Ch. 9 Cellular Respiration
• Living cells require energy from outside sources• Heterotrophs and autotrophs
• 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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Figure 9.2
Lightenergy
ECOSYSTEM
Photosynthesisin chloroplasts
Cellular respirationin mitochondria
CO2 H2O O2
Organicmolecules
ATP powersmost cellular workATP
Heatenergy
• Cellular respiration is often used to refer to aerobic respiration
• C6H12O6 + 6 O2 6 CO2 + 6 H2O + Energy (ATP + heat)
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Redox Reactions
• During cellular respiration, the fuel (such as glucose) is oxidized, and O2 is reduced
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Figure 9.UN03
becomes oxidized
becomes reduced
NAD+
• 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 will eventually synthesize ATP
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Figure 9.4
Nicotinamide(oxidized form)
NAD
(from food)
Dehydrogenase
Reduction of NAD
Oxidation of NADH
Nicotinamide(reduced form)
NADH
• NADH passes the electrons to the electron transport chain
• O2 pulls electrons down the chain in an energy-yielding tumble
• The energy yielded is used to regenerate ATP
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Three stages of respiration
• Harvesting of energy from glucose has three stages– Glycolysis (breaks down glucose into two
molecules of pyruvate)– The citric acid cycle and oxidation of pyruvate
(completes the breakdown of glucose)– Oxidative phosphorylation – electron transport
(accounts for most of the ATP synthesis)
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Figure 9.UN05
Glycolysis (color-coded teal throughout the chapter)1.
Pyruvate oxidation and the citric acid cycle(color-coded salmon)
2.
Oxidative phosphorylation: electron transport andchemiosmosis (color-coded violet)
3.
Figure 9.6-1
Electronscarried
via NADH
Glycolysis
Glucose Pyruvate
CYTOSOL MITOCHONDRION
ATP
Substrate-levelphosphorylation
Figure 9.6-2
Electronscarried
via NADH
Electrons carriedvia NADH and
FADH2
Citricacidcycle
Pyruvateoxidation
Acetyl CoA
Glycolysis
Glucose Pyruvate
CYTOSOL MITOCHONDRION
ATP ATP
Substrate-levelphosphorylation
Substrate-levelphosphorylation
Figure 9.6-3
Electronscarried
via NADH
Electrons carriedvia NADH and
FADH2
Citricacidcycle
Pyruvateoxidation
Acetyl CoA
Glycolysis
Glucose Pyruvate
Oxidativephosphorylation:electron transport
andchemiosmosis
CYTOSOL MITOCHONDRION
ATP ATP ATP
Substrate-levelphosphorylation
Substrate-levelphosphorylation
Oxidative phosphorylation
• The process that generates most of the ATP is called oxidative phosphorylation because it is powered by redox reactions
• A smaller amount of ATP is formed in glycolysis and the citric acid cycle by substrate-level phosphorylation
• For each molecule of glucose degraded to CO2 and water by respiration, the cell makes up to 32 molecules of ATP
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BioFlix: Cellular Respiration
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
• Glycolysis occurs whether or not O2 is present
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Figure 9.8
Energy Investment Phase
Glucose
2 ADP 2 P
4 ADP 4 P
Energy Payoff Phase
2 NAD+ 4 e 4 H+
2 Pyruvate 2 H2O
2 ATP used
4 ATP formed
2 NADH 2 H+
NetGlucose 2 Pyruvate 2 H2O
2 ATP
2 NADH 2 H+ 2 NAD+ 4 e 4 H+
4 ATP formed 2 ATP used
Figure 9.9a
Glycolysis: Energy Investment Phase
ATPGlucose Glucose 6-phosphate
ADP
Hexokinase
1
Fructose 6-phosphate
Phosphogluco-isomerase
2
Figure 9.9b
