Download free energy

Gibbs free energy and equilibrium
constant
Gibbs Free Energy ,G
Is the thermodynamic function that is most
useful for biochemistry.
G is a function of
Enthalpy, H, a measure of the energy (heat
content) of the system at constant pressure,
and
Entropy, S, a measure of the randomness
(disorder) of the system.
The change in Gibbs free energy )ΔG)
for a reaction quantitatively measure
the energy available to do useful work.
It is related to the change in enthalpy
and the change in entropy:
ΔG=ΔH-TΔS
The actual free energy change )ΔG)
depends on 2 parameters:
the standard free energy change for that
reaction )ΔG°) (and thus to Keq ,defining
where equilibrium for this reaction lies),
and
the actual mass action ratio, reflecting the
actual starting conditions, the actual
concentrations of reactants and products
actual ΔG = ΔG° + RTln {actual mass action
ratio}
A + B >=< C + D
All reactions/processes proceed in direction
required to go TOWARD EQUILIBRIUM.
For this reaction, the mass action ratio is
given by: [C] [D]/ [A] [B]
The mass action ratio at equilibrium is the
equilibrium constant for the reaction
Keq =[ C] [D] / [A] [B]
The standard free energy change for a
reaction) ΔG°) is the change in free energy
under STANDARD CONDITIONS.
It is
related to the equilibrium constant by the
equation:
ΔG° = - RT ln Keq
Where R : is the natural gas constant equal
8.315 joules or 1.987 Cal
T :is the absolute temperature
ln : natural logarithm.
Keq : equilibrium constant
At equilibrium ΔG = 0
and K eq = [C] [D]/ [A] [B]
hence
0 = ΔG° + RT ln Keq
ΔG °= -RT ln Keq
Continuation of life requires continuous
chemical reaction
Reactions that reach equilibrium have stopped, can't get out
of that state without external change.
Consumption of food provides a continued supply of
substrates for reactions yielding net negative ΔG. Net
negative ΔG ensures that reactions proceed in the required
direction for continuation of life
.
Energy made available from breakdown of some
compounds, e.g. sugars, fats, amino acids, can be used to
drive the synthesis of other molecules, e.g .structural
components of cells, or compounds such as
polysaccharides that store energy for future needs.
Free energy change is dependent on
concentration
Δ G Actual free energy change under specified conditions,
including concentration of reactants and products .
Δ G ° Standard Free energy change, all reactants and
products in their standard states, i.e. 1 mol / L
concentration.Unfortunately, if H+ concentration is 1 mol /
L ,pH = 0 ,which is not consistent with biochemical
processes .
Standard Free Energy change Δ G°´ for the biochemical
standard state ,all reactants and products at 1 mol / L
except [H+] = [OH-] = 10 -7mol / L, which allows pH = 7.
phosphorylation of glucose to produce
glucose-6-phosphate
Very important reaction in the cell.
first reaction in metabolism of glucose that enters a
cell from the blood
.
Reaction 1: condensation of glucose (alcohol)
with inorganic phosphate ion (acid) to make
glucose-6-phosphate (an ester)
Glucose + Pi >=< glucose-6-phosphate+ H2O
ΔG°= + 13.8 KJ/mole.
Endergonic reaction
Reaction 2: hydrolysis of ATP, a phosphoanhydride, to
generate ADP and inorganic phosphate.
ATP +H2O >=<ADP + Pi
ΔG°= - 30.5 Kj/mole.
Exergonic reaction
To couple the 2 reactions (which requires some
chemical
mechanism,
of
course),
add reactants on left, add products on right, and add
ΔGo 'values to get Δ Go 'for coupled reaction :
Glucose + ATP >=< glucose-6-phosphate + ADP
Δ Go = - 16.7 = kJ/mol
The coupled reaction is exergonic, it will go
spontaneously (forward, left to right) in the cell, but
will it proceed at a rate consistent with cellular
needs ‫؟‬
Most biological reactions would proceed at a very
slow rate indeed if they're not catalyzed. The
biological catalyst enabling the coupled reaction
above to proceed on a biological timescale is an
enzyme, hexokinase.
Free energy coupling, with enzymes as catalysts,
is the strategy used in metabolic pathways.