Biology
Sylvia S. Mader
Michael Windelspecht
Chapter 6
Metabolism:
Energy and
Enzymes
Lecture Outline
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Outline
• 6.1 Cells and the Flow of Energy
• 6.2 Metabolic Reactions and Energy
Transformations
• 6.3 Metabolic Pathways and Enzymes
• 6.4 Organelles and the Flow of Energy
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6.1 Cells and the Flow of
Energy
• Energy – The ability to do work or bring
about a change
 Kinetic energy
• Energy of motion
• Mechanical
 Potential energy
• Stored energy
• Chemical energy
3
Flow of Energy
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solar
energy
heat
heat
heat
Chemical
energy
Mechanical energy
4
Two Laws of
Thermodynamics
• First law:
 Law of conservation of energy
 Energy cannot be created or destroyed, but can be
changed from one form to another
• Second law:
 Law of entropy
 When energy is changed from one form to another,
there is a loss of usable energy
 Waste energy goes to increase disorder
5
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heat
CO2
sun
H 2O
carbohydrate
solar energy
producer
Carbohydrate Metabolism
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heat
carbohydrate
uncontracted muscle
contracted muscle
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Cells and Entropy
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H2O
C6H12O6
CO2
Glucose
Carbon dioxide
and water
• more organized
• more potential energy
• less stable (entropy)
a.
kinetic
energy
• less organized
• less potential energy
• more stable (entropy)
H+
H+
channel protein
H+
H+
H+
H+
H+
H+
H+
Unequal distribution
of hydrogen ions
• more organized
• more potential energy
• less stable (entropy)
b.
H+
H+
H+
H+
H+
H+
H+
H+
H+
Equal distribution
of hydrogen ions
• less organized
• less potential energy
• more stable (entropy)
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6.2 Metabolic Reactions and Energy
Transformations
• Metabolism
 Sum of cellular chemical reactions in cell
 Reactants participate in a reaction
 Products form as result of a reaction
• Free energy is the amount of energy available
to perform work
 Exergonic Reactions - Products have less free
energy than reactants (release energy)
 Endergonic Reactions - Products have more free
energy than reactants (require energy input)
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ATP: Energy for Cells
• Adenosine triphosphate (ATP)
 High energy compound used to drive metabolic
reactions
 Constantly being generated from adenosine
diphosphate (ADP)
• Composed of:
 Adenine, ribose (together = adenosine), and three
phosphate groups
• Coupled reactions
 Energy released by an exergonic reaction
captured in ATP
 ATP is used to drive an endergonic reaction
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The ATP Cycle
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adenosine triphosphate
ATP is unstable and has
a high potential energy.
P
P
P
ATP
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The ATP Cycle
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adenosine triphosphate
ATP is unstable and has
a high potential energy.
P
P
P
ATP
ATP +
P
Endergonic Reaction:
• The hydrolysis of ATP releases
previously stored energy, allowing
the change in free energy to do
work and drive other processes.
• Has negative delta G.
• Examples: protein synthesis, nerve
conduction, muscle contraction
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The ATP Cycle
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adenosine triphosphate
ATP is unstable and has
a high potential energy.
P
P
P
ATP
ADP
+
P
Endergonic Reaction:
• The hydrolysis of ATP releases
Previously stored energy, allowing
the change in free energy to do
work and drive other processes.
• Has negative delta G.
• Examples: protein synthesis, nerve
conduction, muscle contraction
P
P
+
P
+
adenosine diphosphate
phosphate
ADP is more stable and has lower potential energy than ATP.
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The ATP Cycle
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adenosine triphosphate
ATP is unstable and has
a high potential energy.
P
P
P
Exergonic Reaction:
• Creation of ATP from
ADP and Prequires
input of energy from
Other sources.
ATP
• Has positive delta G.
• Example: cellular
respiration
ADP
+
P
Endergonic Reaction:
• The hydrolysis of ATP releases
Previously stored energy, allowing
the change in free energy to do
work and drive other processes.
• Has negative delta G.
