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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
• 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
• Organic compounds possess potential energy
as a result of their arrangement of atoms.
• With the help of enzymes, a cell systematically
degrades complex organic molecules that are
rich in potential energy to simpler waste
products that have less energy.
• Some of the energy taken out of chemical
storage can be used to do work; the rest is
dissipated as heat.
1. Distinguish between:
• The breakdown of organic molecules is exergonic
• Fermentation is a partial degradation of sugars
that occurs without O2
• Aerobic respiration (most prevalent) consumes
organic molecules and O2 and yields ATP
• Anaerobic respiration is similar to aerobic
respiration but consumes compounds other than
O2
• Cellular respiration includes both aerobic and
anaerobic respiration but is often used to refer to
aerobic respiration
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2. Write down the formula for the breakdown of glucose in
cellular 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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3. How do the catabolic pathways that decompose glucose
and other organic fuels yield energy?
• The relocation of electrons during the chemical
reactions of respiration releases energy stored
in organic molecules, and this energy ultimately
is used to synthesize ATP.
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4. What is a redox reaction? Define the following:
• 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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• The electron donor is called the reducing
agent
• The electron receptor 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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 9-UN1
becomes oxidized
(loses electron)
becomes reduced
(gains electron)
Fig. 9-UN2
becomes oxidized
becomes reduced
Fig. 9-3
Reactants
Products
becomes oxidized
becomes reduced
Methane
(reducing
agent)
Oxygen
(oxidizing
agent)
Carbon dioxide
Water
• An electron loses potential energy when it
shifts from a less electronegative atom toward
a more electronegative one, just like a ball
rolling downhill.
• A redox reaction that moves electrons closer to
oxygen, such as the burning of methane,
therefore releases chemical energy that can be
put to work.
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5. In general, why are organic molecules that have an
abundance of hydrogen atoms excellent fuels?
• They are excellent fuels because their bonds
are a source of “hilltop” electrons.
• Their energy may be released as these
electrons “fall” down an energy gradient when
they are transferred to oxygen.
• In respiration, the oxidation of glucose transfers
electrons to a lower energy state, liberating
energy that becomes available for ATP
synthesis.
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6. Describe the difference between igniting glucose and
burning it, and how we break it down in our bodies.
• Cellular respiration does not oxidize glucose in
a single explosive step.
• Rather, we breakdown glucose in a step-bystep process, releasing energy in small
increments rather than in one large quantity.
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7. What is NAD+? What does it become when it is reduced?
What is its role in cellular respiration?
• 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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Fig. 9-4
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)
8. What is an electron transport chain? Why is it so important
in regards to the release of energy?
• 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
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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
9. What element captures electrons at the bottom of the ETC.
What is formed?
• Oxygen
10. In summary, during cellular respiration, most electrons
travel the following “downhill” route:
glucose
NADH
ETC
oxygen
11. Briefly list and describe the three stages of cellular
respiration.
• Cellular respiration 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)
For each molecule of glucose, between about 3638 ATP, although 36 is most widely used.
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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
• Oxidative phosphorylation accounts for almost
90% of the ATP generated by cellular
respiration
BioFlix: Cellular Respiration
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• A smaller amount of ATP is formed in
glycolysis and the citric acid cycle by
substrate-level phosphorylation
Enzyme
Enzyme
ADP
P
Substrate
+
Product
ATP
13. What does the word “glycolysis” mean?
• 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
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14. List and describe the two phases of glycolysis.
• Glycolysis occurs in the cytoplasm and has two major
phases:
– Energy investment phase
• The cell spends 2 ATP to get the process rolling
– Energy payoff phase
• ATP is produced during substrate-level
phosphorylation
• NAD+ is reduced to NADH by electrons released
from the oxidation of glucose
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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+
15. In the end, where has all the carbon from glucose gone?
• In the end, all of the carbon originally present in glucose is
accounted for in the two molecules of pyruvate.
• No CO2 is released during glycolysis.
• Glycolysis occurs whether or not O2 is present
• If O2 is present, the chemical energy stored in pyruvate
and NADH can be extracted by the citric acid cycle and
oxidative phosphorylation.
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Question 16
• 16. Glycolysis releases less than a quarter of the chemical
energy stored in glucose; most of the energy remains
stockpiled in the two molecules of pyruvate. If molecular
oxygen is present, the pyruvate enters the mitochondria,
where the enzymes of the citric acid cycle complete the
oxidation of glucose.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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
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Fig. 9-10
CYTOSOL
MITOCHONDRION
NAD+
NADH + H+
2
1
Pyruvate
Transport protein
3
CO2
Coenzyme A
Acetyl CoA
17. For each acetyl group entering the cycle, please tally the number of
NADH, FADH, and ATP molecules that are produce.
