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Gibbs Free Energy and ATP

  1. Oct 6, 2015 #1
    Hello,
    Gibbs free energy describes the maximum of useful work that can be done by a chemical reaction. My question is(Sorry I am not into chemistry and biochemistry) : during a chemical reaction that creates ATP, we say that ATP stores energy between its chemical bonds . For exemple this is the equation of creating ATP: ADP + Pi + energy ----> ATP and of course this reaction has DG. What is the relation of DG with the stored energy? Is it the same?

    Thank you
     
  2. jcsd
  3. Oct 7, 2015 #2

    Ygggdrasil

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    The full, net equation for the synthesis of ATP is: ADP + Pi --> ATP + H2O and similarly, for the hydrolysis of ATP the equation is: ATP + H2O --> ADP + Pi

    It's somewhat misleading to say that ATP stores energy in its chemical bonds. In general, it takes a net input of energy to break a chemical bond (or else the chemical bond would not form in the first place). So, breaking the P-O bond between the terminal phosphate (called the gamma phosphate) and its neighboring phosphate (the beta phosphate) takes a net input of energy. You can get energy from the hydrolysis of ATP because forming new bonds releases energy, and if those new bonds are more stable than the bonds you broke, you'll have a net release of energy. So, in the hydrolysis of ATP, you are forming a bond between the water and gamma phosphate releases more energy than breaking the linkage between the gamma and beta phosphates. You can rationalize this by thinking about charge. In ATP, you have three negatively charged groups next to each other. ATP hydrolysis allows one of these negatively-charged groups to move away from the molecule, lessening the electrostatic repulsion. (for an extended discussion of this point with figures see http://biowiki.ucdavis.edu/Biochemi...d_Oxidative_Phosphorylation/Properties_of_ATP)

    The discussion above deals with the change in enthalpy (ΔH) of ATP hydrolysis, which is one component of the change in free energy (ΔG). Another component of ΔG is the change in entropy of the reaction. In the case of ATP hydrolysis, cell maintain a high ratio of ATP to ADP, which makes conversion of ATP into ADP even more thermodynamically favorable. The exact relation between entropy, enthalpy, and free energy is: ΔG = ΔH - TΔS
     
  4. Oct 7, 2015 #3
    Thanks,

    When talking about hydrolysis of ATP and energy release, What is the form of this energy?
     
  5. Oct 7, 2015 #4

    Ygggdrasil

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    The release of energy by ATP hydrolysis can take many forms.

    In many metabolic processes, ATP is hydrolyzed not by water, but by metabolites, creating a metabolite linked to either a phosphate or ADP/AMP. The linkage of phosphate or the nucleotide to the metabolite can weaken other bonds in the metabolite, making it easier for the cell to perform other chemical reactions with that molecule. Here, you're transfering chemical potential energy from one molecule to another.

    In molecular motor proteins, the hydrolysis of ATP generates mechanical force by causing a conformational change in the ATP binding pocket of the enzyme that gets transmitted to other parts of the protein (e.g. the lever arms of motors like kinesin). Here, ATP hydrolysis is used to perform mechanical work. Similarly, ATP hydrolysis by ion pumps in the membrane can couple ATP hydrolysis to the transport substances across the cell membrane against their concentration gradient. Here, ATP hydrolysis converts the chemical potential energy from bonds an electrical potential across the membrane which, for example, enables the electrical activity of neurons.
     
  6. Oct 7, 2015 #5
    So we can say for exemple that the DG = -7.3Kcal/mol of hydrolysis of the ATP, is a chemical potential energy that helps performing another chemical reaction right?
     
  7. Oct 7, 2015 #6

    Ygggdrasil

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    Yes. For example, say a reaction has a ΔG = +6 kcal/mol. Because the ΔG of the reaction is positive, it is endergonic and will not occur spontaneously. Coupling that reaction to the hydrolysis of ATP, however, would make the overall reaction thermodynamically favorable and proceed with a ΔG = - 1.3 kcal/mol. Now because the overall ΔG of the process is negative, the reaction will proceed spontaneously.
     
  8. Oct 7, 2015 #7
    Just last question: DG can represent heat released or no?
     
  9. Oct 7, 2015 #8

    Ygggdrasil

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    No. ΔH represents the amount of heat released by a chemical reaction (that occurs at constant pressure). ΔH is related to ΔG by the equation I referred to in post #2.
     
  10. Oct 7, 2015 #9
    Ok but for exemple for a combustion reaction or any exothermal reaction, there is release of heat then DG represent what in this case? is It also chemical potential energy?
     
  11. Oct 7, 2015 #10

    Ygggdrasil

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    Chemical potential energy is a somewhat inexact term that can have multiple meanings (enthalpy or free energy), so I apologize for using it earlier in the discussion. To clarify, enthalpy is a form of chemical potential energy that relates to the energy stored in chemical bonds.

