Where ATP's metabolic power comes from and how it's used
The actual breaking of a bond is always going to require energy (that is, it's endergonic) - but you have to think about what you get in return. In the case of ATP hydrolysis, this includes:
* Freeing of electric repulsion (think of holding negatively-charged phosphate groups together as being like clamping a spring - breaking a bond is like releasing the clamp - you had to do a little work to take off that clamp, but you get a lot of energy after)
* Resonance stabilization - there is better resonance stabilization possible in a free-floating, inorganic phosphate (orthophosphate) than is possible when phosphate groups are linked together. Resonance makes molecules happy. And when you make molecules happy, you lower their free energy, so they release energy during the process
* Hydration - you broke one bond, which took energy, but you also form some non-covalent interactions like H-bonds & ion-dipole bonds with water. And those will give you some energy (although they’re not true bonds, but rather attractions, they still count towards the enthalpy part of the Gibbs’ free energy equation - that is, the ΔH in ΔG = ΔH - TΔS)
* Ionization - the phosphate group you release can ionize, which is again energetically favorable. Energetically favorable = releases energy
* If you break off a pyrophosphate (PPi)(2 phosphates connected) you get even more energy because the released pyrophosphate can break up into 2 orthophosphate
Those combine to give you a large negative ΔG’° (standard free energy change) which tells you that this reaction (ATP + H₂O → ADP + Pi) is intrinsically favorable. If you mixed equal amounts of them at 25°C, you’d end up with way more ADP at equilibrium.
But we don’t want to be at equilibrium. Or we couldn’t exist. At equilibrium, there’s no drive to go either way (forward or reverse in that equation). Instead, if we want the biggest forward drive, we want to maintain high [reactants] compared to [products], as reviewed below. That is to say, we need to have high [ATP]/[ADP] ratios. So we need to stock up on ATP and keep ADP low. And that’s what our bodies do! We spend a lot of energy to constantly make ATP so we can keep those ratios high.
A large source of ATP’s power comes not from that intrinsic favorability alone (which could theoretically be overcome by having way more ADP than ATP) - but rather from the high [ATP]/[ADP] ratios in our body.
Mathematically, this comes down to the equation
ΔG = ΔG’° + RT ln([products]/[reactants])
When [products]/[reactants] is less than 1 (that is, you have more reactants than products), this second part of the equation will be negative. And thus favorable. And this favorability will get “added on” to that intrinsic favorability (expressed as ΔG’°)
We can also write this as saying that our Q ([products]/[reactants] at any given point in time) is less than our Keq ([products]/[reactants] at equilibrium) so there’s a drive to make more products to go towards equilibrium.
blog: https://bit.ly/whyATPsgood ; YouTube: • Why "burning" ATP's a great source of...
Much more on this here: • Metabolic logic & how ATP gets its po...
And for a longer post on ATP, see: https://bit.ly/atpenergymoney
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