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Lectures by
Gregory AhearnUniversity of North Florida
Chapter 6
Energy Flow in the Life of a Cell
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5.1 What Is Energy?
Energy is the capacity to do work.• Synthesizing molecules• Moving objects• Generating heat and light
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5.1 What Is Energy?
Types of energy• Kinetic: energy of movement• Potential: stored energy
Fig. 5-1
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5.1 What Is Energy?
First Law of Thermodynamics• “Energy cannot be created nor destroyed, but
it can change its form.”• Example: potential energy in gasoline can be
converted to kinetic energy in a car, but the energy is not lost
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5.1 What Is Energy?
Second Law of Thermodynamics• “When energy is converted from one form to
another, the amount of useful energy decreases.”
• No process is 100% efficient.• Example: more potential energy is in the
gasoline than is transferred to the kinetic energy of the car moving
• Where is the rest of the energy? It is released in a less useful form as heat—the total energy is maintained.
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5.1 What Is Energy?
Matter tends to become less organized.• There is a continual decrease in useful
energy, and a build up of heat and other non-useful forms of energy.
• Entropy: the spontaneous reduction in ordered forms of energy, and an increase in randomness and disorder as reactions proceed
• Example: gasoline is made up of an eight-carbon molecule that is highly ordered
• When broken down to single carbons in CO2, it is less ordered and more random.
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5.1 What Is Energy?
In order to keep useful energy flowing in ecosystems where the plants and animals produce more random forms of energy, new energy must be brought in.
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5.1 What Is Energy?
Sunlight provides an unending supply of new energy to power all plant and animal reactions, leading to increased entropy.
Fig. 5-2
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5.2 How Does Energy Flow In Chemical Reactions? Chemical reaction: the conversion of one
set of chemical substances (reactants) into another (products)• Exergonic reaction: a reaction that releases
energy; the products contain less energy than the reactants
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energyreleased
reactants
products
Exergonic reaction
+
+
(a)
5.2 How Does Energy Flow In Chemical Reactions? Exergonic reaction
Fig. 5-3a
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5.2 How Does Energy Flow In Chemical Reactions? Endergonic reaction: a reaction that requires
energy input from an outside source; the products contain more energy than the reactants
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energyused
products
reactants
Endergonic reaction
+
+
(b)
5.2 How Does Energy Flow In Chemical Reactions? Endergonic reaction
Fig. 5-3b
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5.2 How Does Energy Flow In Chemical Reactions? Exergonic reactions release energy.
• Example: sugar burned by a flame in the presence of oxygen produces carbon dioxide (CO2) and water
• Sugar and oxygen contain more energy than the molecules of CO2 and water.
• The extra energy is released as heat.
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5.2 How Does Energy Flow In Chemical Reactions? Burning glucose releases energy.
Fig. 5-4
energyreleased
C6H12O6 6 O2
(glucose) (oxygen)
+
6 CO2
(carbondioxide)
6 H2O
(water)
+
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5.2 How Does Energy Flow In Chemical Reactions? Endergonic reactions require an input of
energy.• Example: sunlight energy + CO2 + water in
photosynthesis produces sugar and oxygen• The sugar contains far more energy than the
CO2 and water used to form it.
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5.2 How Does Energy Flow In Chemical Reactions? Photosynthesis requires energy.
Fig. 5-5
C6H12O6 6 O2
(glucose) (oxygen)
+
6 CO2
(carbondioxide)
6 H2O
(water)
+
energy
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high
low
progress of reaction progress of reaction
energycontent
ofmolecules
Activation energy neededto ignite glucose
Energy level of reactants
glucose + O2
CO2 + H2O CO2 + H2O
glucose
Activationenergycapturedfromsunlight
Energy level of reactants
Burning glucose (sugar): an exergonic reaction Photosynthesis: an endergonic reaction(a) (b)
5.2 How Does Energy Flow In Chemical Reactions? All reactions require an initial input of energy.
• The initial energy input to a chemical reaction is called the activation energy.
Fig. 5-6
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5.2 How Does Energy Flow In Chemical Reactions? The source of activation energy is the
kinetic energy of movement when molecules collide.
