Overview: The Energy of Life
Living cell = miniature chemical factory where thousands of reactions occur
Cell extracts energy applies it to perform work Some organisms convert energy light,
bioluminescence
Concept 8.1: An organism’s metabolism transforms matter and energy, subject to
the laws of thermodynamics
Metabolism: totality of organism’s chemical reactions an emergent property of life that arises from interactions
between molecules within the cell
Organization of the Chemistry of Life into Metabolic Pathways
Metabolic pathway: begins with specific molecule and ends with a product Each step catalyzed by a specific enzyme
Catabolic pathways: release energy by breaking down complex molecules into simpler compounds Ex: Cellular respiration= the breakdown of glucose in the
presence of oxygen Anabolic pathways:
consume energy to build complex molecules from simpler ones Ex: synthesis of
protein from amino acids
Bioenergetics: study of how organisms manage their energy resources
Organization of the Chemistry of Life into Metabolic Pathways
Forms of Energy
Energy: the capacity to cause change exists in various forms, some can perform work
Kinetic energy: energy associated with motion
Heat (thermal energy): kinetic energy associated with random movement of atoms or molecules
Potential energy: energy that matter possesses because of its location or structure
Chemical energy: potential energy available for release in a chemical reaction
Energy can be converted from one form to another
Fig. 8-2
Climbing up converts the kineticenergy of muscle movementto potential energy.
A diver has less potentialenergy in the waterthan on the platform.
Diving convertspotential energy tokinetic energy.
A diver has more potentialenergy on the platformthan in the water.
The Laws of Energy Transformation
Thermodynamics: study of energy transformations
Closed system is isolated from its surroundings Ex: liquid in a thermos,
In open system energy and matter can be transferred between the system and its surroundings Ex: Organisms!
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The First Law of Thermodynamics
First law of thermodynamics: energy of the universe is constant: – Energy can be transferred and transformed, but it cannot be
created or destroyed
Also called the principle of conservation of energy
The Second Law of Thermodynamics
Second law of thermodynamics: during every energy transfer or transformation, some energy is unusable, and is often lost as heat Every energy transfer or transformation increases the
entropy (disorder) of the universe Living cells unavoidably
convert organized energy forms heat
Spontaneous processes occur without energy input quickly or slowly
For a process to occur without energy input, it must increase the entropy of universe
Fig. 8-3
(a) First law of thermodynamics (b) Second law of thermodynamics
Chemicalenergy
Heat CO2
H2O
+
Biological Order and Disorder
Cells create ordered structures from less ordered materials
Organisms replace ordered forms of matter and energy with less ordered forms
Energy: Flows into an ecosystem
as light and Exits as heat Explain this.
Evolution of more complex organisms does not violate the second law of thermodynamics Why not?
Entropy (disorder) may decrease in an organism, but the universe’s total entropy increases
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 8-4
50 µm
Root of buttercup plant; As open system, plants can increase their order as long
as order of surroundings decrease
Concept 8.2: The free-energy change of a reaction tells us whether or not the reaction
occurs spontaneously
Biologists want to know which reactions occur spontaneously and which require input of energy THUS! they need to
determine energy changes that occur in chemical reactions
Free-Energy Change, G
Free energy: energy that can do work when temperature and pressure are uniform like in a living cell
Change in free energy (∆G) during a process is related to change in enthalpy/change in total energy (∆H), change in entropy (∆S), and temperature in Kelvin (T):
∆G = ∆H – T∆S• Only processes with a negative ∆G are spontaneous• Spontaneous processes can be harnessed to perform
work Can you think of one?
