In school, we’re taught the general balanced formula for aerobic respiration:
**C6H12O6** **+ 6O2** **→ 6CO2** **+ 6H2O (+ energy)**
Later, the idea of ATP as ‘energy’ is introduced.
If the above equation is balanced, then how on earth are we just getting another 36 molecules of ATP? How do we use that ATP to do useful, energy-comsuming things?
What in God’s name is the Kreb’s Cycle?? Aaaaaaaaaahhhhhh!!!
In: 6
Well, the good news is that your two questions are related (as I assume you already knew): the Krebs cycle is how humans can extract 32-36 ATP from a single glucose molecule! Essentially the Krebs cycle represents a series of small, stepwise reactions that our bodies do to the glucose, and each step breaks it down a tiny bit and releases a bit of energy. By doing this in steps, we maximize the amount of energy we are able to capture! This HIGHLY efficient process is why humans create 36 ATP per glucose.
Still not really clicking? Don’t fixate on the chemical formula, think about the type of chemical reaction: oxidation. The rapid oxidation of carbohydrates is a combustion reaction! As I’m sure you know, combustion reactions are very exothermic and release a lot of energy. The reaction equation you have written is very similar to burning octane, yes if I told you I was able to butn gasoline and use that energy you wouldn’t bat an eye. Our bodies do this very same thing, releasing a small amount of energy at a time and storing that energy as a molecule called ATP.
__How ATP is made:__
ATP is made the same way a hydroelectric dam makes electricity, there is pressure on one side of a wall, which pushes through and spins a generator making electricity.
The water is protons, the generator is [ATP synthase](https://s3.amazonaws.com/labsterim/CACHE/images/media/uploads/CRL/ATP_synthasa/3ce771df286b913bdd26a2ba502d2ad0.jpg) (which actually spins and looks like a generator), and the wall/dam is the inner membrane of the mitochondria.
__What is the Krebs cycle:__
The Krebs cycle is a bunch of chemical reactions that ultimately help set up the proton gradient (water pressure)*. The Krebs cycle is like rain that builds up the water pressure on one side of a dam.
**To be more specific the Krebs cycle is actually like the water evaporating, the electron carriers are the clouds, and the electron transport chain is the rain.*
__How do we use ATP:__
ATP is something called Adenosine with 3 phosphates attached, it looks like this Adenosine-P-P-P (don’t worry about what adenosine is, it doesn’t matter). The phosphate groups are like repelling magnets, they want to shoot apart, this stores potential energy. If you hold two repelling magnets together then let go you will see they shoot apart, releasing the stored energy (which your body can use to do things). Then it will look like this Adenosine-P-P, another P can be added (by that hydro dam generator called ATP synthase) and you can keep doing this all day (and you literally do).
Edit: the best example of ATP in use imo is muscle contraction, if you ignore the jargon [in this short clip](https://m.youtube.com/watch?v=ZTgs-vzfzWc) you can see how ATP re-cocks the “spring loaded” parts of muscles allowing them to contract.
You’re asking “where does the physical matter of the ATP molecules come from?”
The full balanced equation is C6H12O6 + 6O2 + **n ADP + n P** -> 6 CO2 + 6 H2O + **n ATP**, where n is in principle as high as 38 but in practice is usually ~30 or so because the body doesn’t have perfect control of the reaction. The “energy” is the difference between ADP + P and ATP; the ATP doesn’t just appear, it’s assembled by the reactions.
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