Glycolysis Gluconeogensis Spontaneity

[Avicenna]

As I understand it, both processes are spontaneous; meaning no energy input is required - both will lead to a decrease in Gibbs Free Energy. Yet they are the same reaction just in reverse (taking account of only product and reactant since pathways are unimportant for state functions such as Gibbs Free energy).

Where am I mistaken?

---

Members don't see this ad.

 

[Cotterpin]

There are steps of glycolysis that definitely require energy input. Those steps are irreversible and gluconeogenesis has to take alternate pathways around those steps. So it's not the same reaction in both directions.

 

[Avicenna]

There are steps of glycolysis that definitely require energy input. Those steps are irreversible and gluconeogenesis has to take alternate pathways around those steps. So it's not the same reaction in both directions.

I'm not talking about the pathway. Gibbs free energy is a state function, and pathway independent. All that matters are the reactants and the products, right?

Also, the irreversible steps are the ones that have a negative delta G (the ones that release energy)

 

[OrdinaryDO]

No, glycolysis has a net gain in energy molecules, thus being an overall negative delta G. Gluconeogenesis is not the same reaction in reverse due to its use of alternative enzymes and such. Also, gluconeogenesis requires energy, as does any anabolic process in the body, for the most part.

 

[malamia]

The overall delta G is is negative due to reaction coupling with ATP and GTP. Otherwise it wouldn't be spontaneous. So it does require energy both ways

 

[Jelly Bones]

There are steps of glycolysis that definitely require energy input. Those steps are irreversible and gluconeogenesis has to take alternate pathways around those steps. So it's not the same reaction in both directions.

Actually, the fact that the 3 glycolysis steps are irreversible means they have a large negative deltaG. The reason these steps have to be bypassed in gluconeogenesis is because they would require too much energy input (large positive deltaG) to go in the reverse direction so alternative enzymes are used. These steps also serve as the regulatory points because they are so called committed steps. The reversible steps have smaller changes in deltaG and so can go either way based on substrate availabilities.

Overall these pathways BOTH have a negative deltaG but be careful because if glycolysis were to be completely reversed as is, it would result in a large positive deltaG. For this reason, in gluconeogenesis the cell bypasses the irreversible glycolytic steps to have an overall negative deltaG.

Always remember that the reverse of a reaction is the opposite sign of deltaG, same magnitude. However, if a different path (enzyme) is used, then it will be a unique deltaG. Hope this clears things up.