1. Metabolism: Energy Release and Conservation
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Chapter 9
Metabolism: Energy Release and Conservation

2. An Overview of Metabolism
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An Overview of Metabolism
metabolism
total of all chemical reactions occurring in cell
catabolism
breakdown of larger, more complex molecules into smaller, simpler ones
energy is released and some is trapped and made available for work
anabolism
synthesis of complex molecules from simpler ones with the input of energy

3. Sources of energy electrons released
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Sources of energy
electrons released
during oxidation of chemical energy sources must be accepted by an electron acceptor
microorganisms vary in terms of the
acceptors they use
Figure 9.1

4. Electron acceptors for chemotrophic processes
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Electron acceptors for chemotrophic processes
Figure 9.2
exogenous electron acceptors

5. Chemoorganotrophic metabolism
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Chemoorganotrophic metabolism
fermentation
energy source oxidized and degraded using endogenous electron acceptor
often occurs under anaerobic conditions
limited energy made available

6. Chemoorganotrophic metabolism
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Chemoorganotrophic metabolism
aerobic respiration
energy source degraded using oxygen as exogenous electron acceptor
yields large amount of energy, primarily by electron transport activity

7. Chemoorganotrophic metabolism
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Chemoorganotrophic metabolism
anaerobic respiration
energy source oxidized and degraded using molecules other than oxygen as exogenous electron acceptors
can yield large amount of energy (depending on reduction potential of energy source and electron acceptor), primarily by electron transport activity

8. Overview of aerobic catabolism
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Overview of aerobic catabolism
three-stage process
large molecules (polymers)  small molecules (monomers)
initial oxidation and degradation to pyruvate
oxidation and degradation of pyruvate by the tricarboxylic acid cycle (TCA cycle)

9. many different energy sources are funneled into common degradative
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many
different
energy
sources
are
funneled
into
common
degradative
pathways
ATP made
primarily by
oxidative
phosphory-
lation
Figure 9.3

10. Two functions of organic energy sources
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Two functions of organic energy sources
oxidized to release energy
supply carbon and building blocks for anabolism
amphibolic pathways
function both as catabolic and anabolic pathways
Figure 9.4

11. The Breakdown of Glucose to Pyruvate
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The Breakdown of Glucose to Pyruvate
Three common routes
glycolysis
pentose phosphate pathway
Entner-Doudoroff pathway

12. The Glycolytic Pathway
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The Glycolytic Pathway
also called Embden-Meyerhof pathway
occurs in cytoplasmic matrix of both procaryotes and eucaryotes

13. addition of phosphates “primes the pump”
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addition of phosphates
“primes the pump”
oxidation step – generates NADH
high-energy molecules –
used to synthesize ATP
by substrate-level
phosphorylation
Figure 9.5

14. 2 pyruvate + 2ATP + 2NADH + 2H+
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Summary of glycolysis
glucose + 2ADP + 2Pi + 2NAD+ 
2 pyruvate + 2ATP + 2NADH + 2H+

15. The Pentose Phosphate Pathway
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The Pentose Phosphate Pathway
also called hexose monophosphate pathway
can operate at same time as glycolytic or Entner-Doudoroff pathways
can operate aerobically or anaerobically
an amphibolic pathway

16. oxidation sugar steps trans- formation reactions produce NADPH,
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oxidation
steps
sugar
trans-
formation
reactions
produce
NADPH,
which is
needed for
biosynthesis
produce
sugars
needed
for
biosynthesis
sugars can
also be
further
degraded
Figure 9.6

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Figure 9.7

18. Summary of pentose phosphate pathway
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Summary of pentose phosphate pathway
glucose-6-P + 12NADP+ + 7H2O 
6CO2 + 12NADPH + 12H+ Pi

19. The Entner-Doudoroff Pathway
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The Entner-Doudoroff Pathway
reactions of
pentose
phosphate
pathway
yield per glucose molecule:
1 ATP
1 NADPH
1 NADH
reactions of
glycolytic
pathway
Figure 9.8

20. Fermentations oxidation of NADH produced by glycolysis
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Fermentations
oxidation of NADH produced by glycolysis
pyruvate or derivative used as endogenous electron acceptor
ATP formed by substrate-level phosphorylation
Figure 9.9

21. alcoholic homolactic fermentation fermenters heterolactic fermenters
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alcoholic
fermentation
homolactic
fermenters
heterolactic
fermenters
alcoholic
beverages,
bread, etc.
food
spoilage
yogurt,
sauerkraut,
pickles, etc.
Figure 9.10

22. methyl red test – detects pH change in media caused by
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methyl red test – detects pH change in media caused by
mixed acid fermentation

23. Butanediol fermentation
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Butanediol fermentation
Voges-Proskauer test –
detects intermediate acetoin
Methyl red test and Voges-
Proskauer test important for
distinguishing pathogenic
members of
Enterobacteriaceae

24. Fermentations of amino acids
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Fermentations of amino acids
Strickland reaction
oxidation of one amino acid with use of second amino acid as electron acceptor
Figure 9.11

