[Rachelyoonji (OP)]

Hi, I'm a new member 
I love biology and recently I have been studying the electron transport chain. I'm learning it on my own, so I don't have anyone to ask questions to. The concept itself is alright, but it the end...
I was wondering: how is ATP used?
So I searched it up on google and it told me that it split it back into ADP and a Pi, and that's when ATP is referred to as being 'used'. However, if we had ADPs left as 'waste' from the usage of ATP, why don't we have an unlimited supply of ATP? (Since the production of ATP requires ADP...)

A bunch of other questions kind of branch from the main one...
1) Where does the very very first ADP come from if it is, indeed recycled back into the cellular respiration cycle?
2) If the ADP is not recycled where does it go?
3) What happens to the Oxaloacetate in the Krebs cycle? (Unrelated, but... excuse me)
4) Where does NADH come from? What exactly is it?

I'm in year 9, and this is just out of pure curiosity- please don't bombard me with a bunch of chemistry as I don't understand!! (sorry  )

Thanks in advance to everyone who answers!

[Kryptid]

ATP is indeed recycled. However, the energy available from it is not unlimited. Just as breaking phosphate groups off of ATP releases energy, adding phosphate groups to AMP and ADP requires energy. That energy must come from the food we eat.

[puppypower]

ATP is a unique molecule in terms of how it stores energy. It also has an additional role that nobody talks about. These two points may help address the question of this topic.



The three phosphate groups of ATP are electron withdrawing. The central phosphorous atom of each of the three phosphate group has a charge of +5. These phosphorus atoms are attached to highly electronegative oxygen atoms. Therefore, instead of ATP being an energy hill, the phosphate group cause ATP to be more like an election energy vacuum; energy hole.

The triphosphate wants to absorb an election to gain its energy and dig itself out of the energy hole. ATP is an oxidizing agent that wishes to pull an election, from an energy hill source, to fill in its own energy hole. It is very similar to O2. If the cell had endless ATP, this would suck up all the energy, the mother cell  is trying to store,  so she can replicate. It is only during the latter part of cell cycles, will the ATP concentration be allowed to increase. This change helps to drive the needs of an active cell cycle. The ATP increase opens the drain, and sucks out the energy stored in the cells reserves, to put that energy into play.

Another aspect of ATP, that is rarely discussed, is the potential energy within a molecule of ATP, is not sufficient to drive most chemical reactions in the cell. The potential of ATP is equal to about the strength of 1-2 hydrogen bonds. Say you needed to use ATP, to drive an enzyme reaction, with the enzyme covered in hydrogen bonded water molecules. ATP can only provide enough energy to get rid of two water molecules leaving hundreds of hydrogen bonded water to gage the enzyme, so it cannot move to change conformation, to allow reaction.

ATP, by being such a strong oxidizer; energy sink, has a trick up its sleeve. When water surround and cage an enzyme, the water forms what is called cooperative hydrogen bonding. Cooperative hydrogen bonding is where all the hydrogen bonds cooperate, like a team, to form a type of resonance structure loosely similar to benzene. This makes all the hydrogen bonds even stronger due to the sharing. This should make it harder for ATP.

The way ATP works around this is when ATP goes to ADP, it needs to absorb a water molecule. The ATP does not pick just any water molecules, but rather acts like a bolt cutter, ripping a water out of the cooperative. The affect is like a run in nylon stocking with all the cooperative water molecules displaced from their organization. This displacement will increase cooperative water entropy, causing water to act like an energy sink, which then uses hundreds of water to drive the enzyme; suck out the energy.

If ATP concentration was too high, it would be hard to form water cooperatives, since ATP would bolt cut the water near enzymes. Enzymes, without the cooperative, would not be able to become and maintain perfect folds for repeatable reactions. Instead randomness would rule life and life would not be able to evolve based on an evolving foundation.