Post on 08-Mar-2018
Chapter 5
Ground Rules of
Metabolism Sections 1-5
5.1 A Toast to Alcohol Dehydrogenase
• In the liver, the enzyme alcohol dehydrogenase breaks down
toxic ethanol to acetaldehyde, an organic molecule even more
toxic than ethanol
• Ethanol breakdown interferes with normal processes of
metabolism – as a result, fats tend to accumulate as large
globules in the tissues of heavy drinkers
• Ethanol breakdown damages liver cells and can lead to
alcoholic hepatitis and cirrhosis of the liver
Binge Drinking
alcohol
dehydrogenase
Alcoholic Liver Disease
5.2 Energy in the World of Life
• Sustaining life’s organization requires ongoing energy inputs
• Assembly of the molecules of life starts with energy input into
living cells
Energy
• We define energy as the capacity to do work
• One form of energy can be converted to another
• Familiar forms of energy include light, heat, electricity, and
motion (kinetic energy)
• The energy in chemical bonds is a type of potential energy,
because it can be stored
Energy Disperses
• First law of thermodynamics
• Energy is neither created nor destroyed, but can be
transferred from one form to another
• Second law of thermodynamics
• Entropy (a measure of dispersal of energy in a system)
increases spontaneously
• The entropy of two atoms decreases when a bond forms
between them (endergonic reaction)
En
tro
py
Time
heat
energy
Stepped Art
Figure 5-2 p78
Energy Conversion
• Only about 10% of the
energy in food goes
toward building body
mass, most is lost in
energy conversions
Energy’s One Way Flow
• The total amount of energy available in the universe to do
work is always decreasing
• Each time energy is transferred, some energy escapes as
heat (not useful for doing work)
• On Earth, energy flows from the sun, through producers, then
consumers
• Living things need a constant input of energy
Figure 5-4 p79
sunlight
energy
A Energy In. Sunlight energy reaches
environments on Earth. Producers in
those environments capture some of
the energy and convert it to other
forms that can drive cellular work.
Producers
B Some of the
energy captured by
producers ends up
in the tissues of
consumers.
Nutrient
Cycling
Consumers
C Energy Out. With each energy transfer,
some energy escapes into the environment,
mainly as heat. Living things do not use
heat to drive cellular work, so energy flows
through the world of life in one direction
overall.
ANIMATED FIGURE: One-way energy flow
and materials cycling
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Potential Energy
Take-Home Message:
What is energy?
• Energy is the capacity to do work; it can be transferred
between systems or converted from one form to another, but
it cannot be created or destroyed
• Energy disperses spontaneously
• Some energy is lost during every transfer or conversion
• Organisms can maintain their complex organization only as
long as they replenish themselves with energy they harvest
from someplace else
ANIMATION: Total energy remains constant
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5.3 Energy in the Molecules of Life
• All cells store and retrieve energy in chemical bonds of the
molecules of life
Chemical Bond Energy
• Reaction
• A chemical change that occurs when atoms, ions, or
molecules interact
• Reactant
• Atoms, ions, or molecules that enter a reaction
• Product
• Atoms, ions, or molecules remaining at the end of a
reaction
Equations Represent Chemical Reactions
ANIMATED FIGURE: Chemical
bookkeeping
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Reactions Require or Release Energy
• We can predict whether a reaction requires or releases
energy by comparing the bond energies of reactants with
those of products
• Endergonic (“energy in”) • Reactions that require a net input of energy
• Exergonic (“energy out”) • Reactions that end with a net release of energy
Endergonic and Exergonic Reactions
Why the Earth Doesn’t Go Up in
Flames
• Activation energy
• The minimum amount of energy needed to get a reaction
started
• Some reactions require a lot of activation energy, others
do not
Activation Energy
Reactants:
2H2 O2
Activation energy
Difference between free energy
of reactants and products
Products: 2H2O
Time
Fre
e e
nerg
y
ANIMATED FIGURE: Activation energy
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Energy In, Energy Out
• Cells store free energy by running endergonic reactions that
build organic compounds
• Example: photosynthesis
• Cells harvest free energy by running exergonic reactions that
break the bonds of organic compounds
• Example: aerobic respiration
Energy In
Energy Out
Take-Home Message:
How do cells use energy?
