Download AP UNIT 3

Chapter 8
An Introduction to
Metabolism
PowerPoint® Lecture Presentations for
Biology
Eighth Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Concept 8.1: An organism’s metabolism
transforms matter and energy, subject to the laws
of thermodynamics
• Metabolism is the totality of an organism’s
chemical reactions
• Metabolism is an emergent property of life that
arises from interactions between molecules
within the cell
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Organization of the Chemistry of Life into
Metabolic Pathways
• A metabolic pathway begins with a specific
molecule and ends with a product
• Each step is catalyzed by a specific enzyme
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 8-UN1
Enzyme 1
A
Reaction 1
Starting
molecule
Enzyme 2
B
Enzyme 3
C
Reaction 2
D
Reaction 3
Product
Forms of Energy
• Energy is the capacity to cause change
• Energy exists in various forms, some of which
can perform work
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• Kinetic energy is energy associated with motion
• Heat (thermal energy) is kinetic energy
associated with random movement of atoms or
molecules
• Potential energy is energy that matter possesses
because of its location or structure
• Chemical energy is potential energy available for
release in a chemical reaction
• Energy can be converted from one form to another
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
The Laws of Energy Transformation
• Thermodynamics is the study of energy
transformations
• A closed system, such as that approximated by
liquid in a thermos, is isolated from its
surroundings
• In an open system, energy and matter can be
transferred between the system and its
surroundings
• Organisms are open systems
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
The First Law of Thermodynamics
• According to the first law of
thermodynamics, the energy of the universe
is constant:
– Energy can be transferred and transformed,
but it cannot be created or destroyed
• The first law is also called the principle of
conservation of energy
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
The Second Law of Thermodynamics
• During every energy transfer or transformation,
some energy is unusable, and is often lost as
heat
• According to the second law of
thermodynamics:
– Every energy transfer or transformation
increases the entropy (disorder) of the
universe
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• Living cells unavoidably convert organized
forms of energy to heat
• Spontaneous processes occur without energy
input; they can happen quickly or slowly
• For a process to occur without energy input, it
must increase the entropy of the universe
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Biological Order and Disorder
• Cells create ordered structures from less
ordered materials
• Organisms also replace ordered forms of
matter and energy with less ordered forms
• Energy flows into an ecosystem in the form of
light and exits in the form of heat
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• Catabolic pathways release energy by
breaking down complex molecules into simpler
compounds
• Cellular respiration, the breakdown of glucose
in the presence of oxygen, is an example of a
pathway of catabolism
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• Anabolic pathways consume energy to build
complex molecules from simpler ones
• The synthesis of protein from amino acids is an
example of anabolism
• Bioenergetics is the study of how organisms
manage their energy resources
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• The evolution of more complex organisms does
not violate the second law of thermodynamics
• Entropy (disorder) may decrease in an
organism, but the universe’s total entropy
increases
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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
• To do so, they need to determine energy
changes that occur in chemical reactions
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Free-Energy Change, G
• A living system’s free energy is energy that
can do work when temperature and pressure
are uniform, as in a living cell
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• The change in free energy (∆G) during a
process is related to the change in enthalpy, or
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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Free Energy, Stability, and Equilibrium
• Free energy is a measure of a system’s
instability, its tendency to change to a more
stable state
• During a spontaneous change, free energy
decreases and the stability of a system
increases
• Equilibrium is a state of maximum stability
• A process is spontaneous and can perform
work only when it is moving toward equilibrium
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 8-5
• 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
(a) Gravitational motion
(b) Diffusion
(c) Chemical reaction
Fig. 8-5a
• 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
Exergonic and Endergonic Reactions in Metabolism
• An exergonic reaction proceeds with a net
release of free energy and is spontaneous
• An endergonic reaction absorbs free energy
from its surroundings and is nonspontaneous
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 8-6
Reactants
Free energy
Amount of
energy
released
(∆G < 0)
Energy
Products
Progress of the reaction
(a) Exergonic reaction: energy released
Free energy
Products
Amount of
energy
required
(∆G > 0)
Energy
Reactants
Progress of the reaction
(b) Endergonic reaction: energy required
Concept 8.3: ATP powers cellular work by
coupling exergonic reactions to endergonic
reactions
• A cell does three main kinds of work:
– Chemical
– Transport
– Mechanical
• To do work, cells manage energy resources by
energy coupling, the use of an exergonic process
to drive an endergonic one
• Most energy coupling in cells is mediated by ATP
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
The Structure and Hydrolysis of ATP
• ATP (adenosine triphosphate) is the cell’s
energy shuttle
• ATP is composed of ribose (a sugar), adenine
(a nitrogenous base), and three phosphate
groups
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 8-8
Adenine
Phosphate groups
Ribose
• The bonds between the phosphate groups of
ATP’s tail can be broken by hydrolysis
• Energy is released from ATP when the terminal
phosphate bond is broken
• This release of energy comes from the
chemical change to a state of lower free
energy, not from the phosphate bonds
themselves
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 8-9
P
P
P
Adenosine triphosphate (ATP)
H2O
Pi
+
Inorganic phosphate
P
P
+
Adenosine diphosphate (ADP)
Energy
How ATP Performs Work
• The three types of cellular work (mechanical,
transport, and chemical) are powered by the
hydrolysis of ATP
• In the cell, the energy from the exergonic
reaction of ATP hydrolysis can be used to drive
an endergonic reaction
• Overall, the coupled reactions are exergonic
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• ATP drives endergonic reactions by
phosphorylation, transferring a phosphate
group to some other molecule, such as a
reactant
• The recipient molecule is now phosphorylated
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 8-10
NH2
Glu
Glutamic
acid
NH3
+
∆G = +3.4 kcal/mol
Glu
Ammonia
Glutamine
(a) Endergonic reaction
1 ATP phosphorylates
glutamic acid,
making the amino
acid less stable.
