cellular respiration and...
TRANSCRIPT
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LECTURE PRESENTATIONS
For CAMPBELL BIOLOGY, NINTH EDITION Jane B. Reece, Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, Robert B. Jackson
© 2011 Pearson Education, Inc.
Lectures by
Erin Barley
Kathleen Fitzpatrick
Cellular Respiration and
Fermentation
Chapter 9
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Overview: Life Is Work
• Living cells require energy from outside
sources
• Some animals, such as the chimpanzee, obtain
energy by eating plants, and some animals
feed on other organisms that eat plants
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• 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
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Figure 9.2
Light energy
ECOSYSTEM
Photosynthesis in chloroplasts
Cellular respiration in mitochondria
CO2 H2O O2 Organic
molecules
ATP powers most cellular work
ATP
Heat energy
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Concept 9.1: Catabolic pathways yield
energy by oxidizing organic fuels
• Several processes are central to cellular
respiration and related pathways
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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
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• 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)
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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
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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)
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Oxidation of Organic Fuel Molecules During
Cellular Respiration
• During cellular respiration, the fuel (such as
glucose) is oxidized, and O2 is reduced
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Figure 9.UN03
becomes oxidized
becomes reduced
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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
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Figure 9.4
Nicotinamide (oxidized form)
NAD
(from food)
Dehydrogenase
Reduction of NAD
Oxidation of NADH
Nicotinamide (reduced form)
NADH
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Figure 9.UN04
Dehydrogenase
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• 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 energy-
yielding tumble
• The energy yielded is used to regenerate ATP
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Figure 9.5
(a) Uncontrolled reaction (b) Cellular respiration
Explosive release of
heat and light energy
Controlled release of energy for
synthesis of ATP
Fre
e e
ne
rgy,
G
Fre
e e
ne
rgy,
G
H2 1/2 O2 2 H 1/2 O2
1/2 O2
H2O H2O
2 H+ 2 e
2 e
2 H+
ATP
ATP
ATP
(from food via NADH)
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The Stages of Cellular Respiration:
A Preview
• Harvesting of energy from glucose has three
stages
– Glycolysis (breaks down glucose into two
molecules of pyruvate)
– The citric acid cycle (completes the
breakdown of glucose)
– Oxidative phosphorylation (accounts for
most of the ATP synthesis)
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Figure 9.UN05
Glycolysis (color-coded teal throughout the chapter) 1.
Pyruvate oxidation and the citric acid cycle
(color-coded salmon)
2.
Oxidative phosphorylation: electron transport and
chemiosmosis (color-coded violet)
3.
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Figure 9.6-3
Electrons
carried
via NADH
Electrons carried
via NADH and
FADH2
Citric
acid
cycle
Pyruvate
oxidation
Acetyl CoA
Glycolysis
Glucose Pyruvate
Oxidative
phosphorylation:
electron transport
and
chemiosmosis
CYTOSOL MITOCHONDRION
ATP ATP ATP
Substrate-level
phosphorylation Substrate-level
phosphorylation
Oxidative
phosphorylation
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• The process that generates most of the ATP is
called oxidative phosphorylation because it is
powered by redox reactions
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• 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
• For each molecule of glucose degraded to CO2
and water by respiration, the cell makes up to
32 molecules of ATP
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Figure 9.7
Substrate
Product
ADP
P
ATP
Enzyme Enzyme
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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
• Glycolysis occurs whether or not O2 is present
