Role of ATP/ADP

Adenosine diphosphate (ADP)/Adenosine triphosphate (ATP)
ATP is one of the principal chemical compounds that cells use to store and release energy. ATP consites of adenine, a five carbon sugar called ribose, and three phosphate groups.

external image ATP.gif

ATP has enough energy to power a variety of cellular active transport across cell membranes, protein synthesis, and muscle contraction. The energy in ATP is released by breaking the bond between the second and third phosphate. Because a cell can subtract that third phosphate group, it can release energy whenever it is needed.

ADP is a compound that looks almost like ATP, except that it has two phosphate groups instead of three.
Picture of ADP molocule
This stores energy made from cellular resperation. When a cell has energy avalible, it can store small amounts of it by adding a phosghate group to ADP molocules, producing ATP.

What is the difference between ATP and ADP?

ATP has 3 phosphates and ADP only has 2