• Examples: protein synthesis, nerve
conduction, muscle contraction
P
P
+
P
+
adenosine diphosphate
phosphate
ADP is more stable and has lower potential energy than ATP.
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Coupled Reactions
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1
Myosin assumes its
resting shape when
It combines with ATP.
actin
myosin
ATP
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Coupled Reactions
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1
Myosin assumes its
resting shape when
It combines with ATP.
2
ATP splits into ADP
and p , causing
myosin to change its
shape and allowing it
to attach to actin.
actin
myosin
ATP
P
ADP
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Coupled Reactions
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1
Myosin assumes its
resting shape when
It combines with ATP.
2
ATP splits into ADP
and p , causing
myosin to change its
shape and allowing it
to attach to actin.
3
Release of ADP and
p cause myosin to
again change shape
and pull again
stactin, generating
force and motion.
actin
myosin
ATP
P
ADP
17
Coupled Reactions
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1
2
Myosin assumes its
resting shape when
It combines with ATP.
ATP splits into ADP
and p , causing
myosin to change its
shape and allowing it
to attach to actin.
3
Release of ADP and
p cause myosin to
again change shape
and pull against actin,
generating force and
motion.
actin
myosin
ATP
P
ADP
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6.3 Metabolic Pathways and
Enzymes
• Reactions usually occur in a sequence
 Products of an earlier reaction become reactants of a
later reaction
 Such linked reactions form a metabolic pathway
• Begins with a particular reactant, proceeds through several
intermediates, and terminates with a particular end product
AB C D E FG
“A” is Initial
Reactant
B, C, D, E, and F
are Intermediates
“G” is End
Product
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6.3 Metabolic Pathways and
Enzymes
• Enzyme
 Protein molecules that function as catalysts
 The reactants of an enzymatically catalyzed reaction
are called substrates
 Each enzyme accelerates a specific reaction
 Each reaction in a metabolic pathway requires a
unique and specific enzyme
 The end product will not be formed unless ALL
enzymes in the pathway are present and functional
E1
E2
E3
E4
E5
E6
A B  C  D  E  F  G
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Energy of Activation
• Molecules frequently do not react with one
another unless they are activated in some way
 Energy must be added to at least one reactant to
initiate the reaction
• Energy of activation
• Enzyme Operation:
 Enzymes operate by lowering the energy of
activation
 Accomplished by bringing substrates into contact with
one another
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Energy of Activation
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energy of
activation
(Ea)
energy of
reactant
Free Energy
energy of
activation
(Ea)
energy of
product
enzyme not present
enzyme present
Progress of the Reaction
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Enzyme-Substrate Complex
• The active site complexes with the
substrates
 Causes the active site to change shape
 Shape change forces substrates together,
initiating bond
 Induced fit model
• Enzyme is induced to undergo a slight alteration to
achieve optimum fit for the substrates
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Enzyme-Substrate Complex
• Degradation:
 Enzyme complexes with a single substrate molecule
 Substrate is broken apart into two product molecules
• Synthesis:
 Enzyme complexes with two substrate molecules
 Substrates are joined together and released as a
single product molecule
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Enzymatic Actions
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products
enzyme
substrate
enzyme-substrate
complex
active site
Degradation
The substrate is broken down
to smaller products.
enzyme
a.
product
enzyme
substrates
enzyme-substrate
complex
active site
b.
enzyme
Synthesis
The substrates are combined
to produce a larger product.
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Factors Affecting Enzymatic
Speed
• Substrate concentration
 Enzyme activity increases with substrate
concentration due to more frequent collisions
between substrate molecules and the enzyme
• Temperature
 Enzyme activity increases with temperature
 Warmer temperatures cause more effective collisions
between enzyme and substrate
 However, hot temperatures can denature and
destroy enzymes
• pH
 Most enzymes are optimized for a particular pH
26
The Effect of Temperature on Rate of
Reaction
Rate of Reaction
(product per unit of time)
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0
10
20
30
40
50
60
Temperature C
a. Rate of reaction as a function of
temperature
b. Body temperature of ectothermic animals
often limits rates of reactions.
