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I was recently studying about ATP and how it functions as an energy carrier to all biological processes; however, I came across a confusing and paradoxical statement from two sources: (at 7:36) and (the free response question in the bottom of the page)

Basically, in the video, it says that ATP hydrolysis has a high activation energy which is why all of the ATP isn't randomly used by the cell processes.

And in the free response question, the answer is that ATP hydrolysis has a low activation energy and low free energy making it a spontaneous reaction.

A follow up question is: How then does the ATP of the cell used in a selective way to certain processes only?

Thanks for your help.

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High and low are not very descriptive since they are relative. ATP hydrolysis may have a high activation energy compared to some reactions and low when compared to others. The important point is that the activation energy is sufficiently high enough such that ATP is not rapidly hydrolysed under physiological conditions before it can do useful work. In other words ATP hydrolysis is kinetically unfavourable but thermodynamically favourable. The reaction rate is increased by enzymes (by lowering the activation energy), which couple ATP hydrolysis to other processes to do useful work.

Also, a note on reaction spontaneity: it has nothing to do with activation energy but rather depends on the change in free energy; ATP hydrolysis is spontaneous because it is exergonic.

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  • $\begingroup$ If we take a look at the Sodium-Potasium pump (action potential), it looks like the ATP activation energy is low for that process. Otherwise it can't support the speed of the signal propogation, can it? Perhaps it's a very integrated complex chain reaction where a receptor actives within the cell, that literally tells, "for this particular work, the ATP activation energy should be pretty low" and so on. I am curious to know how it exactly applies in this scenario. $\endgroup$ – bonCodigo Jul 19 '15 at 4:44
  • $\begingroup$ @bonCodigo I'm not really sure what you're getting at. Enzymes lower the activation energy of reactions. $\endgroup$ – canadianer Jul 19 '15 at 4:52
  • $\begingroup$ So the video explanation is more accurate. Yes? And I am also confused in something (I am not sure if it was addressed in the answer or not though ) :How do cells control how ATP is used? Because on a wiki page I read that if ATP and ADP are placed in a water container at equillibrium then all the ATP will be converted to ADP. $\endgroup$ – mariam.. Jul 19 '15 at 15:03
  • $\begingroup$ @mariam Enzymes control how ATP is used. $\endgroup$ – canadianer Jul 19 '15 at 16:40
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The video response is more accurate. In particular, saying that the activation barrier is high enough for the reaction not to occur spontaneously in water means that the activation energy is much higher than the thermal energy, $k_B T$, where $k_B$ is Boltzmann's constant and $T$ is the temperature.

You ask, "How do cells control how ATP is used?". As mentioned in the other answer, this energy barrier is lowered by enzymes, in particular ATPases. These are 'machines', such as for example molecular motors or ion pumps, which have a binding pocket for an ATP. When the ATP comes inside this pocket, the activation barrier is lowered and hydrolysis can happen. And, crucially, these 'machines' then use the released energy to perform some useful task. If hydrolysis happened by itself in water, the released energy would be 'lost' in the form of heat.

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