1. Chapter 9 Cellular Respiration: Harvesting Chemical Energy

2. 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
For the Discovery Video Space Plants, go to Animation and Video Files.
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3. 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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4. Organic molecules Cellular respiration in mitochondria
Fig. 9-2
Light
energy
ECOSYSTEM
Photosynthesis
in chloroplasts
Organic
molecules
CO2 + H2O
+ O2
Cellular respiration
in mitochondria
Figure 9.2 Energy flow and chemical recycling in ecosystems
ATP
ATP powers most cellular work
Heat
energy

5. Concept 9.1: Catabolic pathways yield energy by oxidizing organic fuels
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6. Catabolic Pathways and Production of ATP
Catabolic Pathways: The breakdown of organic molecules (exergonic)
2 types:
Fermentation is a partial degradation of sugars that occurs without O2 (anaerobic)
Aerobic respiration consumes organic molecules and O2 and yields ATP
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7. 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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8. 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 from ADP + Pi which is regenerated by burning fuels during respiration
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9. 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 (loses energy)
In reduction, a substance gains electrons, or is reduced (the amount of positive charge is reduced), gains energy
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10. becomes oxidized (loses electron) becomes reduced (gains electron)
Fig. 9-UN1
becomes oxidized
(loses electron)
becomes reduced
(gains electron)

11. becomes oxidized becomes reduced
Fig. 9-UN2
becomes oxidized
becomes reduced

12. Methane (reducing agent) Oxygen (oxidizing agent)
Fig. 9-3
Reactants
Products
becomes oxidized
becomes reduced
Figure 9.3 Methane combustion as an energy-yielding redox reaction
Methane
(reducing
agent)
Oxygen
(oxidizing
agent)
Carbon dioxide
Water
The electron donor is called the reducing agent
The electron receptor is called the oxidizing agent

13. 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
Dehydrogenase
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14. 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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15. NADH H+ NAD+ + 2[H] + H+ 2 e– + 2 H+ 2 e– + H+ Dehydrogenase
Fig. 9-4
2 e– + 2 H+
2 e– + H+
NADH H+
Dehydrogenase
Reduction of NAD+
NAD+ +
2[H] + H+
Oxidation of NADH
Nicotinamide
(reduced form)
Nicotinamide
(oxidized form)
Figure 9.4 NAD+ as an electron shuttle
Figure 9.4 NAD+ as an electron shuttle

16. 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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17. (a) Uncontrolled reaction (b) Cellular respiration
Fig. 9-5
H2 + 1/2 O2
2 H +
1/2 O2
(from food via NADH)
Controlled
release of
energy for
synthesis of
ATP
2 H e–
ATP
Explosive
release of
heat and light
energy
ATP
Electron transport
chain
Free energy, G
Free energy, G
ATP
2 e–
Figure 9.5 An introduction to electron transport chains
1/2 O2
2 H+
H2O
H2O
(a) Uncontrolled reaction
(b) Cellular respiration

18. The Stages of Cellular Respiration: A Preview
Cellular respiration has three stages:
Glycolysis (breaks down glucose into two molecules of pyruvate), cytosol
The citric acid cycle (completes the breakdown of glucose), mitochondria
Oxidative phosphorylation (accounts for most of the ATP synthesis), inner mitochondrial membrane
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19. Electrons carried via NADH ATP Substrate-level phosphorylation
Fig
Electrons
carried
via NADH
Glycolysis
Glucose
Pyruvate
Cytosol
Figure 9.6 An overview of cellular respiration
ATP
Substrate-level
phosphorylation

20. Electrons carried via NADH Electrons carried via NADH and FADH2
Fig
Electrons
carried
via NADH
Electrons carried
via NADH and
FADH2
Glycolysis
Citric
acid
cycle
Glucose
Pyruvate
Mitochondrion
Cytosol
Figure 9.6 An overview of cellular respiration
ATP
ATP
Substrate-level
phosphorylation
Substrate-level
phosphorylation

