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Chapter 6
Energy Flow in the Life of
a Cell
Lecture Outlines by Gregory Ahearn,
University of North Florida
Copyright © 2011 Pearson Education Inc.
Chapter 6 At a Glance
 6.1 What Is Energy?
 6.2 How Does Energy Flow in Chemical
Reactions?
 6.3 How Is Energy Transported Within Cells?
 6.4 How Do Enzymes Promote Biochemical
Reactions?
 6.5 How Do Cells Regulate Their Metabolic
Reactions?
Biology: Life on Earth, 9e
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6.1 What Is Energy?
 Energy is the capacity to do work
 Work is a force acting on an object that causes
the object to move
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6.1 What Is Energy?
 Chemical energy is the energy that is
contained in molecules and released by
chemical reactions
– Molecules that provide chemical energy include
sugar, glycogen, and fat
– Cells use specialized molecules such as ATP to
accept and transfer energy from one chemical
reaction to the next
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6.1 What Is Energy?
 There are two fundamental types of energy
– Potential energy is stored energy
–For example, the chemical energy in bonds,
the electrical charge in a battery, and a
penguin poised to plunge
– Kinetic energy is the energy of movement
–For example, light, heat, electricity, and the
movement of objects
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Author Animation: Types of Energy
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From Potential to Kinetic Energy
Fig. 6-1
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6.1 What Is Energy?
 The laws of thermodynamics describe the basic
properties of energy
– The laws describe the quantity (the total amount) and the
quality (the usefulness) of energy
– Energy can neither be created nor destroyed (the first
law of thermodynamics), but can change form
– The first law is often called the law of conservation
of energy
– The total amount of energy within a closed system
remains constant unless energy is added or removed
from the system
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6.1 What Is Energy?
 The laws of thermodynamics describe the basic
properties of energy (continued)
– The amount of useful energy decreases when
energy is converted from one form to another
(the second law of thermodynamics)
– Entropy (disorder) is the tendency to move
toward a loss of complexity and of useful energy
and toward an increase in randomness, disorder,
and less-useful energy
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6.1 What Is Energy?
 The laws of thermodynamics describe the basic
properties of energy (continued)
– Useful energy tends to be stored in highly
organized matter, and when energy is used in a
closed system (such as the world in which we
live), there is an overall increase in entropy
– For example, when gasoline is burned, the
orderly arrangement of eight carbons bound
together in a gasoline molecule are converted to
eight randomly moving molecules of carbon
dioxide
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Energy Conversions Result in a Loss of Useful
Energy
Combustion
by engine
100 units chemical energy
(concentrated)
75 units heat  25 units kinetic energy
energy
(motion)
Fig. 6-2
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6.1 What Is Energy?
 Living things use the energy of sunlight to
create the low-entropy conditions of life
– The highly organized low-entropy systems of life
do not violate the second law of thermodynamics
because they are achieved through a continuous
influx of usable light energy from the sun
– In creating kinetic energy in the form of sunlight,
the sun also produces vast entropy as heat
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6.2 How Does Energy Flow in Chemical Reactions?
 A chemical reaction is a process that forms or
breaks the chemical bonds that hold atoms
together
– Chemical reactions convert one set of chemical
substances, the reactants, into another set, the
products
– All chemical reactions require a net input of
energy
– Exergonic reactions release energy
– Endergonic reactions require an input of energy
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Author Animation: Exergonic and Endergonic
Reactions
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An Exergonic Reaction
energy
+
reactants
+
products
Fig. 6-3
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An Endergonic Reaction
energy
+
+
products
reactants
Fig. 6-4
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6.2 How Does Energy Flow in Chemical Reactions?