Molecular collisions force electron shells of atoms to mingle and interact, resulting in chemical reactions.
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5.2 How Does Energy Flow in Chemical Reactions? Exergonic reactions may be linked with
endergonic reactions.• Endergonic reactions obtain energy from
energy-releasing exergonic reactions in coupled reactions.
• Example: the exergonic reaction of burning gasoline in a car provides the endergonic reaction of moving the car
• Example: exergonic reactions in the sun release light energy used to drive endergonic sugar-making reactions in plants
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5.3 How Is Energy Carried Between Coupled Reactions? The job of transferring energy from one
place in a cell to another is done by energy-carrier molecules.• ATP (adenosine triphosphate) is the main
energy carrier molecule in cells, and provides energy for many endergonic reactions.
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ADP
ATP
phosphate
energy
+
A P P P
A P P P
5.3 How Is Energy Carried Between Coupled Reactions? ATP is made from ADP (adenosine
diphosphate) and phosphate plus energy released from an exergonic reaction (e.g., glucose breakdown) in a cell.
Fig. 5-7
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5.3 How Is Energy Carried Between Coupled Reactions? ATP is the principal energy carrier in cells.
• ATP stores energy in its phosphate bonds and carries the energy to various sites in the cell where energy-requiring reactions occur.
• ATP’s phosphate bonds then break yielding ADP, phosphate, and energy.
• This energy is then transferred to the energy-requiring reaction.
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phosphateADP
energy
ATP+
A
A P P P
P P P
5.3 How Is Energy Carried Between Coupled Reactions? Breakdown of ATP releases energy.
Fig. 5-8
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5.3 How Is Energy Carried Between Coupled Reactions? To summarize:
• Exergonic reactions (e.g., glucose breakdown) drive endergonic reactions (e.g., the conversion of ADP to ATP).
• ATP moves to different parts of the cell and is broken down exergonically to liberate its energy to drive endergonic reactions.
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5.3 How Is Energy Carried Between Coupled Reactions? Coupled reactions
Fig. 5-9
endergonic(ATP synthesis)
exergonic(ATP breakdown)
exergonic(glucose breakdown)
endergonic(protein synthesis)CO2 + H2O + heat
glucose
aminoacids
protein
+ PP P
P PPA
A
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5.3 How Is Energy Carried Between Coupled Reactions? A biological example of coupled reactions
• Muscle contraction (an endergonic reaction) is powered by the exergonic breakdown of ATP.
• During energy transfer in this coupled reaction, heat is given off, with overall loss of usable energy.
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5.3 How Is Energy Carried Between Coupled Reactions? ATP breakdown is coupled with muscle
contraction.
Fig. 5-10
Energy released from ATPbreakdown exceeds theenergy used for musclecontraction, so the overallcoupled reaction is exergonic
++100 unitsenergyreleased
+
+
+ +80 unitsenergy releasedas heat
20 unitsenergy
+
contractedmuscle
contractedmuscle
relaxedmuscle
relaxedmuscle
Exergonic reaction:
Endergonic reaction:
Coupled reaction:
PADPATP
ADP PATP
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5.3 How Is Energy Carried Between Coupled Reactions?
Animation—Energy and Chemical ReactionsPLAYPLAY
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5.3 How Is Energy Carried Between Coupled Reactions? Electron carriers also transport energy
within cells.• Besides ATP, other carrier molecules
transport energy within a cell.• Electron carriers capture energetic electrons
transferred by some exergonic reaction.• Energized electron carriers then donate these
energy-containing electrons to endergonic reactions.
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low-energyproducts
energized
depleted
low-energyreactants
high-energyreactants
NADH
high-energyproducts
e–
e–
NAD+ + H+
5.3 How Is Energy Carried Between Coupled Reactions? Common electron carriers are NAD+ and
FAD.
Fig. 5-11
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5.3 How Is Energy Carried Between Coupled Reactions?
