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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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• Energy flows into an
ecosystem as sunlight and
leaves as heat
Light
energy
ECOSYSTEM
• The chemical elements
essential to life are recycled:
–
–
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
Photosynthesis
in chloroplasts
CO2 + H2O
Organic
molecules
Cellular respiration
in mitochondria
ATP powers most cellular work
Heat
energy
+O2
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 and they can therefore act as fuels.
• Two important catabolic pathways are:
– Fermentation is a partial degradation of
sugars that occurs without O2
– Aerobic respiration consumes organic
molecules and O2 and yields ATP
• Anaerobic respiration is also a catabolic
pathway. It 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 just 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)
• How does a catabolic pathway like this, that decomposes
glucose, yield energy? The answer is based on the transfer
of electrons during the chemical reactions.
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Redox Reactions: Oxidation and Reduction
• The transfer, or relocation, of electrons during
chemical reactions releases energy stored in
organic molecules
• This released energy is ultimately used to
synthesize ATP
– The exergonic release of energy from glucose
is used to phosphorylate ADP to ATP. Life
processes constantly consume ATP; cellular
respiration burns fuels and uses the energy to
regenerate 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)
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 9-UN1
Specific Example:
becomes oxidized
(loses electron)
becomes reduced
(gains electron)
Fig. 9-UN2
Generalized redox reaction:
becomes oxidized
becomes reduced
– The electron donor (Xe-)is called the reducing
agent
– The electron receptor (Y) is called the
oxidizing agent
• Some redox reactions do not transfer electrons
but change the degree of electron sharing in
covalent bonds
– An example is the reaction between methane
and O2
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Fig. 9-3
Reactants
Products
becomes oxidized
becomes reduced
Methane
(reducing
agent)
Oxygen
(oxidizing
agent)
Carbon dioxide
Water
This reaction releases energy to the surroundings because the electrons lose potential energy when
they end up being shared unequally, spending more time near electronegative atoms such as oxygen.
Fig. 9-UN3
Oxidation of Organic Fuel Molecules During
Cellular Respiration
• During cellular respiration, the fuel (such as
glucose) is oxidized, and O2 is reduced:
becomes oxidized
becomes reduced
• The electrons lose potential energy along the way,
and energy is released.
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;
each step catalyzed by an enzyme.
1. Electrons are stripped from glucose—but they
travel with a proton (i.e. as a hydrogen atom).
2. The hydrogen atoms are not transferred directly to
oxygen, but instead are usually passed to an electron
carrier; the coenzyme NAD+ (nicotinamide adenine
dinucleotide)
• As an electron acceptor, NAD+ functions as an
oxidizing agent during cellular respiration
Fig. 9-UN4
• Dehydrogenase enzymes remove a pair of hydrogen atoms
from glucose (2 electrons and 2 protons). This oxidizes it.
The coenzyme NAD+ accepts the 2 electrons and one proton.
The other proton is released as a hydrogen ion (H+).
Dehydrogenase
By receiving 2 negatively charged electrons but only 1 positively
charged proton, NAD+ is neutralized and reduced to NADH.
Each NADH (the reduced form of NAD+) represents stored
energy that is tapped to synthesize ATP.
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)
3. 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
4. O2 pulls electrons down the chain in an
energy-yielding tumble
5. The energy yielded is used to regenerate ATP
Summary: In cellular respiration, electrons travel the following “downhill” route:
Glucose
NADH
ETC
Oxygen
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Fig. 9-5
The controlled transfer of hydrogen atoms to oxygen atoms can be used to do work. Uncontrolled cannot.
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 (completes the
breakdown of glucose)
– Oxidative phosphorylation (accounts for
most of the ATP synthesis)
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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
• Read page 166.
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
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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
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Fig. 9-8
Energy investment phase: (cell spends 2ATP’s)
Glucose
2 ADP + 2 P
2 ATP
used
Energy payoff phase: (cell regains its investment plus interest)
4 ADP + 4 P
2 NAD+ + 4 e– + 4 H+
4 ATP
formed
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+
Fig. 9-9-1
Glucose
ATP
1
Hexokinase
ADP
Glucose
Glucose-6-phosphate
ATP
1
Hexokinase
ADP
Glucose-6-phosphate
Fig. 9-9-2
Glucose
ATP
1
Hexokinase
ADP
Glucose-6-phosphate
2
Phosphoglucoisomerase
Fructose-6-phosphate
Glucose-6-phosphate
2
Phosphoglucoisomerase
Fructose-6-phosphate
Fig. 9-9-3
Glucose
ATP
1
Hexokinase
ADP
Fructose-6-phosphate
Glucose-6-phosphate
2
Phosphoglucoisomerase
ATP
3
Phosphofructokinase
Fructose-6-phosphate
ATP
3
Phosphofructokinase
ADP
ADP
Fructose1, 6-bisphosphate
Fructose1, 6-bisphosphate
Fig. 9-9-4
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
Fig. 9-9-5
2 NAD+
2 NADH
+ 2 H+
6
Triose phosphate
dehydrogenase
2 Pi
2 1, 3-Bisphosphoglycerate
Glyceraldehyde3-phosphate
2 NAD+
2 NADH
6
Triose phosphate
dehydrogenase
2 Pi
+ 2 H+
2 1, 3-Bisphosphoglycerate
The 2’s
indicate
that this
happens
twice
Fig. 9-9-6
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
2 ATPs are
produced
because
this happens
twice. The
ATPs are
produced by
substratelevel
phosphorylation
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
Fig. 9-9-8
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
Enolase
2 H2O
2-Phosphoglycerate
9
Enolase
2 H2O
2
Phosphoenolpyruvate
2
Phosphoenolpyruvate
(PEP)
Fig. 9-9-9
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
2ATPs are
again
produced by
substrate-level
Phosphorylation.
