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Chapter 8
An Introduction to Metabolism
Overview: The Energy of Life
• The living cell is a miniature chemical factory
where thousands of reactions occur
• The cell extracts energy and applies energy to
perform work
• Some organisms even convert energy to light, as
in bioluminescence
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Metabolism
• Metabolism is the entire collection of an
organism’s chemical reactions
• A metabolic pathway begins with a specific
molecule and ends with a product
• Each step is catalyzed by a specific enzyme
Enzyme 1
Enzyme 2
B
A
Reaction 1
Starting
molecule
Enzyme 3
C
Reaction 2
D
Reaction 3
Product
Two Types of Metabolism
• Catabolic pathways release energy by
breaking down complex molecules into
simpler compounds
GLUCOSE
CO2 + H2O + ATP
• Anabolic pathways consume energy to build
complex molecules from simpler ones
Amino acids
Proteins
Energy
• Energy is the capacity to cause change
• Types of energy
– Kinetic energy is energy associated with motion
– Potential energy is energy that matter possesses
because of its location or structure
• Energy can be converted from one form to
another
Fig. 8-2
A diver has more potential
energy on the platform
than in the water.
Climbing up converts the kinetic
energy of muscle movement
to potential energy.
Diving converts
potential energy to
kinetic energy.
A diver has less potential
energy in the water
than on the platform.
The Laws of Energy Transformation
• Thermodynamics is the study of energy transformations
• First law of thermodynamics
– Energy can be transferred and transformed, but it
cannot be created or destroyed
• Second law of thermodynamics
– During every energy transfer or transformation, some
energy is unusable, and is often lost as heat (entropy)
Free Energy
• Free Energy (∆G) - measures a system’s energy
that is available to perform work.
∆G = ∆H – T∆S
• Spontaneous
– a process that can occur without the input of energy;
∆G must be a negative value
• Nonspontaneous
– a process that requires the input of energy; ∆G will be
a positive value or zero
As Biologists, we want to know which rxns of life are spontaneous. These rxns can
be harnessed to do work (to drive the nonspontaneous rxns.
When we apply the free energy concept to processes
of life and with respect to metabolism, we describe
rxns as exergonic or endergonic.
• An exergonic reaction proceeds with a net
release of free energy and is spontaneous
Glucose + O2
CO2 + H2O
∆G = -686 kcal/mol
• An endergonic reaction absorbs free energy
from its surroundings and is nonspontaneous
CO2 + H2O
Glucose + O2
∆G = +686 kcal/mol
3 Main Types of Cellular Work
• Mechanical
• Transport
• Chemical
Fig. 8-8
Adenine
Phosphate groups
Ribose
Fig. 8-9
P
P
P
Adenosine triphosphate (ATP)
H2O
Pi
+
Inorganic phosphate
P
P
+
Adenosine diphosphate (ADP)
Energy
Fig. 8-11
Membrane protein
P
Solute
Pi
Solute transported
(a) Transport work: ATP phosphorylates
transport proteins
ADP
+
ATP
P
Vesicle
Cytoskeletal track
ATP
Motor protein
Protein moved
(b) Mechanical work: ATP binds noncovalently
to motor proteins, then is hydrolyzed
i
Fig. 8-12
ATP + H2O
Energy from
catabolism (exergonic,
energy-releasing
processes)
ADP + P i
Energy for cellular
work (endergonic,
energy-consuming
processes)
Fig. 8-14
A
B
C
D
Transition state
A
B
C
D
EA
Reactants
A
B
∆G < O
C
D
Products
Progress of the reaction
Enzyme Inhibitors