Food molecules are the $1,000 dollar bills of energy storage. They function as fuel molecules, storing large quantities of energy in a stable form over long periods of time. They are the long-term energy currency of the cell.
For "pocket change", cells require a molecule that stores much smaller quantities of energy, that can be used in synthetic reactions like the formation of sucrose, and that can be used over and over again.
Such a molecule is adenosine triphosphate (ATP).
This molecule acts as the short-term energy currency of the cell and provides the source of energy used in individual synthetic (nonspontaneous) reactions.
ATP collects small packets of energy from the food-burning power plants of the cell and transports this energy to where it is needed.
Some energy in ATP is released to do work, such as move muscles or force a seedling out of the ground. At other times, ATP gives up its energy to a nonspontaneous synthetic reaction, such as the formation of sucrose.
ATP is used to close the energy gap between energy-releasing reactions (food breakdown) and energy-requiring reactions (synthesis).
When a molecule of fatty acid is burned, energy is given off. Some of this energy is trapped in molecules of ATP, and some is lost in the form of heat. Each ATP molecule can then be transported elsewhere within the cell and used where needed.
The energy-carrying part of an ATP molecule is the triphosphate "tail". Three phosphate groups are joined by covalent bonds. The electrons in these bonds carry energy.
Within the power plants of the cell (mitochondria), energy is used to add one molecule of inorganic phosphate (P) to a molecule of adenosine diphosphate (ADP).
The amount of energy stored is about 7,300 calories for every mole of ATP formed.
At the energy-requiring site, the last phosphate group in the tail is broken off and the energy in the bond liberated.
Again, about 7,300 calories of energy per mole is released. The ADP and the phosphate are then free to return to the power plant and be rejoined. In this way, ATP and ADP are constantly being recycled.