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Free Energy, Stability, and Equilibrium Free energy is a measure of a system’s instability or its
tendency to change to a more stable state During spontaneous change:
free energy decreases stability of system increases
Equilibrium: state of maximum stability A process is spontaneous and can perform work only
when it is moving toward equilibrium
Fig. 8-5
(a) Gravitational motion (b) Diffusion (c) Chemical reaction
• More free energy (higher G)• Less stable• Greater work capacity
In a spontaneous change• The free energy of the system decreases (∆G < 0)• The system becomes more stable• The released free energy can be harnessed to do work
• Less free energy (lower G)• More stable• Less work capacity
Fig. 8-5a
• Less free energy (lower G)• More stable• Less work capacity
• More free energy (higher G)• Less stable• Greater work capacity
In a spontaneous change• The free energy of the system decreases (∆G < 0)• The system becomes more stable• The released free energy can be harnessed to do work
Fig. 8-5b
Spontaneouschange
Spontaneouschange
Spontaneouschange
(b) Diffusion (c) Chemical reaction(a) Gravitational motion
Free Energy and Metabolism
Concept of free energy can be applied to chemistry of life’s processes
Exergonic reaction: net release of free energy; spontaneous
Endergonic reaction: absorbs free energy from its surroundings; nonspontaneous
Fig. 8-6a
Energy
(a) Exergonic reaction: energy released
Progress of the reaction
Fre
e en
erg
y
Products
Amount ofenergy
released(∆G < 0)
Reactants
Fig. 8-6b
Energy
(b) Endergonic reaction: energy required
Progress of the reaction
Fre
e en
erg
y
Products
Amount ofenergy
required(∆G > 0)
Reactants
Equilibrium and Metabolism
Reactions in closed system eventually reach equilibrium and then do no work
Cells not in equilibrium; they are open systems with constant flow of materials
Defining feature of life = metabolism is never at equilibrium
Catabolic pathway: releases free energy in a series of reactions
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Fig. 8-7
(a) An isolated hydroelectric system
∆G < 0 ∆G = 0
(b) An open hydroelectric system ∆G < 0
∆G < 0
∆G < 0
∆G < 0
(c) A multistep open hydroelectric system
Fig. 8-7a
(a) An isolated hydroelectric system
∆G < 0 ∆G = 0
Fig. 8-7b
(b) An open hydroelectric system
∆G < 0
Fig. 8-7c
(c) A multistep open hydroelectric system
∆G < 0
∆G < 0
∆G < 0
Concept 8.3: ATP powers cellular work by coupling exergonic reactions to endergonic
reactions
Cell does three main kinds of work: Chemical Transport Mechanical
Cells manage energy resources by energy coupling the use of an
exergonic process to drive an endergonic one
Most energy coupling in cells mediated by ATPCopyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
The Structure and Hydrolysis of ATP
ATP (adenosine triphosphate): cell’s energy shuttle
ATP is composed of : ribose (a sugar) adenine (a nitrogenous base) three phosphate groups
Bonds between phosphate groups of ATP’s tail can be broken by hydrolysis
Energy released from ATP when terminal phosphate bond broken comes from chemical change
to a state of lower free energy, not from the phosphate bonds themselves
How ATP Performs Work Three types of cellular work (mechanical, transport, and
chemical) powered by the hydrolysis of ATP In cell energy from exergonic reaction of ATP hydrolysis
can be used to drive a endergonicreaction
Overall: coupled reactions are exergonic
ATP drives endergonic reactions by phosphorylation transferring a
phosphate group to some other molecule, such as a reactant
recipient molecule is now phosphorylated
Fig. 8-10
(b) Coupled with ATP hydrolysis, an exergonic reaction
Ammonia displacesthe phosphate group,forming glutamine.
(a) Endergonic reaction
(c) Overall free-energy change
PP
GluNH3
NH2
Glu i
GluADP+
PATP+
+
Glu
ATP phosphorylatesglutamic acid,making the aminoacid less stable.
GluNH3
NH2
Glu+
Glutamicacid
GlutamineAmmonia
∆G = +3.4 kcal/mol
+2
1
The Regeneration of ATP ATP renewable resource that is regenerated by addition
of phosphate group to adenosine diphosphate (ADP) Energy to phosphorylate ADP comes from catabolic
reactions in the cell Chemical potential energy temporarily stored in ATP
drives most cellular work
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Concept 8.4: Enzymes speed up metabolic reactions by lowering energy barriers
Catalyst: chemical agent that speeds up a reaction without being consumed by the reaction
Enzyme: catalytic protein
Enzyme-catalyzed reaction Ex: Hydrolysis of
sucrose by enzyme sucrase
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
The Activation Energy Barrier
Every chemical reaction between molecules involves bond breaking AND bond forming
Free energy of activation, or activation energy (EA): initial energy needed to start chemical reaction often supplied in
form of heat from the surroundings
How Enzymes Lower the EA Barrier
Enzymes catalyze reactions by lowering EA barrier Enzymes do not affect change in free energy (∆G);
instead, they hasten reactions that would occur eventually
Substrate Specificity of Enzymes
Substrate: reactant that an enzyme acts on Enzyme-substrate complex: formed when enzyme
binds to its substrate Active site: region on the enzyme where the substrate
binds Induced fit of substrate brings chemical groups of active
site into positions that enhance their ability to catalyze the reaction
Catalysis in the Enzyme’s Active Site
In enzymatic reaction the substrate binds to the active site of the enzyme
Active site can lower an EA barrier by Orienting substrates
correctly Straining substrate bonds Providing a favorable
microenvironment Covalently bonding to the
substrate
Fig. 8-17
Substrates
Enzyme
Products arereleased.
Products
Substrates areconverted toproducts.
Active site can lower EA
and speed up a reaction.
Substrates held in active site by weakinteractions, such as hydrogen bonds andionic bonds.