P
+
Glu
ATP
Glu
+ ADP
NH2
2 Ammonia displaces
the phosphate group,
forming glutamine.
P
Glu
+
NH3
Glu
+ Pi
(b) Coupled with ATP hydrolysis, an exergonic reaction
(c) Overall free-energy change
The Regeneration of ATP
• ATP is a renewable resource that is
regenerated by addition of a phosphate group
to adenosine diphosphate (ADP)
• The energy to phosphorylate ADP comes from
catabolic reactions in the cell
• The chemical potential energy temporarily
stored in ATP drives most cellular work
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 8-12
ATP + H2O
Energy from
catabolism (exergonic,
energy-releasing
processes)
ADP + P i
Energy for cellular
work (endergonic,
energy-consuming
processes)
Concept 8.4: Enzymes speed up metabolic
reactions by lowering energy barriers
• A catalyst is a chemical agent that speeds up
a reaction without being consumed by the
reaction
• An enzyme is a catalytic protein
• Hydrolysis of sucrose by the enzyme sucrase
is an example of an enzyme-catalyzed reaction
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 8-13
Sucrose (C12H22O11)
Sucrase
Glucose (C6H12O6)
Fructose (C6H12O6)
The Activation Energy Barrier
• Every chemical reaction between molecules
involves bond breaking and bond forming
• The initial energy needed to start a chemical
reaction is called the free energy of
activation, or activation energy (EA)
• Activation energy is often supplied in the form
of heat from the surroundings
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 8-14
A
B
C
D
Transition state
A
B
C
D
EA
Reactants
A
B
∆G < O
C
D
Products
Progress of the reaction
How Enzymes Lower the EA Barrier
• Enzymes catalyze reactions by lowering the EA
barrier
• Enzymes do not affect the change in free
energy (∆G); instead, they hasten reactions
that would occur eventually
Animation: How Enzymes Work
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 8-15
Course of
reaction
without
enzyme
EA
without
enzyme
EA with
enzyme
is lower
Reactants
Course of
reaction
with enzyme
∆G is unaffected
by enzyme
Products
Progress of the reaction
Substrate Specificity of Enzymes
• The reactant that an enzyme acts on is called
the enzyme’s substrate
• The enzyme binds to its substrate, forming an
enzyme-substrate complex
• The active site is the region on the enzyme
where the substrate binds
• Induced fit of a substrate brings chemical
groups of the active site into positions that
enhance their ability to catalyze the reaction
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 8-17
1 Substrates enter active site; enzyme
changes shape such that its active site
enfolds the substrates (induced fit).
2 Substrates held in
active site by weak
interactions, such as
hydrogen bonds and
ionic bonds.
Substrates
Enzyme-substrate
complex
6 Active
site is
available
for two new
substrate
molecules.
Enzyme
5 Products are
released.
4 Substrates are
converted to
products.
Products
3 Active site can lower EA
and speed up a reaction.
