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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: Life Is Work
• Living cells require energy from outside
sources.
• Some animals, such as the giant panda, 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 C-H-O, which are used in cellular
respiration.
• Cells use chemical energy stored in organic
molecules C-H-O to regenerate ATP, which
powers work.
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Energy Flow
and
IN: Light Energy
Chemical
Recycling
ECOSYSTEM
in
Ecosystems
Photosynthesis
in chloroplasts
CO2 + H2O
Organic + O2
Molecules
Cellular respiration
in mitochondria
ATP
ATP powers most cellular work
OUT:
Heat energy
Catabolic pathways yield energy by oxidizing
organic fuels: C-H-O molecules to Produce 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 without 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
• Oxidation is a LOSS
(of H or electrons).
• Reduction is a GAIN
(of H or electrons).
• 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 / Reduction Reactions
becomes oxidized
(loses electron)
becomes reduced
(gains electron)
Oxidation / Reduction Reactions
becomes oxidized
becomes reduced
• The electron donor is oxidized and is called
the reducing agent
• The electron receptor is reduced and is called
the oxidizing agent
• Some redox reactions do not transfer
electrons but change the electron sharing in
covalent bonds.
• An example is the reaction between methane
and O2
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Methane combustion as an energy-yielding redox reaction
Reactants
Products
becomes oxidized
becomes reduced
Methane
(reducing
agent)
Oxygen
(oxidizing
agent)
Carbon dioxide
Water
Oxidation of Organic Fuel Molecules During
Cellular Respiration
During cellular respiration:
• The fuel C-H-O (such as glucose) is oxidized,
looses H’s
• and O2 is reduced, gains H’s
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During Cellular Respiration During
cellular respiration, the
fuel (such
becomes oxidized
becomes reduced
se) is oxidized, and O2 i
NAD+ is Reduced to NADH + H+
Dehydrogenase
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.
• NADH = the reduced form of NAD+ . Each
NADH represents stored energy that is tapped
to synthesize ATP.
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NAD+ as an electron shuttle
2 e– + 2 H+
2 e– + H+
NADH
H+
Dehydrogenase
Reduction of NAD+
NAD+
+
+ H+
2[H]
Oxidation of NADH
Nicotinamide
(reduced form)
Nicotinamide
(oxidized form)
• NADH passes the electrons to the electron
transport chain ETC.
• 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 ETC chain in an
energy-yielding tumble.
• The energy yielded is used to regenerate ATP.
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An introduction to electron transport chains
H2 + 1/2 O2
2H
(from food C-H-O 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: step-wise
controlled reaction
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: Krebs Cycle
(completes the breakdown of glucose)
– Oxidative phosphorylation (accounts for
most of the ATP synthesis) by chemiosmosis.
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An overview of cellular respiration - Glycolysis
Electrons
carried
via NADH
Glycolysis
Pyruvate
Glucose
Cytosol
ATP
Substrate-level
phosphorylation
An overview of cellular respiration: Glycolysis & Krebs Cycle
Electrons carried
via NADH and
FADH2
Electrons
carried
via NADH
Glycolysis
Pyruvate
Glucose
Krebs
Cycle
Mitochondrion
Cytosol
ATP
ATP
Substrate-level
phosphorylation
Substrate-level
phosphorylation
An overview of Cellular Respiration Basics
Electrons carried
via NADH and
FADH2
Electrons
carried
via NADH
Krebs
Cycle
Glycolysis
Pyruvate
Glucose
Oxidative
phosphorylation:
electron transport
and
chemiosmosis
Mitochondrion
Cytosol
ATP
ATP
Substrate-level
phosphorylation
Substrate-level
phosphorylation
ATP
Oxidative
phosphorylation
• The process in cell respiration that generates
most of the ATP is oxidative phosphorylation
(powered by redox reactions).
• 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 / Krebs Cycle by substratelevel phosphorylation.
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Substrate-level phosphorylation
Enzyme
Enzyme
ADP
P
Substrate
+
Product
ATP
Glycolysis harvests chemical energy by oxidizing
glucose to pyruvate
• Glycolysis (“sugar splitting”) breaks down
glucose into two molecules of pyruvate.