Substrates enter active site; enzyme changes shape such that its active siteenfolds the substrates (induced fit).
Activesite is
availablefor two new
substratemolecules.
Enzyme-substratecomplex
5
3
21
6
4
Effects of Local Conditions on Enzyme Activity
Enzyme’s activity can be affected by General
environmental factors Temperatur
e pH
Chemicals that specifically influence enzyme
Fig. 8-18
Ra
te o
f re
ac
tio
n
Optimal temperature forenzyme of thermophilic
(heat-tolerant) bacteria
Optimal temperature fortypical human enzyme
(a) Optimal temperature for two enzymes
(b) Optimal pH for two enzymes
Ra
te o
f re
ac
tio
n
Optimal pH for pepsin(stomach enzyme)
Optimal pHfor trypsin(intestinalenzyme)
Temperature (ºC)
pH543210 6 7 8 9 10
0 20 40 80 60 100
Cofactors Cofactors: nonprotein enzyme helpers
may be inorganic (such as a metal in ionic form) or organic
Coenzyme: organic cofactor include vitamins
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Enzyme Inhibitors Competitive inhibitors: bind to active site of enzyme,
competing with the substrate
Noncompetitive inhibitors: bind to another part of enzyme, causing enzyme to change shape and making active site less effective
Examples: toxins, poisons, pesticides, and antibiotics
Fig. 8-19
(a) Normal binding (c) Noncompetitive inhibition(b) Competitive inhibition
Noncompetitive inhibitor
Active siteCompetitive inhibitor
Substrate
Enzyme
Concept 8.5: Regulation of enzyme activity helps control metabolism
Chemical chaos would result if cell’s metabolic pathways were not tightly regulated
Cell does this by: 1. switching on/off genes that encode specific enzymes
2. regulating the activity of enzymes
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Allosteric Regulation of Enzymes
Allosteric regulation may either inhibit or stimulate an enzyme’s activity occurs when a regulatory molecule binds to protein at
one site and affects the protein’s function at another site
Allosteric Activation and
Inhibition Most allosterically regulated enzymes
made from polypeptide subunits Each enzyme has active and inactive
forms Binding of activator stabilizes the
active form of the enzyme Binding of inhibitor stabilizes the
inactive form of the enzyme Cooperativity: form of allosteric
regulation that can amplify enzyme activity binding by substrate to one active
site stabilizes favorable conformational changes at all other subunits
Fig. 8-20a
(a) Allosteric activators and inhibitors
InhibitorNon-functionalactivesite
Stabilized inactiveform
Inactive form
Oscillation
Activator
Active form Stabilized active form
Regulatorysite (oneof four)
Allosteric enzymewith four subunits
Active site(one of four)
Fig. 8-20b
(b) Cooperativity: another type of allosteric activation
Stabilized activeform
Substrate
Inactive form
Identification of Allosteric Regulators
Allosteric regulators = attractive drug candidates for enzyme regulation
Inhibition of proteolytic enzymes called caspases may help management of inappropriate inflammatory responses
Fig. 8-21a
SH
Substrate
Hypothesis: allostericinhibitor locks enzymein inactive form
Active form canbind substrate
S–SSH
SH
Activesite
Caspase 1
Known active form
Known inactive form
Allostericbinding site
Allostericinhibitor
EXPERIMENT
Fig. 8-21b
Caspase 1
RESULTS
Active form
Inhibitor
Allostericallyinhibited form
Inactive form
Feedback Inhibition
Feedback inhibition: end product of a metabolic pathway shuts down the pathway Prevents cell
from wasting chemical resources by synthesizing more product than needed
Specific Localization of Enzymes Within the Cell
Structures within cell help bring order to metabolic pathways
Some enzymes act as structural components of membranes
In eukaryotic cells, some enzymes reside in specific organelles Ex: enzymes for
cellular respiration located in mitochondria
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
You should now be able to:
1. Distinguish between the following pairs of terms: 1. catabolic and anabolic pathways2. kinetic and potential energy3. open and closed systems 4. exergonic and endergonic reactions
2. In your own words, explain the second law of thermodynamics and explain why it is not violated by living organisms
3. Explain in general terms how cells obtain the energy to do cellular work
4. Explain how ATP performs cellular work 5. Explain why an investment of activation energy is
necessary to initiate a spontaneous reaction6. Describe the mechanisms by which enzymes lower
activation energy7. Describe how allosteric regulators may inhibit or
stimulate the activity of an enzyme
Fig. 8-UN2
Progress of the reaction
Products
Reactants
∆G is unaffectedby enzyme
Course ofreactionwithoutenzyme
Fre
e en
erg
y
EA
withoutenzyme EA with
enzymeis lower
Course ofreactionwith enzyme
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