During oxidation within a cell, all the energy contained in respiratory substrates is not released free in a single step. It is released in a series of slow step-wise reactions controlled by enzymes and it is trapped as chemical energy in the form of ATP. The significance of this step-wise release of energy is that some energy is used to synthesise ATP. This ATP is stored for the later utilisation wherever, required. Hence, ATP acts as the energy currency of cell.
The energy stored in ATP can be utilised following
(i) In various energy requiring processes of organisms.
(ii) The carbon skeleton produced during respiration is used as precursors for the synthesis of other molecules.