21. Electrons carried via NADH Electrons carried via NADH and FADH2
Fig
Electrons
carried
via NADH
Electrons carried
via NADH and
FADH2
Oxidative
phosphorylation:
electron transport
and
chemiosmosis
Glycolysis
Citric
acid
cycle
Glucose
Pyruvate
Mitochondrion
Cytosol
Figure 9.6 An overview of cellular respiration
ATP
ATP
ATP
Substrate-level
phosphorylation
Substrate-level
phosphorylation
Oxidative
phosphorylation

22. Oxidative phosphorylation accounts for almost 90% of the ATP generated by cellular respiration (powered by redox reactions)
A smaller amount of ATP is formed in glycolysis and the citric acid cycle by substrate-level phosphorylation
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23. Enzyme Enzyme ADP P Substrate + ATP Product
Fig. 9-7
Enzyme
Enzyme
ADP P
Substrate +
ATP
Figure 9.7 Substrate-level phosphorylation
Product
Figure 9.7 Substrate-level phosphorylation

24. Glycolysis occurs in the cytoplasm and has two major phases:
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/consuming phase
Energy producing/payoff phase
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25. Energy investment phase
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 e– + 4 H+
2 NADH
+ 2 H+
Figure 9.8 The energy input and output of glycolysis
2 Pyruvate + 2 H2O
Net
Glucose
2 Pyruvate + 2 H2O
4 ATP formed – 2 ATP used
2 ATP
2 NAD e– + 4 H+
2 NADH + 2 H+

26. Glucose ATP 1 Hexokinase ADP Glucose-6-phosphate Fig. 9-9-1
Figure 9.9 A closer look at glycolysis
Glucose-6-phosphate

27. Glucose-6-phosphate 2 Phosphogluco- isomerase Fructose-6-phosphate
Fig
Glucose
ATP 1
Hexokinase
ADP
Glucose-6-phosphate
Glucose-6-phosphate 2
Phosphoglucoisomerase 2
Phosphogluco-
isomerase
Fructose-6-phosphate
Figure 9.9 A closer look at glycolysis
Fructose-6-phosphate

28. Fructose- 1, 6-bisphosphate
Fig
Glucose
ATP 1 1
Hexokinase
ADP
Fructose-6-phosphate
Glucose-6-phosphate 2 2
Phosphoglucoisomerase
ATP 3
Phosphofructo-
kinase
Fructose-6-phosphate
ATP 3 3
ADP
Phosphofructokinase
ADP
Figure 9.9 A closer look at glycolysis
Fructose-
1, 6-bisphosphate
Fructose-
1, 6-bisphosphate

29. Aldolase Isomerase Fructose- 1, 6-bisphosphate 4 5 Dihydroxyacetone
Fig
Glucose
ATP 1
Hexokinase
ADP
Glucose-6-phosphate 2
Phosphoglucoisomerase
Fructose-
1, 6-bisphosphate 4
Fructose-6-phosphate
Aldolase
ATP 3
Phosphofructokinase
ADP
Figure 9.9 A closer look at glycolysis 5
Isomerase
Fructose-
1, 6-bisphosphate 4
Aldolase 5
Isomerase
Dihydroxyacetone
phosphate
Glyceraldehyde-
3-phosphate
Dihydroxyacetone
phosphate
Glyceraldehyde-
3-phosphate

30. Glyceraldehyde- 3-phosphate
Fig
2 NAD+ 6
Triose phosphate
dehydrogenase 2
NADH 2 P i
+ 2 H+ 2 2
1, 3-Bisphosphoglycerate
Glyceraldehyde-
3-phosphate
2 NAD+ 6
Triose phosphate
dehydrogenase 2 P 2
NADH i
+ 2 H+
Figure 9.9 A closer look at glycolysis 2
1, 3-Bisphosphoglycerate

31. 2 2 ADP 2 ATP 2 3-Phosphoglycerate 1, 3-Bisphosphoglycerate 7
Fig
2 NAD+ 6
Triose phosphate
dehydrogenase 2
NADH 2 P i
+ 2 H+ 2
1, 3-Bisphosphoglycerate
2 ADP 7
Phosphoglycerokinase
2 ATP 2
1, 3-Bisphosphoglycerate
2 ADP 2
3-Phosphoglycerate 7
Phosphoglycero-
kinase
2 ATP
Figure 9.9 A closer look at glycolysis 2
3-Phosphoglycerate

