Topic 4.
Metabolism
September 28, 2005
Biology 1001
4.1 Introduction to Metabolism
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Metabolism is the totality of an organism’s chemical reactions
Reactions are arranged in metabolic pathways or series where
each step is catalyzed by an enzyme
Metabolic pathways are either catabolic or anabolic
 Catabolic pathways are degradative, and release energy
 Anabolic pathways consume energy to build larger
molecules from smaller ones
Chemistry & Thermodynamics
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Bioenergetics is the study of how organisms manage their
energy resources
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Energy is the capacity to cause change. Forms of energy include
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Thermodynamics is the study of energy transformation
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Kinetic – the energy of motion
Thermal – the random kinetic energy of atoms or molecules
Potential – the energy of location or structure
Chemical – the potential energy of molecules during a chemical reaction
The first law of TD - the energy of the universe is constant
The second law of TD – energy transformation increases the entropy or
randomness of the universe
Living organisms increase entropy despite biological order
Chemistry & Thermodynamics
PE & KE
4.2 - ATP Powers Cellular Work
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Work involves using energy to move matter against
opposing forces
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Organisms perform three kinds of work
 Mechanical,
transport, & chemical
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To do work organisms couple endergonic reactions
with exergonic ones, a process called energy coupling
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The molecule used to power cellular work by energy
coupling is adenosine triphosphate - ATP
ATP
A “high-energy” bond
Figure 8.9 ATP Hydrolysis
Figure 8.8 The Structure of ATP
An exergonic reaction
Energy Coupling
The exergonic
hydrolysis of ATP is
coupled to an
endergonic reaction
by way of a
phosphorylated
intermediate
The Regeneration of ATP
The ATP Cycle
 ADP gets phosphorylated back to ATP using energy released by
catabolic reactions in the cell and inorganic phosphate
 This shuttling of inorganic phosphate and energy from ATP to an
intermediate and back to ATP is the ATP cycle
Fig. 8.12
4.4 Enzymes and Metabolism
(We will do Topic 4.3 with Topic 9)
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A catalyst is a chemical agent that speeds up a reaction
without itself being consumed
An enzyme is a protein that functions as a biological catalyst
Both exergonic and endergonic reactions require enzymes to
occur at appropriate rates in the cell
This is because of the activation energy required to get a
reaction started
**Enzymes lower the activation energy barrier and allow
reactions to proceed more quickly**
How Enzymes
Work
Figures 8.14 & 8.15
How Enzymes Work - The Catalytic Cycle
Figure 8.17 - The active site
and the catalytic cycle of an
enzyme
Features of the Catalytic Cycle
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The reactant that an enzyme acts on is called a substrate
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The enzyme binds to the substrate forming an enzymesubstrate complex
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Only a restricted part of the enzyme usually binds to the
substrate – the active site
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Binding to the active site may involve induced fit
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Enzymes use a variety of mechanisms to lower activation
energy
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When the products are released the enzyme is free to enter
another cycle
More About Enzymes
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Enzymes are substrate- and reaction-specific
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Enzymes catalyze reactions in both directions, depending on
the relative concentration of reactants vs. products
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Enzymes do not change the overall energy of the reaction
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Enzymes may require cofactors, called coenyzmes if they are
organic molecules
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Enzymes are affected by environmental factors such as
temperature and pH
Temperature and pH affect enzyme activity
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Each enzyme has an optimal
temperature at which its
reaction rate is greatest, one
that allows the greatest number
of molecular collisions but
does not denature the enzyme
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35-40oC for humans, 70oC for
prokaryotes living in hot springs
Similarly, enzymes operate
best at their optimal pH,
usually between 6 and 8