On a molecular level, why does exercise/shivering->heat?

[TheEugenius]

I understand that myosin requires ATP to move the actin filaments to cause contraction of the muscles, and so once the energy from the ATP is turned into mechanical energy, how does that energy become heat energy? Does it occur during the release of the actin filaments causing them to slide back and result in friction?

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[aldol16]

Yes, it's friction that causes heat energy. Sliding back and forth always induces friction 😉

 

[iBS1972]

A quick review of the actin-myosin crossbridge cycle:
1. ATP binds to myosin, allowing myosin to detach from actin and relax
2. ATP is hydrolyzed (by myosin) into ADP+P, which allows myosin to reorient into power position
3. release of P allows myosin to bind to actin in that power position
4. release of ADP allows myosin to do power stroke
5. new ATP binds to myosin allowing to release and relax

To answer your question, heat is released when ATP is hydrolyzed/when its bonds are broken because catabolism (bond breaking) releases heat (exothermic), while anabolic processes (bond making) requires energy. Why? think about the energy vs. reaction process graphs you learned about in chemistry/physics with reactants being up higher and products being lower energy for catabolic reactions. To go into further detail is out of scope and would require some PhD student.

 

[aldol16]

To answer your question, heat is released when ATP is hydrolyzed/when its bonds are broken because catabolism (bond breaking) releases heat (exothermic), while anabolic processes (bond making) requires energy. Why? think about the energy vs. reaction process graphs you learned about in chemistry/physics with reactants being up higher and products being lower energy for catabolic reactions. To go into further detail is out of scope and would require some PhD student.

The energy released by ATP hydrolysis is directly coupled to produce mechanical movement of the filaments - in other words, that energy is used to perform work. It's not simply released as heat energy, which would be a waste of ATP. The heat comes from the friction caused by the sliding of the filaments.

 

[iBS1972]

Part of the energy is used to perform work. However, some of the energy is converted to heat (often as a by-product of friction). Usually we regard this heat energy as useless and waste, because it can't be used for work. But often-times heat energy is exactly what we want, in cases of shivering.
I guess what I'm trying to say is that friction and heat energy are two different things. Or would you say that heat energy only comes from friction of molecules? (I'm reaching the limits of my understanding of physics now haha).

 

[aldol16]

Part of the energy is used to perform work. However, some of the energy is converted to heat (often as a by-product of friction). Usually we regard this heat energy as useless and waste, because it can't be used for work. But often-times heat energy is exactly what we want, in cases of shivering or the brown fat in babies.
I guess what I'm trying to say is that friction and heat energy are two different things. Or would you say that heat energy only comes from friction of molecules? (I'm reaching the limits of my understanding of physics now haha).

Friction and heat energy are different only in that friction is a force and heat energy is energy. So we're talking about Newtons versus Joules. But frictional forces produce heat energy - and that's the important piece of the puzzle here. The "heat" released from ATP hydrolysis is not really "heat" in the quotidian sense of the term. As chemists, we use "heat" in a very specific context. That is, as a thermodynamic quantity that characterizes the energy released from a reaction without regard to whether that energy is used to produce meaningful work or not. But let's take a step back. Energy can be broadly classified into two types - "ordered" energy and "disordered" energy. Work is the culmination of ordered energy, where energy is used to effect a certain purpose. Thermal heat is disordered energy, where that energy is dissipated without producing any work.

Now, when ATP is hydrolyzed in your muscle, it produces "heat" in the chemical sense of the term. It would be more accurate to say that it releases energy. That energy is then used to produce movement - it's not simply released as thermal heat. If it were, that would be very wasteful. Nature has evolved over billions of years to very efficiently couple the energy released from ATP hydrolysis to produce work in the form of moving filaments. It's not a 100% efficient process, but it converges on it - nature has the advantage of billions of years. Now, the movement of those filaments produces friction, which produces thermal energy which is dissipated. So it's incorrect to say that ATP hydrolysis is what produces "heat" during shivering.

