The phosphate group is a molecule containing one atom of phosphorus covalently bound to four oxygen residues, two of which may be expressed as a hydroxyl group. They are relatively reactive molecules that readily form phophoester bonds by the interaction with hydroxyl groups.
Phosphates as an Energy store
Phosphates act as the main feature of the energy sources ATP and GTP. Due to the negative electrostatic charges on each phosphate group, it is difficult to cause them to bind three together in the arrangement seen in ATP and GTP. As a result, large amounts of energy are needed to form a bond between them. it is this high energy bond that acts as the storage system. Cleavage of this bond allows the release of this energy, to drive cellular activities, such as conformational changes in proteins or activation of an ATPase.
Phosphates in Protein Activity
Here phosphates act as the on/off switch for a protein. This process is phosphorylation and enables them to switch to an active state and perform their designated function. It is one of most commonly observed methods for cells alter protein activity on a short-term basis, and is a method for rapid change in protein activity compared to synthesis of new proteins.
Phosphate addition can act as either an on or off switch mechanism, it is dependant on the protein as to which activates. The process is controlled by two types of enzymes; the kinases (which adds a phosphate) and the phosphatases (which oppose this action and remove a phosphate).