Bioenergetics
Bryant Miles
Basis of Thermodynamics
Every living cell and organism must perform work to stay alive, to grow and to reproduce.
The
ability to harvest energy from nutrients or photons of light and to channel it into biological work
is the miracle of life.
Living organisms carry out a remarkable variety of energy transductions.
The biological energy transductions obey the physical laws that govern all natural processes,
including the laws of thermodynamics.
1
st
Law of Thermodynamics
The energy of the universe remains constant.
2
nd
Law of Thermodynamics
All spontaneous processes increase the entropy of the universe.
State functions
depend only on the initial and final conditions not on path taken between the
initial and final conditions.
They are independent of path
.
The important state functions for
the study of biological systems are:
G,
the
Gibbs free energy
which is equal to the total amount of energy capable of doing work
during a process at constant temperature and pressure.
•
If
Δ
G is negative, then the process is spontaneous and termed exergonic.
•
If
Δ
G is positive, then the process is nonspontaneous and termed endergonic.
•
If
Δ
G is equal to zero, then the process has reached equibrium.
H
, the
Enthalpy
which is the heat content of the system.
•
When
Δ
H is negative the process produces heat and is termed exothermic.
•
When
Δ
H is positive the process absorbs heat and is termed endothermic.
S
, the
Entropy
is a quantitative expression of the degree of randomness or disorder of the
system.
•
When
Δ
S is positive then the disorder of the system has increased.
•
When
Δ
S is negative then the disorder of the system has decreased.
The conditions of biological systems are constant temperature and pressure.
Under such
conditions the relationships between the change in free energy, enthalpy and entropy can be
described by the expression where T is the temperature of the system in Kelvin.
Δ
G =
Δ
H
−
T
Δ
S
Equilibrium Constants
All spontaneous processes proceed until equilibrium is reached.
Consider the following
chemical reaction.
A
+
B
C
+
D
k
1
k
2
The forward rate of product formation is = k
1
[A][B]
The reverse rate of reactant formation is = k
2
[C][D]
At equilibrium the concentrations of products and reactants are such that forward and reverse
rates are equal
k
1
[A
eq
][B
eq
] = k
2
[C
eq
][D
eq
].
A little algebra and presto

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