You have a lot of information available to you on ATP in your textbook. For example, there is information about ATP and a general overview of how it is made on pages 106 - 108. There's also some good stuff on it (really good) in the CD under the "muscle metabolism" chapter... but only look at slides number 1 - 5. OK? You can also go to this good site on ATP. Here's an interesting little tidbit on how ATP (the adenosine portion) and caffeine are related in keeping us awake!
Meanwhile, let's talk about ATP... You can review what we discussed about ATP back in unit 2 on the nucleic acid page first if that helps you to remember what nucleic acids and nucleotides are before continuing.
Here's an image of ATP taken from RasMol. ATP is a nucleotide, but it has 3 phosphate groups attached to it (each phosphorus atom is yellow). Normally, nucleotides that are already within nucleic acids only have one. There is a chemical characteristic of the bonds between the phosphate groups (PO42-) in ATP that they are unusually strong. This is not something that I expect for you to figure out in detail, because you'd have to learn a lot of chemistry to understand it fully. But I do want you to realize that it is true.
Here's another way to look at it... ATP means . If we draw it out more simply, it could be written as:
where there are three phosphate groups stuck off the end of adenosine. This string of three phosphate groups is held together, as you would expect, by covalent bonds. That's nothing new. All macromolecules and monomers are held together by covalent bonds. But for some reason, phosphate groups in a string need a really, really strong bond to hold them together. So the ones within the string are . I can redraw the above to show which ones are really strong:
The yellow bonds are very strong. Now think of a bond in another way. Think about it like a rope in a tug-of-war. If two people are pulling on a rope in a tug-of-war, and they are pulling very hard, if someone else comes along and cuts the rope, the two people will go flying! If the two people playing tug-of-war are not pulling hard at all, when the rope is cut, nothing will really happen to them.
This example of tug-of-war with the rope is an analogy to molecules being attached by covalent bonds. When the rope is cut, and the people go flying off (and get hurt-- oh no!) that is because lots of energy was being stored in the rope, and so when it was cut, the energy was released as the people fell. With a covalent bond, if it is holding molecules together with a lot of energy, when the bond is cut, the energy is released! Since it takes a lot of energy to hold the phosphate groups together, when the bond between them is cut (a "high energy bond"), the energy is released. When this happens to our ATP as it was written above, it becomes ADP (adenosine diphosphate) and an inorganic phosphate group goes off as does energy:
Can you understand why we call PO42- inorganic phosphate? Remember, in order for a molecule to be organic it has to contain carbon atoms and hydrogen atoms. PO42- does not contain any carbon, so it is considered inorganic. It's that simple. Don't let a word here or there throw you, because it is always rather simple in the end.
Now think about muscle. Every time it needs to contract, it will need thousands of these ATP molecules to provide energy for the powerstroke of the myosin heads... that's a lot of ATP! So, in order to get the energy, the myosin heads have to break down (hydrolyze) ATP into ADP + inorganic phosphate to release the energy. ADP, although I have shown one high energy bond in it, doesn't really offer as much energy as ATP, so it is not really too useful for our muscle.
The products of ATP hydrolysis, ADP and inorganic phosphate, can be stuck back together to make ATP. But this is not as simple as breaking it was. How our muscle cells (fibers) re-form ATP is the topic for the last web page in this unit.
Meanwhile, as myosin heads hydrolyze ATP (myosin heads are ATPases, remember?), they release the energy of that high energy bond they broke from ATP. This energy is immediately used by the myosin head itself to get itself into its high-energy state (or high-energy conformation). This is going to power muscle contraction.
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