair mechanisms and gene regulation, helping cells respond to damage and adapt to new conditions.