 Exergonic reactions release energy
– Reactants contain more energy than products in
exergonic reactions
– An example of an exergonic reaction is the burning of
glucose
– As glucose is burned, the sugar (C6H12O6) combines
with oxygen (O2) to produce carbon dioxide (CO2) and
water (H2O), releasing energy
– Because molecules of sugar contain more energy
than do molecules of carbon dioxide and water, the
reaction releases energy
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Reactants and End Products of Burning Glucose
energy
C6H12O6  6 O2
(glucose) (oxygen)
6 CO2  6 H2O
(carbon
(water)
dioxide)
Fig. 6-5
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Activation Energy in Exergonic Reactions
Activation energy needed
to ignite glucose
high
energy level of reactants
energy
content
of
molecules
glucose + O2
CO2 + H2O
low
progress of reaction
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Fig. 6-6
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6.2 How Does Energy Flow in Chemical Reactions?
 Exergonic reactions release energy (continued)
– All chemical reactions require an initial energy
input (activation energy) to get started
–The negatively charged electron shells of
atoms repel one another and inhibit bond
formation
–Molecules need to be moving with sufficient
collision speed to overcome electronic
repulsion and react
–Increasing the temperature will increase
kinetic energy and, thus, the rate of reaction
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6.2 How Does Energy Flow in Chemical Reactions?
 Endergonic reactions require a net input of
energy
– The reactants in endergonic reactions contain
less energy than the products
– An example of an endergonic reaction is
photosynthesis
–In photosynthesis, green plants add the
energy of sunlight to the lower-energy
reactants water and carbon dioxide to produce
the higher-energy product sugar
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Photosynthesis
energy
C6H12O6  6 O2
(glucose) (oxygen)
6 CO2  6 H2O
(carbon
(water)
dioxide)
Fig. 6-7
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6.3 How Is Energy Transported Within Cells?
 Most organisms are powered by the breakdown
of glucose
 Energy in glucose cannot be used directly to
fuel endergonic reactions
 Energy released by glucose breakdown is first
transferred to an energy-carrier molecule
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6.3 How Is Energy Transported Within Cells?
 Energy-carrier molecules are high-energy,
unstable molecules that are synthesized at the
site of an exergonic reaction, capturing some of
the released energy
– These high-energy molecules then transfer the
energy to an endergonic reaction elsewhere in
the cell
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6.3 How Is Energy Transported Within Cells?
 ATP is the principal energy carrier in cells
– Adenosine triphosphate (ATP) is the most
common energy-carrying molecule
– ATP is composed of the nitrogen-containing base
adenine, the sugar ribose, and three phosphates
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The Interconversion of ADP and ATP
energy
P
P
P

P
P
P
ATP
phosphate
ADP
(a) ATP synthesis: Energy is stored in ATP
energy
P
P
P
ATP
P
ADP
(b) ATP breakdown: Energy is released
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P

P
phosphate
Fig. 6-8
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6.3 How Is Energy Transported Within Cells?
 ATP is the principal energy carrier in cells
(continued)
– Energy is released in cells during glucose
breakdown and is used to combine the relatively
low-energy molecules adenosine diphosphate
(ADP) and phosphate (P) into ATP
– Energy is stored in the high-energy phosphate
bonds of ATP
– The formation of ATP is an endergonic reaction
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6.3 How Is Energy Transported Within Cells?
 ATP is the principal energy carrier in cells
(continued)
– At sites in the cell where energy is needed, ATP
is broken down into ADP + P and its stored
energy is released
– This energy is then transferred to endergonic
reactions through coupling
– Unlike glycogen and fat, ATP stores energy very
briefly before being broken down
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6.3 How Is Energy Transported Within Cells?
 Electron carriers also transport energy within cells
– ATP is not the only energy-carrier molecule in cells
– Energy can be transferred to electrons in glucose
metabolism and photosynthesis
– Electron carriers like nicotinamide adenine dinucleotide
(NAD+) and flavin adenine dinucleotide (FAD) transport
high-energy electrons
– Electron carriers donate their high-energy electrons to
other molecules, often leading to ATP synthesis
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6.3 How Is Energy Transported Within Cells?
