10/18/11 Chapter 9: Cellular Respiration
10/18/11
Chapter 9: Cellular Respiration
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)
Fig. 9-UN1
becomes oxidized(loses electron)
becomes reduced(gains electron)
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
Oxidation of Organic Fuel Molecules During Cellular
Respiration• During cellular respiration, the fuel (such as
glucose) is oxidized, and O2 is reduced:
Fig. 9-UN3
becomes oxidized
becomes reduced
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
• 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
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)
Fig. 9-6-3
Mitochondrion
Substrate-levelphosphorylation
ATP
Cytosol
Glucose Pyruvate
Glycolysis
Electronscarried
via NADH
Substrate-levelphosphorylation
ATP
Electrons carriedvia NADH and
FADH2
Oxidativephosphorylation
ATP
Citricacidcycle
Oxidativephosphorylation:electron transport
andchemiosmosis
• 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
• A smaller amount of ATP is formed in glycolysis and the citric acid cycle by substrate-level phosphorylation
Fig. 9-7
Enzyme
ADP
PSubstrate
Enzyme
ATP+
Product
Glycolysis
• 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
Fig. 9-8
Energy investment phase
Glucose
2 ADP + 2 P 2 ATP used
formed4 ATP
Energy payoff phase
4 ADP + 4 P
2 NAD+ + 4 e– + 4 H+ 2 NADH + 2 H+
2 Pyruvate + 2 H2O
2 Pyruvate + 2 H2OGlucoseNet
4 ATP formed – 2 ATP used 2 ATP
2 NAD+ + 4 e– + 4 H+ 2 NADH + 2 H+
Fig. 9-9-1
ATP
ADP
Hexokinase1
ATP
ADP
Hexokinase1
Glucose
Glucose-6-phosphate
Glucose
Glucose-6-phosphate
Glucose enters the cell and is phosphorylated by hexokinase , which transfers a phosphate group to glucose
Fig. 9-9-2
Hexokinase
ATP
ADP
1
Phosphoglucoisomerase2
Phosphogluco-isomerase
2
Glucose
Glucose-6-phosphate
Fructose-6-phosphate
Glucose-6-phosphate
Fructose-6-phosphate
Glucose 6 phosphate is converted to its isomer fructose 6 phosphate by phosphoglucoisomerase
1
Fig. 9-9-3
Hexokinase
ATP
ADP
Phosphoglucoisomerase
Phosphofructokinase
ATP
ADP
2
3
ATP
ADP
Phosphofructo-kinase
Fructose-1, 6-bisphosphate
Glucose
Glucose-6-phosphate
Fructose-6-phosphate
Fructose-1, 6-bisphosphate
1
2
3
Fructose-6-phosphate
3
Phosphofructokinase transfers a phosphate group from ATP to the sugar investing another ATP, sugar is now ready to be split
Fig. 9-9-4
Glucose
ATP
ADP
Hexokinase
Glucose-6-phosphate
Phosphoglucoisomerase
Fructose-6-phosphate
ATP
ADP
Phosphofructokinase
Fructose-1, 6-bisphosphate
Aldolase
Isomerase
Dihydroxyacetonephosphate
Glyceraldehyde-3-phosphate
1
2
3
4
5
Aldolase
Isomerase
Fructose-1, 6-
bisphosphate
Dihydroxyacetonephosphate
Glyceraldehyde-
3-phosphate
4
5
Aldolase cleaves sugar into 2 3- carbon sugars
Fig. 9-9-6
NADH2
+ 2 H+
2 P i
2
2 ADP
2 ATP
2
6
7
2
This enzyme:1.Oxidizes the sugar by transfer of electrons and H+ to NAD+ forming NADH2.Exergonic and enzyme uses energy to transfer phosphate to substrate
Fig. 9-9-52 NAD+
NADH2
+ 2 H+
2
2 P i
