ATP is short for adenosine triphosphate, and it is a modified RNA molecule, used as an immediate energy source. It is composed of adenine (nitrogenous base), ribose and three phosphate groups.
When the terminal phosphate is removed from ATP, the molecule only contains two phosphate groups - therefore it is called adenosine diphosphate.
Comparison - ATP and Glucose
Glucose is also used for the storage of energy - but is is released through a long series of reactions. Therefore, it takes longer to release and is not a good source of immediate energy.
However, during cellular respiration, glucose is metabolised which releases 'free energy' - this is used to form a chemical bond. This joins ADP and Pi to produce ATP.
If glucose is used to provide energy for a reaction, the excess energy is lost as heat and as a result it is not good for low-energy reactions.
By storing energy in ATP, it allows energy to be transferred to other molecules. There is can be used as chemical energy.
The net yield from aerobic respiration is around 38 molecules of ATP per glucose molecule that enters aerobic respiration. However, it is not a good storage molecule as it naturally reverts back to ADP
Purpose
ATP is needed for a wide variety of processes, these include:
- Metabolism
- Movement
- Active transport
- Thermoregulation
- Molecular synthesis
- Cell maintenance and repair
Where ATP is used - further detail
Metabolic processes are processes such as the building of macromolecules e.g. starch and polypeptides
Movement e.g. when the filaments of a muscle can slide past one another, shortening the muscle fibres
Active transport allows the movement of molecules against their concentration gradient e.g. changing the shape of carrier proteins in the plasma membrane
Secretion is the formation of lysosomes
Activation of molecules - the inorganic phosphate released during the hydrolysis of ATP can phosphorylate other compounds. This makes them more reactive and can lower the activation energy needed for a reaction to occur
The reactions
Hydrolysis
When energy is required for a reaction, ATP is hydrolysed. The symbol equation for the reaction is:
ATP + H20 --> ADP + Pi + E
Alternatively, the word equation is:
ATP + water --> ADP + inorganic phosphate + energy
Condensation
ATP is synthesised during this reaction, using energy that is released during respiration. The symbol equation for the reaction is:
ADP + Pi + E --> ATP + H2O
Alternatively, the word equation is:
ADP + inorganic phosphate + energy --> ATP + water