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Chapter 6
An Introduction to Metabolism
p. 116-134
Concept 6.1: An organism’s metabolism
transforms matter and energy
 Metabolism is…
 Metabolism is an emergent property of life that
arises from interactions between molecules within
the cell
 A metabolic pathway begins with a specific
molecule and ends with a product, and each step is
catalyzed by a specific enzyme
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Figure 6.UN01
Enzyme 1
Starting
molecule
A
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Enzyme 2
C
B
Reaction 1
Enzyme 3
Reaction 2
Reaction 3
D
Product
Catabolism
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Anabolism
Forms of Energy
 Energy
 Kinetic energy
 Thermal energy
 Heat
 Potential energy
 Chemical energy
Figure 6.2
A diver has more potential
energy on the platform.
Climbing up converts the kinetic
energy of muscle movement
to potential energy.
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Diving converts
potential energy to
kinetic energy.
A diver has less potential
energy in the water.
The Laws of Energy Transformation
 Thermodynamics=
Isolated System
Open System
According to the first law of thermodynamics, the energy of
the universe is constant
Energy can be transferred and transformed, but it cannot
be created or destroyed
Chemical
energy
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(a) First law of thermodynamics
According to the second law of thermodynamics:
Every energy transfer or transformation increases the
entropy of the universe
Heat
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(b) Second law of thermodynamics
Biological Order and Disorder
 Cells create ordered structures from less ordered
materials
 Organisms also replace ordered forms of matter and
energy with less ordered forms
 How does energy flow into an ecosystem?
 How does it flow out?
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Concept 6.2: The free-energy change of a reaction
tells us whether or not the reaction occurs
spontaneously
 A living system’s free energy (G) is energy that can
do work when temperature and pressure are
uniform
 What is ΔG?
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Figure 6.5
• 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
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(b) Diffusion
(c) Chemical
reaction
Exergonic and Endergonic Reactions in Metabolism
Exergonic Reactions
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Endergonic Reactions
Figure 6.6
(a) Exergonic reaction: energy released, spontaneous
Free energy
Reactants
Amount of
energy
released
(G  0)
Energy
Products
Progress of the reaction
(b) Endergonic reaction: energy required,
nonspontaneous
Free energy
Products
Energy
Reactants
Progress of the reaction
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Amount of
energy
required
(G  0)
Figure 6.7
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
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Concept 6.3: ATP powers cellular work by coupling
exergonic reactions to endergonic reactions
 A cell does three main kinds of work
 Chemical
 Transport
 Mechanical
 Cells utilize energy coupling
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The Structure and Hydrolysis of ATP
Adenine
Phosphate groups
Ribose
(a) The structure of ATP
Adenosine triphosphate (ATP)
Energy
Inorganic
phosphate
Figure 6.8
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Adenosine diphosphate (ADP)
(b) The hydrolysis of ATP
Figure 6.9: How ATP drives chemical work!
GGlu  3.4 kcal/mol
Glutamic acid Ammonia
Glutamine
(a) Glutamic acid conversion to glutamine
Phosphorylated
intermediate
Glutamic acid
Glutamine
(b) Conversion reaction coupled with ATP hydrolysis
GGlu  3.4 kcal/mol
GGlu  3.4 kcal/mol
 GATP  −7.3 kcal/mol
Net G  −3.9 kcal/mol
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GATP  −7.3 kcal/mol
(c) Free-energy change for coupled reaction
Figure 6.10
How ATP drives
transport and
mechanical work
Transport protein
Solute
Solute transported
(a) Transport work: ATP phosphorylates transport proteins.
Vesicle
Motor protein
Cytoskeletal track
Protein and
vesicle moved
(b) Mechanical work: ATP binds noncovalently to motor proteins
and then is hydrolyzed.
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The Regeneration of ATP
Energy from
catabolism
(exergonic, energyreleasing processes)
Figure 6.11
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Energy for cellular
work (endergonic,
energy-consuming
processes)
Concept 6.4: Enzymes speed up metabolic reactions by
lowering energy barriers
Sucrase
Sucrose
(C12H22O11)
Figure 6.UN02
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Glucose
(C6H12O6)
Fructose
(C6H12O6)
Figure 6.12: An energy profile of an exergonic reaction
A
B
C
D
Free energy
Transition state
A
B
C
D
EA
Reactants
A
B
G  0
C
D
Products
Progress of the reaction
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Figure 6.13: The effect of an enzyme on activation energy
Free energy
Course of
reaction
without
enzyme
EA
without
enzyme
EA with
enzyme
is lower
Reactants
G is unaffected
by enzyme
Course of
reaction
with enzyme
Products
Progress of the reaction
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Figure 6.14: Induced fit between an enzyme and its substrate
Substrate
Active site
Enzyme
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Enzyme-substrate
complex
Figure 6.15-4
2 Substrates are
held in active site by
weak interactions.
1 Substrates enter
active site.
Substrates
Enzyme-substrate
complex
5 Active
site is
available
for new
substrates.
Enzyme
4 Products are
released.
Products
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3 Substrates are
converted to
products.
Effects of Local Conditions on Enzyme Activity
 An enzyme’s activity can be affected by:
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Figure 6.16:
Factors that
affect enzyme
activity
Rate of reaction
Optimal temperature for
typical human enzyme
(37C)
0
Optimal temperature for
enzyme of thermophilic
(heat-tolerant)
bacteria (77C)
40
80
60
Temperature (C)
(a) Optimal temperature for two enzymes
20
Rate of reaction
Optimal pH for pepsin
(stomach
enzyme)
0
1
2
3
5
pH
(b) Optimal pH for two enzymes
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4
120
100
Optimal pH for trypsin
(intestinal
enzyme)
6
7
8
9
10
Cofactors and Enzyme Inhibitors
 What are Cofactors?
 What are enzyme inhibitors?
 Two types
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Figure 6.17 Inhibition of Enzyme Activity
(a) Normal binding
(b) Competitive inhibition
(c) Noncompetitive
inhibition
Substrate
Active site
Competitive
inhibitor
Enzyme
Noncompetitive
inhibitor
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Concept 6.5: Regulation of enzyme activity helps
control metabolism
 Chemical chaos would result if a cell’s metabolic
pathways were not tightly regulated
 How does a cell regulate its pathways?
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Figure 6.18a
(a) Allosteric activators and inhibitors
Allosteric enzyme
with four subunits
Regulatory
site (one
of four)
Active site
(one of four)
Activator
Active form
Stabilized
active form
Oscillation
Nonfunctional
active site
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Inactive
form
Inhibitor
Stabilized
inactive form
Figure 6.18b
(b) Cooperativity: another type of allosteric
activation
Substrate
Inactive form
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Stabilized
active form
Feedback Inhibition
 In feedback inhibition, the end product of a
metabolic pathway shuts down the pathway
 WHY is it important?
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Figure 6.19
Feedback Inhibition
Active site available
Isoleucine
used up by
cell
Threonine
in active site
Enzyme 1
(threonine
deaminase)
Intermediate A
Feedback
inhibition
Enzyme 2
Intermediate B
Enzyme 3
Isoleucine
binds to
allosteric
site.
Intermediate C
Enzyme 4
Intermediate D
Enzyme 5
End product
(isoleucine)
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Figure 6.20
Mitochondria
The matrix contains
enzymes in solution
that are involved in
one stage of cellular
respiration.
Enzymes for another
stage of cellular
respiration are
embedded in the
inner membrane.
1 m
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