So in terms of energy conservation, it's more useful to think of it this way. ATP has chemical energy stored up in the form of high energy phosphodiester bonds. During hydrolysis, that chemical energy is transformed into mechanical energy by driving the motion of the filaments. In other words, that energy is used to perform work. Then, as the filaments move, friction acts on them and that finally converts the mechanical energy into thermal energy, which cannot be used to do any more work and is dissipated into the environment. That's what happens when you "shiver."

 

[iBS1972]

Wow, that was a great explanation--thank you. I hope you do some sort of active teaching in your job.

 

[desperadoflakes]

Friction and heat energy are different only in that friction is a force and heat energy is energy. So we're talking about Newtons versus Joules. But frictional forces produce heat energy - and that's the important piece of the puzzle here. The "heat" released from ATP hydrolysis is not really "heat" in the quotidian sense of the term. As chemists, we use "heat" in a very specific context. That is, as a thermodynamic quantity that characterizes the energy released from a reaction without regard to whether that energy is used to produce meaningful work or not. But let's take a step back. Energy can be broadly classified into two types - "ordered" energy and "disordered" energy. Work is the culmination of ordered energy, where energy is used to effect a certain purpose. Thermal heat is disordered energy, where that energy is dissipated without producing any work.

Now, when ATP is hydrolyzed in your muscle, it produces "heat" in the chemical sense of the term. It would be more accurate to say that it releases energy. That energy is then used to produce movement - it's not simply released as thermal heat. If it were, that would be very wasteful.

Would you say that the mechanism for heat production in brown fat is an exception to this? Brown fat cells have channels in the mitochondrial inner membrane that allow for facilitated diffusion of protons across the membrane, just like the ATP synthase but without the elements necessary for using the energy of that proton gradient for ATP synthesis, essentially creating energy to be used for nothing. Here's where it gets a bit fuzzy for me though. Animals like polar bears and [human] babies have a ton of brown fat because this uncoupling process produces so much heat compared to the normal ATP synthase function (babies haven't yet developed the ability to shiver). What is it about the uncoupling process that produces this heat? Is it just because the ATP synthase chemical reaction is endothermic, so by doing the electron transport you're creating the same amount of heat but the loss of ATP synthesis prevents it from being absorbed endothermically?

And if so, why does the function of the electron transport chain produce heat at all? Does the creation of an electrochemical gradient increase the kinetic energy of the protons moving down it? That's a bit fuzzy for me, would love to hear what you think.

 

[aldol16]

Would you say that the mechanism for heat production in brown fat is an exception to this? Brown fat cells have channels in the mitochondrial inner membrane that allow for facilitated diffusion of protons across the membrane, just like the ATP synthase but without the elements necessary for using the energy of that proton gradient for ATP synthesis, essentially creating energy to be used for nothing. Here's where it gets a bit fuzzy for me though. Animals like polar bears and [human] babies have a ton of brown fat because this uncoupling process produces so much heat compared to the normal ATP synthase function (babies haven't yet developed the ability to shiver). What is it about the uncoupling process that produces this heat? Is it just because the ATP synthase chemical reaction is endothermic, so by doing the electron transport you're creating the same amount of heat but the loss of ATP synthesis prevents it from being absorbed endothermically?

Brown fat contains an uncoupling protein that specifically allows protons that are pumped out to return into the matrix without going through ATP synthase. The reason you need protons to go through ATP synthase is because the protons actually mechanically power the "pump" action of ATP synthase. You might look into this, as there is a cool experiment where they attached a fluorescent reporter onto the end of ATP synthase and show that it actually rotates. There is a video in the supporting information of that particular paper that shows this beautifully.

With respect to the actual generation of heat, it can be best seen from a conservation of energy argument. Before, you had a proton motive force whose energy is translated into the mechanical energy of the ATP synthase pump. But now you have this thermogenin protein that uncouples the proton motive force from ATP synthase - it doesn't absorb the energy itself, so where does it go? It goes to the generation of heat, or dissipative energy. In other words, the proton motive force can be expressed the following way: proton(out) ---> proton(in) + energy. In ATP synthase, that energy is used to spin the lower part of the protein to force formed ATP out (rate-determining step in ATP synthesis). In thermogenin, that energy is released as heat. How? The normally exothermic reactions are now uncoupled from endothermic ones (ATP synthesis) and therefore release disordered heat into the surroundings.

