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Chapter 8
METABOLISM
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The living cell
– Is a miniature factory
– Where thousands of reactions occur
– Converts energy in many ways
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• Metabolism
– The sum of all chemical reactions in an
organism
– Arises from interactions between molecules
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Organization of the Chemistry of Life into
Metabolic Pathways
• A metabolic pathway has many steps:
– Begin with a specific molecule, and
– End with a product
– Each is catalyzed by a specific enzyme
Enzyme 1
A
Enzyme 2
D
C
B
Reaction 1
Enzyme 3
Reaction 2
Starting
molecule
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Reaction 3
Product
• Catabolic pathways
– Break down complex molecules into simpler
compounds
– Release energy
• Anabolic pathways
– Build complicated molecules from simpler ones
– Consume energy
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Forms of Energy
• Energy
– Is the capacity to cause change
– Exists in various forms
– Some forms can perform work
• Kinetic energy
– Associated with motion
• Potential energy
– Stored in the location of matter
– Includes chemical energy:
• tored in molecular structure
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Energy can be converted
– From one form to another
On the platform, a diver
has more potential energy.
Climbing up converts kinetic
energy of muscle movement
Figure 8.2
to potential energy.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Diving converts potential
energy to kinetic energy.
In the water, a diver has
less potential energy.
• An example of energy conversion
Chemical
energy
Figure 8.3
(a) First law of thermodynamics: Energy
can be transferred or transformed but
Neither created nor destroyed. For
example, the chemical (potential) energy
in food will be converted to the kinetic
energy of the cheetah’s movement in (b).
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Energy of Food
• The energy value of food:
– is measured in kilocalories (Kcal)
– Kilocalories are also called “large calories”(C)
• One kilocalorie is:
– The amount of heat energy needed to raise
the temperature of one kilogram of water 1 oC
(1.8 oF)
– Equal to 1000 calorie (c)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Laws of Energy Transformation
• Thermodynamics
– Is the study of energy transformations
• First law of thermodynamics:
– Energy can be transferred and transformed
– Energy cannot be created or destroyed
• Second law of thermodynamics:
– Every energy transfer or transformation
increases the entropy “measure of randomness or
disorder” of the universe
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Second Law of Thermodynamics
Heat
co2
+
H2O
b)
Figure 8.3
Second law of thermodynamics: Every energy transfer or transformation increases
the disorder (entropy) of the universe. For example, disorder is added to the cheetah’s
surroundings in the form of heat and the small molecules that are the by-products
of metabolism.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Free-Energy Change, G
• The free-energy change of a reaction tells us
whether the reaction occurs spontaneously
• A living system’s free energy is:
– The portion of energy that can perform work
under cellular conditions
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Free Energy and Metabolism
 Base on their free energy changes, chemical
reaction are classified into:
 Exergenic reactions:
 Proceed with release of energy
 Are spontaneous
 Endergenic reaction:
 Absorb energy
 nonspontaneous
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Exergonic and Endergonic Reactions in Metabolism
• An exergonic reaction
Reactants
Free energy
Amount of
energy
released
(∆G <0)
Energy
Products
Progress of the reaction
Figure 8.6
(a) Exergonic reaction: energy released
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• An endergonic reaction
Free energy
Products
Energy
Reactants
Progress of the reaction
Figure 8.6
(b) Endergonic reaction: energy required
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Amount of
energy
released
(∆G>0)
Equilibrium and Metabolism
• Reactions in a closed system
– Eventually reach equilibrium
∆G < 0
Figure 8.7 A
∆G = 0
(a) A closed hydroelectric system. Water flowing downhill turns a turbine
that drives a generator providing electricity to a light bulb, but only until
the system reaches equilibrium.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Cells in our body (open system)
– Are open system
– Experience a constant flow of materials in and
out
– This prevents metabolic pathways from
reaching equilibrium
(b) An open hydroelectric
system. Flowing water
keeps driving the generator
because intake and outflow
of water keep the system
from reaching equlibrium.
Figure 8.7
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∆G < 0
• An analogy for cellular respiration
∆G < 0
∆G < 0
∆G < 0
Figure 8.7
(c) A multistep open hydroelectric system. Cellular respiration is
analogous to this system: Glucoce is brocken down in a series
of exergonic reactions that power the work of the cell. The product
of each reaction becomes the reactant for the next, so no reaction
reaches equilibrium.
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• ATP powers cellular work by coupling:
– Exergonic reactions to endergonic reactions
• A cell does three main kinds of work
– Mechanical
– Transport
– Chemical
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• Energy coupling
– Is a key feature in the way cells manage their
energy resources to do this work
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Structure and Hydrolysis of ATP
• ATP (adenosine triphosphate)
– Is the cell’s energy shuttle
– Provides energy for cellular functions
Adenine
N
O
O
-O
O
-
O
-
O
O
C
C
N
HC
O
O
O
NH2
-
Phosphate groups
Figure 8.8
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N
CH2
O
H
N
H
H
H
OH
CH
C
OH
Ribose
• Energy is released from ATP
– When the terminal phosphate bond is broken
P
P
P
Adenosine triphosphate (ATP)
H2O
P
i
+
Figure 8.9 Inorganic phosphate
P
P
Adenosine diphosphate (ADP)
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Energy
How ATP Performs Work
• ATP drives endergonic reactions
– By phosphorylation (transferring a phosphate)
to other molecules
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings