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Homework Help: Krebs Cycle

  1. Oct 20, 2011 #1
    Having some trouble understanding if the Kreb cycle is exergonic or endergonic?
    I know it has to be exergonic because it's releasing ATP?
  2. jcsd
  3. Oct 21, 2011 #2
    GTP is actually is released, which from a potential chemical energy perspective, is equivalent to ATP. The Krebs cycle reduces the electron carrier NAD+ to NADH, which is used in the electron transport chain after the Krebs cycle to create ATP. The overall process must be exergonic, or else we would not have made far past being protoplasm. Individual endergonic reactions in the overall process are generally overcome through the use of highly energetic phophate-containing compounds; glycolysis is the best example of this, where 2 ATPs are consumed but 4 are gained, with a net of two ATP out.
  4. Oct 21, 2011 #3


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    What makes a process exergonic? Well, it is helpful to consider the equation for Gibbs free energy: ΔG = ΔH - TΔS. Since exergonic processes bring your system to a state of lower free energy (i.e. ΔG < 0), processes that lower the potential energy of your system (ΔH < 0) or increase its disorder (ΔS > 0) will help a process be more exergonic.

    How does one lower the potential energy of the system? Well, for carbon-based compounds, the answer is basically by increasing the number of bonds carbon has to oxygen. This is true because carbon-oxygen bonds are much more stable (i.e. have a lower potential energy) than carbon-hydrogen or carbon-carbon bonds.

    What about the disorder of the system? One easy way to increase the disorder of a system is to break large molecules into smaller molecules.

    Now, consider the Krebs cycle. The cell feeds acetyl-CoA into the Krebs cycle and it releases two molecules of carbon dioxide and regenerates free coenzyme A (CoA). Does this process create products that have a lower potential energy than the reactants? Does the process increase the entropy of the system?
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