Triose phosphatedehydrogenase
1, 3-Bisphosphoglycerate
6
2 NAD+
Glyceraldehyde-3-phosphate
Triose phosphatedehydrogenase
NADH2
+ 2 H+
2 P i
1, 3-Bisphosphoglycerate
6
2
2
Fig. 9-9-62 NAD+
NADH2
Triose phosphatedehydrogenase
+ 2 H+
2 P i
2
2 ADP
1, 3-Bisphosphoglycerate
Phosphoglycerokinase2 ATP
2 3-Phosphoglycerate
6
7
2
2 ADP
2 ATP
1, 3-Bisphosphoglycerate
3-Phosphoglycerate
Phosphoglycero-kinase
2
7
Glycolysis produces some ATP by substrate-level phophorylation
Total of 2 ATP because there are 2 sugar molecules
Fig. 9-9-7
3-Phosphoglycerate
Triose phosphatedehydrogenase
2 NAD+
2 NADH+ 2 H+
2 P i
2
2 ADP
Phosphoglycerokinase
1, 3-Bisphosphoglycerate
2 ATP
3-Phosphoglycerate2
Phosphoglyceromutase
2-Phosphoglycerate2
2-Phosphoglycerate2
2
Phosphoglycero-mutase
6
7
8
8
Fig. 9-9-82 NAD+
NADH2
2
2
2
2
+ 2 H+
Triose phosphatedehydrogenase2 P i
1, 3-Bisphosphoglycerate
Phosphoglycerokinase
2 ADP
2 ATP
3-Phosphoglycerate
Phosphoglyceromutase
Enolase
2-Phosphoglycerate
2 H2O
Phosphoenolpyruvate
9
8
7
6
2 2-Phosphoglycerate
Enolase
2
2 H2O
Phosphoenolpyruvate
9
Fig. 9-9-9
Triose phosphatedehydrogenase
2 NAD+
NADH2
2
2
2
2
2
2 ADP
2 ATP
Pyruvate
Pyruvate kinase
Phosphoenolpyruvate
Enolase2 H2O
2-Phosphoglycerate
Phosphoglyceromutase
3-Phosphoglycerate
Phosphoglycerokinase
2 ATP
2 ADP
1, 3-Bisphosphoglycerate
+ 2 H+
6
7
8
9
10
2
2 ADP
2 ATP
Phosphoenolpyruvate
Pyruvate kinase
2 Pyruvate
10
2 P i
If Oxygen is present…
• 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
Fig. 9-10
CYTOSOL MITOCHONDRION
NAD+ NADH + H+
2
1 3
Pyruvate
Transport protein
CO2Coenzyme A
Acetyl CoA
Fig. 9-11
Pyruvate
NAD+
NADH
+ H+Acetyl CoA
CO2
CoA
CoA
CoA
Citricacidcycle
FADH2
FAD
CO22
3
3 NAD+
+ 3 H+
ADP + P i
ATP
NADH
Fig. 9-12-1
Acetyl CoA
Oxaloacetate
CoA—SH
1
Citrate
Citricacidcycle
Fig. 9-12-2
Acetyl CoA
Oxaloacetate
Citrate
CoA—SH
Citricacidcycle
1
2
H2O
Isocitrate
Fig. 9-12-3
Acetyl CoA
CoA—SH
Oxaloacetate
Citrate
H2O
Citricacidcycle
Isocitrate
1
2
3
NAD+
NADH
+ H+
-Keto-glutarate
CO2
Fig. 9-12-4
Acetyl CoA
CoA—SH
Oxaloacetate
Citrate
H2O
IsocitrateNAD+
NADH
+ H+
Citricacidcycle
-Keto-glutarate
CoA—SH
1
2
3
4
NAD+
NADH
+ H+SuccinylCoA
CO2
CO2
Fig. 9-12-5
Acetyl CoA
CoA—SH
Oxaloacetate
Citrate
H2O
IsocitrateNAD+
NADH
+ H+
CO2
Citricacidcycle
CoA—SH -Keto-
glutarate
CO2NAD+
NADH
+ H+SuccinylCoA
1
2
3
4
5
CoA—SH
GTP GDP
ADP
P iSuccinate
ATP
Fig. 9-12-6
Acetyl CoA
CoA—SH
Oxaloacetate
H2O
CitrateIsocitrate
NAD+
NADH
+ H+
CO2
Citricacidcycle
CoA—SH -Keto-
glutarate
CO2NAD+
NADH
+ H+
CoA—SH
P
SuccinylCoA
i
GTP GDP
ADP
ATP
Succinate
FAD
FADH2
Fumarate
1
2
3
4
5
6
Fig. 9-12-7
Acetyl CoA
CoA—SH
Oxaloacetate
Citrate
H2O
IsocitrateNAD+
NADH
+ H+
CO2
-Keto-glutarate
CoA—SH
NAD+
NADH
SuccinylCoA
CoA—SH
PP
GDPGTP
ADP
ATP
Succinate
FAD
FADH2
Fumarate
CitricacidcycleH2O
Malate
1
2
5
6
7
i
CO2
+ H+
3
4
Fig. 9-12-8
Acetyl CoA
CoA—SH
Citrate
H2O
IsocitrateNAD+
NADH
+ H+
CO2
-Keto-glutarate
CoA—SH
CO2NAD+
NADH
+ H+SuccinylCoA
CoA—SH
P i
GTP GDP
ADP
ATP
Succinate
