ADP-ATP system  of energy  exchange :

In 1929 Lohmann, Fiske and  subbarow  published  their  discovery  that the energy  exchanges  compounds in all biological  systems  are two  phosphorylated  derivatives  of adenosine  monophosphate  AMP. Viz. Adenosine diphosphate ADP and adenosine  triphosphate ATP  AMP is a d nucleotide  formed by the  combination  ofadenine  ribosome sugar and a phosphate molecule .It takes  part in the  composition  of DNA and RNA also  when  a second   phosphate molecule is  added  to AMP  it become ADP and this  addition of  a third  phosphate molecule  is added  to AMP., It molecule of ADP.

The terminal  phosphate bonds  to one in ADP  two in ATP depicted by  instead of are  high energy bonds because these bind about 2.5 times  more energy then ordinary phosphate bonds. An ordinary  phosphate bond binds about 3000calories of energy, whereas  each  of the  high energy terming  phosphate  bonds  of ATP binds  about 7300 calories of energy under  standard  conditions and about1200 calories under  conditions normally found in the body to make a distinction the C-C and C-H  bond energy of  glucose  and other fuel molecules released in oxidation  processes,  is called   liberate energy where the energy  stored in  terminal  high  energy bonds of ATP is called bond  energy  the latter is the Biological energy.

A molecule of ADP is always  coupled with  catabolic  reaction  in which liberate  energy is  to be  released. Hence  the released  energy  is directly  transferred ADP is  stored in a high energy  bond  that  links  a new  phosphate molecule to the ADP molecule  by dehydration .forming  a molecule of ATP. Similarly  molecules of ATP are coupled  with  anabolic  reactions  so that energy  is transferred      directly  to the   reactions by  breaking  off the  terminal  phosphate bond  of ATP is consequently  reduced to  ADP. That  is how ATP  serves  as the common  obligatory  intermediate  compound, or energy  broker ( broker of energy  currency)  or bioenergy  shuttle.  That connects  exergonic  to endergonic  reactions. Thus  ADP =ADP  is the  most  important energy transfer  in living  systems.

Power  house of cell  factory: The  chief  sites of catabolic breakdown  of fuel compounds  for  energy production and ATP formation  in the cells  fuel mitochondria .The  latter  are therefore  called  power house  or power  plants  of the cell factory  .Form  mitochondria  the ATP molecules are  transported  into  cytoplasm which  is the  chief  site  of their  utilization.

Use  of bioligical  energy  : Cellular  processes that  consume  energy  include.

Active transport: or materials into or out  of the cells biosynthesis of new macromolecules   for repair  growth  and multiplication.

Secretion  of specialized products  like hormones  enzyme or excretions 

Mechanical work : like contraction  streaming  or cyclosis  movement  of  parts or particles  within  the cytoplasm.

Electric  work  like never impulse transmission and  production  of electricity  or bio luminescence

When skeletal  muscles  are at work .these  spend  a large  amount  of energy  in a short time .Thererfore  a compound creating phosphate serves  in these  muscles  as a reservoir  of high energy phosphate  bonds. It apidly  transfers  its phosphate bond to ADP to ensure  a regular supply  of ATP to the working  muscle fibres.