Here is a very informative review if you are interested (beyond the scope of the MCAT): http://physrev.physiology.org/content/84/1/277

 

[desperadoflakes]

Brown fat contains an uncoupling protein that specifically allows protons that are pumped out to return into the matrix without going through ATP synthase. The reason you need protons to go through ATP synthase is because the protons actually mechanically power the "pump" action of ATP synthase. You might look into this, as there is a cool experiment where they attached a fluorescent reporter onto the end of ATP synthase and show that it actually rotates. There is a video in the supporting information of that particular paper that shows this beautifully.

With respect to the actual generation of heat, it can be best seen from a conservation of energy argument. Before, you had a proton motive force whose energy is translated into the mechanical energy of the ATP synthase pump. But now you have this thermogenin protein that uncouples the proton motive force from ATP synthase - it doesn't absorb the energy itself, so where does it go? It goes to the generation of heat, or dissipative energy. In other words, the proton motive force can be expressed the following way: proton(out) ---> proton(in) + energy. In ATP synthase, that energy is used to spin the lower part of the protein to force formed ATP out (rate-determining step in ATP synthesis). In thermogenin, that energy is released as heat. How? The normally exothermic reactions are now uncoupled from endothermic ones (ATP synthesis) and therefore release disordered heat into the surroundings.

Here is a very informative review if you are interested (beyond the scope of the MCAT): http://physrev.physiology.org/content/84/1/277

Awesome, that's exactly what I would've inferred. Super helpful, thank you!

 

[desperadoflakes]

Brown fat contains an uncoupling protein that specifically allows protons that are pumped out to return into the matrix without going through ATP synthase. The reason you need protons to go through ATP synthase is because the protons actually mechanically power the "pump" action of ATP synthase. You might look into this, as there is a cool experiment where they attached a fluorescent reporter onto the end of ATP synthase and show that it actually rotates. There is a video in the supporting information of that particular paper that shows this beautifully.

With respect to the actual generation of heat, it can be best seen from a conservation of energy argument. Before, you had a proton motive force whose energy is translated into the mechanical energy of the ATP synthase pump. But now you have this thermogenin protein that uncouples the proton motive force from ATP synthase - it doesn't absorb the energy itself, so where does it go? It goes to the generation of heat, or dissipative energy. In other words, the proton motive force can be expressed the following way: proton(out) ---> proton(in) + energy. In ATP synthase, that energy is used to spin the lower part of the protein to force formed ATP out (rate-determining step in ATP synthesis). In thermogenin, that energy is released as heat. How? The normally exothermic reactions are now uncoupled from endothermic ones (ATP synthesis) and therefore release disordered heat into the surroundings.

Here is a very informative review if you are interested (beyond the scope of the MCAT): http://physrev.physiology.org/content/84/1/277

ATP Synthase is one of my favorite things in biology ever - so incredible how something like that works on that kind of scale

 

[iBS1972]

Brown fat contains an uncoupling protein that specifically allows protons that are pumped out to return into the matrix without going through ATP synthase. The reason you need protons to go through ATP synthase is because the protons actually mechanically power the "pump" action of ATP synthase. You might look into this, as there is a cool experiment where they attached a fluorescent reporter onto the end of ATP synthase and show that it actually rotates. There is a video in the supporting information of that particular paper that shows this beautifully.

With respect to the actual generation of heat, it can be best seen from a conservation of energy argument. Before, you had a proton motive force whose energy is translated into the mechanical energy of the ATP synthase pump. But now you have this thermogenin protein that uncouples the proton motive force from ATP synthase - it doesn't absorb the energy itself, so where does it go? It goes to the generation of heat, or dissipative energy. In other words, the proton motive force can be expressed the following way: proton(out) ---> proton(in) + energy. In ATP synthase, that energy is used to spin the lower part of the protein to force formed ATP out (rate-determining step in ATP synthesis). In thermogenin, that energy is released as heat. How? The normally exothermic reactions are now uncoupled from endothermic ones (ATP synthesis) and therefore release disordered heat into the surroundings.

