ATP is arguably one of the most important molecules in all of biology. ATP is an abbreviation for adenosine triphosphate. That sounds very posh. But all you have to remember is that any time you see ATP floating around in a biochemical reaction, something in your brain should tell you that you're dealing with biological energy. ATP can also be thought of as the currency of biological energy. So, how does it function as an energy currency? ATP, on the other hand, stores energy in its bonds. And before we learn what an adenosine group or a 3-phosphate group looks like, you can take a leap of faith and imagine that ATP is made up of something called an adenosine group. Then there will be three phosphates attached to it. You will, not might. Just like that, you'll have three phosphates attached to it. And this is what ATP is. Adenosine triphosphate is an abbreviation for adenosine triphosphate. The term tri- refers to the presence of three phosphate groups.
Now, if you take adenosine triphosphate and hydrolyze the bond, that is, if you take it in the presence of water. So let me just pour some water into this. And I'll explain why in more detail later. But first, let me give you a big picture. What you have now is an adenosine group with two phosphates on it. This is known as adenosine diphosphate or ADP. We used to have triphosphate, which stands for three phosphates. We now have diphosphate, adenosine triphosphate, so instead of a tri, we write a di here. That is, you have two phosphate groups. So either the ATP has been hydrolyzed or one of these phosphate groups has been broken off. So either the ATP has been hydrolyzed or one of these phosphate groups has been broken off. As a result, you now have ADP and an extra phosphate group right here. And this is the entire key to everything we discuss when we're dealing with ATP and you have some energy. This is why I refer to ATP as the currency of biological energy. Is that if you have ATP and you pop off this phosphate through some chemical reaction, it will generate energy. That energy can be used for nothing more than general heating. You could also combine this reaction with others that require energy. Then those reactions will be able to progress.
ATP has energy, and When you break a phosphate off, it generates energy. And then if you want to go from ADP and phosphate back to ATP, you have to use energy up again. So if you have ATP, that's a source of energy. If you have ADP and you want ATP, you need to use energy. And so far I've just drawn a circle with an A around it and said that's adenosine. But sometimes I think it's satisfying to see what the molecule looks like. ATP is the currency of energy, I think is fairly straightforward. When it has three phosphates, one phosphate can break off. And then that'll result in some energy being put into the system. Or if you want to attach that phosphate you have to use up energy. That's just the basic principle of ATP. But this is its actual structure. But even here we can break it down and see that it's not too bad. We said adenosine. Let me draw the adenosine group. We have adenosine. This right here is adenosine. This part of the molecule is right there. That is adenosine. And for those of you that have paid attention to some of the other videos, you might recognize that this part of adenosine.
So some of these molecules in biological systems have more than one use. This is the same adenine where we talk about adenine and guanine. This is a purine. And there's also the pyrimidines, but I won't go into that much. But that's the same molecules that are just an interesting thing. The same thing that makes up DNA is also part of what makes up these energy currency molecules. So the adenine makes part of the adenosine part of ATP. And then the other part right here is ribose. Which you might also recognize from RNA, ribonucleic acid. That's because you have ribose dealing in the whole situation. But I won't go into that much. But ribose is just a 5-carbon sugar. When they don't draw the molecule, it's implied that it's carbon. So this is one carbon right there, two carbons, three carbons, four carbons, five carbons. And that's just nice to know. It's nice to know that they share parts of their molecules with DNA. And these are familiar building blocks that we see over and over again. But I want to emphasize that knowing this, or memorizing this, in no way will help you understand the simpler understanding of ATP just being what drives biological reactions. And then here I drew 3-phosphate groups, and this is their actual molecular structure. Their Lewis structures right here. That's one phosphate group. This is the second phosphate group. And this is a third phosphate group. Just like that. When I first learned this, my first question was, OK I can take this as a leap of faith that if you take one of these phosphate groups off or if this bond is hydrolyzed, that somehow that releases energy. And then I kind of went on and answered all the questions that I had to answer. But why does it release energy? What is it about this bond that releases energy?
