Chapter 5B:
Energy & Enzymes
1. What is Energy?
2. Energy & Chemical Reactions
3. ATP
4. Enzymes & Metabolic Pathways
1. What is Energy?
2 Basic Forms of Energy
Kinetic Energy (KE)
• energy in motion or “released” energy:
heat (molecular motion)
*electric current (flow of charged particles)
*light energy (radiation of photons)
*mechanical energy (structural movement)
*chemical energy (breaking covalent bonds,
flow from high to low concentration)
*forms of KE cells use to “do things”
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Potential Energy (PE)
• stored energy (i.e., not yet released):
gravitational potential
*chemical bonds
*chemical, charge gradients
*sources of PE cells rely on
Illustration of Kinetic &
Potential Energy
KE highest at B, lowest at A & C
PE highest at A & C, lowest at B
Conservation of energy
1st Law of Thermodynamics:
“energy is neither created nor
destroyed, but may be converted
to other forms”
Energy Conversion
2nd Law of Thermodynamics:
“in any energy conversion,
some usable energy is
always lost”
NO energy conversion is 100% efficient
“Lost” energy = energy released as heat to the
surroundings, energy that can’t be used
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2. Energy & Chemical
Reactions
gain of PE
Chemical
Reactions
Endergonic
• net consumption
of energy
• increases PE
Exergonic
loss of PE
• net release of
energy
• loss of PE,
converted to KE
gain of PE Photosynthesis
is Endergonic
• energy input
from sunlight
• stored in glucose
Respiration is
Exergonic
• releases PE
stored in glucose
loss of PE
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Activation Energy (Ea)
Whether endergonic or exergonic, all chemical
reactions require some energy input for the
reaction to proceed: the activation energy (Ea)
• all reactions require some sort of “spark”
• this is why sources of chemical PE are “stable”
Mechanical Model of
Activation Energy
The upright bottle falling over is analogous to an
exergonic reaction, yet it still requires some
energy input for the bottle to tip over.
3. ATP
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Adenosine Triphosphate (ATP)
Chemical PE in
molecules of ATP is
the principal source
of directly useable
energy in cells.
• “food energy” is
transferred to ATP
by respiration
gain of PE
ATP synthesis
is Endergonic
• energy input is
needed to add
a 3rd phosphate
ATP “hydrolysis”
is Exergonic
loss of PE
• breaking the
covalent bond
with the 3rd PO4
releases energy
Coupling of Biochemical Reactions
Exergonic reactions fuel (provide energy for)
endergonic reactions in cells (i.e, they are “coupled”)
• breakdown of
glucose fuels
ATP production
exergonic
endergonic
exergonic
endergonic
• ATP hydrolysis
fuels most
cellular activities
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Examples of ATP-powered “Work”
ATP
Chemical work
Mechanical work
Transport work
membrane
protein
P +
motor
protein
solute
P
P
Reactants
P
P
P
Product
Molecule formed
Protein moved
Solute transported
ADP+ P
4. Enzymes &
Metabolic Pathways
Enzymes are Biological Catalysts
Enzymes are
proteins that
increase the
rates of
reactions by
lowering the
Ea
• the enzymes
themselves are
not changed
• allows reactions
to occur that
that otherwise
could not
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The catalytic cycle of an enzyme
1 Enzyme available
with empty active
site
Active site
• every enzyme has a
unique substrate &
thus catalyzes a
specific reaction
Substrate
(sucrose)
2 Substrate binds to
enzyme with induced fit
Enzyme
(sucrase)
Glucose
Fructose
H2O
4 Products are
released
3 Substrate is
converted to
products
• cells produce
1000s of different
enzymes, all of
which are proteins
encoded by a
particular gene
Metabolic Pathways
Most biological processes, whether anabolic
(building) or catabolic (breaking down), require
a series of chemical reactions (i.e., a pathway)
• each step in a metabolic pathway is catalyzed by a
specific enzyme
A Simple Metabolic Pathway
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Regulation of
Metabolic Pathways
Most metabolic pathways are regulated so that
“too much” is not produced:
• e.g., Feedback Inhibition: excess product inhibits
the activity of an enzyme early in the pathway
Inhibitors prevent substrate binding
Substrate
Active site
Enzyme
• provides a
means to
regulate
enzyme
activity
Normal binding of substrate
Competitive
inhibitor
Noncompetitive
inhibitor
• many
poisons
inhibit
enzyme
activity
Enzyme inhibition
Key Terms for Chapter 5B (5.10-16)
• kinetic, potential energy
• endergonic, exergonic, coupling of reactions
• activation energy
• ATP, enzyme, catalyst
• substrate, active site
• competitive, noncompetitive inhibition
Relevant Review Questions:
2, 4, 7, 12, 14, 17
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