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.