FAD
FADH2
Fumarate
CitricacidcycleH2O
Malate
Oxaloacetate
NADH
+H+
NAD+
1
2
3
4
5
6
7
8
The Pathway of Electron Transport
• The electron transport chain is in the cristae of the mitochondrion
• Most of the chain’s components are proteins and multiprotein complexes
• Electrons release energy as they go down the chain and are finally passed to O2, forming H2O
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Fig. 9-13
NADH
NAD+2FADH2
2 FADMultiproteincomplexesFAD
Fe•S
FMN
Fe•S
Q
Fe•S
Cyt b
Cyt c1
Cyt c
Cyt a
Cyt a3
IV
Fre
e en
erg
y (G
) r e
lat i
ve t
o O
2 (
kcal
/mo
l)
50
40
30
20
10 2
(from NADHor FADH2)
0 2 H+ + 1/2 O2
H2O
e–
e–
e–
Fig. 9-16
Protein complexof electroncarriers
H+
H+H+
Cyt c
Q
V
FADH2 FAD
NAD+NADH
(carrying electronsfrom food)
Electron transport chain
2 H+ + 1/2O2H2O
ADP + P i
Chemiosmosis
Oxidative phosphorylation
H+
H+
ATP synthase
ATP
21
http://www.youtube.com/watch?v=3y1dO4nNaKY
• Electrons are transferred from NADH or FADH2 to the electron transport chain
• Electrons are passed through a number of proteins including cytochromes to O2
• The electron transport chain generates no ATP
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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 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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 9-14
INTERMEMBRANE SPACE
Rotor
H+
Stator
Internalrod
Cata-lyticknob
ADP+P ATP
iMITOCHONDRIAL 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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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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 9-17
Maximum per glucose: About36 or 38 ATP
+ 2 ATP+ 2 ATP + about 32 or 34 ATP
Oxidativephosphorylation:electron transport
andchemiosmosis
Citricacidcycle
2AcetylCoA
Glycolysis
Glucose2
Pyruvate
2 NADH 2 NADH 6 NADH 2 FADH2
2 FADH2
2 NADHCYTOSOL Electron shuttles
span membrane
or
MITOCHONDRION
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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• 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 OverviewAnimation: Fermentation Overview
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 9-18a
2 ADP + 2 P i 2 ATP
Glucose Glycolysis
2 Pyruvate
2 NADH2 NAD+
+ 2 H+CO2
2 Acetaldehyde2 Ethanol
(a) Alcohol fermentation
2
Fig. 9-18
2 ADP + 2 Pi 2 ATP
Glucose Glycolysis
2 NAD+ 2 NADH
2 Pyruvate
+ 2 H+
2 Acetaldehyde2 Ethanol
(a) Alcohol fermentation
2 ADP + 2 Pi2 ATP
Glucose Glycolysis
2 NAD+ 2 NADH+ 2 H+
2 Pyruvate
2 Lactate
(b) Lactic acid fermentation
2 CO2
• 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
Glucose
2 ADP + 2 P i 2 ATP
Glycolysis
2 NAD+ 2 NADH+ 2 H+
2 Pyruvate
2 Lactate
(b) Lactic acid fermentation
• 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. 9-19Glucose
Glycolysis
Pyruvate
CYTOSOL
No O2 present:Fermentation
O2 present:
Aerobic cellular respiration
MITOCHONDRION
Acetyl CoAEthanolor
lactateCitricacidcycle