BCM.18 - FREE ENERGY

Gibbs free energy formula

∆G=∆H-T∆S

What is Gibbs free energy

- A way of determining if a reaction will occur (‘be spontaneous’).

- G is the maximum energy available to do useful chemical work at constant pressure and temperature.

Change in G MUST be

-ve for reaction to occur

Exergonic

releases free energy

Endergonic

requires free energy

A reaction is driven by

Whichever of entropy or enthalpy has a -ve value

Standard free energy change (∆G0) is defined at

standard temperature and pressure, for 1 M concentrations.

25°C (298 K)

•1 atm (101 325 Pa)

•All the chemicals in the system are in their reference states: for soluble things, that means at a concentration of 1 molar.

Biologically modified version of ∆G0

∆G0'

–[H+] = 10−7 M (pH must be 7)

–[H2O] = 55.6 M ( water take up whatever volume is left after getting all the other components to 1 M )

Why do we use ∆G0'

Raw ∆G0' values ...

1 M conc for metabolites is unrealistic

Impossible to get 1M of water as its pure concentration is 55.6M

Are not used to directly predict anything

∆G depends on

the relative concentrations of reactants and products.

Free energy calculation for realistic conditions

ΔG = ΔG0' + RT ln[C][D]/[A][B]

[products]/[reactants] =

Mass action ratio (Γ)

G can be negative even if ∆G0 positive ....

if [reactants] sufficiently > [products]

Equilibrium constant (Keq) is the

value of the mass action ratio when ∆G=0

What does if mean if...

•Γ < Keq

•Γ > Keq

•Γ = Keq

1)“Too many reactants”

∆G negative - exergonic

Forwards reaction spontaneous

2) “Too many products”

∆G positive - endergonic

Forward reaction not spontaneous

Backwards reaction favoured

3)“Just right”

∆G zero - equilibrium

No net reaction

If ∆G=∆G0+RT ln([C]eq [D]eq)/([A]eq [B]eq )=0

Therefore ..

∆G0=-RT ln(Keq)

Adenosine triphosphate (ATP) has a high....

“phosphoryl transfer potential”

∆G for reaction is very negative

ATP hydrolysis (here to AMP) can be coupled to...

Nonspontaneous reactions to drive them

Ion gradients are also a .....

Energy is derived from their...

store of free energy

collapse eg H+ gradients forming ATP

Two things that drive ion movements into cells

Chemical potential ( conc gradient )

Electrical potential ( voltage gradient )

In relation to the nerst equation, reactions are exergonic if

1) More ions on outside of membrane than inside

2) If outside of membrane is more positive than inner

Nerst Equation

The Nernst equation can predict

φ from equilibrium ion distributions and vice versa.

Assumption of Nerst equation

- Membrane is permeable to one ion

- G = 0, assuming system is at equilibrium

- neutral concentration gradient of the permeant ion (i.e. until the ions start crossing the membrane, there is no voltage difference across it)

Other ways to write nerst

If 2 ions are equally permeable and of opposite charge,

the membrane potential will be 0.