Cofactors
• Cofactors are nonprotein enzyme helpers
• Cofactors may be inorganic (such as a metal in
ionic form) or organic
• An organic cofactor is called a coenzyme
• Coenzymes include vitamins
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Effects of Local Conditions on Enzyme Activity
• An enzyme’s activity can be affected by
– General environmental factors, such as
temperature and pH
– Chemicals that specifically influence the
enzyme
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Effects of Temperature and pH
• Each enzyme has an optimal temperature in
which it can function
• Each enzyme has an optimal pH in which it can
function
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 8-18
Rate of reaction
Optimal temperature for
typical human enzyme
Optimal temperature for
enzyme of thermophilic
(heat-tolerant)
bacteria
40
60
80
Temperature (ºC)
(a) Optimal temperature for two enzymes
0
20
Optimal pH for pepsin
(stomach enzyme)
100
Optimal pH
for trypsin
Rate of reaction
(intestinal
enzyme)
4
5
pH
(b) Optimal pH for two enzymes
0
1
2
3
6
7
8
9
10
Enzyme Inhibitors
• Competitive inhibitors bind to the active site
of an enzyme, competing with the substrate
• Noncompetitive inhibitors bind to another
part of an enzyme, causing the enzyme to
change shape and making the active site less
effective
• Examples of inhibitors include toxins, poisons,
pesticides, and antibiotics
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 8-19
Substrate
Active site
Competitive
inhibitor
Enzyme
Noncompetitive inhibitor
(a) Normal binding
(b) Competitive inhibition
(c) Noncompetitive inhibition
Concept 8.5: Regulation of enzyme activity helps
control metabolism
• Chemical chaos would result if a cell’s
metabolic pathways were not tightly regulated
• A cell does this by switching on or off the genes
that encode specific enzymes or by regulating
the activity of enzymes
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Allosteric Regulation of Enzymes
• Allosteric regulation may either inhibit or
stimulate an enzyme’s activity
• Allosteric regulation occurs when a regulatory
molecule binds to a protein at one site and
affects the protein’s function at another site
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Allosteric Activation and Inhibition
• Most allosterically regulated enzymes are
made from polypeptide subunits
• Each enzyme has active and inactive forms
• The binding of an activator stabilizes the active
form of the enzyme
• The binding of an inhibitor stabilizes the
inactive form of the enzyme
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 8-20a
Allosteric enzyme
with four subunits
Active site
(one of four)
Regulatory
site (one
of four)
Activator
Active form
Stabilized active form
Oscillation
NonInhibitor
Inactive
form
functional
active
site
(a) Allosteric activators and inhibitors
Stabilized inactive
form
• Cooperativity is a form of allosteric regulation
that can amplify enzyme activity
• In cooperativity, binding by a substrate to one
active site stabilizes favorable conformational
changes at all other subunits
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 8-20b
Substrate
Inactive form
Stabilized active
form
(b) Cooperativity: another type of allosteric activation
Identification of Allosteric Regulators
• Allosteric regulators are attractive drug
candidates for enzyme regulation
• Inhibition of proteolytic enzymes called
caspases may help management of
inappropriate inflammatory responses
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 8-21a
EXPERIMENT
Caspase 1
Active
site
Substrate
SH
Known active form
SH Allosteric
binding site
Allosteric
Known inactive form
inhibitor
SH
Active form can
bind substrate
S–S
Hypothesis: allosteric
inhibitor locks enzyme
in inactive form
Feedback Inhibition
• In feedback inhibition, the end product of a
metabolic pathway shuts down the pathway
• Feedback inhibition prevents a cell from
wasting chemical resources by synthesizing
more product than is needed
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 8-22
Initial substrate
(threonine)
Active site
available
Isoleucine
used up by
cell
Threonine
in active site
Enzyme 1
(threonine
deaminase)
Intermediate A
Feedback
inhibition
Isoleucine
binds to
allosteric
site
Enzyme 2
Active site of
enzyme 1 no
longer binds Intermediate B
threonine;
pathway is
Enzyme 3
switched off.