• The citric acid cycle, also called the Krebs
cycle, takes place within the mitochondrial
matrix
• The cycle oxidizes organic fuel derived from
pyruvate, generating 1 ATP, 3 NADH, and 1
FADH2 per turn
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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
• 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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Fig. 9-12-1
Acetyl CoA
CoA—SH
1
Oxaloacetate
Citrate
Citric
acid
cycle
Fig. 9-12-2
Acetyl CoA
CoA—SH
H2O
1
Oxaloacetate
2
Citrate
Isocitrate
Citric
acid
cycle
Fig. 9-12-3
Acetyl CoA
CoA—SH
1
H2O
Oxaloacetate
2
Citrate
Isocitrate
NAD+
Citric
acid
cycle
3
NADH
+ H+
CO2
-Ketoglutarate
Fig. 9-12-4
Acetyl CoA
CoA—SH
1
H2O
Oxaloacetate
2
Citrate
Isocitrate
NAD+
Citric
acid
cycle
NADH
+ H+
3
CO2
CoA—SH
-Ketoglutarate
4
NAD+
Succinyl
CoA
NADH
+ H+
CO2
Fig. 9-12-5
Acetyl CoA
CoA—SH
1
H2O
Oxaloacetate
2
Citrate
Isocitrate
NAD+
Citric
acid
cycle
NADH
+ H+
3
CO2
CoA—SH
-Ketoglutarate
4
CoA—SH
5
NAD+
Succinate
GTP GDP
ADP
ATP
Pi
Succinyl
CoA
NADH
+ H+
CO2
Fig. 9-12-6
Acetyl CoA
CoA—SH
H2O
1
Oxaloacetate
2
Citrate
Isocitrate
NAD+
Citric
acid
cycle
NADH
+ H+
3
CO2
Fumarate
CoA—SH
6
-Ketoglutarate
4
CoA—SH
5
FADH2
NAD+
FAD
Succinate
GTP GDP
ADP
ATP
Pi
Succinyl
CoA
NADH
+ H+
CO2
Fig. 9-12-7
Acetyl CoA
CoA—SH
H2O
1
Oxaloacetate
2
Malate
Citrate
Isocitrate
NAD+
Citric
acid
cycle
7
H2O
NADH
+ H+
3
CO2
Fumarate
CoA—SH
-Ketoglutarate
4
6
CoA—SH
5
FADH2
NAD+
FAD
Succinate
GTP GDP
ADP
ATP
Pi
Succinyl
CoA
NADH
+ H+
CO2
Fig. 9-12-8
Acetyl CoA
CoA—SH
NADH
+H+
H2O
1
NAD+
8
Oxaloacetate
2
Malate
Citrate
Isocitrate
NAD+
Citric
acid
cycle
7
H2O
NADH
+ H+
3
CO2
Fumarate
CoA—SH
6
-Ketoglutarate
4
CoA—SH
5
FADH2
NAD+
FAD
Succinate
GTP GDP
ADP
ATP
Pi
Succinyl
CoA
NADH
+ H+
CO2
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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Question 18
• Glycolysis and the citric acid cycle produce only 4 ATP
molecules per glucose molecule; 2 net ATP from glycolysis
and 2 ATP from the citric acid cycle. At this point,
molecules of NADH and FADH2 account for most of the
energy extracted from the glucose. The electron escorts
link glycolysis and the citric acid cycle to the machinery of
oxidative phosphorylation, which uses energy released
from the electron transport chain to power ATP synthesis.
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The Pathway of Electron Transport
•
The electron transport chain is in the 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
•
Electrons are transferred from NADH or FADH2 to the electron
transport chain
•
The electron transport chain generates no ATP
•
The chain’s function is to break the large free-energy drop from food to
O2 into smaller steps that release energy in manageable amounts
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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
19. Define the following:
• ATP synthase – a protein complex (enzyme) that
makes ATP from ADP and inorganic phosphate
• Chemiosmosis – the process in which energy
stored in the form of a hydrogen ion gradient
across a membrane is used to drive cellular work
such as the synthesis of ATP.
• Proton-motive force – the H+ gradient formed
across a membrane that is capable of performing
work
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Fig. 9-14
INTERMEMBRANE SPACE
H+
Stator
Rotor
Internal
rod
Catalytic
knob
ADP
+
P
i
ATP
MITOCHONDRIAL MATRIX
Fig. 9-15
EXPERIMENT
Magnetic bead
Electromagnet
Sample
Internal
rod
Catalytic
knob
Nickel
plate
RESULTS
Rotation in one direction
Number of photons
detected (103)
Rotation in opposite direction
No rotation
30
25
20
0
Sequential trials
Fig. 9-15a
EXPERIMENT
Magnetic bead
Electromagnet
Sample
Internal
rod
Catalytic
knob
Nickel
plate
Fig. 9-15b
RESULTS
Rotation in one direction
Rotation in opposite direction
No rotation
30
25
20
0
Sequential trials
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
Question 21
• 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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Fig. 9-17
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
and
chemiosmosis
+ about 32 or 34 ATP
24. What are the two mechanisms cells can use to
generate ATP when oxygen is not present?
• In the absence of O2, glycolysis couples with
fermentation or anaerobic respiration to produce ATP
• The distinction between these two is based on
whether an ETC is present.
• 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
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25. Fermentation is a way of harvesting chemical
energy without using either oxygen or any electron
transport chain.
•
In other words, without cellular respiration.
26. As an alternative to respiratory oxidation or
organic nutrients, fermentation is an expansion of
glycolysis that allows continuous generation of ATP
by the substrate-level phosphorylation of glycolysis.
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27. Briefly describe the following two processes”
• In alcohol fermentation, pyruvate is
converted to ethanol in two steps
– The first step releases carbon dioxide from the
pyruvate, which is converted to the two-carbon
compound acetaldehyde
– In the second step, acetaldehyde is reduced
by NADH to ethanol, regenerating the supply
of NAD+
• Alcohol fermentation by yeast is used in
brewing, winemaking, and baking
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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
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
• It was thought that lactic acid caused muscle
soreness, but recent research suggests that it
might be increased levels of potassium ions.
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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 38 ATP per
glucose molecule; fermentation produces 2
ATP per glucose molecule
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29. What is the difference between obligate anaerobes and
facultative anaerobes?.
• 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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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
30. What is the evolutionary significance of glycolysis?
• Glycolysis occurs in nearly all organisms
• Glycolysis probably evolved in ancient
prokaryotes before there was oxygen in the
atmosphere
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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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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
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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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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