32. 2 3-Phosphoglycerate 8 Phosphoglycero- mutase 2 2-Phosphoglycerate
Fig
2 NAD+ 6
Triose phosphate
dehydrogenase 2
NADH 2 P i
+ 2 H+ 2
1, 3-Bisphosphoglycerate
2 ADP 7
Phosphoglycerokinase
2 ATP 2
3-Phosphoglycerate 2
3-Phosphoglycerate 8
Phosphoglyceromutase 8
Phosphoglycero-
mutase 2
2-Phosphoglycerate
Figure 9.9 A closer look at glycolysis 2
2-Phosphoglycerate

33. 2 2-Phosphoglycerate Enolase 2 H2O 2 Phosphoenolpyruvate 9 Fig. 9-9-8
2 NAD+ 6
Triose phosphate
dehydrogenase 2
NADH 2 P i
+ 2 H+ 2
1, 3-Bisphosphoglycerate
2 ADP 7
Phosphoglycerokinase
2 ATP 2
2-Phosphoglycerate 2
3-Phosphoglycerate 8
Phosphoglyceromutase 9
Enolase
2 H2O 2
2-Phosphoglycerate 9
Enolase
Figure 9.9 A closer look at glycolysis
2 H2O 2
Phosphoenolpyruvate 2
Phosphoenolpyruvate

34. 2 Phosphoenolpyruvate 2 ADP 10 Pyruvate kinase 2 ATP 2 Pyruvate
Fig
2 NAD+ 6
Triose phosphate
dehydrogenase 2
NADH 2 P i
+ 2 H+ 2
1, 3-Bisphosphoglycerate
2 ADP 7
Phosphoglycerokinase
2 ATP 2
Phosphoenolpyruvate
2 ADP 2
3-Phosphoglycerate 8 10
Phosphoglyceromutase
Pyruvate kinase
2 ATP 2
2-Phosphoglycerate 9
Enolase
2 H2O
Figure 9.9 A closer look at glycolysis 2
Phosphoenolpyruvate
2 ADP 10
Pyruvate kinase
2 ATP 2
Pyruvate 2
Pyruvate

35. In the presence of O2, pyruvate enters the mitochondrion
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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36. CYTOSOL MITOCHONDRION NAD+ NADH + H+ 2 1 3 Acetyl CoA Pyruvate
Fig. 9-10
CYTOSOL
MITOCHONDRION
NAD+
NADH
+ H+ 2 1 3
Acetyl CoA
Figure 9.10 Conversion of pyruvate to acetyl CoA, the junction between glycolysis and the citric acid cycle
Pyruvate
Coenzyme A
CO2
Transport protein
Figure 9.10 Conversion of pyruvate to acetyl CoA, the junction between glycolysis and the citric acid cycle

37. Fig. 9-11
Pyruvate
CO2
NAD+
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 1 ATP, 3 NADH, and 1 FADH2 per turn
NADH
+ H+
Acetyl CoA
CoA
CoA
Citric
acid
cycle 2
CO2
Figure 9.11 An overview of the citric acid cycle
FADH2
3 NAD+
FAD 3
NADH
+ 3 H+
ADP + P i
ATP

39. 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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40. Citric acid cycle Succinyl CoA
Fig
Acetyl CoA
CoA—SH
NADH
+H+ 1
H2O
NAD+ 8
Oxaloacetate 2
Malate
Citrate
Isocitrate
NAD+
Citric
acid
cycle
NADH 3 7
+ H+
H2O
CO2
Fumarate
CoA—SH
-Keto-
glutarate
Figure 9.12 A closer look at the citric acid cycle 4 6
CoA—SH
FADH2 5
CO2
NAD+
FAD
Succinate P
NADH i
GTP
GDP
Succinyl
CoA
+ H+
ADP
ATP

41. 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
For the Cell Biology Video ATP Synthase 3D Structure — Side View, go to Animation and Video Files.
For the Cell Biology Video ATP Synthase 3D Structure — Top View, go to Animation and Video Files.
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42. The Pathway of Electron Transport
The electron transport chain is in the cristae of the mitochondrion
Most of the chain’s components are proteins
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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43. Figure 9.13 Free-energy change during electron transport
NADH 50 2 e–
NAD+
FADH2 2 e–
FAD
Multiprotein
complexes  40
FMN
FAD
Fe•S 
Fe•S Q

