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Sal: In the previous video we talked about how an ATP molecule can, in the presence of water, hydrolysis will take place, and one of the phosphoryl groups could be plunked off, and how that would release energy because these electrons are going to be able to go into a lower energy state. You could imagine that this was already not that stable of a bond, that all these negative charges wanted to get away from each other, and once this is plunked off, then of course when they get into a more comfortable state energy is released. But you might say, "I want more. I want to actually see "the mechanism by which the hydrolysis takes place." That's what I'm going to do in this video. Let's start with our ATP molecule, and let's throw some water in there, H2O. Let's say this is water right here, oxygen with two hydrogens. I'll do the two pairs of oxygen that aren't in bonds right over here, in the outermost shell. Actually let me draw one more water molecule right over here. There's multiple way that you could actually depict this right over here. Let's say that one of these, let's say this water molecule right here, and obviously no chemical reaction happens this cleanly. This is showing how it could happen if they just bump into each other in the exact right way. This has got this pair of electrons. Let's say this pair of electrons is essentially given by this oxygen to this hydrogen proton. We could draw it like this right there. It nabs just the hydrogen, then both of these electrons that are in this pair, that are in this bond I should say, go back to the oxygen to form, essentially you could think of this as a pair of electrons attached to that oxygen. Then that gives the oxygen license to allow these two electrons to form a bond with the phosphorous. The phosphorous isn't in the mood to form six bonds, it's already got five, and this is a fairly uncomfortable situation for it. That allows these two electrons right over here to go, these two electrons to go to this oxygen, just like that. As a result of everything I've just depicted happening, what does it look like? Let me draw a little arrow here. Now you're going to have your adenosine diphosphate. Let me put it over here. And just to be clear, this thing has now gained, this oxygen right over here, one way you can think about it, it was party to a bond so it was sharing two electrons. Now it's getting both of the electrons, so now it's going to have a negative charge. It had half of-- Actually it had a little bit more than half, it's more electronegative than the phosphorous. Now it's going to get both of them. Now this is going to have a negative charge. This is the adenosine diphosphate. This phosphoryl group over here, let me just redraw it. It's going to look like this. Double bond to that oxygen. You have this oxygen right over here. That's there. You have that oxygen right over there. And of course the water, the water molecule, or I guess now it's just going to be an OH group, is going to be, it has ... Let me see if I can make the colors interesting. These two electrons have now formed a bond and you have the oxygen and of course this hydrogen here. I haven't draw in any other oxygens but this thing still has two lone pairs. And of course you have this character right over here, who gained a proton. This one you can depict like this. It's oxygen, hydrogen, hydrogen. It had one lone pair, but now it gave half of this lone pair to form a bond with that hydrogen, and hydrogen without an electron is just a proton. Actually let me draw it like this just so you can see it. These two are now the two electrons in this bond with this, with this hydrogen proton. This right over here, this is a positively charged, this is a positively charged molecule right over here. And you can imagine maybe this thing breaks off and it could be viewed as a proton, or you could view this as a positive charged molecule, but either way this is the reaction that we just depicted. You have ATP being, hydrolysis takes place. You're left with ADP, you're left with a phosphate, a released phosphate molecule, and then you're left with a positive charge. You could either view this as kind of a proton or the proton attaches and forms a hydronium ion right over here. And of course in the process of doing all of this, as these electrons got into a more comfortable situation just sitting right over here and allowing this thing to break off, it releases energy. It releases, it releases energy, which is in most biological systems the whole point of having the ATP molecules around.