Exergonic and Endergonic Reac ons in Metabolism

Fig.
8‐5
Fig.
8‐5
• More free energy (higher G)
• Less stable
• Greater work capacity
In a spontaneous change
• The free energy of the system
decreases (∆G < 0)
• The system becomes more
stable
• The released free energy can
be harnessed to do work
• Less free energy (lower G)
• More stable
• Less work capacity
(a) Gravitational motion
(b) Diffusion
(c) Chemical reaction
Exergonic
and
Endergonic
Reac.ons
in
Metabolism
• Free
energy
changes
in
reac1ons
• Exergonic
reac.on
•
proceeds
with
a
net
release
of
free
energy
and
is
spontaneous
•
Results
in
lower
energy,
more
stable
products
• Endergonic
reac.on
•
absorbs
free
energy
from
its
surroundings
and
is
nonspontaneous
•
Results
in
higher
energy,
less
stable
products
Fig.
8‐6a
Fig.
8‐6a
Free energy
Reactants
Amount of
energy
released
(∆G < 0)
Energy
Products
Progress of the reaction
(a) Exergonic reaction: energy released
Fig.
8‐6b
Fig.
8‐6b
Free energy
Products
Amount of
energy
required
(∆G > 0)
Energy
Reactants
Progress of the reaction
(b) Endergonic reaction: energy required
Equilibrium
and
Metabolism
•
Closed
systems
•
Cells
are
open
systems
•
•
eventually
reach
equilibrium
and
then
do
no
more
work
•
Therefore,
not
in
equilibrium
•
Experiencing
a
constant
flow
of
materials
Metabolism
is
never
at
equilibrium
•
A
defining
feature
of
life
•
A
catabolic
pathway
in
a
cell
releases
free
energy
in
a
series
of
reac1ons
•
Closed
and
open
hydroelectric
systems
can
serve
as
analogies
Fig.
8‐7
Fig.
8‐7
∆G < 0
∆G = 0
(a) An isolated hydroelectric system
(b) An open hydroelectric
system
∆G < 0
∆G < 0
∆G < 0
∆G < 0
(c) A multistep open hydroelectric system
Coupled
Reac.ons
•
A
cell
does
three
main
kinds
of
work:
•
Chemical
•
Forced
endergonic
rxns
•
•
•
Ac1ve
transport
•
•
Monomers
‐>
polymers
Transport
Across
cell
membrane
against
concentra1on
gradients
Mechanical
•
Movement
•
Muscle
contrac1on,
bea1ng
of
flagella
or
cilia
•
Energy
coupling
•
Most
energy
coupling
in
cells
is
mediated
by
ATP
•
Cells
use
energy
of
an
exergonic
process
to
drive
an
endergonic
one
The
Structure
and
Hydrolysis
of
ATP
• ATP
(adenosine
triphosphate)
•
Energy
currency
of
the
cell
•
composed
of
• ribose
(a
sugar)
• adenine
(a
nitrogenous
base)
• three
phosphate
groups
Adenine
Phosphate groups
Ribose
The
Structure
and
Hydrolysis
of
ATP
• Harves1ng
power
from
ATP
•
Break
high
energy
phosphate
bonds
• By
hydrolysis
•
Energy
released
when
terminal
phosphate
bond
is
broken
• Release
of
energy
comes
from
chemical
change
to
state
of
lower
free
energy
•
not
from
the
phosphate
bonds
themselves
Fig.
8‐9
Fig.
8‐9
P
P
P
Adenosine triphosphate (ATP)
H2 O
Pi
+
P
Inorganic phosphate
P
+
Energy
Adenosine diphosphate (ADP)
How
ATP
Performs
Work
• The
three
types
of
cellular
work
(mechanical,
transport,
and
chemical)
•
powered
by
the
hydrolysis
of
ATP
• Energy
from
exergonic
ATP
hydrolysis
•
used
to
power
endergonic
rxns
• coupled
• overall
exergonic
Fig.
8‐10
Fig.
8‐10
NH2
Glu
Glutamic
acid
NH3
+
∆G = +3.4 kcal/mol
Glu
Ammonia
Glutamine
(a) Endergonic reaction
1 ATP phosphorylates
glutamic acid,
making the amino
acid less stable.
P
Glu
+
ATP
+
NH3
Glu
+ ADP
NH2
2 Ammonia displaces
the phosphate group,
forming glutamine.
P
Glu
Glu
+ Pi
(b) Coupled with ATP hydrolysis, an exergonic reaction
(c) Overall free-energy change
ATP
• Phosphoryla.on
•
Process
of
transferring
a
phosphate
group
to
some
other
molecule,
such
as
a
reactant
•
Drives
endergonic
reac1ons
• The
recipient
molecule
is
now
phosphorylated
Fig.
8‐11
Fig.
8‐11
Membrane protein
P
Pi
Solute transported
Solute
(a) Transport work: ATP phosphorylates
transport proteins
ADP
+
ATP
Vesicle
Cytoskeletal track
Pi
ATP
Protein moved
Motor protein
(b) Mechanical work: ATP binds noncovalently
to motor proteins, then is hydrolyzed
Enzymes
lower
energy
barriers
• Catalyst
•
Chemical
agent
that
speeds
up
a
reac1on
without
being
consumed
by
the
reac1on
• Enzyme
•
Cataly1c
protein
• Hydrolysis
of
sucrose
by
the
enzyme
sucrase
is
an
example
of
an
enzyme‐catalyzed
reac1on
Fig.
8‐13
Fig.
8‐13
Sucrose (C12H22O11)
Sucrase
Glucose (C6H 12O6)
Fructose (C6H12O6)
The
Ac.va.on
Energy
Barrier
• Every
chemical
reac1on
between
molecules
involves
bond
breaking
and
bond
forming
• Ac.va.on
energy
(EA),
or
free
energy
of
ac.va.on
The
ini1al
energy
needed
to
start
a
chemical
reac1on
• OYen
supplied
in
the
form
of
heat
from
the
surroundings
Fig.
8‐14
Fig.
8‐14
A
B
C
D
Transition state
Free energy
•
A
B
C
D
EA
Reactants
A
B
C
D
∆G < O
Products
Progress of the reaction