• Glycolysis occurs in the cytoplasm and has two
major phases:
– Energy investment phase = EA
– Energy payoff phase = ATP and NADH
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The
energy
input
and
output
of
glycolysis
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+
A
closer
look
at
glycolysis
Glucose
ATP
1
Hexokinase
ADP
Glucose-6-phosphate
2
Phosphoglucoisomerase
Fructose-6-phosphate
Glucose-6-phosphate
2
Phosphoglucoisomerase
Fructose-6-phosphate
Glucose
ATP
1
Hexokinase
ADP
Fructose-6-phosphate
Glucose-6-phosphate
2
Phosphoglucoisomerase
ATP
3
Phosphofructokinase:
Fructose-6-phosphate
allosteric enzyme
ATP
3
Phosphofructokinase
ADP
ADP
Fructose1, 6-bisphosphate
Fructose1, 6-bisphosphate
Glucose
ATP
1
Hexokinase
ADP
Glucose-6-phosphate
2
Phosphoglucoisomerase
Fructose1, 6-bisphosphate
4
Fructose-6-phosphate
ATP
Aldolase
3
Phosphofructokinase
ADP
5
Isomerase
Fructose1, 6-bisphosphate
4
Aldolase
5
Isomerase
Dihydroxyacetone
phosphate
Dihydroxyacetone
phosphate
Glyceraldehyde3-phosphate
Glyceraldehyde3-phosphate
2 NAD+
2 NADH
+ 2 H+
6
Triose phosphate
dehydrogenase
2 Pi
2 1, 3-Bisphosphoglycerate
2 ADP
7
Phosphoglycerokinase
2 ATP
2 1, 3-Bisphosphoglycerate
2 ADP
2
3-Phosphoglycerate
2 ATP
2
7
Phosphoglycerokinase
3-Phosphoglycerate
Fig. 9-9-7
2 NAD+
2 NADH
+ 2 H+
6
Triose phosphate
dehydrogenase
2 Pi
2 1, 3-Bisphosphoglycerate
2 ADP
7 Phosphoglycerokinase
2 ATP
2
3-Phosphoglycerate
8
2
3-Phosphoglycerate
Phosphoglyceromutase
2
8
Phosphoglyceromutase
2-Phosphoglycerate
2
2-Phosphoglycerate
2 NAD+
2 NADH
+ 2 H+
6
Triose phosphate
dehydrogenase
2 Pi
2 1, 3-Bisphosphoglycerate
2 ADP
7 Phosphoglycerokinase
2 ATP
2
3-Phosphoglycerate
2
2-Phosphoglycerate
8
Phosphoglyceromutase
9
2
2 H2O
2-Phosphoglycerate
Enolase
9
Enolase
2 H2O
2
Phosphoenolpyruvate
2
Phosphoenolpyruvate
2 NAD+
6
Triose phosphate
dehydrogenase
2 Pi
2 NADH
+ 2 H+
2 1, 3-Bisphosphoglycerate
2 ADP
7 Phosphoglycerokinase
2 ATP
2
Phosphoenolpyruvate
2 ADP
2
3-Phosphoglycerate
8
Phosphoglyceromutase
2 ATP
2
10
Pyruvate
kinase
2-Phosphoglycerate
9
2 H2O
Enolase
2 Phosphoenolpyruvate
2 ADP
10
Pyruvate kinase
2 ATP
2
2
Pyruvate
Pyruvate
The Citric Acid Cycle = Krebs Cycle: completes the
energy-yielding oxidation of organic molecules
• If O2 is present, pyruvates (3 carbon C-H-O)
enter the mitochondria.
• Before the Krebs Cycle can begin, the pyruvate
must be converted to acetyl CoA
• Acetyl CoA = Acetate + CoEnzyme A
Acetate = a 2 carbon C-H-O
CoEnzyme A = a carrier molecule
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Conversion of pyruvate to acetyl CoA, the junction between
glycolysis and the citric acid cycle
CYTOSOL
MITOCHONDRION
NAD+
NADH + H+
2
1
Pyruvate
Transport protein
3
CO2
Coenzyme A
Acetyl CoA
• The citric acid cycle / Krebs cycle takes place
within the mitochondrial matrix.
• The Krebs cycle oxidizes organic fuel derived
from pyruvate, generating 1 ATP, 3 NADH, and
1 FADH2 per turn (two turns per glucose
molecule from glycolysis).
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citric acid /
Krebs
Cycle
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
• The citric acid / Krebs cycle has eight steps, each
catalyzed by a specific enzyme.
• The acetyl group of acetyl CoA joins the cycle by
combining with oxaloacetate, OAA, forming citrate
(citric acid).
• The next seven steps break down the citrate and
regenerate oxaloacetate,OAA, making the process a
cycle.
• The NADH and FADH2 produced by the Krebs Cycle
carry electrons extracted from food (C-H-O) to the
electron transport chain in the mitochondrial cristae
membrane.
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A closer
look at
the
citric acid /
Krebs cycle
Acetyl CoA
CoA—SH
NADH +
+H
H2O
1
NAD+
8
Oxaloacetate
OAA
2
Malate
Citrate
Isocitrate
NAD+
Citric
Acid
Cycle
7
H2O
Fumarate
6
3
CO2
CoA—SH
4
CoA—SH
5
FADH2
NAD+
FAD
Succinate
GTP GDP
ADP
ATP
NADH
+ H+
Pi
Succinyl
CoA
NADH
+ H+
-Ketoglutarate
CO2
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: NADH and FADH2
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 cristae
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,
redox.
• Electrons drop in free energy as they go down
the chain and are finally passed to O2, forming
H2O (waste).
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ETC cristae
Free-energy change
during electron transport
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
waste
• Electrons are transferred from NADH or FADH2
to the electron transport chain, ETC.
• Electrons are passed along the cristae
membrane 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.
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Chemiosmosis The Energy-Coupling Mechanism
• Electron transfer, redox, in the electron transport
chain causes proteins to pump H+ from the
mitochondrial matrix to the intermembrane space
 creating a proton H+ gradient.