Intermediate C
Enzyme 4
Intermediate D
Enzyme 5
End product
(isoleucine)
Specific Localization of Enzymes Within the Cell
• Structures within the cell help bring order to
metabolic pathways
• Some enzymes act as structural components
of membranes
• In eukaryotic cells, some enzymes reside in
specific organelles; for example, enzymes for
cellular respiration are located in mitochondria
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Chapter 9
Cellular Respiration:
Harvesting Chemical Energy
PowerPoint® Lecture Presentations for
Biology
Eighth Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
OVERVIEW
• Energy flows into an ecosystem as sunlight
and leaves as heat
• Photosynthesis generates O2 and organic
molecules, which are used in cellular
respiration
• Cells use chemical energy stored in organic
molecules to regenerate ATP, which powers
work
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 9-2
Light
energy
ECOSYSTEM
Photosynthesis
in chloroplasts
CO2 + H2O
Organic
+O
molecules 2
Cellular respiration
in mitochondria
ATP
ATP powers most cellular work
Heat
energy
Catabolic Pathways and Production of ATP
• The breakdown of organic molecules is
exergonic
• Fermentation is a partial degradation of
sugars that occurs without O2
• Aerobic respiration consumes organic
molecules and O2 and yields ATP
• Anaerobic respiration is similar to aerobic
respiration but consumes compounds other
than O2
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• Cellular respiration includes both aerobic and
anaerobic respiration but is often used to refer
to aerobic respiration
• Although carbohydrates, fats, and proteins are
all consumed as fuel, it is helpful to trace
cellular respiration with the sugar glucose:
C6H12O6 + 6 O2  6 CO2 + 6 H2O + Energy
(ATP + heat)
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Redox Reactions: Oxidation and Reduction
• The transfer of electrons during chemical
reactions releases energy stored in organic
molecules
• This released energy is ultimately used to
synthesize ATP
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
The Principle of Redox
• Chemical reactions that transfer electrons
between reactants are called oxidation-reduction
reactions, or redox reactions
• In oxidation, a substance loses electrons, or is
oxidized
• In reduction, a substance gains electrons, or is
reduced (the amount of positive charge is
reduced)
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 9-UN1
becomes oxidized
(loses electron)
becomes reduced
(gains electron)
Oxidation of Organic Fuel Molecules During
Cellular Respiration
• During cellular respiration, the fuel (such as
glucose) is oxidized, and O2 is reduced:
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 9-UN3
becomes oxidized
becomes reduced
Stepwise Energy Harvest via NAD+ and the Electron
Transport Chain
• In cellular respiration, glucose and other
organic molecules are broken down in a series
of steps
• Electrons from organic compounds are usually
first transferred to NAD+, a coenzyme
• As an electron acceptor, NAD+ functions as an
oxidizing agent during cellular respiration
• Each NADH (the reduced form of NAD+)
represents stored energy that is tapped to
synthesize ATP
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• NADH passes the electrons to the electron
transport chain
• Unlike an uncontrolled reaction, the electron
transport chain passes electrons in a series of
steps instead of one explosive reaction
• O2 pulls electrons down the chain in an energyyielding tumble
• The energy yielded is used to regenerate ATP
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 9-5
H2 + 1/2 O2
2H
(from food via NADH)
Controlled
release of
+
–
2H + 2e
energy for
synthesis of
ATP
1/
2 O2
Explosive
release of
heat and light
energy
1/
(a) Uncontrolled reaction
(b) Cellular respiration
2 O2
The Stages of Cellular Respiration: A Preview
• Cellular respiration has three stages:
– Glycolysis (breaks down glucose into two
molecules of pyruvate)
– The citric acid cycle or Krebs cycle
(completes the breakdown of glucose)
– Oxidative phosphorylation (accounts for
most of the ATP synthesis)
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 9-6-1
Electrons
carried
via NADH
Glycolysis
Pyruvate
Glucose
Cytosol
ATP
Substrate-level
phosphorylation
Fig. 9-6-2
Electrons carried
via NADH and
FADH2
Electrons
carried
via NADH
Citric
acid
cycle
Glycolysis
Pyruvate
Glucose
Mitochondrion
Cytosol
ATP
ATP
Substrate-level
phosphorylation
Substrate-level
phosphorylation
Fig. 9-6-3
Electrons carried
via NADH and
FADH2
Electrons
carried
via NADH
Citric
acid
cycle
Glycolysis
Pyruvate
Glucose
Oxidative
phosphorylation:
electron transport
and
chemiosmosis
Mitochondrion
Cytosol
ATP
ATP
ATP
Substrate-level
phosphorylation
Substrate-level
phosphorylation
Oxidative
phosphorylation
• The process that generates most of the ATP is
called oxidative phosphorylation because it is
powered by redox reactions
BioFlix: Cellular Respiration