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Figure 9.8
Energy Investment Phase
Glucose
2 ADP 2 P
4 ADP 4 P
Energy Payoff Phase
2 NAD+ 4 e 4 H+
2 Pyruvate 2 H2O
2 ATP used
4 ATP formed
2 NADH 2 H+
Net Glucose 2 Pyruvate 2 H2O
2 ATP
2 NADH 2 H+ 2 NAD+ 4 e 4 H+
4 ATP formed 2 ATP used
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Figure 9.9-4
Glycolysis: Energy Investment Phase
ATP ATP Glucose Glucose 6-phosphate Fructose 6-phosphate Fructose 1,6-bisphosphate
Dihydroxyacetone phosphate
Glyceraldehyde 3-phosphate
To step 6
ADP ADP
Hexokinase Phosphogluco-
isomerase Phospho-
fructokinase
Aldolase
Isomerase
1 2 3 4
5
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Figure 9.9-9
Glycolysis: Energy Payoff Phase
2 ATP 2 ATP
2 NADH
2 NAD + 2 H
2 P i
2 ADP
1,3-Bisphospho- glycerate
3-Phospho- glycerate
2-Phospho- glycerate
Phosphoenol- pyruvate (PEP)
Pyruvate
2 ADP 2 2 2
2 H2O
Phospho-
glycerokinase
Phospho-
glyceromutase
Enolase Pyruvate
kinase
6 7 8
9 10
Triose
phosphate
dehydrogenase
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Concept 9.3: After pyruvate is oxidized, the
citric acid cycle completes the energy-
yielding oxidation of organic molecules
• In the presence of O2, pyruvate enters the
mitochondrion (in eukaryotic cells) where the
oxidation of glucose is completed
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Oxidation of Pyruvate to Acetyl CoA
• Before the citric acid cycle can begin, pyruvate
must be converted to acetyl Coenzyme A
(acetyl CoA), which links glycolysis to the citric
acid cycle
• This step is carried out by a multienzyme
complex that catalyses three reactions
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Figure 9.10
Pyruvate
Transport protein
CYTOSOL
MITOCHONDRION
CO2 Coenzyme A
NAD + H NADH Acetyl CoA
1
2
3
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• The citric acid cycle, also called the Krebs
cycle, completes the break down of pyruvate
to CO2
• The cycle oxidizes organic fuel derived from
pyruvate, generating 1 ATP, 3 NADH, and 1
FADH2 per turn
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The Citric Acid Cycle
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Figure 9.11
Pyruvate
NAD
NADH
+ H Acetyl CoA
CO2
CoA
CoA
CoA
2 CO2
ADP + P i
FADH2
FAD
ATP
3 NADH
3 NAD
Citric
acid
cycle
+ 3 H
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• 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
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Figure 9.12-8
NADH
1
Acetyl CoA
Citrate Isocitrate
-Ketoglutarate
Succinyl
CoA
Succinate
Fumarate
Malate
Citric
acid
cycle
NAD
NADH
NADH
FADH2
ATP
+ H
+ H
+ H
NAD
NAD
H2O
H2O
ADP
GTP GDP
P i
FAD
3
2
4
5
6
7
8
CoA-SH
CO2
CoA-SH
CoA-SH
CO2
Oxaloacetate
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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
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The Pathway of Electron Transport
• The electron transport chain is in the inner
membrane (cristae) 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
H2O
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Figure 9.13
NADH
FADH2
2 H + 1/2 O2
2 e
2 e
2 e
H2O
NAD
Multiprotein
complexes
(originally from
NADH or FADH2)
I II
III
IV
50
40
30
20
10
0
Fre
e e
nerg
y (
G)
rela
tive t
o O
2 (
kca
l/m
ol)
FMN
FeS FeS
FAD
Q
Cyt b
Cyt c1
Cyt c
Cyt a
Cyt a3
FeS
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• 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
directly
• It breaks the large free-energy drop from food
to O2 into smaller steps that release energy in
manageable amounts
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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 the proton, 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
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Figure 9.14
INTERMEMBRANE SPACE
Rotor
Stator H
Internal
rod
Catalytic
knob
ADP
+
P i ATP
MITOCHONDRIAL MATRIX
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Figure 9.15
Protein complex of electron carriers
(carrying electrons from food)
Electron transport chain
Oxidative phosphorylation
Chemiosmosis
ATP synth- ase
I
II
III
IV Q
Cyt c
FAD FADH2
NADH ADP P i NAD
H
2 H + 1/2O2
H
H H
2 1
H
H2O
ATP
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• 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 proton-
motive force, emphasizing its capacity to do
work
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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 34% of the energy in a glucose molecule
