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.

  • $\begingroup$ Energy can't release energy, because energy is... energy. Energy has to be contained otherwise it's useless. You can't pour a handful of charged electrons in your pc can you? Plants can't store rays of sunlight in their leaves, right? They store.sun energy as glucose later to be converted into ATP to release energy. Google ATP: first line ATP wiki: called the "molecular unit of (...) intracellular energy transfer $\endgroup$ – AliceD Mar 16 '16 at 10:31
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    $\begingroup$ I'm voting to close this question as off-topic because it's too trivial $\endgroup$ – AliceD Mar 16 '16 at 10:31
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    $\begingroup$ Because there is so much slack usage in relation to ATP and energy, it might be worth reformulating the question title, e.g. "What is ATP and why is it said to be a source of energy?" And then reworking the question to its essentials. $\endgroup$ – David Mar 16 '16 at 13:10
  • $\begingroup$ "Energy can't release energy because energy is ... energy". But Gibbs free energy (the type 'released' by ATP) is not energy in this sense, surely? $\endgroup$ – user1136 Mar 16 '16 at 15:50
  • $\begingroup$ See The Demise of the Squiggle and Why Is ATP an Important Energy Currency in Biochemistry? and A Long Life in Times of Great Upheaval for very stimulating articles about the question posed. $\endgroup$ – user1136 Mar 16 '16 at 16:30

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.

Further Information

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/


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  • $\begingroup$ (+1) And, of course the hydrolysis of $ADP$ to $P_i$ + $AMP$ proceeds with an almost identical free of hydrolysis to that of $ATP$ to $ADP$ + $P_i $. When dealing with ATP, 'energy changes' invariably refer to changes in Gibbs free energy of hydrolysis. $\endgroup$ – user1136 Mar 16 '16 at 15:43
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    $\begingroup$ Yes. In fact I as going to add that for the ATP to AMP + PPi but I was trying to keep it short. However I think it better to do that. $\endgroup$ – David Mar 16 '16 at 16:10

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