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• Oxidative phosphorylation accounts for almost
90% of the ATP generated by cellular
respiration
• A smaller amount of ATP is formed in glycolysis
and the citric acid cycle by substrate-level
phosphorylation
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 9-7
Enzyme
Enzyme
ADP
P
Substrate
+
Product
ATP
Concept 9.2: Glycolysis harvests chemical energy
by oxidizing glucose to pyruvate
• Glycolysis (“splitting of sugar”) breaks down
glucose into two molecules of pyruvate
• Glycolysis occurs in the cytoplasm and has two
major phases:
– Energy investment phase
– Energy payoff phase
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 9-8
Energy investment phase
Glucose
2 ADP + 2 P
2 ATP
used
4 ATP
formed
Energy payoff phase
4 ADP + 4 P
2 NAD+ + 4 e– + 4 H+
2 NADH + 2 H+
2 Pyruvate + 2 H2O
Net
Glucose
4 ATP formed – 2 ATP used
2 NAD+ + 4 e– + 4 H+
2 Pyruvate + 2 H2O
2 ATP
2 NADH + 2 H+
Concept 9.3: The citric acid cycle completes the
energy-yielding oxidation of organic molecules
• In the presence of O2, pyruvate enters the
mitochondrion
• Before the citric acid cycle can begin, pyruvate
must be converted to acetyl CoA, which links
the cycle to glycolysis
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 9-10
CYTOSOL
MITOCHONDRION
NAD+
NADH + H+
2
1
Pyruvate
Transport protein
3
CO2
Coenzyme A
Acetyl CoA
• The citric acid cycle, also called the Krebs
cycle, takes place within the mitochondrial
matrix (inside the inner membrane)
• The cycle oxidizes organic fuel derived from
pyruvate, generating 1 ATP, 3 NADH, and 1
FADH2 per turn
• Two CO2 molecules are also produced per turn
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• The citric acid cycle has eight steps, each
catalyzed by a specific enzyme
• The acetyl group of acetyl CoA joins the cycle
by combining with oxaloacetate, forming citrate
• The next seven steps decompose the citrate
back to oxaloacetate, making the process a
cycle
• The NADH and FADH2 produced by the cycle
relay electrons extracted from food to the
electron transport chain
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 9-11
Pyruvate
CO2
NAD+
CoA
NADH
+ H+
Acetyl CoA
CoA
CoA
Citric
acid
cycle
FADH2
2 CO2
3 NAD+
3 NADH
FAD
+ 3 H+
ADP + P i
ATP
Concept 9.4: During oxidative phosphorylation,
chemiosmosis couples electron transport to ATP
synthesis
• Following glycolysis and the citric acid cycle,
NADH and FADH2 account for most of the
energy extracted from food
• These two electron carriers donate electrons to
the electron transport chain, which powers ATP
synthesis via oxidative phosphorylation
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
The Pathway of Electron Transport
• The electron transport chain is in the cristae
(inner membrane) of the mitochondrion
• Most of the chain’s components are proteins,
which exist in multiprotein complexes
• The carriers alternate reduced and oxidized
states as they accept and donate electrons
• Electrons drop in free energy as they go down
the chain and are finally passed to O2, forming
H 2O
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 9-13
NADH
50
2 e–
NAD+
FADH2
2 e–
40

FMN
FAD
Multiprotein
complexes
FAD
Fe•S 
Fe•S
Q

Cyt b
30
Fe•S
Cyt c1
I
V
Cyt c
Cyt a
Cyt a3
20
10
2 e–
(from NADH
or FADH2)
0
2 H+ + 1/2 O2
H2O
• Electrons are transferred from NADH or FADH2
to the electron transport chain
• Electrons are passed through a number of
proteins including cytochromes (each with an
iron atom) to O2
• The electron transport chain generates no ATP
• The chain’s function is to break the large freeenergy drop from food to O2 into smaller steps
that release energy in manageable amounts
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Chemiosmosis: The Energy-Coupling Mechanism
• Electron transfer in the electron transport chain
causes proteins to pump H+ from the
mitochondrial matrix to the intermembrane space
• H+ then moves back across the membrane,
passing through channels in ATP synthase
• ATP synthase uses the exergonic flow of H+ to
drive phosphorylation of ATP
• This is an example of chemiosmosis, the use of
energy in a H+ gradient to drive cellular work
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 9-14
INTERMEMBRANE SPACE
H+
Stator
Rotor
Internal
rod
Catalytic
knob
ADP
+
P
i
ATP
MITOCHONDRIAL MATRIX
• The energy stored in a H+ gradient across a
membrane couples the redox reactions of the
electron transport chain to ATP synthesis
• The H+ gradient is referred to as a protonmotive force, emphasizing its capacity to do
work
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 9-16
H+
H+
H+
H+
Protein complex
of electron
carriers
Cyt c
V
Q


ATP
synthase

FADH2
NADH
2 H+ + 1/2O2
H2O
FAD
NAD+
ADP + P i
(carrying electrons
from food)
ATP
H+
1 Electron transport chain
Oxidative phosphorylation
2 Chemiosmosis
An Accounting of ATP Production by Cellular
Respiration
• During cellular respiration, most energy flows in