Again, the
2 shows that
this happens
twice.
Final Results of Glycolysis
• Overall, glycolysis has used 2 ATP in the energy
investment phase (steps 1 and 3) and produced 4 ATP in
the energy payoff phase (steps 7 and 10), for a net gain of
2 ATP.
• Additional energy was stored in 2NADH ( step 6), which
can be used to make ATP by oxidative phosphorylation if
oxygen is present.
• Glucose has been oxidized to two molecules of pyruvate,
the end product of the glycolytic pathway.
• If oxygen is present, the chemical energy in pyruvate can
be extracted by the citric acid cycle.
• If oxygen is not present, fermentation may occur.
Net Results of Glycolysis
Glucose
4 ATP formed – 2 ATP used
2 NAD+ + 4 e- +4 H+
2 Pyruvate + 2H2O
2 ATP
2 NADH + 2 H+
Concept 9.3: The citric acid cycle completes the
energy-yielding oxidation of organic molecules
• Glycolysis releases less that ¼ of the energy
stored in glucose. Most of it remains as
potential energy stored in the two pyruvate
molecules.
• In the presence of O2, pyruvate enters the
mitochondrion (Note: in prokaryotic cells this
process will occur in the cytosol).
• 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
3
CO2
Coenzyme A
Acetyl CoA
Transport protein
Pyruvate uses a transport protein to enter the mitochondrion by active transport. 1) The carboxyl group is
removed and released as CO2. 2) The remaining 2-Carbon compound is oxidized and the stripped
electrons are transferred to NAD+ to form NADH. 3) Co A is attached to form acetyl CoA.
• The citric acid cycle, also called the Krebs cycle,
takes place within the mitochondrial matrix
• The cycle oxidizes organic fuel derived from
pyruvate, generating 3 NADH, 1 ATP, and
1 FADH2 per turn. 2CO2 molecules are released
• **Note: One glucose is split into two
pyruvates..each one goes through the Krebs
cycle…so the net gain is 6NADH, 2ATP, 2FADH2
and 4CO2
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Fig. 9-11
Pyruvate
CO2
NAD+
CoA
NADH
+ H+
Acetyl CoA
CoA
Krebs Cycle goes
around twice!!
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
2 carbon Acetyl CoA
joins w/ 4 carbon
oxaloacetate to make
the 6 carbon citrate
Fig. 9-12-2
Acetyl CoA
CoA—SH
H2O
1
Citrate is converted
to its isomer by the
removal and addition
of a water molecule
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
Isocitrate is oxidized,
reducing NAD+ to
NADH. One CO2
is lost
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
Another CO2 is lost.
Another NAD+ is
reduced to NADH.
CoA is attached
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+
One ATP is generated
by the addition and
transfer of a phosphate
group. CoA is
displaced
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
Succinate is oxidized
when two hydrogens
are transferred to FAD,
forming FADH2
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
Addition of a water
molecule rearranges
bonds in the substrate
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
Malate is oxidized,
reducing NAD+ to
NADH and regenrating
oxaloacetate
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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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
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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
• 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
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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 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
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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
• It is in place because the inner membrane of
the mitochondria is impermeable to hydrogen
ions. They are like water behind a dam—with
their only exit being ATP synthase.
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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
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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
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 electronegative oxygen to pull electrons
down the ETC, oxidative phosphorylation ceases.
• 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
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– Anaerobic respiration uses an electron
transport chain with an electron acceptor
other than O2, for example sulfate (SO42-)
• It takes place in some prokaryotic cells that
live in environments without oxygen.
• Hydrogen sulfide (H2S) is produced as a byproduct instead of water.
– Fermentation uses 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
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 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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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
Fig. 9-18
2 ADP + 2 Pi
Glucose
2 ATP
Glycolysis
2 Pyruvate
2 NAD+
2 NADH
+ 2 H+
2 CO2
2 Acetaldehyde
2 Ethanol
(a) Alcohol fermentation
2 ADP + 2 Pi
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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• 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
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
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
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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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 9-21
Glucose
AMP
Glycolysis
Fructose-6-phosphate
–
Stimulates
+
Phosphofructokinase
–
Fructose-1,6-bisphosphate
Inhibits
Inhibits
Pyruvate
ATP
Citrate
Acetyl CoA
Citric
acid
cycle
Oxidative
phosphorylation
Phosphofructokinase is
the enzyme that catalyzes
the 3rd step of glycolysis
Fig. 9-UN5
Outputs
Inputs
2
ATP
Glycolysis
+
2 NADH
Glucose
2
Pyruvate
Fig. 9-UN6
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
Fig. 9-UN7
INTERMEMBRANE
SPACE
H+
ATP
synthase
ADP + P i
MITOCHONDRIAL
MATRIX
ATP
H+
Fig. 9-UN8
Time
Fig. 9-UN9
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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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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