why do we used standard free energies?

[::Princess::]

Hi
im stuck on gibsbs free energy and i must finish it by tonight coz i havent made notes or tried to understand like 20 lectures so please help me

why do we use the standard free energy and what exactly is it doing in this equation

DeltaG = DeltG0' -RTInKeq'

why cant we just work out the Delta G using the equation without the standard free energy?
in the text book theres reference to some 'activity' but it doesnt explain it properly.

is there a way of understanding this or do i have to just memorize what happens? b/c most of the people that i asked didnt understand it either
thanks

[shahana]

all i know is that Gibbs free energy is the energy that is available to do work..and entropy (disease) is negative and the equation above...i dont think you need to learn it in that much detail..but more on the basis of to with catabolic=exergonic
anabolic=endergonic

sorry i couldnt be more help...

[Pammy]

G0 refers to "standard conditions". That means, the Gibbs free energy was measured when the concentrations of all reactants and products were 1 molar. Now this doesn't usually apply in the cell. Imagine the reaction for hydrolysis of ATP to ADP. There is a lot more ATP than ADP in the cell, which favours the direction of ATP hydrolysis to ADP rather than synthesis from ADP. Hence, if you want to really know which direction a reaction will go, you need to not just know it for the rare case where all concentrations are 1 M (in which case e.g. for ATP hydrolysis dG0 is around -30 kJ/mol) but for the case when you plug in actual concentrations (these go into the equilibrium constant, K, which comes in as RTlnK in the equation). So here, K would be determined from the concentrations of ATP, ADP, and Pi (I presume you know how to calculate an equlibrium constant?). When you now work out the dG under cellular conditions from dG0 (-30) and RTlnK, you end up with about -45 kJ/mol, meaning ATP hydrolysis is very favourable (more favourable than when all concentrations are 1 M).
Hope that makes sense...
Pammy