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Biology
A Guide to the Natural World
Chapter 6 • Lecture Outline
Life’s Mainspring: An Introduction to Energy
Fifth Edition
David Krogh
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6.1 Energy Is Central to Life
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Energy Is Central to Life
• All living things require energy.
• The sun is the ultimate source of energy for
most living things.
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Energy Is Central to Life
• The sun’s energy is captured on Earth by
photosynthesizing organisms (such as
plants), which then pass this energy on to
other organisms in the form of food.
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6.2 The Nature of Energy
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The Nature of Energy
• Energy can be defined as the capacity to
bring about movement against an opposing
force.
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Forms of Energy
• Energy can be conceptualized as either
potential energy, meaning stored energy; or
kinetic energy, meaning energy in motion.
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Thermodynamics
• The study of energy is known as
thermodynamics.
• The laws of thermodynamics are
fundamental principles of energy.
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Thermodynamics
• The first law of thermodynamics states that
energy is never created or destroyed but is
only transformed.
• The second law of thermodynamics states
that energy transfer will always result in a
greater amount of disorder in the universe.
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Tranformations of Energy
• In line with the second law of
thermodynamics, in every energy
transaction, some energy will be lost to the
most disordered form of energy, heat.
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Tranformations of Energy
• Thus, in the operation of a car engine, only
part of the energy released by the
combustion of gasoline actually helps
propel the car; the rest of the energy
released in the combustion is lost to heat.
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Tranformations of Energy
• Entropy is a measure of the amount of
disorder in a system; the greater the
entropy, the greater the disorder.
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Transformations of Energy
3. Mechanical energy
2. Heat energy
piston-driven
flywheel
heat
coal
1. Chemical
bond energy
heat
steam
motion
Total energy is constant.
Entropy increases.
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Figure 6.2
6.3 How Is Energy Used by
Living Things?
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How Is Energy Used by Living Things?
• Living things can bring about local
increases in order (in themselves) through
their metabolic processes.
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Macromolecular Synthesis
• They can, for example, build up moreordered molecules (starches, proteins) from
less ordered molecules (simple sugars,
amino acids).
• However, it takes energy to do this.
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Endergonic and Exergonic Reactions
• Energy in living things is stored away in
endergonic (uphill) reactions in which the
products of the reaction contain more
energy than the starting substances (or
reactants).
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Endergonic and Exergonic Reactions
• Conversely, energy is released in exergonic
(downhill) reactions, in which the reactants
contain more energy than the products.
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Endergonic and Exergonic Reactions
• The linkage of simple sugars to form a
complex carbohydrate is an endergonic
reaction.
• Such a reaction will not occur without an
input of energy.
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Endergonic and Exergonic Reactions
• Conversely, the breakdown of a complex
carbohydrate into simple sugars is an
exergonic reaction.
• Such a reaction releases energy.
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Endergonic and Exergonic Reactions
• Endergonic and exergonic reactions are
linked in coupled reactions—reactions in
which an energy yielding exergonic reaction
powers an energy-requiring endergonic
reaction.
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Energy Stored and Released
Product (glycogen) contains
more energy than the
reactants (glucose)
glycogen
molecule
endergonic
reaction
energy
in
energy
out
exergonic
reaction
glucose
molecules
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Product (glucose) contains
less energy than the
reactants (glycogen)
Figure 6.4
ATP
• The molecule most often used in living
things to power coupled reactions is ATP.
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6.4 The Energy Dispenser: ATP
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The Energy Dispenser: ATP
• Adenosine triphosphate (ATP) is the most
important energy transfer molecule in living
things.
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ATP
• In animals, energy that is extracted from
food is transferred to ATP, and this energy
is then used to drive a vast array of
metabolic processes.
• Energy supplied by ATP is used, for
example, to power muscle contraction and
nerve signal transmission.
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ATP
• ATP’s energy transfer powers stem from
the fact that it contains three phosphate
groups, each of which is negatively
charged, meaning these groups repel each
other.
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The ATP/ADP Cycle
• ATP drives chemical reactions by donating
its third phosphate group to them.
