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Bioenergetics
Energy is the ability to do work
Types of Energy: Electrical, Chemical, thermal, etc.
Unit of energy (Heat) is cal/kcal OR J/kJ.
Calorie (cal) is the amount of energy required to
raise the temperature of one gram of water by 1 °C.
Joule (J) is the amount of energy needed to apply a
1-newton force over a distance of 1 meter.
1 kJ = 0.239 kcal
Energy can be converted from one form to another
Energy is transferred through the universe from
one system to another
Sun
CO2 + H2O
Energy
Photosynthetic Cell
Heterotrophic Cell
plants
animals
CHO
CO2 + H2O
C6H12O6
O2
ATP
ADP + Pi
Stages of energy extraction
Production of
ATP
Photosynthesis
Plants,
Some algae,
Oxidative Phosphorylation
All living organisms
Why we need Energy (e.g. ATP)
To do work that can be used as:
1. Chemical work; e.g. to build
macromolecules
2. To preserve ionic equilibrium across
membranes; e.g. active transport.
3. Mechanical work; e.g. for movement.
Steady State & Equilibrium
•
The glucose ingested is circulating in blood and will be taken by cells. Now, less
glucose in blood.
•
A fresh supply of glucose is produced by the liver, so that your blood glucose
concentration is more or less constant over the whole day.
•
The constancy of concentration is the result of a dynamic steady state, which is
different from the term equilibrium.
Closed and Open system
Closed system is a theoretical assumption that we
can separate the system completely from its
surroundings.
Open system is the actual life or the reality of
reactions.
Definition of Entropy
The key descriptors of entropy are randomness and disorder, manifested in different
ways.
Case 1: The Teakettle and the Randomization of Heat
(teakettle (system) + kitchen (surrounding) = universe)
We know that steam generated from boiling water can
do useful work.
Heat passes from the teakettle to the surroundings,
raising the temperature of the surroundings (the kitchen)
by an infinitesimally small amount until complete
equilibrium is attained.
At this point all parts of the teakettle and the kitchen are at precisely the same
temperature.
Moreover, the increase in entropy of the kitchen (the surroundings)
is irreversible.
G =  H - TS
 = difference
Free energy content, G,
Enthalpy, H,
The absolute temperature, T (in degrees Kelvin)
Entropy, S
G > 0  endergonic
G < 0  exergonic
A process tends to occur spontaneously if  G is negative.
To carry out thermodynamically unfavorable, energy-requiring (endergonic)
reactions, cells couple them to other reactions that liberate free energy (exergonic
reactions), so that the overall process is exergonic.
The sum of the free energy changes is negative.
•When a system is at equilibrium, the rate of product formation exactly equals the rate
at which product is converted to reactant.
•The energy change as the system moves from its initial state to equilibrium, with no
changes in temperature or pressure, is given by the free-energy change, G.
• The magnitude of G depends on the particular chemical reaction and on how far
from equilibrium the system is initially. Each compound involved in a chemical reaction
contains a certain amount of potential energy, related to the kind and number of its
bonds.
Also means that amount of
energy needed by the
muscle cell to make ATP, ie
to reverse the reaction
Also means that amount of
energy needed by the muscle
cell to make ATP, ie to reverse
the reaction
Because the concentrations of ATP, ADP, and Pi differ from one cell type
to another, G for ATP hydrolysis likewise differs among cells.
Moreover, in any given cell, G can vary from time to time, depending on
the metabolic conditions in the cell and how they influence the
concentrations of ATP, ADP, Pi, and H+ (pH).
To further complicate the issue, the total concentrations of ATP, ADP, Pi,
and H may be substantially higher than the free concentrations, which are
the thermodynamically relevant values. The difference is due to tight
binding of ATP, ADP, and Pi to cellular proteins.
For example, the concentration of free ADP in resting muscle has been
variously estimated at between 1 and 37 M.
Thus, the energy released by ATP hydrolysis is greater than the standard
free-energy change, Go`.
G = - RT ln Keq
A
+
B
(substrates or reactants)
Keq

C
+
D
(products)
High G means that the substrates or reactants are far
from equilibrium state.