this sequence:
glucose  NADH  electron transport chain
 proton-motive force  ATP
• About 40% of the energy in a glucose molecule
is transferred to ATP during cellular respiration,
making about 38 ATP
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Concept 9.6: Glycolysis and the citric acid cycle
connect to many other metabolic pathways
• Glycolysis and the citric acid cycle are major
intersections to various catabolic and anabolic
pathways
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
The Versatility of Catabolism
• Catabolic pathways funnel electrons from many
kinds of organic molecules into cellular
respiration
• Glycolysis accepts a wide range of
carbohydrates
• Proteins must be digested to amino acids;
amino groups can feed glycolysis or the citric
acid cycle
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• Fats are digested to glycerol (used in
glycolysis) and fatty acids (used in generating
acetyl CoA)
• Fatty acids are broken down by beta oxidation
and yield acetyl CoA
• An oxidized gram of fat produces more than
twice as much ATP as an oxidized gram of
carbohydrate
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 9-20
Proteins
Carbohydrates
Amino
acids
Sugars
Glycolysis
Glucose
Glyceraldehyde-3- P
NH3
Pyruvate
Acetyl CoA
Citric
acid
cycle
Oxidative
phosphorylation
Fats
Glycerol
Fatty
acids
Biosynthesis (Anabolic Pathways)
• The body uses small molecules to build other
substances
• These small molecules may come directly from
food, from glycolysis, or from the citric acid
cycle
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Regulation of Cellular Respiration via Feedback
Mechanisms
• Feedback inhibition is the most common
mechanism for control
• If ATP concentration begins to drop, respiration
speeds up; when there is plenty of ATP,
respiration slows down
• Control of catabolism is based mainly on
regulating the activity of enzymes at strategic
points in the catabolic pathway
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Concept 9.5: Fermentation and anaerobic
respiration enable cells to produce ATP without
the use of oxygen
• Most cellular respiration requires O2 to produce
ATP
• Glycolysis can produce ATP with or without O2
(in aerobic or anaerobic conditions)
• In the absence of O2, glycolysis couples with
fermentation or anaerobic respiration to
produce ATP
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• Anaerobic respiration uses an electron
transport chain with an electron acceptor other
than O2, for example sulfate
• Fermentation uses phosphorylation instead of
an electron transport chain to generate ATP
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Types of Fermentation
• Fermentation consists of glycolysis plus
reactions that regenerate NAD+, which can be
reused by glycolysis
• Two common types are alcohol fermentation
and lactic acid fermentation
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• In alcohol fermentation, pyruvate is
converted to ethanol in two steps, with the first
releasing CO2
• Alcohol fermentation by yeast is used in
brewing, winemaking, and baking
Animation: Fermentation Overview
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 9-18a
2 ADP + 2 P i
Glucose
2 ATP
Glycolysis
2 Pyruvate
2 NAD+
2 Ethanol
(a) Alcohol fermentation
2 NADH
+ 2 H+
2 CO2
2 Acetaldehyde
• In lactic acid fermentation, pyruvate is
reduced by NADH, forming lactate as an end
product, with no release of CO2
• Lactic acid fermentation by some fungi and
bacteria is used to make cheese and yogurt
• Human muscle cells use lactic acid
fermentation to generate ATP when O2 is
scarce
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 9-18b
2 ADP + 2 P i
Glucose
2 ATP
Glycolysis
2 NAD+
2 NADH
+ 2 H+
2 Pyruvate
2 Lactate
(b) Lactic acid fermentation
Fermentation and Aerobic Respiration Compared
• Both processes use glycolysis to oxidize
glucose and other organic fuels to pyruvate
• The processes have different final electron
acceptors: an organic molecule (such as
pyruvate or acetaldehyde) in fermentation and
O2 in cellular respiration
• Cellular respiration produces 36 (net) ATP per
glucose molecule; fermentation produces 2
(net) ATP per glucose molecule
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• Obligate anaerobes carry out fermentation or
anaerobic respiration and cannot survive in the
presence of O2
• Yeast and many bacteria are facultative
anaerobes, meaning that they can survive
using either fermentation or cellular respiration
• In a facultative anaerobe, pyruvate is a fork in
the metabolic road that leads to two alternative
catabolic routes
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 9-19
Glucose
CYTOSOL
Glycolysis
Pyruvate
No O2 present:
Fermentation
O2 present:
Aerobic cellular
respiration
MITOCHONDRION
Ethanol
or
lactate
Acetyl CoA
Citric
acid
cycle
The Evolutionary Significance of Glycolysis
• Glycolysis occurs in nearly all organisms
• Glycolysis probably evolved in ancient
