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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Fig. 9-1
• 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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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

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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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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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Fig. 9-UN3

becomes oxidized

becomes reduced
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

Glucose Pyruvate

Cytosol

ATP

Substrate-level
phosphorylation
Fig. 9-6-2

Electrons Electrons carried

carried via NADH and
via NADH FADH2

Glycolysis Citric
acid
Glucose Pyruvate cycle

Mitochondrion
Cytosol

ATP ATP

Substrate-level Substrate-level
phosphorylation phosphorylation
Fig. 9-6-3

Electrons Electrons carried

carried via NADH and
via NADH FADH2

Oxidative
Glycolysis Citric phosphorylation:
acid electron transport
Glucose Pyruvate cycle and
chemiosmosis

Mitochondrion
Cytosol

ATP ATP ATP

Substrate-level Substrate-level Oxidative

phosphorylation phosphorylation phosphorylation
• 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

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Fig. 9-7

Enzyme Enzyme

ADP
P
Substrate + ATP
Product
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

Glucose

2 ADP + 2 P 2 ATP used

Energy payoff phase

4 ADP + 4 P 4 ATP formed

2 NAD+ + 4 e– + 4 H+ 2 NADH + 2 H+

2 Pyruvate + 2 H2O

Net
Glucose 2 Pyruvate + 2 H2O
4 ATP formed – 2 ATP used 2 ATP
2 NAD+ + 4 e– + 4 H+ 2 NADH + 2 H+
 Glucose

ATP
1
Hexokinase

ADP

Glucose-6-phosphate

2
Phosphoglucoisomerase
Fructose-
1, 6-bisphosphate

4
Fructose-6-phosphate
Aldolase
ATP
3
Phosphofructokinase

ADP

5
Isomerase
Fructose-
1, 6-bisphosphate

4
Aldolase

5
Isomerase
Dihydroxyacetone Glyceraldehyde-
phosphate 3-phosphate
Dihydroxyacetone Glyceraldehyde-
phosphate 3-phosphate
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 10
Pyruvate
2 ATP kinase
2 2-Phosphoglycerate

9
Enolase
2 H2O

2 Phosphoenolpyruvate
2 ADP
10
Pyruvate kinase
2 ATP

2 Pyruvate

2 Pyruvate
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+

1 3
Acetyl CoA
Pyruvate CO2 Coenzyme A

Transport protein
• 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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Pyruvate

CO2
NAD+
CoA
NADH
+ H+ Acetyl CoA
CoA

CoA

Citric
acid
cycle 2 CO2

FADH2 3 NAD+

FAD 3 NADH
+ 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

Oxaloacetate

Citrate

Citric
acid
cycle
Fig. 9-12-2

Acetyl CoA
CoA—SH

1 H2O

Oxaloacetate
2

Citrate
Isocitrate

Citric
acid
cycle
Fig. 9-12-3

Acetyl CoA
CoA—SH

1 H2O

Oxaloacetate
2

Citrate
Isocitrate
NAD+
Citric NADH
3
acid + H+
cycle
CO2

-Keto-
glutarate
Fig. 9-12-4

Acetyl CoA
CoA—SH

1 H2O

Oxaloacetate
2

Citrate
Isocitrate
NAD+
Citric 3
NADH
acid + H+
cycle
CO2

CoA—SH
-Keto-
glutarate
4

CO2
NAD+

NADH
Succinyl + H+
CoA
Fig. 9-12-5

Acetyl CoA
CoA—SH

1 H2O

Oxaloacetate
2

Citrate
Isocitrate
NAD+
Citric NADH
3
acid + H+
cycle
CO2

CoA—SH
-Keto-
glutarate
4
CoA—SH

5
CO2
NAD+

Succinate Pi NADH
GTP GDP Succinyl + H+
CoA
ADP

ATP
Fig. 9-12-6

Acetyl CoA
CoA—SH

1 H2O

Oxaloacetate
2

Citrate
Isocitrate
NAD+
Citric NADH
3
acid + H+
cycle
CO2

Fumarate CoA—SH
-Keto-
glutarate
6 4
CoA—SH

FADH2 5
CO2
NAD+
FAD
Succinate Pi NADH
GTP GDP Succinyl + H+
CoA
ADP

ATP
Fig. 9-12-7

Acetyl CoA
CoA—SH

1 H2O

Oxaloacetate
2

Malate Citrate
Isocitrate
NAD+
Citric 3
NADH
7
acid + H+
H2O cycle
CO2

Fumarate CoA—SH
-Keto-
glutarate
6 4
CoA—SH

FADH2 5
CO2
NAD+
FAD
Succinate Pi NADH
GTP GDP Succinyl + H+
CoA
ADP

ATP
Fig. 9-12-8

Acetyl CoA
CoA—SH

NADH
+H+ 1 H2O

NAD+
8 Oxaloacetate
2

Malate Citrate
Isocitrate
NAD+
Citric 3
NADH
7
acid + H+
H2O cycle
CO2

Fumarate CoA—SH
-Keto-
glutarate
6 4
CoA—SH

FADH2 5
CO2
NAD+
FAD
Succinate Pi NADH
GTP GDP Succinyl + H+
CoA
ADP

ATP
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– FAD
Multiprotein
40  FAD complexes
FMN
Fe•S Fe•S 
Q

Cyt b
Fe•S
30
Cyt c1 I
Cyt c V
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 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 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

Cata-
lytic
knob

ADP
+
P ATP
i

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 proton-
motive force, emphasizing its capacity to do
work

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Fig. 9-16

H+
H+

H+
H+
Protein complex Cyt c
of electron
carriers

V
Q
 
ATP
synthase

2 H+ + 1/2O2 H2O
FADH2
FAD

NADH NAD+
ADP + P i ATP
(carrying electrons
from food)
H+

1 Electron transport chain 2 Chemiosmosis

Oxidative phosphorylation
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

CYTOSOL Electron shuttles MITOCHONDRION

span membrane 2 NADH
or
2 FADH2

2 NADH 2 NADH 6 NADH 2 FADH2

Glycolysis Oxidative
2 2 Citric phosphorylation:
Glucose Pyruvate Acetyl acid electron transport
CoA cycle and
chemiosmosis

+ 2 ATP + 2 ATP + 34 ATP

Maximum per glucose: 38 ATP

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

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• 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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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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Fig. 9-18a

2 ADP + 2 P i 2 ATP

Glucose Glycolysis

2 Pyruvate

2 NAD+ 2 NADH 2 CO2

+ 2 H+

2 Ethanol 2 Acetaldehyde

(a) Alcohol 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

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Fig. 9-18b

2 ADP + 2 P i 2 ATP

Glucose 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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Fig. 9-19
Glucose

Glycolysis
CYTOSOL

Pyruvate

No O2 present: O2 present:
Fermentation Aerobic cellular
respiration

MITOCHONDRION
Ethanol Acetyl CoA
or
lactate
Citric
acid
cycle