25. The Tricarboxylic Acid Cycle
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The Tricarboxylic Acid Cycle
also called citric acid cycle and Kreb’s cycle
completes oxidation and degradation of glucose and other molecules
common in aerobic bacteria, free-living protozoa, most algae, and fungi
amphibolic
provides carbon skeletons for biosynthesis

26. energy drives condensation oxidation of acetyl steps – form group with
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energy drives
condensation
of acetyl
group with
oxaloacetate
oxidation
steps – form
NADH and
FADH2
high-energy
molecule
oxidation and
decarbox-
ylation steps
complete
oxidation and
degradation
substrate-
level
phosphory-
lation
also form
NADH
Figure 9.12

27. Summary for each acetyl-CoA molecule oxidized, TCA cycle generates:
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Summary
for each acetyl-CoA molecule oxidized, TCA cycle generates:
2 molecules of CO2
3 molecules of NADH
one FADH2
one GTP

28. Electron Transport and Oxidative Phosphorylation
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Electron Transport and Oxidative Phosphorylation
only 4 ATP molecules synthesized directly from oxidation of glucose to CO2
most ATP made when NADH and FADH2 (formed as glucose degraded) are oxidized in electron transport chain (ETC)

29. The Electron Transport Chain
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The Electron Transport Chain
series of electron carriers that operate together to transfer electrons from NADH and FADH2 to a terminal electron acceptor
electrons flow from carriers with more negative E0 to carriers with more positive E0

30. Electron transport chain…
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Electron transport chain…
as electrons transferred, energy released
some released energy used to make ATP by oxidative phosphorylation
as many as 3 ATP molecules made per NADH using oxygen as acceptor
P/O ratio = 3
P/O ratio for FADH2 is 2
i.e., 2 ATP molecules made

31. large difference in E0 of NADH and E0 of O2 large amount of
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large difference in
E0 of NADH and
E0 of O2
large amount of
energy released
Figure 9.13

32. Mitochondrial ETC electron transfer accompanied by
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Mitochondrial ETC
Figure 9.14
electron transfer accompanied by
proton movement across inner
mitochondrial membrane

33. Procaryotic ETCs located in plasma membrane
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Procaryotic ETCs
located in plasma membrane
some resemble mitochondrial ETC, but many are different
different electron carriers
may be branched
may be shorter
may have lower P/O ratio

34. ETC of E. coli branched pathway upper branch – stationary phase and
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ETC of E. coli
branched pathway
upper branch –
stationary phase and
low aeration
lower branch – log
phase and high
aeration
Figure 9.15

35. ETC of Paracoccus denitrificans - aerobic
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ETC of Paracoccus denitrificans - aerobic
Figure 9.16a

36. ETC of P. denitrificans - anaerobic
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ETC of P. denitrificans - anaerobic
Figure 9.16b
example of anaerobic respiration

37. Oxidative Phosphorylation
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Oxidative Phosphorylation
chemiosmotic hypothesis
most widely accepted explanation of oxidative phosphorylation
postulates that energy released during electron transport used to establish a proton gradient and charge difference across membrane
called proton motive force (PMF)

38. PMF drives ATP synthesis
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PMF drives ATP synthesis
diffusion of protons back across membrane (down gradient) drives formation of ATP
ATP synthase
enzyme that uses proton movement down gradient to catalyze ATP synthesis

39. movement of protons establishes PMF ATP synthase uses proton flow down
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movement of protons
establishes
PMF
ATP synthase
uses proton
flow down
gradient to
make ATP
Figure 9.17

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Figure 9.19a

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Figure 9.19b

42. Inhibitors of ATP synthesis
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Inhibitors of ATP synthesis
blockers
inhibit flow of electrons through ETC
uncouplers
allow electron flow, but disconnect it from oxidative phosphorylation
many allow movement of ions, including protons, across membrane without activating ATP synthase
destroys pH and ion gradients
some may bind ATP synthase and inhibit its activity directly

43. Importance of PMF Figure 9.18
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Importance of PMF
Figure 9.18

44. The Yield of ATP in Glycolysis and Aerobic Respiration
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The Yield of ATP in Glycolysis and Aerobic Respiration
aerobic respiration provides much more ATP than fermentation
Pasteur effect
decrease in rate of sugar metabolism when microbe shifted from anaerobic to aerobic conditions
occurs because aerobic process generates greater ATP per sugar molecule

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46. Copyright © The McGraw-Hill Companies, Inc
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ATP yield…
amount of ATP produced during aerobic respiration varies depending on growth conditions and nature of ETC
under anaerobic conditions, glycolysis only yields 2 ATP molecules

47. Anaerobic Respiration
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Anaerobic Respiration
uses electron carriers other than O2
generally yields less energy because E0 of electron acceptor is less positive than E0 of O2

48. An example dissimilatory nitrate reduction
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An example
dissimilatory nitrate reduction
use of nitrate as terminal electron acceptor
denitrification
reduction of nitrate to nitrogen gas
in soil, causes loss of soil fertility