prokaryotes before there was oxygen in the
atmosphere
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Chapter 10
Photosynthesis
PowerPoint® Lecture Presentations for
Biology
Eighth Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Overview: The Process That Feeds the Biosphere
• Photosynthesis is the process that converts
solar energy into chemical energy
• Directly or indirectly, photosynthesis nourishes
almost the entire living world
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• Autotrophs sustain themselves without eating
anything derived from other organisms
• Autotrophs are the producers of the biosphere,
producing organic molecules from CO2 and
other inorganic molecules
• Almost all plants are photoautotrophs, using
the energy of sunlight to make organic
molecules from H2O and CO2
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Concept 10.1: Photosynthesis converts light energy
to the chemical energy of food
• Chloroplasts are structurally similar to and
likely evolved from photosynthetic bacteria
• The structural organization of these cells allows
for the chemical reactions of photosynthesis
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Chloroplasts: The Sites of Photosynthesis in Plants
• Leaves are the major locations of
photosynthesis
• Their green color is from chlorophyll, the
green pigment within chloroplasts
• Light energy absorbed by chlorophyll drives the
synthesis of organic molecules in the
chloroplast
• CO2 enters and O2 exits the leaf through
microscopic pores called stomata
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• Chloroplasts are found mainly in cells of the
mesophyll, the interior tissue of the leaf
• A typical mesophyll cell has 30–40 chloroplasts
• The chlorophyll is in the membranes of
thylakoids (connected sacs in the chloroplast);
thylakoids may be stacked in columns called
grana
• Chloroplasts also contain stroma, a dense fluid
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 10-3b
Chloroplast
Outer
membrane
Thylakoid
Stroma
Granum
Thylakoid
space
Intermembrane
space
Inner
membrane
1 µm
The Two Stages of Photosynthesis: A Preview
• Photosynthesis consists of the light reactions
(the photo part) and Calvin cycle (the synthesis
part)
• The light reactions (in the thylakoids):
– Split H2O
– Release O2
– Reduce NADP+ to NADPH
– Generate ATP from ADP by
photophosphorylation
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• The Calvin cycle (in the stroma) forms sugar
from CO2, using ATP and NADPH
• The Calvin cycle begins with carbon fixation,
incorporating CO2 into organic molecules
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Concept 10.2: The light reactions convert solar
energy to the chemical energy of ATP and NADPH
• Chloroplasts are solar-powered chemical
factories
• Their thylakoids transform light energy into the
chemical energy of ATP and NADPH
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
A Photosystem: A Reaction-Center Complex
Associated with Light-Harvesting Complexes
• A photosystem consists of a reaction-center
complex (a type of protein complex)
surrounded by light-harvesting complexes
• The light-harvesting complexes (pigment
molecules bound to proteins) funnel the energy
of photons to the reaction center
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• A primary electron acceptor in the reaction
center accepts an excited electron from
chlorophyll a
• Solar-powered transfer of an electron from a
chlorophyll a molecule to the primary electron
acceptor is the first step of the light reactions
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 10-12
Photosystem
STROMA
Light-harvesting Reaction-center
complex
complexes
Primary
electron
acceptor
Thylakoid membrane
Photon
e–
Transfer
of energy
Special pair of
chlorophyll a
molecules
Pigment
molecules
THYLAKOID SPACE
(INTERIOR OF THYLAKOID)
• There are two types of photosystems in the
thylakoid membrane
• Photosystem II (PS II) functions first (the
numbers reflect order of discovery) and is best at
absorbing a wavelength of 680 nm
• The reaction-center chlorophyll a of PS II is
called P680
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• Photosystem I (PS I) is best at absorbing a
wavelength of 700 nm
• The reaction-center chlorophyll a of PS I is
called P700
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Linear Electron Flow
• During the light reactions, there are two
possible routes for electron flow: cyclic and
linear
• Linear electron flow, the primary pathway,
involves both photosystems and produces ATP
and NADPH using light energy
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• A photon hits a pigment and its energy is
passed among pigment molecules until it
excites P680
• An excited electron from P680 is transferred to
the primary electron acceptor
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 10-13-1
Primary
acceptor
e–
2
P680
1 Light
Pigment
molecules
Photosystem II
(PS II)
