What is the relation between adenosine and adenosine triphosphate?

ATP is adenosine bonded to three phosphate groups. As phosphate groups are removed, ATP becomes ADP, then AMP, and after the last phosphate is removed, adenosine remains. 1

• ATP breakdown: Energy-consuming activity can break ATP down to ADP or AMP, releasing energy; further phosphate removal leaves adenosine. 2
• Adenosine and sleepiness: Adenosine can bind to receptors associated with sleepiness, contributing to the drive to sleep. 3
• Caffeine’s effect: Caffeine competes for and blocks those receptors, reducing the feeling of sleepiness without removing the accumulated adenosine. 4