Low G means that the substrates or reactants are
near to equilibrium state
0 G means that the substrates or reactants are at
equilibrium state
The sign of G (+ or -) means exorgenic or endorgenic.
Also, means the reaction spontaneous (-) or not (+, i.e.,
should go in the reverse direction)
Go` = - RT ln Keq
The standard free-energy change of a chemical
reaction is simply an alternative mathematical way
of expressing its equilibrium constant.
A+BC+D
I’m confused G or Go`
Go` is the standard
G is the actual
Reaction is spontaneous (means favor to happen.
Also, means it will occur in the forward direction) is
due to G or Go`?
Due to G. Why? Because G = - RT ln Keq
Remember that activation energy is DIFFERENT.
ATP hydrolysis gives two benefits:
(ATP  ADP + Pi + E)
1. Amount of energy to be coupled with
another reaction (must be endergonic
(G > 0 OR +G) &
2. To release heat into the surrounding so
the universe (total) is now increased in
entropy (S).
So, What is efficiency?
Free energy change (G) only tells us the
difference and has no connection with the path
of the reaction or number of steps involved.
S
Keq1
G1
Keq3
Keq5
G3
G5
X
Keq2
G2
P
Y
Keq4
G4
Goverall = G1 + G2= G3 + G4 = G5
Keqoverall = Keq1 x Keq2 = Keq3 x Keq4 = Keq5
Example
The conversion of glucose to lactic acid has an
overall G of -52,000 cal/mol. In an anaerobic cell,
this conversion is coupled to the synthesis of 2
moles of ATP per mole of glucose. (a) calculate the
G of the overall coupled reaction. (b) calculate the
efficiency of energy conservation in the anaerobic
cell. (c) At the same efficiency, how many moles of
ATP per mole of glucose could be obtained in an
aerobic organism in which glucose is completely
oxidized to CO2 and H2O (G = -686,000 cal/mol)?
(a) Glucose  2 lactic acid G1 = -52,000 cal/mol
2ADP + 2Pi  2ATP
G2 = +7,700 cal/mol x 2
= +15,400 cal/mol
(Sum) glucose + 2ADP + 2Pi  2 lactic acid + 2ATP
G3 = G1 + G2
= (-52,000 cal/mol) + (+15,400 cal/mol)
= -36,600 cal/mol
energy conserved
(b) Efficiency = energy
made available
Efficiency = 29.6 %
x 100% =
15,400 cal/mol
x 100%
52,000 cal/mol
(c) Glucose + 6O2  6CO2 + 6H2O G4 = -686,000 cal/mol
nADP + nPi  nATP
G1 = n (+7,700 cal/mol)
(Sum) glucose + 6O2 + nADP + nPi  6CO2 + 6H2O + nATP
At 29.6% efficiency,
Total energy that can be conserved is
0.296 X 686,000 cal/mol = 203,000 cal/mol
If each mole of ATP requires 7,700 cal for its synthesis, Then:
203,000 cal/mol = 26.4 moles of ATP
7,700 cal/mol
= 26 moles of ATP (nearest whole number)
Why ATP?
ATP = Adenosine Triphosphate
Hydrolysis: -30.5 kJ mol-1 OR -7.3 kcal mol-1
ATP  ADP + Pi + E
10
6
7


8

5`
4`
3`
ATP4-
1
5
2
9
4
1`
2`
3
Mg2+ in the cytosol binds to ATP and ADP, and for most
enzymatic reactions that involve ATP as phosphoryl group
donor, the true substrate is MgATP2-.
ATP Donates Phosphoryl, Pyrophosphoryl, and Adenylyl Groups
The group transferred from ATP is a phosphoryl (PO32-), not a phosphate (OPO32-).
ATP Provides Energy by
Group Transfers, Not by
Simple Hydrolysis.
ATP hydrolysis per se usually
accomplishes nothing but the
liberation of heat, which cannot
drive a chemical process in an
isothermal system (means that our
temp stay at 37oC).
ATP conatins 4 negative charge
ADP conatins two phosphate groups
Go for ATP is -7.3 kcal/mole
The terminal PO4- can easily be released because:
Electrostatic Repulsion & Resonance Stabilization
Advantages of ATP:
1. The released energy (-7.3 kcal/mol) is moderate amongst
the biochemical compounds inside the cell.