• P680+ (P680 that is missing an electron) is a
very strong oxidizing agent
• H2O is split by enzymes, and the electrons are
transferred from the hydrogen atoms to P680+,
thus reducing it to P680
• O2 is released as a by-product of this reaction
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 10-13-2
Primary
acceptor
2 H+
+
1/ O
2
2
H2O
e–
2
3
e–
e–
P680
1 Light
Pigment
molecules
Photosystem II
(PS II)
• Each electron “falls” down an electron transport
chain from the primary electron acceptor of PS
II to PS I
• Energy released by the fall drives the creation
of a proton gradient across the thylakoid
membrane
• Diffusion of H+ (protons) across the membrane
drives ATP synthesis
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 10-13-3
4
Primary
acceptor
1/
2
H+
2
+
O2
H2O
e–
2
Pq
Cytochrome
complex
3
Pc
e–
e–
5
P680
1 Light
ATP
Pigment
molecules
Photosystem II
(PS II)
• In PS I (like PS II), transferred light energy
excites P700, which loses an electron to an
electron acceptor
• P700+ (P700 that is missing an electron)
accepts an electron passed down from PS II
via the electron transport chain
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 10-13-4
4
Primary
acceptor
1/
2
H+
2
+
O2
H2O
e–
2
Primary
acceptor
e–
Pq
Cytochrome
complex
3
Pc
e–
e–
P700
5
P680
Light
1 Light
6
ATP
Pigment
molecules
Photosystem II
(PS II)
Photosystem I
(PS I)
• Each electron “falls” down an electron transport
chain from the primary electron acceptor of PS
I to the protein ferredoxin (Fd)
• The electrons are then transferred to NADP+
and reduce it to NADPH
• The electrons of NADPH are available for the
reactions of the Calvin cycle
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 10-13-5
4
Primary
acceptor
2
H+
+
1/ O
2
2
H2O
e–
2
Primary
acceptor
e–
Pq
Cytochrome
complex
7
Fd
e–
e–
8
NADP+
reductase
3
NADPH
Pc
e–
e–
P700
5
P680
Light
1 Light
6
ATP
Pigment
molecules
Photosystem II
(PS II)
NADP+
+ H+
Photosystem I
(PS I)
A Comparison of Chemiosmosis in Chloroplasts
and Mitochondria
• Chloroplasts and mitochondria generate ATP
by chemiosmosis, but use different sources of
energy
• Mitochondria transfer chemical energy from
food to ATP; chloroplasts transform light energy
into the chemical energy of ATP
• Spatial organization of chemiosmosis differs
between chloroplasts and mitochondria but
also shows similarities
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• In mitochondria, protons are pumped to the
intermembrane space and drive ATP synthesis
as they diffuse back into the mitochondrial
matrix
• In chloroplasts, protons are pumped into the
thylakoid space and drive ATP synthesis as
they diffuse back into the stroma
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 10-16
Mitochondrion
Chloroplast
MITOCHONDRION
STRUCTURE
CHLOROPLAST
STRUCTURE
H+
Intermembrane
space
Inner
membrane
Diffusion
Electron
transport
chain
Thylakoid
space
Thylakoid
membrane
ATP
synthase
Stroma
Matrix
Key
ADP + P i
[H+]
Higher
Lower [H+]
H+
ATP
• ATP and NADPH are produced on the side
facing the stroma, where the Calvin cycle takes
place
• In summary, light reactions generate ATP and
increase the potential energy of electrons by
moving them from H2O to NADPH
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 10-17
STROMA
(low H+ concentration)
Cytochrome
Photosystem I
complex
Light
Photosystem II
4 H+
Light
Fd
NADP+
reductase
NADP+ + H+
NADPH
Pq
H2O
THYLAKOID SPACE
(high H+ concentration)
e–
1
e–
1/
Pc
2
2
3
O2
+2 H+
4 H+
To
Calvin
Cycle
Thylakoid
membrane
STROMA
(low H+ concentration)
ATP
synthase
ADP
+
Pi
ATP
H+
Concept 10.3: The Calvin cycle uses ATP and
NADPH to convert CO2 to sugar
• The Calvin cycle, like the citric acid cycle,
regenerates its starting material after
molecules enter and leave the cycle
• The cycle builds sugar from smaller molecules
by using ATP and the reducing power of
electrons carried by NADPH
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• Carbon enters the cycle as CO2 and leaves as
a sugar named glyceraldehyde-3-phospate
(G3P)
• For net synthesis of 1 G3P, the cycle must take
place three times, fixing 3 molecules of CO2
• The Calvin cycle has three phases:
– Carbon fixation (catalyzed by rubisco)
– Reduction
– Regeneration of the CO2 acceptor (RuBP)
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 10-18-3
Input 3
(Entering one
at a time)
CO2
Phase 1: Carbon fixation
Rubisco
3 P
Short-lived
intermediate
P
6
P
3-Phosphoglycerate
3P
P
Ribulose bisphosphate
(RuBP)
6
ATP
6 ADP
3 ADP
3
Calvin
Cycle
6 P
P
1,3-Bisphosphoglycerate
ATP
6 NADPH
Phase 3:
Regeneration of
the CO2 acceptor
(RuBP)
6 NADP+
6 Pi
P
5
G3P
6
P
Glyceraldehyde-3-phosphate
(G3P)
1
Output
P
G3P
(a sugar)
Glucose and
other organic
compounds
Phase 2:
Reduction
Fig. 10-21
H2O
CO2
Light
NADP+
ADP
+ P
i
Light
Reactions:
Photosystem II
Electron transport chain
Photosystem I
Electron transport chain
RuBP
ATP
NADPH
3-Phosphoglycerate
Calvin
Cycle
G3P
Starch
(storage)
Chloroplast
O2
Sucrose (export)