When we discuss on how are atp and adp related, we should focus in three aspects like their role in the body, their chemistry and finally their synthesis and location.
ATP and ADP are both involved in energy generation to the body, Both of them are energy molecules which can impart required energy for different physiological processes.
However, the ATP molecule (Adenosine triphosphate) has high levels of energy. ADP which is adenosine diphosphate is the parent molecule to ATP and it has low energy comparatively.
Chemically both have adenosine in their structure which is a nitrogenous base adenine, a five-carbon sugar namely ribose, and the phosphate groups. ATP molecule has one phosphate group more than its precursor ADP.
Both ATP and ADP have one monomer of adenine nitrogen base and also one carbohydrate monomer i.e. ribose a 5 carbon sugar.
The phosphate groups are present in the α, β and γ positions. When hydrolyzed, the energy is released, due to removal of phosphate group on γ position.
The phosphoanhydride bonds present in these molecules are the reason for high energy generation. These bonds are present in both ATP and ADP. So, the energy is stored in the bonds.
I Mole of ATP releases 30 kilo joules of energy by single bond break. When ATP converts directly to AMP, the energy released is 45 KJ/ Mol. ATP is broken down to form APD and ADP is made into ATP in a constant cycle.