Is ATP a molecule or a kind of energy. When I studied the active transport, it's said the ATP would release energy to change the carrier protein shape. So confused. Thanks for your help.
ATP is adenosine 5'-triphosphate, a molecule consisting of the base adenine attached to which is attached a ribose (at its 1 position) and (at its 5 position) three phosphates, alpha-, beta- and -gamma (see edited image from Wikipedia below). The (Gibbs) free energy of hydrolysis of the phosphoanydride bonds between the gamma and beta phosphates and betweeen the alpha and beta phosphates is relatively large in terms of biological reactions. This means that coupling of the hydrolysis of ATP (usually, but not always, to ADP — adenosine 5'-diphosphate) to a reaction that is energetically unfavourable can often produce an overall reaction that is energetically favourabe (has an overall negative free energy change). Enzymes have evolved to catalyse such coupled reactions involving ATP.
Although ATP is often referred to as having ‘energy-rich bonds’ this is not the best way of thinking of things. First, and perhaps most important, you cannot think of energy in abstract, and even referring to ‘breaking a bond’ is insufficient. One is talking about free energy change which must relate to a specific reaction, in this case hydrolysis:
ATP + H2O → ADP + PO43– or less frequently ATP + H2O → AMP + pyrophosphate
Second the free energy change is only ‘high’ in relation to cellular reactions. Third the energy is only of any use if it can be coupled to these reactions (or processes like active cation transport).
It is perhaps better to refer to ATP as having a ‘high phosphate group transfer potential’.
The coupling of the hydrolysis of ATP to mechanical processes like muscle contraction or the change of shape of transporters (if that is what happens) is more complex, but obeys essentially the same principles. The ATP hydrolysis reaction with its favourable negative change in free energy is coupled to the mechanical process with its unfavourable positive free energy change.
ATP and Free Energy: http://www.ncbi.nlm.nih.gov/books/NBK21737/#A360
‘High Energy Phosphate’: https://en.wikipedia.org/wiki/High-energy_phosphate
ATP and Cation Transport: http://www.ncbi.nlm.nih.gov/books/NBK21481/