is transferred to ATP during cellular respiration,
making about 32 ATP
• There are several reasons why the number of
ATP is not known exactly
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Figure 9.16
Electron shuttles span membrane
MITOCHONDRION 2 NADH
2 NADH 2 NADH 6 NADH
2 FADH2
2 FADH2
or
2 ATP 2 ATP about 26 or 28 ATP
Glycolysis
Glucose 2 Pyruvate
Pyruvate oxidation
2 Acetyl CoA
Citric acid cycle
Oxidative phosphorylation: electron transport
and chemiosmosis
CYTOSOL
Maximum per glucose: About
30 or 32 ATP
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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
• Without O2, the electron transport chain will
cease to operate
• In that case, glycolysis couples with
fermentation or anaerobic respiration to
produce ATP
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• Anaerobic respiration uses an electron
transport chain with a final electron acceptor
other than O2, for example sulfate
• Fermentation uses substrate-level
phosphorylation instead of an electron
transport chain to generate ATP
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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
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• 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
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Animation: Fermentation Overview Right-click slide / select “Play”
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Figure 9.17
2 ADP 2 ATP
Glucose Glycolysis
2 Pyruvate
2 CO2 2
2 NADH
2 Ethanol 2 Acetaldehyde
(a) Alcohol fermentation (b) Lactic acid fermentation
2 Lactate
2 Pyruvate
2 NADH
Glucose Glycolysis
2 ATP 2 ADP 2 P i
NAD
2 H
2 P i
2 NAD 2 H
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• In lactic acid fermentation, pyruvate is reduced
to 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
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Comparing Fermentation with Anaerobic
and Aerobic Respiration
• All use glycolysis (net ATP = 2) to oxidize glucose and harvest chemical energy of food
• In all three, NAD+ is the oxidizing agent that accepts electrons during glycolysis
• 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 32 ATP per glucose molecule; fermentation produces 2 ATP per glucose molecule
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• 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
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Figure 9.18
Glucose
CYTOSOL Glycolysis
Pyruvate
No O2 present:
Fermentation
O2 present:
Aerobic cellular
respiration
Ethanol,
lactate, or
other products
Acetyl CoA
MITOCHONDRION
Citric
acid
cycle
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The Evolutionary Significance of Glycolysis
• Ancient prokaryotes are thought to have used
glycolysis long before there was oxygen in the
atmosphere
• Very little O2 was available in the atmosphere
until about 2.7 billion years ago, so early
prokaryotes likely used only glycolysis to
generate ATP
• Glycolysis is a very ancient process
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Concept 9.6: Glycolysis and the citric acid
cycle connect to many other metabolic
pathways
• Gycolysis and the citric acid cycle are major
intersections to various catabolic and anabolic
pathways
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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
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• 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
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Figure 9.19
Carbohydrates Proteins
Fatty
acids
Amino
acids
Sugars
Fats
Glycerol
Glycolysis
Glucose
Glyceraldehyde 3- P
NH3 Pyruvate
Acetyl CoA
Citric
acid
cycle
Oxidative
phosphorylation
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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
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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
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Figure 9.20
Phosphofructokinase
Glucose
Glycolysis AMP
Stimulates
Fructose 6-phosphate
Fructose 1,6-bisphosphate
Pyruvate
Inhibits Inhibits
ATP Citrate
Citric
acid
cycle
Oxidative
phosphorylation
Acetyl CoA
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Figure 9.UN06
Inputs Outputs
Glucose
Glycolysis
2 Pyruvate 2 ATP 2 NADH
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Figure 9.UN07
Inputs Outputs
2 Pyruvate 2 Acetyl CoA
2 Oxaloacetate Citric
acid
cycle
2
2 6
8 ATP NADH
FADH2 CO2