 Coupled reactions link exergonic with endergonic
reactions
– In a coupled reaction, an exergonic reaction provides
the energy needed to drive an endergonic reaction
– Sunlight energy stored in glucose by plants is transferred
to other organisms by the exergonic breakdown of the
sugar and its use in endergonic processes such as
protein synthesis
– The two reactions may occur in different parts of the cell,
so energy-carrier molecules carry the energy from one to
the other
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Author Animation: Coupled Reactions
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Coupled Reactions Within Living Cells
high-energy
reactants
(glucose)
ATP
exergonic
(glucose breakdown)
low-energy
products
(CO2, H2O)
high-energy
products
(protein)
endergonic
(protein synthesis)
ADP  P
low-energy
reactants
(amino acids)
Fig. 6-9
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6.4 How Do Enzymes Promote Biochemical
Reactions?
 Enzyme structures allow biochemical reactions
to catalyze specific reactions
 Enzymes, like all catalysts, lower activation
energy
 Enzymes control the rate of energy release and
capture some energy in ATP
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6.4 How Do Enzymes Promote Biochemical
Reactions?
 At body temperatures, spontaneous reactions
proceed too slowly to sustain life
– Reaction speed is generally determined by the
activation energy required; that is, how much
energy is required to start the process
–Reactions with low activation energies
proceed rapidly at body temperature
–Reactions with high activation energies (e.g.,
sugar breakdown) move very slowly at body
temperature, even if exergonic overall
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6.4 How Do Enzymes Promote Biochemical
Reactions?
 At body temperatures, spontaneous reactions
proceed too slowly to sustain life (continued)
– Enzymes are employed to catalyze (speed up)
chemical reactions in cells by lowering the
activation energy needed to start the reaction
– Enzymes are biological catalysts and regulate all
the reactions in living cells
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6.4 How Do Enzymes Promote Biochemical
Reactions?
 Catalysts reduce activation energy
– Catalysts speed up the rate of a chemical
reaction without themselves being used up
– All catalysts have three important properties:
1. They speed up reactions by lowering the
activation energy required for the reaction to
begin
2. They speed up only exergonic reactions
3. They are not consumed or changed by the
reactions they promote
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6.4 How Do Enzymes Promote Biochemical
Reactions?
 Catalysts reduce activation energy (continued)
– Catalytic converters in cars facilitate the
conversion of carbon monoxide (CO) to carbon
dioxide (CO2)
– 2 CO + O2  2 CO2 + heat energy
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Author Animation: Activation Energy
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Catalysts Such As Enzymes Lower Activation
Energy
high
Activation energy
without catalyst
energy
content
of
molecules
Activation energy
with catalyst
reactants
products
low
progress of reaction
Biology: Life on Earth, 9e
Fig. 6-10
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6.4 How Do Enzymes Promote Biochemical
Reactions?
 Enzymes are biological catalysts
– Enzymes are composed primarily of protein
synthesized by living organisms and may require
small nonprotein helper molecules called
coenzymes in order to function
– Many water-soluble vitamins (certain B vitamins)
are essential to humans because they are used
by the body to synthesize coenzymes
– Enzymes orient, distort, and reconfigure
molecules in the process of lowering activation
energy
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6.4 How Do Enzymes Promote Biochemical
Reactions?
 Enzymes are biological catalysts (continued)
– Enzymes (proteins) have two attributes that set
them apart from nonbiological catalysts:
1. Enzymes are very specific for the reactions
they catalyze
2. Enzyme activity is regulated
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6.4 How Do Enzymes Promote Biochemical
Reactions?
 Enzyme structures allow them to catalyze specific
reactions
– Each enzyme has a pocket called an active site into
which one or more reactant molecules, called substrates,
can enter
– The amino acid sequence of the enzyme protein and
the way the protein chains are folded create in the
active site a distinctive shape and distribution of
electrical charge
– The distinctive shape of the active site is both
complementary and specific to the substrate
– Active site amino acids bind to the substrate and
distort bonds to facilitate a reaction
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6.4 How Do Enzymes Promote Biochemical
Reactions?