• Activation energy is the minimum amount of energy required
to start a chemical reaction
• Endergonic reactions cannot run without a net input of energy
• Exergonic reactions end with a net release of energy
• Cells store energy in chemical bonds by running endergonic
reactions that build organic compounds; they harvest energy
by breaking the bonds
ANIMATION: Energy changes in chemical
work
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5.4 How Enzymes Work
• Enzyme
• In a process called catalysis, an enzyme makes a specific
reaction occur much faster than it would on its own
• Enzymes are not consumed or changed by participating in
a reaction
• Most are proteins, some are RNA
• Substrate
• The specific reactant acted upon by an enzyme
The Transition State
• Enzymes lower the activation energy required to bring on the
transition state, when substrate bonds break and reactions
run spontaneously
• Active sites
• Locations on the enzyme molecule where substrates bind
and reactions proceed
• Complementary in shape, size, polarity and charge to the
substrate
Active Site of an Enzyme
Figure 5-10a p82
active site
enzyme
A Like other enzymes, hexokinase
has active sites that bind and alter
specific substrates. A model of the
whole enzyme is shown to the left.
Figure 5-10b p82
reactant(s)
B A close-up shows glucose and
phosphate meeting in the active
site. The microenvironment of the
site favors a reaction between the
two molecules.
Figure 5-10c p82
product(s)
c Here, the glucose has bonded with
the phosphate. The product of this
reaction, glucose-6-phosphate, is
shown leaving the active site.
Mechanisms of
Enzyme-Mediated Reactions
• Binding at enzyme active sites may bring on the transition
state by four mechanisms
• Helping substrates get together
• Orienting substrates in positions that favor reaction
• Inducing a fit between enzyme and substrate (induced-fit
model)
• Shutting out water molecules
Effects of Temperature, pH, and Salinity
• Raising the temperature boosts reaction rates by increasing a
substrate’s energy
• But very high temperatures denature enzymes
• Each enzyme has an optimum pH range
• In humans, most enzymes work at ph 6 to 8
• Salt levels affect the hydrogen bonds that hold enzymes in
their three-dimensional shape
Figure 5-12 p83
rate of
enzyme
activity
increases
activity rate
decreases as
enzyme
denatures
Figure 5-13 p83
glycogen
phosphorylase
trypsin
pepsin
Take-Home Message:
How do enzymes work?
• Enzymes greatly enhance the rate of specific reactions.
• Binding at an enzyme’s active site causes a substrate to
reach its transition state. In this state, the substrate’s bonds
are at the breaking point
• Each enzyme works best at certain temperatures, pH, and
salt concentration
ANIMATED FIGURE: Enzymes and their
role in lowering activation energy
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5.5 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 enzyme-mediated
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
intermediate
reactant
enzyme 1
enzyme 2
intermediate
enzyme 3
product
X
Stepped Art
Figure 5-14 p84
Figure 5-15 p84
active site substrate in
active site
A Inactive form. B Active form.
regulatory
molecules
ANIMATED FIGURE: Allosteric activation
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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)
Figure 5-16 p85
glucose
+
oxygen carbon
dioxide
+
water
Figure 5-16 p85
glucose
1
carbon
dioxide
+
water
oxygen
+
2
3
e–
e–
H+
ANIMATED FIGURE: Controlling energy
release
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Take-Home Message:
What are metabolic pathways?
• Metabolic pathways are sequences of enzyme-mediated
reactions; some are biosynthetic; others are degradative
• Control mechanisms enhance or inhibit the activity of many
enzymes; the adjustments help cells produce only what they
require in any given interval
• Many metabolic pathways involve electron transfers, or redox
reactions.
• Redox reactions occur in electron transfer chains; the chains
are important sites of energy exchange in photosynthesis and
aerobic respiration
ANIMATION: Types of reaction sequences
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