Animation—Energy and LifePLAYPLAY
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PATHWAY 1
Initial reactant Intermediates Final products
enzyme 1 enzyme 2 enzyme 3 enzyme 4
PATHWAY 2
enzyme 5 enzyme 6
A B D E
F
C
G
5.4 How Do Cells Control Their Metabolic Reactions? Cell metabolism: the multitude of chemical
reactions going on at any specific time in a cell
Metabolic pathways: the sequence of cellular reactions (e.g., photosynthesis and glycolysis)
Fig. 5-12
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5.4 How Do Cells Control Their Metabolic Reactions? At body temperature, many spontaneous
reactions proceed too slowly to sustain life.• A reaction can be controlled by controlling its
activation energy (the energy needed to start the reaction).
• At body temperature, reactions occur too slowly because their activation energies are too high.
• Molecules called catalysts are able to gain access to energy that is not produced spontaneously.
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progress of reaction
low
high
energycontent
ofmolecules
Activation energywith catalyst
Activation energywithout catalyst
reactants
products
5.4 How Do Cells Control Their Metabolic Reactions? Catalysts reduce activation energy.
• Catalysts are molecules that speed up a reaction without being used up or permanently altered.
• They speed up the reaction by reducing the activation energy.
Fig. 5-13
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5.4 How Do Cells Control Their Metabolic Reactions? Three important principles about all
catalysts• Catalysts speed up a reaction.• They speed up reactions that would occur
anyway, if their activation energy could be surmounted.
• Catalysts are not altered by the reaction.
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5.4 How Do Cells Control Their Metabolic Reactions? Enzymes are biological catalysts.
• Almost all enzymes are proteins.• Enzymes are highly specialized, generally
catalyzing only a single reaction.• In metabolic pathways involving multiple
reactions, each reaction is catalyzed by a different enzyme.
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5.4 How Do Cells Control Their Metabolic Reactions? The structure of enzymes allows them to
catalyze specific reactions.• Enzymes have an active site where the
reactant molecules, called substrates, enter and undergo a chemical change as a result.
• The specificity of an enzyme reaction is due to the distinctive shape of the active site, which only allows proper substrate molecules to enter.
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5.4 How Do Cells Control Their Metabolic Reactions? How does an enzyme catalyze a reaction?
• Both substrates enter the enzyme’s active site.
• Substrates enter an enzyme’s active site, changing both of their shapes.
• The chemical bonds are altered in the substrates, promoting the reaction.
• The substrates change into a new form that will not fit the active site, and so are released.
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5.4 How Do Cells Control Their Metabolic Reactions? The cycle of enzyme–substrate interactions
Fig. 5-14
substrates
active siteof enzyme
enzyme
Substrates enterthe active site in aspecific orientation
1
The substrates, bondedtogether, leave the enzyme;the enzyme is ready for anew set of substrates
3 The substrates andactive site change shape,promoting a reactionbetween the substrates
2
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5.4 How Do Cells Control Their Metabolic Reactions?
Animation—EnzymesPLAYPLAY
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5.4 How Do Cells Control Their Metabolic Reactions? Cells regulate metabolism by controlling
enzymes.• Allosteric regulation can increase or decrease
enzyme activity.• In allosteric regulation, an enzyme’s activity
is modified by a regulator molecule.• The regulator molecule binds to a special
regulatory site on the enzyme separate from the enzyme’s active site.
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5.4 How Do Cells Control Their Metabolic Reactions? Binding of the regulator molecule modifies
the active site on the enzyme, causing the enzyme to become more or less able to bind substrate.
Thus, allosteric regulation can either promote or inhibit enzyme activity.
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active site
substrate
enzyme
Enzyme structure
Many enzymes haveboth active sites andallosteric regulatorysites
allostericregulatory site
(a)
5.4 How Do Cells Control Their Metabolic Reactions? Enzyme structure
Fig. 5-15a
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allostericregulatormolecule
Allosteric inhibition
An allosteric regulatormolecule causes theactive site to changeshape, so the substrateno longer fits
(b)
5.4 How Do Cells Control Their Metabolic Reactions? Allosteric inhibition
Fig. 5-15b
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5.4 How Do Cells Control Their Metabolic Reactions? Competitive inhibition can be temporary or
permanent. Some regulatory molecules temporarily bind
directly to an enzyme’s active site, preventing the substrate molecules from binding.
These molecules compete with the substrate for access to the active site, and control the enzyme by competitive inhibition.
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