Which molecule is used by living things to store and transfer energy

This molecule is ATP (adenosine triphosphate). It is a complex composed of the nucleoside adenosine and a tail consisting of three phosphates.

As far as known, all organisms from the simplest bacteria to humans use ATP as their primary energy currency. The energy level it carries is just right amount for most biological reactions.

The total human body content of ATP is only about 50 grams which must be constantly recycled everyday. Energy is usually liberated from the ATP molecule to do work in the cell by a reaction which removes one of the phosphate groups.