Chapter 7 Raven

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Chapter 7Raven Johnson

• Energy, Metabolism, Enzymes

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Energy and Work

• energy
• capacity to do work or to cause particular changes

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Types of work carried out by organisms

• chemical work
• synthesis of complex molecules
• transport work
• take up of nutrients, elimination of wastes, and
  maintenance of ion balances
• mechanical work
• movement of organisms or cells and movement of
  internal structures

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Energy is the capacity to perform work

• Energy is defined as the capacity to do work
• All organisms require energy to stay alive
• Energy makes change possible

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• Kinetic energy is energy that is actually doing
  work

Figure 5.1A

• Potential energy is stored energy

Figure 5.1B
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The cells energy cycle
Figure 8.3
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The Laws of Thermodynamics

• thermodynamics
• a science that analyzes energy changes in a
  collection of matter called a system (e.g., a
  cell)
• all other matter in the universe is called the
  surroundings

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Energy units

• calorie (cal)
• amount of heat energy needed to raise 1 gram of
  water from 14.5 to 15.5C
• joules (J)
• units of work capable of being done by a unit of
  energy
• 1 cal of heat is equivalent to 4.1840 J of work

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First law of thermodynamics

• energy can be neither created nor destroyed
• total energy in universe remains constant
• however energy may be redistributed either within
  a system or between the system and its
  surroundings

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Two laws govern energy conversion

• First law of thermodynamics
• Energy can be changed from one form to another
• However, energy cannot be created or destroyed

Figure 5.2A
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Second law of thermodynamics

• entropy
• amount of disorder in a system
• physical and chemical processes proceed in such a
  way that the disorder of the universe increases
  to the maximum possible

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• Second law of thermodynamics

• Energy changes are not 100 efficient
• Energy conversions increase disorder, or entropy
• Some energy is always lost as heat

Figure 5.2B
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Second law in action

• molecules are redistributed,
• increasing entropy of system

Figure 8.4
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• There are two types of chemical reactions

• Endergonic reactions absorb energy and yield
  products rich in potential energy

Products
Amount of energy INPUT
Potential energy of molecules
Reactants
Figure 5.3A
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• Exergonic reactions release energy and yield
  products that contain less potential energy than
  their reactants

Reactants
Amount of energy OUTPUT
Potential energy of molecules
Products
Figure 5.3B
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ATP shuttles chemical energy within the cell

• In cellular respiration, some energy is stored in
  ATP molecules
• ATP powers nearly all forms of cellular work
• ATP molecules are the key to energy coupling

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Energy currency of cells

• ATP
• used to transfer energy from cells
  energy-conserving systems to the systems that
  carry out cellular work

Figure 8.2
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• How ATP powers cellular work

Reactants
Products
Potential energy of molecules
Work
Protein
Figure 5.4B
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• When the bond joining a phosphate group to the
  rest of an ATP molecule is broken by hydrolysis,
  the reaction supplies energy for cellular work

Adenine
Phosphategroups
Hydrolysis
Energy
Ribose
Adenosine triphosphate
Adenosine diphosphate(ADP)
Figure 5.4A
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Free Energy and Reactions

• ?G ?H - T ?S
• expresses the change in energy that can occur in
  chemical reactions and other processes
• used to indicate if a reaction will proceed
  spontaneously

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?G ?H - T ?S

• ?G
• free energy change
• amount of energy that is available to do work
• ?H
• change in enthalpy (heat content)
• T
• temperature in Kelvin
• ?S
• change in entropy

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Chemical equilibrium

• equilibrium
• consider the chemical reaction
• A B C D
• reaction is at equilibrium when rate of forward
  reaction rate of reverse reaction
• equilibrium constant (Keq)
• expresses the equilibrium concentrations of
  products and reactants to one another

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Standard free energy change (?Go)

• free energy change defined at standard conditions
  of concentration, pressure, temperature, and pH
• ?Go
• standard free energy change at ph 7
• directly related to Keq
• ?Go -2.303RTlogKeq

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The relationship

• Exergonic reactions
• AB CD
• Keq 1
• ?Go is negative
• (reaction proceeds spontaneously)

• Endergonic reactions
• AB CD
• Keq
• ?Go is positive
• (reaction will not proceed spontaneously)

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?Go -2.303RTlogKeq
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The Role of ATP in Metabolism

• exergonic breakdown of ATP is coupled with
  endergonic reactions to make them more favorable

Figure 8.6
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Oxidation/ ReductionREDOX
What do you know?
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Oxidation-Reduction Reactions and Electron
Carriers

• many metabolic processes involve
  oxidation-reduction reactions (electron
  transfers)
• electron carriers are often used to transfer
  electrons from an electron donor to an electron
  acceptor

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Oxidation-reduction (redox) reactions

