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What is the difference between ADP and ATP?
Answer 1:

This is a pretty sophisticated question. ATP is short for "adenosine triphospate" which means there's one adenosine 3 phosphates in this molecule. I'm guessing that you know that ATP is basically a way for the cells of your body to store energy. To make things a little simpler, think of ATP as having 4 parts: the A (adenosine) and 3 P's (the phosphates). The energy is not stored in the A or the P's, but in the BONDS between the molecules. An ATP is kind of like a charged battery. When you take off one P, you get adenosine diphosphate or ADP (which is an A and 2 P's) plus an extra P. You also release a bunch of energy that can be used by the cell to do work. So an ADP is like a battery that has lost some of its charge. You can take off another P and get adenosine monophosphate (AMP) and another spare P, but your body usually uses AMP for sending signals, so let's ignore that. There are pictures of these molecules at:
http://www.bris.ac.uk/Depts/Chemistry/MOTM/atp/atp1.htm.
Actually they're not true pictures because ATP is way too small to see, but they are models that show how we think they are put together.

ATP is a sort of "universal power source" that allows your body to take the energy from pizza, carrots, or milk and turn it all into the same type of energy. It's sort of like the way we use electricity for most of our power needs even though the energy was once in the form of coal, water behind a dam, the inside of an atom, or something else. You wouldn't want to shovel coal into your calculator. In the same way, it's easier for all your cell's systems to use the same kind of power: ATP.
Where does almost all of the energy on Earth come from?

Answer 2:

ATP is a molecule with a lot of chemical energy. ATP stands for Adenosine Tri Phosphate, and that third phosphate is bonded to the other two with a very high energy bond, so a lot of energy is released when that bond is broken. When the third phosphate is removed from ATP, you get ADP, which stands for Adenosine Di Phosphate. With just 2 phosphates left, the molecule has much less chemical energy, because the high energy bond between the last 2 phosphates has been broken. The Adenosine part of ADP and ATP is composed of Adenine, which is one of the 4 letters of the genetic code in DNA (A, C, G, T), bonded to a sugar molecule.


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