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we've talked a lot about ATP being the energy currency of cells but I want to dig a little bit deeper into that in this video and as we'll see when we go from ATP to ADP ADP plus a phosphate group we have a release of free energy if we look at just the system ATP's free energy is over here but once hydrolysis has taken place and now it's adp plus a phosphate group the free energy has dropped by roughly net by it's dropped by roughly 30 and a half kilojoules per mole or our Delta G is negative 30 point 5 kilojoules per mole and if you watch the videos on Gibbs for energy this tells us that this is a spontaneous reaction Delta Delta G is less than zero which tells us that this is going to be this is going to be spontaneous now when I first learned this I was like well it's going to be spontaneous why wouldn't all of the ATP just spontaneously in all the water turn into ADP and just release its energy is as heat or whatever else and the key is it has to get over this hump you have this activation energy it has to go uphill a little bit if it's if there's no enzyme to catalyze it and the reason why we have this uphill hump is it the way that ATP gets broken up is that you have to if we're talking about a water molecule doing the hydrolysis which is kind of typically what people think about hydrolysis although it can so I'm going to be done by a different molecule if you think about the water molecule what needs to happen is you need this lone pair of electrons on this oxygen to be able to do what we call a nucleophilic attack on this phosphorus in this phosphate group and while that happens if that happens it forms this bond and then these electrons these electrons can be taken back by this oxygen which gives its negative charge right over there now you might say okay this might make a lot of sense but you have to remember this electrons are negative and they're surrounded by these negative charges so they have to overcome getting close to these things so as they're approaching these negative charges they want to repel each other so you have to overcome that and the way that's overcome is a class of enzymes called 80 paces ATP ATP ASE's I guess is the best way better way to say it ATP ATPase and what they do is remember these enzymes are these big protein complexes and and the ATP molecule can bind combined in the right place and they essentially try to surround the ATP molecule with some positive ions so let's say there's a positive ion here so it can keep these electrons busy while the water or whatever is doing the nucleophilic attack can react cannot have to worry about these electrons over here so it might have a positive ion here and remember the if we think in three dimensions this thing is all wrapped around in different ways around the ATP molecule so this is the ATP this is the ATPase and so by having the enzyme over here you're lowering the activation energy and so it might actually look something more like this and so the reaction can actually occur so the reason why you just don't see this happening all the time without an enzyme is what you have to overcome this time but once you have the enzymes they can actually allow the the reaction to occur so this attacks this forms this bond and now you say well you have another you have another hydrogen right over here you have a another hydrogen right over here but this could get nabbed up by another water molecule becoming a hydronium ion which is actually what's going to happen and then this character takes away this pair of electrons becomes a negative charge and we're left with we're left with that phosphate group has been broken off and then we have our ADP and we have a release of energy now typically you don't want this to just you know release energy for no good reason most ATP ASE's are going to leverage that energy that released this reaction to phosphorylate a molecule in this case you can kind of think that this hydroxide has been phosphorylated but it might phosphorylate something else or it might change the confirmation of this ATPase so this ATPase so it can do some other type of work transfer molecules against their concentration gradient or their chemo electric gradient whatever it might be you don't want to just release the energy sometimes that might happen if you just want to generate some heat but usually you're going to do some useful work for the actual cell so hopefully this gives you a little bit more appreciation for ATP and the reaction by which again the hydrolysis can occur this is called hydrolysis because at least in this example we're using a water molecule to break it up to take off phosphate group
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