Bioenergetics -- General Principles
• Biological entities obey the laws of thermodynamics.
• Accordingly, they can be viewed as systems that use
energy transformations to perform various types of work
Gibbs Free Energy
Useful for predicting whether or not a process is spontaneous.
Total Energy Change = Macroscopic + Microscopic
Total -- in constant temp/pressure, measured with E (enthalpy)
Macroscopic -- energy associated with change in the macroscopic state of
matter
Microscopic -- change in entropy of system
E  G  TS
or
G  E  TS
Energy Transformations and Coupling
Agents
How do energy transformation processes work?
Proteins as coupling agents -- devices that link spontaneous and
non-spontaneous processes.
A mechanical example (and model):
At this position E 1 = 0 since
M can spontaneously move
no further.
M
G
Fulcrum
Coupling
The Principle of Coupling
Illus trate d by a Le ve r
Assume this lever is supported
L1 = L 2
M
2M
L1
L2
Fulcrum
d
G
E1 = M * G * d
E2 = 2M * G * d
The Concept of Efficiency
Conserved (useful) Energy or Power
E
*100
Available (Total) Energy or Power
In the previous example, if we considered the energy available on the
right lever arm as the energy used to move the load on the left, then
the efficiency of this process was:
m* G* d
E
*100 50%
2m* G* d
The Principle of Coupling
Illus trate d by a Le ve r
Assume this lever is supported
L1 = L 2
M
2M
L1
L2
Fulcrum
d
G
E1 = M * G * d
E2 = 2M * G * d
In this case, 50% of the
available energy was
conserved and the rest
was lost.
Free Energy Equations
Free Energy (G) = Energy available for work, i.e., the work done in
going to equilibrium
GeneralForm:
G  nRT ln
[state 1]
[state 2]
G = # units involved * a measure of useful energy (work) per unit
Simple diffusion as an example:
[state 1]
G  nRT ln
[state 2]
E( J)
V
PV
J
V
R


 J * K 1 * mols 1
nT
nT
nT
Free Energy in a Diffusion
Gradient
[side 1]
GD  n RT ln
[side 2]
… or, restated:
[side 1]
GD  2.3* nRT log
[side 2]
Free Energy and Electrical
Charge
G  nzFE
z = charges per particle
F = Faraday’s constant -- the
amount of charge in coulombs in
one mol of electrons
E -- electrical potential in joules/coulomb
Free Energy from Chemical
Reactions
aA bBcC dD
We can write a description of this reaction at any moment in time
as the reaction quotient (QR) or mass-action ratio (q)
[C]c *[D]d
QR  q 
[A]a *[B]b
At equilibrium we say that:
Continued
[C]c *[D]d
K eq  QR  q 
[A]a *[B]b
Since, by definition, a system at equilibrium can do no work, then
any system displaced from equilibrium can do work in either the
forward or reverse direction (depending on the direction of
displacement from equilibrium.)
Thus:
GC  nRT ln
Keq
QR
or:
GC  2.3* nRT log
K eq
QR
Chemical Free Energy Plot
Absolute Value of G
Review Questions
What are diffusive, electrical and chemical work?
How could diffusing ions be used to drive some other
process?
What would be examples of the coupler?