c. Body temperature of endothermic animals
promotes rates of reactions.
b: © James Watt/Visuals Unlimited; c: © Creatas/PunchStock
27
The Effect of pH on Rate of Reaction
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trypsin
Rate of Reaction
(product per unit of time)
pepsin
0
1
2
3
4
5
6
7
8
9
10
11
12
pH
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Factors Affecting Enzymatic
Speed
• Cells can regulate the presence/absence of an
enzyme
• Cells can regulate the concentration of an
enzyme
• Cells can activate or deactivate some enzymes
 Enzyme Cofactors
• Molecules required to activate enzyme
• Coenzymes are nonprotein organic molecules
• Vitamins are small organic compounds required in the diet
for the synthesis of coenzymes
29
Cofactors at Active Site
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cofactor
active
site
a.
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Cofactors at Active Site
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substrate
b.
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Cofactors at Active Site
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cofactor
active
site
a.
substrate
b.
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Enzyme Inhibition
• Reversible enzyme inhibition
 A substance known as an inhibitor binds to an
enzyme and decreases its activity
• Competitive inhibition – the substrate and the
inhibitor are both able to bind to active site
• Noncompetitive inhibition – the inhibitor does not
bind at the active site, but at an allosteric site
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Noncompetitive Inhibition of an Enzyme
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A
E
enzymes
1
substrates A
1
allosteric site
1
E
B
E
2
C
3
E
4
D
E
5
E
F
(end
product)
Metabolic pathway produces F, the end product.
active site
2
E
E
1
F
(end
product)
F binds to allosteric site and the active site of E1 changes shape.
F
A
3
E
1
(end
product)
A cannot bind to E1; the enzyme has been inhibited by F.
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Enzyme Inhibitors Can Spell
Death
• Materials that irreversibly inhibit an enzyme are
known as poisons
• Cyanide inhibits enzymes required for ATP
production
• Sarin inhibits an enzyme located at the
neuromuscular junction.
• Warfarin inhibits an enzyme responsible for the
blood clotting process
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6.4 Organelles and the Flow
of Energy
• Oxidation-reduction (redox) reactions
 Electrons pass from one molecule to another
• Oxidation - loss of an electron
• Reduction – gain of an electron
 Both take place at same time
 One molecule accepts the electron given up by
the other
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Photosynthesis and Cellular Respiration
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Electron Transport Chain
• Consists of membrane-bound carrier proteins
found in mitochondria and chloroplasts
• Physically arranged in an ordered series
 Starts with high-energy electrons
 Pass electrons from one carrier to another
• Electron energy used to pump hydrogen ions (H+) to one side
of membrane
• Establishes an electrochemical gradient across the
membrane
• The electrochemical gradient is used to make ATP from ADP
– Chemiosmosis
 Ends with low-energy electrons and high-energy ATP
38
ElectronTransport Chain
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e–
high-energy
electrons
High-energy electrons
are unstable and have
high potential energy.
This energy is released
in stages, as kinetic
energy, during the
electron transport chain.
energy for
Synthesis of
ATP
electron
transport chain
As energy is released,
the electrons become
more stable and have
less potential energy.
e-
low-energy
electrons
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Chemiosmosis
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High H+ concentration
H+ pump in electron
transport chain
NADH
Low
NAD +
H+ concentration
ATP
synthase
complex
H+
H+
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Chemiosmosis
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High H+ concentration
H+
H+ pump
in electron
transport chain
NADH
Low
H+
H+
ATP
synthase
complex
NAD + H+
H+ concentration
H+
41
Chemiosmosis
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High H+ concentration
H+
H+ pump
in electron
transport chain
NADH
Low
H+
H+
ATP
synthase
complex
NAD + H+
H+ concentration
H+
42
Chemiosmosis
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High H+ concentration
H+
H+
H+ pump
in electron
transport chain
H+
H+
H+
ATP
ADP + P
ADP + P
NADH
NAD +
Low H+ concentration
H+
H+
ATP
ATP
synthase
complex
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