• 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 ATP synthesis.
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Chemiosomosis
• The energy stored in a H+ (proton) gradient,
across a membrane couples the redox
reactions of the electron transport chain to ATP
synthesis.
• The H+ gradient is a proton-motive force,
emphasizing its capacity to do work.
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Chemiosmosis: Energy Coupling - couples the electron transport chain to ATP synthesis
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: redox
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.
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Aerobic Cellular Respiration
ATP yield per molecule of glucose at each stage of cellular
respiration
Electron shuttles
span membrane
CYTOSOL
2 NADH
Glycolysis
Glucose
2
Pyruvate
MITOCHONDRION
2 NADH
or
2 FADH2
6 NADH
2 NADH
2
Acetyl
CoA
+ 2 ATP
Citric
Acid
Cycle
+ 2 ATP
Maximum per glucose:
About
36 or 38 ATP
2 FADH2
Oxidative
phosphorylation:
electron transport
chemiosmosis
+ about 32 or 34 ATP
Fermentation and anaerobic respiration enable cells
to produce ATP without the use of oxygen
• Most cellular respiration requires O2 to produce
ATP.
• Glycolysis produces ATP without O2 (in aerobic
or anaerobic conditions).
• In the absence of O2, glycolysis couples with
fermentation or anaerobic respiration to
produce ATP.
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Fermentation: Anaerobic / No Oxygen Used
• 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 / $$
commercial uses.
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Fermentation
Regenerates NAD+
for use in Glycolysis
2 ADP + 2 Pi
Glucose
2 ATP
Glycolysis
2 Pyruvate
2 NAD+
2 NADH
+ 2 H+
2 Acetaldehyde
2 Ethanol
(a) Alcohol
2 CO2
fermentation
2 ADP + 2 Pi
Glucose
2 ATP
Glycolysis
2 NAD+
2 NADH
+ 2 H+
2 Pyruvate
2 Lactate
(b) Lactic
acid fermentation
• In lactic acid fermentation, pyruvate is
reduced to NADH, forming lactate as an end
product, with no release of CO2
• Lactic acid fermentation by some fungi and
bacteria is used to make cheese and yogurt $$
• Human muscle cells use lactic acid
fermentation to generate ATP when O2 is
scarce; meaning there is an O2 debt. This
reaction is reversible when O2 is available.
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2 ADP + 2 P i
Glucose
2 ATP
Glycolysis
2 NAD+
2 NADH
+ 2 H+
2 Pyruvate
2 Lactate
Lactic acid fermentation: reversible if oxygen is available
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.
 O2 in aerobic cellular respiration.
• Aerobic cellular respiration nets 38 ATP per
glucose molecule; fermentation nets only 2
ATP per glucose molecule.
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Micro-organisms
• Obligate anaerobes carry out fermentation or
anaerobic respiration and cannot survive in the
presence of O2
• Facultative anaerobes (yeast and many
bacteria) 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.
• Obligate aerobes carry out aerobic cellular
respiration and require O2
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Pyruvate as a key
juncture in catabolism
CYTOSOL
Glucose
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.
• Fermentation evolved to recycle NAD+ back to
glycolysis so ATP production continues in the
absence of O2
• Gycolysis and the Citric Acid Cycle are major
intersections to various catabolic and anabolic
pathways.
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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: breaking down
• 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
which 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 (fat has more calories = unit of
energy for cell work).
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Versatility
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: Building
• The body uses small molecules to build larger
more complex molecules.
• 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, regulation.
• 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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Regulation: Feedback Inhibition
Glucose
The control of cellular respiration
AMP
Glycolysis
Fructose-6-phosphate
–
Stimulates
+
Phosphofructokinase
–
Fructose-1,6-bisphosphate
Inhibits
Inhibits
Pyruvate
ATP
Citrate
Acetyl CoA
Citric
acid
cycle
Oxidative
phosphorylation
Review Glycolysis:
Outputs
Inputs
2
ATP
Glycolysis
+
2 NADH
Glucose
2
Pyruvate
Review: Citric Acid / Krebs Cycle
Inputs
Outputs
S—CoA
C
2
ATP
6
NADH
O
CH3
2
Acetyl CoA
O
C
COO
CH2
COO
2
Oxaloacetate
Citric acid
cycle
2 FADH2
Review:
Chemiosmosis =
Energy Coupling:
INTERMEMBRANE
SPACE
H+
ATP Synthesis
ATP
synthase
ADP + P i
MITOCHONDRIAL
MATRIX
ATP
H+
Proton Motive Force = H+ concentration Gradient. As H’s increase, the pH drops
in the Intermembrane space; so pH difference Increases across the membrane
You should now be able to:
1. Explain in general terms how redox reactions
are involved in energy exchanges.
2. Name the three stages of cellular respiration;
for each, state the region of the eukaryotic
cell where it occurs and the products that
result.
3. In general terms, explain the role of the
electron transport chain in cellular respiration.
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4. Explain where and how the respiratory
electron transport chain creates a proton
gradient.
5. Distinguish between fermentation and
anaerobic respiration.
6. Distinguish between obligate and facultative
anaerobes.
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