Here is a very informative review if you are interested (beyond the scope of the MCAT): http://physrev.physiology.org/content/84/1/277

Hi aldo, I loved your explanations. But it leaves a couple of questions unanswered in my mind that I was hoping you could answer. If the energy released from hydrolysis of ATP is as preserved or "closed" as you explain, then ehat is the point of shivering? How then DO we maintain body temperature? And why does exercise increase our body temperature so fast then?

 

[aldol16]

Hi aldo, I loved your explanations. But it leaves a couple of questions unanswered in my mind that I was hoping you could answer. If the energy released from hydrolysis of ATP is as preserved or "closed" as you explain, then ehat is the point of shivering? How then DO we maintain body temperature? And why does exercise increase our body temperature so fast then?

Shivering moves your muscles past one another and creates friction between them. Heat is always produced as a byproduct of muscle motion - the energy released from ATP hydrolysis is used to drive your muscles but some of that is lost to friction, or heat energy, as your muscles move. That's why your muscles "warm up" when you move around and when you shiver. The difference is, when you're shivering, heat isn't a "byproduct" but rather the intended product. Adults also have brown fat reserves, which is a way to uncouple the proton motive force from ATP synthesis so that the energy contained within the proton-motive force can be dissipated as heat.

 

[Professional Student]

Shivering moves your muscles past one another and creates friction between them. Heat is always produced as a byproduct of muscle motion - the energy released from ATP hydrolysis is used to drive your muscles but some of that is lost to friction, or heat energy, as your muscles move. That's why your muscles "warm up" when you move around and when you shiver. The difference is, when you're shivering, heat isn't a "byproduct" but rather the intended product. Adults also have brown fat reserves, which is a way to uncouple the proton motive force from ATP synthesis so that the energy contained within the proton-motive force can be dissipated as heat.

To think that the heat produced by shivering is due to friction is frankly, pretty silly. The heat is produced by the inefficiency of ANY energy transformation. When ATP is converted to ADP in order for the myosin-actin filament interaction to occur, the MAJORITY of the energy released is lost directly as heat. There is no energy transformation reaction that exists in the world that is more than 30%-40% efficient. The remaining 60%-70% is lost as heat energy. This result is exemplified again in the presence of a fever when the body is fighting infection. A vast amount of energy (ATP) is being used by the body to restore homeostasis, resulting in raised body temperature even though there is no shivering. The heat produced from shivering is also from the ATP being inefficiently consumed by the body, just like during a fever, but this time the reaction is the harmless contraction of muscle fibers and not the inflammatory response of the immune system.

To say that the heat is a result of friction of the sliding filaments is adding an unnecessary step to think about. I'm sure some heat is produced by friction, but the fraction is insignificant compared to the energy lost during the hydrolysis of ATP. You will learn that every chemical reaction is inefficient, but biochemical reactions are especially inefficient. If the filaments were producing that much heat from friction they would quickly degrade from the mechanical stress, similar to wearing a hole through the threads of cloth.

 

[aldol16]

To think that the heat produced by shivering is due to friction is frankly, pretty silly. The heat is produced by the inefficiency of ANY energy transformation. When ATP is converted to ADP in order for the myosin-actin filament interaction to occur, the MAJORITY of the energy released is lost directly as heat. There is no energy transformation reaction that exists in the world that is more than 30%-40% efficient. The remaining 60%-70% is lost as heat energy. This result is exemplified again in the presence of a fever when the body is fighting infection. A vast amount of energy (ATP) is being used by the body to restore homeostasis, resulting in raised body temperature even though there is no shivering. The heat produced from shivering is also from the ATP being inefficiently consumed by the body, just like during a fever, but this time the reaction is the harmless contraction of muscle fibers and not the inflammatory response of the immune system.

Yes, upon looking into it further, I agree that a non-trivial amount of heat is lost due to inefficient coupling. However, less is lost than you think (actually you have the numbers almost reversed) and friction does play a non-trivial role. See here: Bioenergetics—Conversion of Biochemical to Mechanical Energy in the Cardiac Muscle - Springer. In brief, they're looking specifically at heart muscle but the efficiency of energy transfer of the ATP ---> mechanical energy transition is 50-70%, which is relatively high and what you would expect given evolutionary time frame.