 Enzyme structures allow them to catalyze
specific reactions (continued)
– There are three steps of enzyme catalysis
1. Both the shape and the charge of the active
site allow substrates to enter the enzyme only
in specific orientations
2. Upon binding, the substrates and active site
change shape to promote a reaction
3. When the reaction between the substrates is
finished, the product(s) no longer properly fit
into the active site and drift away
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Author Animation: Enzymes and Substrates
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The Cycle of Enzyme-Substrate Interactions
substrates
active site
of enzyme
enzyme
1 Substrates enter
the active site in a
specific orientation
3 The substrates, bonded
together, leave the enzyme;
the enzyme is ready for a
new set of substrates
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2 The substrates and
active site change shape,
promoting a reaction
between the substrates
Fig. 6-11
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6.4 How Do Enzymes Promote Biochemical
Reactions?
 Enzymes, like all catalysts, lower activation
energy
– The breakdown or synthesis of a molecule within
a cell usually occurs in many small steps, each
catalyzed by a different enzyme
– Each of the enzymes lowers the activation
energy for its particular reaction, allowing the
reaction to occur readily at body temperature
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6.4 How Do Enzymes Promote Biochemical
Reactions?
 Enzymes control the rate of energy release and
capture some energy in ATP
– Thanks to a series of chemical transformations,
each catalyzed by a different enzyme, the energy
stored in sugar is released gradually during its
breakdown
– Some energy is lost as heat, while some is
harnessed to power endergonic reactions that
lead to ATP synthesis
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6.5 How Do Cells Regulate Their Metabolic
Reactions?
 The sum of all the chemical reactions inside a
cell is its metabolism
 Many cellular reactions are linked through
metabolic pathways
– In metabolic pathways, an initial reactant
molecule is modified by an enzyme, creating a
slightly different intermediate molecule, which is
modified by another enzyme, and so on, until a
final product is produced
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Simplified Metabolic Pathways
Initial reactant
PATHWAY 1
A
B
enzyme 1
Final products
Intermediates
D
C
enzyme 2
enzyme 3
E
enzyme 4
G
F
PATHWAY 2
enzyme 5
enzyme 6
Fig. 6-12
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6.5 How Do Cells Regulate Their Metabolic
Reactions?
 Reaction rates tend to increase as substrate or
enzyme levels increase
– For a given amount of enzyme, as substrate
levels increase, the reaction rate will increase
until the active sites of all the enzyme molecules
are being continuously occupied by new
substrate molecules
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6.5 How Do Cells Regulate Their Metabolic
Reactions?
 Reaction rates tend to increase as substrate or
enzyme levels increase (continued)
– Metabolic pathways are controlled in several
ways
–Control of enzyme synthesis, which regulates
availability
–Control of enzyme activity
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6.5 How Do Cells Regulate Their Metabolic
Reactions?
 Cells regulate enzyme synthesis
– Genes that code for specific proteins are turned
on and off according to metabolic need
– An increase in substrate can trigger increased
enzyme production, leading to decreased
substrate levels
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6.5 How Do Cells Regulate Their Metabolic
Reactions?
 Cells regulate enzyme activity
– Some enzymes are synthesized in inactive form
–For example, the protein-digesting enzymes
pepsin and trypsin are inactive when
synthesized, but become activated in the
stomach under acidic conditions (pepsin) or in
the small intestine under alkaline conditions
(trypsin)
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6.5 How Do Cells Regulate Their Metabolic
Reactions?
 Cells regulate enzyme activity (continued)
– Some enzymes are inhibited
–In competitive inhibition, a substance that is
not the enzyme’s normal substrate binds to
the active site of the enzyme, competing with
the substrate for the active site
–In noncompetitive inhibition, a molecule
binds to a site on the enzyme distinct from the
active site
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Competitive and Noncompetitive Enzyme Inhibition
substrate
active site
enzyme
noncompetitive
inhibitor site
Fig. 6-13a
(a) A substrate binding to an enzyme
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Competitive and Noncompetitive Enzyme Inhibition
A competitive inhibitor
molecule occupies the
active site and blocks
entry of the substrate
(b) Competitive inhibition
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Fig. 6-13b
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Competitive and Noncompetitive Enzyme Inhibition
A noncompetitive
inhibitor molecule
causes the active site
to change shape, so the
substrate no longer fits
(c) Noncompetitive inhibition
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noncompetitive
inhibitor molecule
Fig. 6-13c
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6.5 How Do Cells Regulate Their Metabolic
Reactions?
