Phosphagens are a group of molecules that includes creatine phosphate (in vertebrates), and arginine phosphate, lombricine, and phosphoopheline (in invertebrates). These molecules have similar functions in different organisms.

What function do these molecules have in common?

Phosphagen provides the needed energy for the muscle tissues which can not be immediately supplied by glycolysis or oxidative phosphorylation. They supply immediate but limited energy as sudden demands for lots of energy by the muscle tissues arise.

Explanation:

Phosphagens are high energy storage compounds majorly found in muscular tissue of animals.

They allow maintenance of the high energy phosphate stores in its normal concentration ranges which discard the problems associated with ATP-consuming reactions in these tissues as against the presence of adenosine triphosphate.

The muscle tissues are actively working and need constant supply of energy and the energy produced by glycolysis and oxidative phosphorylation might not sum up to the needs of the tissues. So therefore, phosphagens serve as a stand by mechanism for energy production for the tissues mostly during sustained muscle activity.

The man, the muscle cells' phosphocreatinine concentration is more than three times the concentration of ATP and represent a ready reserve of high energy phosphate that can be donated directly to Adenosine diphosohate to release energy.

Different organisms use different biomolecule as a phosphagen. Majority of animals use arginine as their phosphagen, chordates use creatinine, annelids use lombricine.

They all perform these similar functions described above.