• transfer of electrons from a donor to an acceptor
  can result in energy release, which can be
  conserved and used to form ATP

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Redox reactions

• Oxidation
• Removal of electrons
• Fe2 ? Fe3 1e-
• Releases energy
• Reduction
• Accepts electrons
• O2 4e- ( 4H) ? 2H2O
• Requires energy

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Redox reactions

• Each oxidation requires a simultaneous reduction
• Therefore, an oxidation half reaction requires a
  reduction half reaction
• 4Fe2 ? 4Fe3 4e-
• O2 4e- ( 4H) ? 2H2O
• 4Fe2 O2 4H ? 4Fe3 2H2O

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Convention for depicting redox couples, and
definitions
or 2H/H2
Electron donor reductant, reducing agent, is
energy rich Electron acceptor oxidant, oxidizing
agent, is energy poor
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Standard reduction potential (E0)

• equilibrium constant for an oxidation-reduction
  reaction
• a measure of the tendency of the reducing agent
  to lose electrons
• more negative E0 ? better electron donor
• more positive E0 ? better electron acceptor

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The greater the difference between the E0 of the
donor and the E0 of the acceptor ? the
more negative the ?Go
Figure 8.7
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Energy and electron flow in metabolism

• flow of electrons down the tower releases energy
• light energy is used to drive electrons up the
  tower during photosynthesis

Figure 8.8
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Electron carriers

• NAD
• nicotinamide adenine dinucleotide
• NADP
• nicotinamide adenine dinucleotide phosphate

Figure 8.9a
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Figure 8.9b
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H2 fumarate ? succinate ?G0 -86 kJ
H2 NO3- ? NO2- H2O ?G0 -163 kJ
H2 1/2O2 ? H2O ?G0 -237 kJ
?G0 -nF?Eo
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Electron Acceptors Over Time
Organic Food Electron Acceptor ? CO2 Reduced
Product
SO42-
O2
NO3-
CH4
Time
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Same with Depth
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Enzymes

• protein catalysts
• have great specificity for the reaction catalyzed
  and the molecules acted on
• catalyst
• substance that increases the rate of a reaction
  without being permanently altered
• substrates
• reacting molecules
• products
• substances formed by reaction

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Structure and Classification of Enzymes

• some enzymes are composed solely of one or more
  polypeptides
• some enzymes are composed of one or more
  polypeptides and nonprotein components

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Enzyme structure

• apoenzyme
• protein component of an enzyme
• cofactor
• nonprotein component of an enzyme
• prosthetic group firmly attached
• coenzyme loosely attached
• holoenzyme apoenzyme cofactor

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Coenzymes

• often act as carriers, transporting substances
  around the cell

Figure 8.13
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The Mechanism of Enzyme Reactions

• a typical exergonic reaction
• A B ? AB ? C D
• transition-state complex
• resembles both the substrates and the products

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activation energy energy required to form
transition- state complex
without enzyme
with enzyme
enzyme speeds up reaction by lowering Ea
Figure 8.14
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Interaction of enzyme and substrate
catalytic site
lock-and-key model
Figure 8.15
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How enzymes lower Ea

• by increasing concentrations of substrates at
  active site of enzyme
• by orienting substrates properly with respect to
  each other in order to form the transition-state
  complex

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The Effect of Environment on Enzyme Activity

• enzyme activity is significantly impacted by
  substrate concentration, pH, and temperature

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Effect of substrate

• rate increases as substrate increases
• no further increase occurs after all enzyme
  molecules are saturated with substrate

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Enzyme Inhibition

• competitive inhibitor
• directly competes with binding of substrate to
  active site

Figure 8.19

• noncompetitive inhibitor
• binds enzyme at site other than active site
  changes enzymes shape so that it becomes less
  active

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The Nature and Significance of Metabolic
Regulation

• conservation of energy and materials
• maintenance of metabolic balance despite changes
  in environment
• three major mechanisms
• metabolic channeling
• control enzyme activity
• control number of enzyme molecules present
  (discussed in Chapter 12)

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Control of Enzyme Activity

• allosteric regulation
• covalent modification

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Allosteric Regulation
enzyme inactive cant bind substrate
allosteric enzyme
effector binding alters shape of active site
enzyme catalyzes reaction
Figure 8.21
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Figure 8.23
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Covalent Modification of Enzymes

• reversible addition or removal of a chemical
  group alters enzyme activity

Figure 8.25
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Feedback Inhibition

• also called end product inhibition
• inhibition of one or more critical enzymes in a
  pathway regulates entire pathway
• pacemaker enzyme
• catalyzes the slowest or rate-limiting reaction
  in the pathway

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each end product regulates its own branch of the
pathway
isoenzymes different enzymes that catalyze
same reaction
each end product regulates the initial pacemaker e
nzyme
Figure 8.27