Energy release/ Energy required

[Medgrant]

I probably already know this, but can't reason out at the moment. I am sure that I am over-complicating it, but I've been curious for the past few hours.

If energy is required to form bonds, how is energy released from breaking bonds in digestion? Assuming the different metabolic processes within cells are not considered.

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[sazerac]

Energy is not required to form bonds - energy is released to form bonds. Otherwise the bond never would have formed in the first place, or the atoms would quickly drift apart after forming a bond.

Energy is released during digestion (breaking bonds) because of the simultaneous bond-forming reactions that occur. Often these reactions are not emphasized.

For instance, when ATP is broken into ADP and Pi, that reaction must have required energy input. Why do biologists say that this reaction releases energy into a usable form? Because simultaneously a water was broken into H and OH (which also required energy), and they bonded to the ADP and the Pi, which released a ton of energy.

Biologists play fast and loose with the rules of chemistry.

 

[lifetothefullest]

If you want a more quantitative explanation and a look at the molecular level of it, take a look at the Morse potential. It's a really simple-minded explanation for big biological molecules but when you break it down, bond by bond, that's what you want to look at. Forming a bond releases energy because the atom is rolling downhill in the energy landscape, so to speak. So take a look at the Morse potential and imagine rolling a marble starting from the top. To break the bond, you essentially have to separate the molecules infinitely. The energy required to do this (to climb out of the so-called potential energy well) is your bond dissociation energy and it's what is needed to break the bond (i.e. to separate the atoms).

 

[NextStepTutor_1]

@sazerac nailed it! That example of ATP hydrolysis was exactly the example I was going to give!
Most content review books (and even some textbooks and online resources) just give the simple info that bond formation releases energy and bond breakage requires energy, but obviously when you look at something like ATP hydrolysis and the overall gain of energy, that info solely within the context of what's happening to ATP can't explain the whole story.

 

[Medgrant]

Thanks for the explanations! I didn't write my first post properly. I meant to say that energy has to be introduced to break bonds, so how is energy released from breaking bonds. Thank you!