 Cells regulate enzyme activity (continued)
– Small regulator molecules can bind to enzymes
and enhance or inhibit activity by allosteric
regulation
– Enzymes that undergo allosteric regulation have
a special regulatory binding site on the enzyme
that is distinct from the enzyme’s active site and
similar to a noncompetitive inhibitor site
– Allosteric regulation can either increase or
decrease enzyme activity, whereas
noncompetitive inhibition only reduces activity
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6.5 How Do Cells Regulate Their Metabolic
Reactions?
 Cells regulate enzyme activity (continued)
– Feedback inhibition is a negative feedback type
of allosteric inhibition that causes a metabolic
pathway to stop producing its product when
quantities reach an optimum level
– An enzyme near the beginning of a metabolic
pathway is inhibited allosterically by the end
product of the pathway
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Allosteric Regulation of an Enzyme by Feedback
Inhibition
intermediates
A
enzyme 1
threonine
(initial reactant)
B
enzyme 2
D
C
enzyme 3
enzyme 4
enzyme 5
As levels of isoleucine rise,
it binds to the regulatory site
on enzyme 1, inhibiting it
enzyme 1
isoleucine
isoleucine
(end product)
Fig. 6-14
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6.5 How Do Cells Regulate Their Metabolic
Reactions?
 Poisons, drugs, and environmental conditions
influence enzyme activity
– Drugs and poisons often inhibit enzymes by
competing with the natural substrate for the
active site
–This process occurs either by competitive or
by noncompetitive inhibition
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6.5 How Do Cells Regulate Their Metabolic
Reactions?
 Poisons, drugs, and environmental conditions
influence enzyme activity (continued)
– Some inhibitors bind permanently to the enzyme
–Some nerve gases and insecticides
permanently block the active site of
acetylcholinesterase
–Arsenic, mercury, and lead bind permanently
to the non-active sites of various enzymes,
inactivating them
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6.5 How Do Cells Regulate Their Metabolic
Reactions?
 Poisons, drugs, and environmental conditions
influence enzyme activity (continued)
– The activity of an enzyme is influenced by the
environment
–The three-dimensional structure of an enzyme
is sensitive to pH, salts, temperature, and the
presence of coenzymes
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6.5 How Do Cells Regulate Their Metabolic
Reactions?
 The activity of an enzyme is influenced by the
environment (continued)
– Enzyme structure is distorted (denatured) and
function is destroyed when pH is too high or low
– Salts in an enzyme’s environment can also
destroy function by altering structure
–Salt ions can bind with key amino acids in
enzymes, influencing three-dimensional
structure and destroying function
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6.5 How Do Cells Regulate Their Metabolic
Reactions?
 The activity of an enzyme is influenced by the
environment (continued)
– Temperature also affects enzyme activity
–Low temperatures slow down molecular
movement
–High temperatures cause enzyme shape to be
altered, destroying function
–Most enzymes function optimally only within a
very narrow range of these conditions
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Human Enzymes Function Best Within Narrow
Ranges of pH and Temperature
fast
rate
of
reaction
For pepsin, maximum
activity occurs at about
pH 2
For trypsin, maximum
activity occurs at about
pH 8
For most cellular
enzymes, maximum
activity occurs
at about pH 7.4
slow
(a) Effect of pH on enzyme activity
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Fig. 6-15a
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Human Enzymes Function Best Within Narrow
Ranges of pH and Temperature
fast
For most human enzymes,
maximum activity occurs
at about 98.6°F (37°C)
rate
of
reaction
slow
(b) Effect of temperature on enzyme activity
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Fig. 6-15b
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