2. Can transfer energy from compounds with high free
energy content to those with less.
3. It acts like a bridge between the chemical reactions
e.g.
A
+ ATP  x
ATP + y
+ ATP
 ADP + y~P
ATP in aqueous solution is thermodynamically unstable (means
easily hydrolysed) and is therefore a good phosphoryl group
donor but it is kinetically stable (means its activation energy is
high).
Because of the huge activation energies (200 to 400 kJ/mol)
required for uncatalyzed cleavage of its phosphoanhydride bonds,
ATP does not spontaneously donate phosphoryl groups to water
or to the hundreds of other potential acceptors in the cell.
Only specific enzymes can lower the energy of activation, so
the phosphoryl group is now transfered from ATP.
The Flow of Electrons Can Do Biological Work
1. Electrons do not move alone. Must be a force to drive them from one
position to another.
2. This force is the affinity of each atom to capture the electrons, which is
called the electronegativity.
3. The order of electronegativity is H < C < S < N < O.
4. Living cells have a biological “circuit,” with a relatively reduced
compound such as glucose as the source of electrons.
5. As glucose is enzymatically oxidized, the released electrons flow
spontaneously through a series of electron-carrier intermediates to
another chemical species, such as O2.
6. This electron flow is exergonic, because O2 has a higher affinity for
electrons than do the electron-carrier intermediates.
H+
H+
High-energy electron
H+
H+
H+
H+
H+
3
Pumping out
e2
NADH+H+
FADH2
H+
6
O2
H+
Low-energy electron
H+
NADPH+H+
Succinate dehydrogenase
1
Inner mitochondrial membrane
(OR Bacterial cell membrane)
Electron-carrier
5
4
(A) The electromotive force comes from the movement of the electron from
metabolites to O2. It results due to O2 has high affinity to electron than any
other compound. This force releases energy that are use to pump out
protons.
eO2
H+
Energy
enough to
make ATP
----------------++++++++++++
(B) Generation of the proton-motive force
H+
i
ii
The resulting electromotive force provides energy to a variety of molecular energy
transducers (enzymes and other proteins) that do biological work.
Examples:
1. In the mitochondrion, membrane-bound enzymes couple electron flow to the
production of a trans-membrane pH difference, accomplishing osmotic
and electrical work.
The proton gradient thus formed has potential energy (the proton-motive
force).
Another enzyme, ATP synthase in the inner mitochondrial membrane,
uses the proton-motive force to do chemical work: synthesis of ATP from
ADP and Pi. as protons flow spontaneously across the membrane.
2. Similarly, membrane-localized enzymes in E. coli convert electromotive force to
proton-motive force, which is then used to power flagellar motion.
more reduced compounds
↑H
↓O
Oxidation states of
carbon in the biosphere
more oxidized compounds
Reduction Potentials (E)
Measure Affinity for Electrons
Eo is +
Eo is -
e-
Reduction Potentials (E)
Measure Affinity for Electrons
Eo is Eo is +
e-
Reduction Potentials (E)
Measure Affinity for Electrons
electrons tend to flow through the
external circuit from lower to higher
standard reduction potential.
Electrons tend to flow to the half-cell
with the more positive Eo, and the
strength of that tendency is
proportional to the difference in
reduction potentials, Eo.
pH=0
By convention, the half-cell with the
stronger tendency to acquire electrons
is assigned a positive value of Eo.
pH=7
• Nernst equation:
F = Farady constant, n = number of electrons
• Standard Reduction (Eo) Potentials Can Be
Used to Calculate the Free-Energy Change
Example
Cellular Oxidation of Glucose to Carbon Dioxide
Requires Specialized Electron Carriers
C6H12O6 + 6O2  6CO2 + 6H2O
The complete oxidation of glucose: has a G of 2,840 kJ/mol.
This is a much larger release of free energy than is required
for ATP synthesis (50 to 60 kJ/mol). Cells convert glucose to
CO2 not in a single, high-energy-releasing reaction, but
rather in a series of controlled reactions, some of which are
oxidations.
Electrons removed in these oxidation steps are transferred to
coenzymes specialized for carrying electrons, such as NAD
and FAD.