49. Catabolism of Carbohydrates and Intracellular Reserves
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Catabolism of Carbohydrates and Intracellular Reserves
many different carbohydrates can serve as energy source
carbohydrates can be supplied externally or internally (from internal reserves)

50. Carbohydrates monosaccharides disaccharides and polysaccharides
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Carbohydrates
monosaccharides
converted to other sugars that enter glycolytic pathway
disaccharides and polysaccharides
cleaved by hydrolases or phosphorylases
Figure 9.20

51. (glucose)n + Pi  (glucose)n-1 + glucose-1-P
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Reserve Polymers
used as energy sources in absence of external nutrients
e.g., glycogen and starch
cleaved by phosphorylases
(glucose)n + Pi  (glucose)n-1 + glucose-1-P
glucose-1-P enters glycolytic pathway
e.g., PHB
PHB    acetyl-CoA
acetyl-CoA enters TCA cycle

52. Lipid Catabolism triglycerides common energy sources
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Lipid Catabolism
triglycerides
common energy sources
hydrolyzed to glycerol and fatty acids by lipases
glycerol degraded via glycolytic pathway
fatty acids often oxidized via β-oxidation pathway
Figure 9.21

53. β-oxidation pathway enters TCA cycle or used for biosynthesis to ETC
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β-oxidation pathway
enters TCA cycle
or used for
biosynthesis
to ETC
Figure 9.22

54. Protein and Amino Acid Catabolism
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Protein and Amino Acid Catabolism
protease
hydrolyzes protein to amino acids
deamination
removal of amino group from amino acid
resulting organic acids converted to pyruvate, acetyl-CoA, or TCA cycle intermediate
can be oxidized via TCA cycle
can be used for biosynthesis

55. deamination often occurs by transamination
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deamination often occurs by transamination
Figure 9.23
transfer of amino group
from one amino acid to α-keto acid

56. Oxidation of Inorganic Molecules
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Oxidation of Inorganic Molecules
carried out by chemolithotrophs
electrons released from energy source
transferred to terminal electron acceptor by ETC
ATP synthesized by oxidative phosphorylation

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usually aerobic

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59. Nitrifying bacteria NH3  NO2  NO3 oxidize ammonia to nitrate
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Nitrifying bacteria
NH3  NO2  NO3
oxidize ammonia to nitrate
requires 2 different genera
NO2  NO3
Figure 9.24

60. Sulfur-oxidizing bacteria
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Sulfur-oxidizing bacteria
ATP can be
synthesized by
both oxidative
phosphorylation
and substrate-
level
Figure 9.25

61. Metabolic flexibility of chemolithotrophs
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Metabolic flexibility of chemolithotrophs
many switch from chemolithotrophic metabolism to chemoorganotrophic metabolism
many switch from autotrophic metabolism (via Calvin cycle) to heterotrophic metabolism

62. Autotrophic growth by chemolithotrophs
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Autotrophic growth by chemolithotrophs
Calvin cycle requires NADH as electron source for fixing CO2
many energy sources used by chemolithotrophs have E0 more positive than NAD/NADH
use reverse electron flow to generate NADH

63. Photosynthesis light reactions dark reactions
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Photosynthesis
light reactions
energy from light trapped and converted to chemical energy
dark reactions
chemical energy used to reduce CO2 and synthesize cell constituents (discussed in Chapter 10)

64. oxygenic photosynthesis – eucaryotes and cyanobacteria
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oxygenic photosynthesis – eucaryotes and cyanobacteria
anoxygenic photosynthesis – all other bacteria

65. The Light Reaction in Eucaryotes and Cyanobacteria
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The Light Reaction in Eucaryotes and Cyanobacteria
chlorophylls
major light-absorbing pigments
accessory pigments
transfer light energy to chlorophylls

66. different chlorophylls have different absorption peaks Figure 9.26
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different
chlorophylls
have different
absorption
peaks
Figure 9.26

67. absorb different wavelengths of light than chlorophylls
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absorb different wavelengths of light than chlorophylls
Figure 9.27

68. Organization of pigments
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Organization of pigments
antennas
highly organized arrays of chlorophylls and accessory pigments
captured light transferred to special reaction-center chlorophyll
directly involved in photosynthetic electron transport
photosystems
antenna and its associated reaction-center chlorophyll

69. Green plant photosynthesis
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Green plant photosynthesis
noncyclic
electron flow –
ATP + NADPH
made
cyclic electron
flow – ATP
made
Figure 9.29

70. electron flow  PMF  ATP
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electron flow  PMF  ATP
Figure 9.30

71. The Light Reaction in Green and Purple Bacteria
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The Light Reaction in Green and Purple Bacteria

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Figure 9.26

73. Purple nonsulfur bacteria
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Purple nonsulfur bacteria
electron source for
generation of NADH
by reverse electron
flow
Figure 9.31

74. Reverse electron flow more positive more negative E0 E0
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Reverse electron flow
more positive E0
more negative E0
Figure 9.32
ATP or PMF used to drive “upward” (reverse) flow
of electrons

75. Green sulfur bacteria Figure 9.33
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Green sulfur bacteria
Figure 9.33