Complex organic food molecules such as sugars, fats and proteins are rich sources of energy for cells because much of the energy used to form these molecules is stored within the chemical bonds that hold them together. The cells release the stored energy through a series of oxidation reactions.

During each oxidation reaction involved in food breakdown, the product of reaction has a lower energy content than the donor molecule. At the same time, electron acceptor molecules capture some of the energy lost during oxidation and store it for later use. Cells convert the energy from oxidation reactions to energy-rich molecules such as ATP which can be used throught the cell to power metabolism and construct new cellular components.

The term energy currency refers to that molecule which provides energy for cellular activities, whenever required. ATP is termed as energy currency because the energy is present in the form of high energy bonds of ATP. Other energy yielding molecules are GTP, CTP, UTP, etc.

The conversion of ATP to ADP yields about $7.3 \mathrm{kcal} / \mathrm{mol}$ of energy. This is the energy source in a variety of biological processes occurring in both plants and animals.

Justification for the term 'energy currency' for ATP can be given as

(i) Store small packets of energy as soon as it is available thus, minimising its wastage.

(ii) Can make energy available to a distant location in cell away from where the site it is produced.

(iii) Can carry out heavy work/activity by continuosly supplying large amount of energy through its accumulation at one place.