Cyt b
Fe•S 30
Cyt c1 IV
Free energy (G) relative to O2 (kcal/mol)
Cyt c
Cyt a
Cyt a3 20
Figure 9.13 Free-energy change during electron transport e– 10 2
(from NADH
or FADH2)
2 H+ + 1/2 O2
H2O

44. The electron transport chain generates no ATP
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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46. 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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47. INTERMEMBRANE SPACE H+ Stator Rotor Internal rod Cata- lytic knob ADP
Fig. 9-14
INTERMEMBRANE SPACE H+
Stator
Rotor
Internal
rod
Figure 9.14 ATP synthase, a molecular mill
Cata-
lytic
knob
ADP + P
ATP i
MITOCHONDRIAL MATRIX

48. 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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49. Electron transport chain 2 Chemiosmosis
Fig. 9-16 H+ H+ H+ H+
Protein complex
of electron
carriers
Cyt c V Q
 
ATP synthase 
2 H+ + 1/2O2
H2O
FADH2
FAD
NADH
NAD+
Figure 9.16 Chemiosmosis couples the electron transport chain to ATP synthesis
ADP + P
ATP i
(carrying electrons
from food) H+ 1
Electron transport chain 2
Chemiosmosis
Oxidative phosphorylation

50. 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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51. Fig. 9-17
CYTOSOL
Electron shuttles
span membrane
MITOCHONDRION
2 NADH or
2 FADH2
2 NADH
2 NADH
6 NADH
2 FADH2
Glycolysis
Oxidative
phosphorylation:
electron transport
and
chemiosmosis 2
Pyruvate 2
Acetyl
CoA
Citric
acid
cycle
Glucose
+ 2 ATP
+ 2 ATP
+ about 32 or 34 ATP
Figure 9.17 ATP yield per molecule of glucose at each stage of cellular respiration
About
36 or 38 ATP
Maximum per glucose:

53. Most cellular respiration requires O2 to produce 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
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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54. 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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55. 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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56. (a) Alcohol fermentation
Fig. 9-18a
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
2 ADP + 2 P
2 ATP i
Glucose
Glycolysis
2 Pyruvate
2 NAD+
2 NADH 2
CO2
+ 2 H+
Figure 9.18a Fermentation
2 Acetaldehyde
2 Ethanol
(a) Alcohol fermentation

57. 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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58. (b) Lactic acid fermentation
Fig. 9-18b
2 ADP + 2 P
2 ATP i
Glucose
Glycolysis
2 NAD+
2 NADH
+ 2 H+
2 Pyruvate
Figure 9.18b 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
2 Lactate
(b) Lactic acid fermentation

59. Fermentation and Aerobic Respiration Compared
Both processes use glycolysis to oxidize glucose and other organic fuels to pyruvate
final electron acceptors:
Fermentation: an organic molecule (such as pyruvate or acetaldehyde)
Aerobic respiration: O2
ATP production:
Cellular respiration produces 38 ATP per glucose molecule
fermentation produces 2 ATP per glucose molecule
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60. 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
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61. Ethanol or lactate Citric acid cycle
Fig. 9-19
Glucose
Glycolysis
CYTOSOL
Pyruvate
O2 present:
Aerobic cellular
respiration
No O2 present:
Fermentation
MITOCHONDRION
Ethanol or
lactate
Acetyl CoA
Figure 9.19 Pyruvate as a key juncture in catabolism
Citric
acid
cycle

62. 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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63. 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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64. 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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65. Fatty acids are broken down by beta oxidation and yield acetyl CoA
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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66. Citric acid cycle Oxidative phosphorylation
Fig. 9-20
Proteins
Carbohydrates
Fats
Amino
acids
Sugars
Glycerol
Fatty
acids
Glycolysis
Glucose
Glyceraldehyde-3- P
NH3
Pyruvate
Acetyl CoA
Figure 9.20 The catabolism of various molecules from food
Citric
acid
cycle
Oxidative
phosphorylation

67. 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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