Ground Rules of Metabolism
Chapter 6 Part 2
6.4 Metabolism – Organized,
Enzyme-Mediated Reactions
 ATP, enzymes, and other molecules interact in
organized pathways of metabolism (activities by
which cells acquire and use energy)
Types of Metabolic Pathways
 A metabolic pathway is any series of enzymemediated reactions by which a cell builds,
rearranges, or breaks down an organic substance
• Anabolic pathways build molecules
• Catabolic pathways break apart molecules
• Cyclic pathways regenerate a molecule from the
first step
Controls Over Metabolism
 Concentrations of reactants or products can
make reactions proceed forward or backward
 Feedback mechanisms can adjust enzyme
production, or activate or inhibit enzymes
 Regulatory molecules can bind to an allosteric
site to activate or inhibit enzymes
• Feedback inhibition
Allosteric Control
Fig. 6-14a, p. 100
allosteric activator
allosteric binding site vacant
enzyme active site
substrate cannot bind
active site altered,
substrate can bind
a
Fig. 6-14a, p. 100
Fig. 6-14b, p. 100
allosteric inhibitor
allosteric
binding
site vacant;
active site
can bind
substrate
active site
altered, can’t
bind substrate
b
Fig. 6-14b, p. 100
allosteric activator
allosteric inhibitor
allosteric binding site vacant
enzyme active site
substrate cannot bind
active site altered,
substrate can bind
allosteric
binding
site vacant;
active site
can bind
substrate
active site
altered, can’t
bind substrate
Stepped Art
Fig. 6-14, p. 100
Animation: Allosteric activation
Animation: Allosteric inhibition
Feedback Inhibition
enzyme 2
enzyme 1
substrate
enzyme 3
enzyme 4
Excess molecules of end
product bind to molecules of
an enzyme that catalyzes this
pathway’s first step. The
greater the excess, the more
enzyme molecules are
inhibited, and the less
product is synthesized.
enzyme 5
end
product
Fig. 6-15, p. 100
Animation: Feedback inhibition
Redox Reactions
 Oxidation-reduction reactions (paired reactions)
• A molecule that gives up electrons is oxidized
• A molecule that accepts electrons is reduced
• Coenzymes can accept molecules in redox
reactions (also called electron transfers)
ATP Synthesis
 Coenzymes deliver electrons to electron transfer
chains for ATP synthesis
 Electron transfer chain
• A series of redox reactions in membrane-bound
enzymes or molecules that release energy in
small, controlled steps
Uncontrolled and Controlled Reactions
Fig. 6-16a, p. 101
glucose
CO2
oxygen
water
spark
A Glucose and oxygen react when
exposed to a spark. Energy is released
all at once as CO2 and water form.
Fig. 6-16a, p. 101
Fig. 6-16b, p. 101
CO2
oxygen
1 Energy input splits glucose
into carbon dioxide, electrons,
and hydrogen ions (H+).
e–
glucose
2 Electrons lose energy as
they move through an
electron transfer chain.
3 Energy released by
electrons is harnessed for
cellular work.
H+
e–
water
4 Electrons, protons, and
oxygen combine to form
water.
B The same overall reaction occurs in small steps with
an electron transfer chain. Energy is released in amounts
that cells can harness for cellular work, such as muscle
contraction or active transport.
Fig. 6-16b, p. 101
CO2
oxygen
1 Energy input splits glucose
into carbon dioxide, electrons,
and hydrogen ions (H+).
e–
glucose
2 Electrons lose energy as
they move through an
electron transfer chain.
3 Energy released by
electrons is harnessed for
cellular work.
H+
e–
water
4 Electrons, protons, and
oxygen combine to form
water.
Stepped Art
Fig. 6-16b, p. 101
Animation: Controlling energy release
Overview: Energy Pathways
nutrients
(carbohydrates,
fats, proteins)
Energy-releasing reactions
ADP
+ Pi
oxidized
coenzymes
ATP
molecules
of life
breakdown
products
(e. g., carbon
dioxide; water)
reduced
coenzymes
Energy-requiring reactions
small organic
molecules
Fig. 6-17, p. 101
6.4 Key Concepts:
The Nature of Metabolism
 Metabolic pathways are energy-driven
sequences of enzyme-mediated reactions
 They concentrate, convert, or dispose of
materials in cells
 Controls over enzymes that govern key steps in
metabolic pathways can shift cell activities fast
6.5 Night Lights
 Bioluminescence is visible evidence of
metabolism
 Bioluminescence
• Light emitted from metabolic reactions in living
organisms
Enzymes of Bioluminescence
 Bioluminescent organisms emit light when
enzymes (luciferases) convert chemical bond
energy in luciferins to light energy
luciferin + ATP → luciferin-ADP + Pi
luciferin-ADP + O2 →
oxyluciferin + AMP + CO2 + light
Bioluminescence: Fireflies
Fig. 6-18a, p. 102
Fig. 6-18b, p. 102
A Research Connection
 Researchers can transfer genes for
bioluminescence from one species to another
• Example: bacteria with jellyfish genes
6.5 Key Concepts:
Metabolism Everywhere
 Knowledge about metabolism, including how
enzymes work, can help you interpret some
natural phenomena
Key Players in Metabolic Reactions
Animation: Chemical bookkeeping
Animation: Chemical equilibrium
Animation: Energy changes in chemical
work
Animation: Enzymes and activation
energy
Animation: Enzymes and temperature
Animation: How catalase works
Animation: One-way energy flow and
materials cycling
Video: Alcohol, enzymes, and your liver