    Free energy is a more difficult concept to explain, but it is basically a measure of how likely a reaction is to proceed. All chemical reactions are reversible, and the forward and backwards reactions have some probability of occurring. The free energy of a reaction (ΔG) is a useful quantity because it relates to the probability of the forward reaction occurring (pfor) relative to the probability of the reverse reaction occurring (prev). The exact relationship is pfor/prev = exp(-ΔGo/kBT). If ΔG < 0 the reaction will proceed spontaneously. For such reactions, their tendency to go forward can be used to power other processes (i.e. perform work) in the same way that water flowing downhill can be used to generate electricity. The water is going downhill anyway, so you can steal some of the energy from its downhill motion to perform work.

    Reactions that release heat are in general thermodynamically favorable because generating heat is a process that increases the entropy of the surroundings. However, you can have processes that are exergonic (ΔG < 0) yet endothermic (ΔH > 0). For example, ice melting is a thermodynamically favorable process at room temperature yet requires heat from the surroundings and is endothermic. In other words, the release of heat is not a prerequisite for performing work nor is the release of heat directly performing any work.
     
  12. Oct 7, 2015 #11
    Ok. But Gibbs free energy is the amount of work that can be performed by a chemical reaction. For exemple the DG of hydrolysis of ATP, this DG is a chemical potential energy right? But for the exemple of melting ice, DG is negative because the reaction is spontaneous but it doesn't have any physical meaning right?
     
  13. Oct 7, 2015 #12

    Ygggdrasil

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    In theory, you could do work with the free energy of the ice melting. For example, it could generate a voltage on a thermocouple.
     
  14. Oct 7, 2015 #13
    so DG represents the chemical potential energy? but for a combustion what is the work that can be performed other than heat? Can we say that the heat released by the combustion, part of it can do work and the other part cannot?
     
  15. Oct 7, 2015 #14

    Ygggdrasil

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    In an internal combustion engine, the heat is not performing any work (in fact, energy released as heat is lost). Rather, the work comes from the expansion that comes from converting some molecules in a dense liquid into many more gas molecules. The expansion of the gas pushes against the piston, which produces mechanical work. Again, the release of heat is not a prerequisite for performing work nor is the release of heat directly performing any work. The chemical reaction itself (converting fuel molecules into CO2) is performing the work.

    Some reactions proceed upon heating, but those are generally exergonic anyway and need the heat to speed up the reaction and are not using the heat to drive the thermodynamics of the reaction.
     
  16. Oct 7, 2015 #15
    Just to clarify, the DG of hydrolysis of ATP is a chemical potential energy right?
     
  17. Oct 7, 2015 #16

    Ygggdrasil

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    The Gibbs free energy is a type of potential energy, but there are others as well. Usually, probably when most people refer to chemical potential energy, they mean internal energy (U) or enthalpy (H) rather than Gibbs free energy (G).
     
  18. Oct 7, 2015 #17
    Great and sorry for disturbing you. What I am confused about is the definition of Gibbs free energy that it is the amount of work that can be done by a chemical reaction but this energy is not well defined like what is the type of this energy (potential, electrical, or simply work done) and it depends a lot of the chemical reaction itself
     
  19. Oct 7, 2015 #18

    Ygggdrasil

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    I think the "definition" of Gibbs free energy as the maximum amount of work that a chemical reaction can do is not a very rigorous one that people use as a low-level way of undestanding Gibbs free energy. Rigorously, Gibbs free energy is defined as G = H - TS, and the properties of free energy all come from that definition (plus, the definitions of enthalpy, temperature, and entropy). Here's a useful, perhaps more intuitive way of thinking about free energy that may be helpful: https://gravityandlevity.wordpress....e-plays-skee-ball-the-meaning-of-free-energy/
     
  20. Oct 7, 2015 #19
    Hello Again :P,

    Look what I have found:http://www.wiley.com/college/pratt/0471393878/student/review/thermodynamics/7_relationship.html

    It is said that DG is maximum amount of work or potential that can be done during a chemical reaction. It gives that DG of hydrolysis of ATP is equal to -30.5 Kcal/mol but due to the increase of entropy only 16.5 Kcal/mol is released as heat and the other part of the energy is lost due to increase of entropy ( or random movement of molecules thus lost work). So heat here is considered useful work?
     
  21. Oct 7, 2015 #20

    Ygggdrasil

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    I don't think that explanation is correct (many biology and biochemistry texts aren't very rigorous with their physical chemistry). They argue that the TΔS part of the equation represents energy that is unavailable to do work but is lost. However, I can think of a few examples (e.g. chemiosmotic processes inside of the cell or salinity gradient power generation) that primarily extract energy from the TΔS portion of ΔG.
     
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