Remember all bonds are are electrons being shared with different atoms. So the best way you could think about it is right here. These electrons are being shared right across this bond, or this electron that's being shared right across this bond, and it's coming from the phosphate. I won't draw the periodic table right now. But you know the phosphate has five electrons to share. It's less electronegative than oxygen, so oxygen will kind of hog the electron. But this electron is very uncomfortable. There's a couple of reasons why it is uncomfortable. It's in a high-energy state. One reason why is, you have all these negative oxygens here. So they kind of want to push away from each other. So these electrons in this bond really can't the kind of get close to the nucleus. They'll go into a kind of a low-energy state. All of this is more of an analogy than reality. We all know that electrons can get quite complex. And there's a whole quantum mechanical world. But that's a good way to think of it. That these molecules want to be away from each other. But you have these bonds, so this electron, it's kind of in a high energy state. It's further from the nuclei of these two atoms than it might want to be. And when you pop this phosphate group off, all of a sudden these electrons can enter into a lower energy state. And that generates energy. So this energy right here is always in any chemical reaction where they say energy is generated, it's always from electrons going to a lower energy state. That's what it's all about. And later in future videos when we do cellular respiration and glycolysis and all that, whenever we show energy, it's really from electrons going from uncomfortable states to more comfortable states. And in the process, they generate energy.
If I'm in a plane or I'm jumping out of a plane, I have a lot of potential energy right when I jump out of the plane. And you can view that as an uncomfortable state. And then when I'm sitting on my couch watching football, I have a lot less potential energy, so that's a very comfortable state. And I could have generated a lot of energy falling to my couch. But I don't know. My analogies always break down at some point. Now, the last thing I want to go over for you is exactly how this reaction happens. So far you could turn off this video and you could already deal with ATP as it is used in 95% of biology, especially AP Bio. But I want you to understand how this reaction happens. So to do that, what I'm going to do is copy and paste parts of these. So that's the phosphate group that breaks off. And then you have the rest of it. You have the ADP that's leftover. So this is the ADP. I don't even have to copy and paste all of this stuff. You can just accept that that's the adenosine group. Just like that. So we've already said that this thing gets hydrolyzed off or gets cut off and that generates energy. But what I want to do is show you the mechanism. A little bit of hand-wavy mechanism of how this happens. So I said this reaction occurs in the presence of water. So let me draw some water here. So I have oxygen and hydrogen. And then I have another hydrogen. That's water right there. So hydrolysis is just a reaction where you say, hey, this guy here, he wants to bond with something or he wants to share someone else's electrons. So maybe this hydrogen right here goes down here and shares its electron with this oxygen right here. And then this phosphorus has an extra electron that it needs to share. Remember it has five valence electrons; it wants to share them with oxygen. It has one, two, three, four being shared right now. Well, if this hydrogen goes to this guy, then you're left with this blue OH right here. And this guy can share one of the phosphorus' extra electrons. So you get the OH just like that. So that's the actual process that happens. And it could go the other way as well. I could've cleaved it here. I could have cleaved the whole thing here. And so this guy would have kept the oxygen and the hydrogen would have gone to him. And then this guy would have taken the OH. It could happen in either order. And so either order would be fine. And there's one other point I want to make. And this is a little bit more complex. And I was even wondering whether I wanted to make it. My whole reason why you're kind of in a lower energy state is, once you break apart
It's in a lower energy state because it's not being stretched. It's not having to spend time between that guy and that guy because this molecule and this molecule want to spread apart because they have negative charges. That's part of the reason. The other reason why, and we'll talk about this in a lot more detail when we learn more about organic chemistry, is that this has more resonance. More resonance structures or resonance configurations. And all that means is that these electrons, these extra electrons here, can kind of move about between the different atoms. And that makes it even more stable. So if you imagine that this oxygen right here has an extra electron with it. So that extra electron right there, could come down here and then form a double bond with the phosphorus. And then this electron right here can then jump back up to that oxygen. And then that could happen on this site and on that side. And I won't go into the details, but that's another reason why it makes it more stable. If you've already taken organic chemistry, you can kind of appreciate that more. But I don't want to get all into the weeds. The most important thing to remember about ATP is that when you leave off a phosphate group it generates energy that can drive all sorts of biological functions, like growth and movement, muscle movement, muscle contraction, electrical impulses in nerves and the brain. So this is the main battery or currency of energy in biological systems. That's the main thing that you just need to remember about ATP.