[Kate]

Hello.

So in biology I'm taught that the break of ATP into ADP and phosphate releases energy and yet I was under the impression, from my chem classes, that to break bonds there has to be an energy input.

Btw, if a molecule is unstable, it can release energy when it breaks apart right? Also, when bonds form is energy always released?

[Corribus]

We had a nice thread about this recently. See if this answers any of your questions.

http://www.chemicalforums.com/index.php?topic=72745.0

When a bond forms between atoms, energy is always released, since by definition a bond is a lower state of energy than the isolated atoms. However, bonds rarely form on their own, meaning: to form some bonds, others typically are broken. Whether there is a net release of energy to or absorption of energy from the environment depends on the relative strengths of the bonds broken and formed.

[Big-Daddy]

Hello.

So in biology I'm taught that the break of ATP into ADP and phosphate releases energy and yet I was under the impression, from my chem classes, that to break bonds there has to be an energy input.

Btw, if a molecule is unstable, it can release energy when it breaks apart right? Also, when bonds form is energy always released?

1) ATP does not just break into ADP and phosphate. It is hydrolysed. Therefore some bonds are formed as well as the ones which are broken, so it is very possible that q (heat) would be negative for the system in this reaction, i.e. that heat is released.

2) Energy is always released when bonds form. Energy is always taken in to break bonds. So an unstable molecule "breaking apart" cannot release energy if by breaking apart you mean heterolytic fission of each bond (which is what is meant when we talk about bond dissociation or formation energies).

However, in practice you also have electrons moving around and "forming bonds" (not chemical bonds but rather new attachments) to various atoms, once the original species has broken apart. This relocation of electrons composes an additional set of reactions which will determine the heat and energy changes when the reaction occurs to go from the original species to the final products of its decomposition, which may well be negative (i.e. heat & energy may well be released from the system in the process of decomposition).

[Kate]

You are correct that energy is usually not directly released by ATP when it is enzymatically converted to ADP, by which I mean energy is not dumped out into the external medium (in which case it would be dispersed as heat). This is not a useful use of energy, so what good would it do the cell? Rather the stored energy is used to directly form other bonds in other biological molecules, or unfavorable changes in protein conformation, and so forth. In this, the ultimate truth is that biology is simply chemistry and chemistry is simply applied thermodynamics. Heat is the final waste product of everything, and each conversion process does release a little heat. There's always a little waste. Where do you think body temperature comes from?

I have question here (this was taken from your response in the thread you mentioned): for the energy not to be released as heat but rather used directly to form other bonds, does that mean that some other molecule becomes phosphorylated? Is that how ATP coupling works?

When a bond forms between atoms, energy is always released, since by definition a bond is a lower state of energy than the isolated atoms. However, bonds rarely form on their own, meaning: to form some bonds, others typically are broken. Whether there is a net release of energy to or absorption of energy from the environment depends on the relative strengths of the bonds broken and formed.

In my textbook it says: "ATP is the prime energy currency in all cells, being generated during exergonic reactions and consumed in endergonic reactions."

So this is my understanding right now, considering what you said and what my textbook says:
In the formation of ATP, even though some energy is released when phosphate and ADP bond, the overall effect is an absorption of energy; therefore, the formation of ATP is coupled with reactions that release energy (exergonic). On the contrary, in the hydrolysis of ATP, even though some energy is necessary to break it into ADP and Pi, the bonding of Pi to some other molecule releases energy that compensates the initial energy expenditure.

Is this it?

Edit: But then my textbook says this: "In the reaction acetyl-S-CoA + H₂O + ADP + Pi  acetate^- + HS-CoA + ATP + H⁺, the energy released in the hydrolysis of coenzyme A is conserved in the synthesis of ATP"



I don't know, this terminology of saying that the hydrolysis of whatever releases energy is confusing.

1) ATP does not just break into ADP and phosphate. It is hydrolysed. Therefore some bonds are formed as well as the ones which are broken, so it is very possible that q (heat) would be negative for the system in this reaction, i.e. that heat is released.

I get what you're saying, but when you calculate the free energy for any reaction, the mechanism is irrelevant.

2) Energy is always released when bonds form. Energy is always taken in to break bonds. So an unstable molecule "breaking apart" cannot release energy if by breaking apart you mean heterolytic fission of each bond (which is what is meant when we talk about bond dissociation or formation energies).

Got it.

[Corribus]

@Kate,

I have to admit that I'm trying to wrap my head around where your point of difficulty is. Hydrolysis of ATP is an exergonic reaction in a normal, aqueous solution, which means that the process is spontaneous. However, realize that this process may not happen in an isolated way. In the cell, the spontaneous hydrolysis of ATP may be used to help drive another process that wouldn't otherwise happen spontaneously - that is, it can be used to do chemical work.

Maybe, think of ATP as the gunpowder in the cannon and some other chemical process as the ball. On its own the ball isn't going to go anywhere because to become fired up in the air takes gravitational work (it is initially in a state of low potential energy and to get to a state of high potential energy - up in the air - takes work). Now take a pile of gun powder and apply a spark. What happens? The gun powder ignites and explodes, but the energy released is wasted. Free chemical energy is released (as heat, sound, whatever) but that energy just dissipates into the environment because it's not engineered to be focused toward a useful endpoint. Now put the gunpowder instead in a cannon, and put the ball in the cannon, and what happens when you light the powder? The same amount of free energy is released, but this time the free energy is directly focused to do work on the ball due to the engineering of the systems - the design of the cannon allows the released energy to fling the heavy ball up in the air and, hopefully, rain unholy terror upon a city or ship. That is, the free energy liberated from the burning of the gunpowder performs useful work on the ball, allowing it to do something it wouldn't otherwise spontaneously do. Of course, there is still waste (heat, fire, sound) but a useful process has been completed.

In this analogy, the cannon, of course, is the cellular architecture which permits the released free energy from the gun powder burning (ATP hydrolysis) to be harnessed in a specific way, that is - flinging the ball up in the air (some needed but unfavorable cellular process).

I think part of the confusion may be that the informal language people use when talking about ATP (e.g., "high energy bonds" is sometimes confusing and not strictly rigorous.  It is confusing to say that ATP releases energy by breaking a phosphate bond, when in fact it takes energy to break a bond. What we really mean to say is that the phosphate bond is not as strong as the bonds which are formed (plus the entropic factors) between the surrounding medium and free phosphate and ADP. It's the energy difference between products and reactants that are "high energy magnitude", not the energy of the ATP bond itself.

If that makes any sense...

[Kate]

Thanks for taking the time to respond.

Well, I probably wasn't too clear before in explaining myself but luckily for me you ended up explaining it here:

I think part of the confusion may be that the informal language people use when talking about ATP (e.g., "high energy bonds" is sometimes confusing and not strictly rigorous.  It is confusing to say that ATP releases energy by breaking a phosphate bond, when in fact it takes energy to break a bond. What we really mean to say is that the phosphate bond is not as strong as the bonds which are formed (plus the entropic factors) between the surrounding medium and free phosphate and ADP. It's the energy difference between products and reactants that are "high energy magnitude", not the energy of the ATP bond itself.

I kept seeing that the free energy for the hydrolysis of ATP is (roughly) -30 kj/mol, but I was having a hard time understanding what sort of bonds happen when you have ADP and Pi that drive the reaction forward. I get it now: between ADP and medium molecules and Pi and other molecules.

Thanks.