• In the process, it becomes the twophosphate molecule adenosine diphosphate
(ADP).
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The ATP/ADP Cycle
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Figure 6.6
The ATP/ADP Cycle
• To again become ATP, it must have a third
phosphate group attached to it.
• This shuttling back and forth between ATP
and ADP takes place constantly in living
things.
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Energy and Biology
Animation 6.1: Energy and Biology
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6.5 Efficient Energy Use in Living
Things: Enzymes
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Efficient Energy Use in Living
Things: Enzymes
• An enzyme is a type of protein that
accelerates the rate at which a chemical
reaction takes place in an organism.
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Enzymes
• Nearly every chemical process that takes
place in living things is facilitated by an
enzyme.
• For example, the enzyme lactase facilitates
the splitting of the sugar lactose into its
component sugars, glucose and galactose.
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Enzymes
• The substance that an enzyme helps
transform through chemical reaction is
called its substrate.
• Lactose is the substrate of the enzyme
lactase.
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Enzymes
• Many activities in living things are
controlled by metabolic pathways, in which
a series of reactions is undertaken in
sequence, each facilitated by its own
enzyme.
• In such a series, the product of one reaction
becomes the substrate for the next.
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Enzyme Action
enzyme
A
enzyme
B
substrates
enzyme
C
product
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Figure 6.8
Metabolism and Enzymes
• The sum of all the chemical reactions that a
cell or larger living thing carries out is its
metabolism.
• Enzymes are active in all facets of the
metabolism of all living things.
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6.6 Enzymes and the Activation
Barrier
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Enzymes and the Activation Barrier
• Enzymes work by lowering activation
energy, which is the energy required to
initiate a chemical reaction.
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Enzymes Accelerate Chemical Reactions
(a) Without enzyme
lactose
glucose + galactose
activation energy
without enzyme
net energy released
from splitting of
lactose
(b) With enzyme
lactase
lactose
glucose + galactose
activation energy
with enzyme
net energy released
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Figure 6.9
Enzymes Accelerate Chemical Reactions
• Enzymes are catalysts.
• They bring about a change in their
substrates without being chemically altered
themselves.
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Enzymes Accelerate Chemical Reactions
• Enzymes generally take the form of
globular or ball-like proteins whose shape
includes a pocket into which the enzyme’s
substrate fits.
• This pocket is the active site—that portion
of an enzyme that binds with a substrate,
thus helping transform it.
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Substrate Binding
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Figure 6.10
Substrate Binding
• Only a few of the hundreds of amino acids
that typically make up an enzyme will be
involved in substrate binding.
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Substrate Binding
• In some cases, substrate binding is
facilitated by coenzymes: molecules other
than amino acids that facilitate the work of
enzymes by binding with them.
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6.7 Regulating Enzymatic Activity
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Regulating Enzymatic Activity
• Enzyme activity can be controlled in several
ways.
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Regulating Enzymatic Activity
• Competitive inhibition is a reduction in the
activity of an enzyme by means of a
compound other than the enzyme’s usual
substrate binding with the enzyme in its
active site.
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Regulating Enzymatic Activity
• Another means of control is allosteric
regulation, in which a molecule binds with
the enzyme at a site other than its active
site.
• Such binding changes the enzyme’s shape,
thereby decreasing or increasing the
enzyme’s ability to bind with its substrate.
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Regulating Enzymatic Activity
• Because of such processes as allosteric
regulation, enzymes are not fated to turn out
product in strict accordance with the
amount of substrate in their environment;
rather, enzyme activity can be finely tuned
in accordance with cellular needs.
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Allosteric Regulation
(a) Substrate becomes product.
1
2
substrate
enzyme
1. Substrate binds
to enzyme.
product 2. Enzyme transforms
substrate to product.
(b) Product feeds back on enzyme.
4
Shape
change
3
3. Product binds to
a different site on the
enzyme, causing the
enzyme to change
shape.
4. The new shape of
the enzyme prevents
it from binding to any
more substrate.
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Figure 6.12
Enzymes Overview
Animation 6.2: Enzymes
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