ATP

ATP is the immediate source of energy in a cell 

Plant and animal cells release energy from glucose - this process is called respiration 

A cell cannot get it's energy directly from glucose 

So, in respiration, the energy released from glucose is used to make ATP (Adenosine triphosphate) 

ATP is made from the nucleotide base adenine, combined with a ribose sugar and three phosphate groups. It is what's known as a nucleotide derivative because it's a modified form of a nucleotide. 

Once the ATP is made, ATP diffuses to the part of the cell of which requires energy. 

The energy in ATP is stored in high energy bonds between the phosphate groups. It is therefore realised via hydrolysis reactions. 

ATP is quickly made and used 

When energy is needed by a cell, ATP is broken down into ADP (adenosine diphosphate) and Pi (inorganic phosphate). 

This reaction that occurs is a hydrolysis reaction. A phosphate bond is broken and energy is released, this reaction is catalysed by the enzyme ATP hydrolase. 

ATP hydrolysis can be 'coupled' to other energy-requiring reactions in the cell - this means the energy released can be used directly to make the coupled reaction happen, rather than being lost as heat. 

The released inorganic phosphate can also be put to use - it can be added to another compound (known as phosphorylation), which often makes the compound more reactive 

ATP can be re-synthesised in a condensation reaction between ADP and Pi. This happens during both respiration and photosynthesis, and is catalysed by the enzyme ATP synthase.