 

[aldol16]

To say that the heat is a result of friction of the sliding filaments is adding an unnecessary step to think about. I'm sure some heat is produced by friction, but the fraction is insignificant compared to the energy lost during the hydrolysis of ATP. You will learn that every chemical reaction is inefficient, but biochemical reactions are especially inefficient. If the filaments were producing that much heat from friction they would quickly degrade from the mechanical stress, similar to wearing a hole through the threads of cloth.

Friction is non-trivial because all of the ATP energy is eventually dissipated. Just because, say, 60% of the ATP hydrolysis energy is transferred directly into mechanical motion doesn't mean that the buck stops there. Otherwise, the mechanical motion can just go on forever. But it can't. It stops and so that energy must go elsewhere - it could go into potential energy if you're flexing but if you think on a longer timescale, the energy transduction becomes clear. You wake up laying in bed (hopefully) in the morning. You move your muscles around and do a lot of stuff throughout the day. Let's say you fast the entire day. You start with 60 moles of ATP. You use 30 of them to move your muscles the entire day. At the end of the day, you return to bed - you're in the same state. So now let's compare your total energy content from the beginning of the day to the end of the day. Now you're 30 moles of ATP less. Where did all that energy that caused the muscles to move go? If it's conserved, then you would be building up energy endlessly in another part of your body. That doesn't happen. That means it's been dissipated.

 

[Professional Student]

Fair enough. But the OP's original question was where does the heat produced by shivering come from. If only 50%-70% of the energy released from ATP hydrolysis is converted to mechanical energy, that means that nearly half of the energy is lost directly as heat, which is a massive amount of energy considering it's nearly the same amount of energy that's used to contract the muscles in the first place. I think it's important to make the distinction between mechanical energy and friction. The 50%-70% of the energy that is converted to mechanical energy is not all, or even primarily, lost as heat produced from friction. The energy is used to initiate movement and overcome the elasticity of the filaments themselves. In fact, I would argue that almost no heat production is the result of friction because the filaments are not actually sliding against one another.

Imagine you are in a tree and are getting down by the means of a rope. Anyone that has ever slid down a rope knows that you can't do it, because the rope burns your hands very quickly because of the friction. If you want to get down without injury you must let go of the rope one hand at a time like your climbing a ladder, lowering yourself each time. The second example is almost identical to the sliding filament theory where the filaments are sliding in relation to each other but not actually against each other. That's a very interesting link you provided, I wish the whole content was free!

 

[aldol16]

Fair enough. But the OP's original question was where does the heat produced by shivering come from. If only 50%-70% of the energy released from ATP hydrolysis is converted to mechanical energy, that means that nearly half of the energy is lost directly as heat, which is a massive amount of energy considering it's nearly the same amount of energy that's used to contract the muscles in the first place. I think it's important to make the distinction between mechanical energy and friction. The 50%-70% of the energy that is converted to mechanical energy is not all, or even primarily, lost as heat produced from friction. The energy is used to initiate movement and overcome the elasticity of the filaments themselves. In fact, I would argue that almost no heat production is the result of friction because the filaments are not actually sliding against one another.

Imagine you are in a tree and are getting down by the means of a rope. Anyone that has ever slid down a rope knows that you can't do it, because the rope burns your hands very quickly because of the friction. If you want to get down without injury you must let go of the rope one hand at a time like your climbing a ladder, lowering yourself each time. The second example is almost identical to the sliding filament theory where the filaments are sliding in relation to each other but not actually against each other. That's a very interesting link you provided, I wish the whole content was free!

If you can get in on your institution's network, the content will likely be free, as that's a fairly often-used publishing company.

Regarding your example, you realize that the very reason why your hands do not slide down is because of the static friction between your hands and the rope holding your hands still right? Gravity will always pull you down, but the reason you experience no acceleration down is because of the frictional force between your hands and the rope. At the end of the day, energy cannot be lost. Your body is like a car. It takes in fuel and burns that fuel to produce movement. But that movement doesn't go on forever - otherwise, you wouldn't ever need to fill up your tank. The reason you need to fill up your tank - both the car and humans - is because the energy is dissipated. The tires heat up against the road due to static friction and you eventually stop moving unless you burn more fuel.