Glycolysis: Energy Investment Phase
ATPFructose 6-phosphate
ADP
3
Fructose 1,6-bisphosphate
Phospho-fructokinase
4
5
Aldolase
Dihydroxyacetonephosphate
Glyceraldehyde3-phosphate
Tostep 6Isomerase
Figure 9.9c
Glycolysis: Energy Payoff Phase
2 NADH2 ATP
2 ADP 2
2
2 NAD + 2 H
2 P i
3-Phospho-glycerate
1,3-Bisphospho-glycerate
Triosephosphate
dehydrogenase
Phospho-glycerokinase
67
Figure 9.9d
Glycolysis: Energy Payoff Phase
2 ATP
2 ADP2222
2 H2O
PyruvatePhosphoenol-pyruvate (PEP)
2-Phospho-glycerate
3-Phospho-glycerate
89
10
Phospho-glyceromutase
Enolase Pyruvatekinase
• In the presence of O2, pyruvate enters the mitochondrion (in eukaryotic cells) where the oxidation of glucose is completed
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Oxidation of Pyruvate to Acetyl CoA
• Before the citric acid cycle can begin, pyruvate must be converted to acetyl Coenzyme A (acetyl CoA), which links glycolysis to the citric acid cycle
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Figure 9.10
Pyruvate
Transport protein
CYTOSOL
MITOCHONDRION
CO2 Coenzyme A
NAD + HNADH Acetyl CoA
1
2
3
• The cycle oxidizes organic fuel derived from pyruvate, generating 1 ATP, 3 NADH, and 1 FADH2 per turn
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The Citric Acid Cycle - Krebs
Figure 9.11Pyruvate
NAD
NADH
+ HAcetyl CoA
CO2
CoA
CoA
CoA
2 CO2
ADP + P i
FADH2
FAD
ATP
3 NADH
3 NAD
Citricacidcycle
+ 3 H
• 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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Figure 9.12-1
1
Acetyl CoA
Citrate
Citricacidcycle
CoA-SH
Oxaloacetate
Figure 9.12-2
1
Acetyl CoA
CitrateIsocitrate
Citricacidcycle
H2O
2
CoA-SH
Oxaloacetate
Figure 9.12-3
1
Acetyl CoA
CitrateIsocitrate
-Ketoglutarate
Citricacidcycle
NADH+ H
NAD
H2O
3
2
CoA-SH
CO2
Oxaloacetate
Figure 9.12-4
1
Acetyl CoA
CitrateIsocitrate
-Ketoglutarate
SuccinylCoA
Citricacidcycle
NADH
NADH
+ H
+ H
NAD
NAD
H2O
3
2
4
CoA-SH
CO2
CoA-SH
CO2
Oxaloacetate
Figure 9.12-5
1
Acetyl CoA
CitrateIsocitrate
-Ketoglutarate
SuccinylCoA
Succinate
Citricacidcycle
NADH
NADH
ATP
+ H
+ H
NAD
NAD
H2O
ADP
GTP GDP
P i
3
2
4
5
CoA-SH
CO2
CoA-SH
CoA-SH
CO2
Oxaloacetate
Figure 9.12-6
1
Acetyl CoA
CitrateIsocitrate
-Ketoglutarate
SuccinylCoA
Succinate
Fumarate
Citricacidcycle
NADH
NADH
FADH2
ATP
+ H
+ H
NAD
NAD
H2O
ADP
GTP GDP
P i
FAD
3
2
4
5
6
CoA-SH
CO2
CoA-SH
CoA-SH
CO2
Oxaloacetate
Figure 9.12-7
1
Acetyl CoA
CitrateIsocitrate
-Ketoglutarate
SuccinylCoA
Succinate
Fumarate
Malate
Citricacidcycle
NADH
NADH
FADH2
ATP
+ H
+ H
NAD
NAD
H2O
H2O
ADP
GTP GDP
P i
FAD
3
2
4
5
6
7
CoA-SH
CO2
CoA-SH
CoA-SH
CO2
Oxaloacetate
Figure 9.12-8
NADH
1
Acetyl CoA
CitrateIsocitrate
-Ketoglutarate
SuccinylCoA
Succinate
Fumarate
Malate
Citricacidcycle
NAD
NADH
NADH
FADH2
ATP
+ H
+ H
+ H
NAD
NAD
H2O
H2O
ADP
GTP GDP
P i
FAD
3
2
4
5
6
7
8
CoA-SH
CO2
CoA-SH
CoA-SH
CO2
Oxaloacetate
Figure 9.12a
Acetyl CoA
Oxaloacetate
CitrateIsocitrate
H2O
CoA-SH
1
2
Figure 9.12b
Isocitrate
-Ketoglutarate
SuccinylCoA
NADH
NADH
NAD
NAD
+ H
CoA-SH
CO2
CO2
3
4
+ H
Figure 9.12c
Fumarate
FADH2
CoA-SH6
SuccinateSuccinyl
CoA
FAD
ADP
GTP GDP
P i
ATP
5
Figure 9.12d
Oxaloacetate8
Malate
Fumarate
H2O
NADH
NAD
+ H
7
Electron Transport Chain
• The electron transport chain is in the inner membrane (cristae) of the mitochondrion
• 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
• 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
• Electrons are passed through a number of proteins including cytochromes (each with an iron atom) to O2
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Figure 9.13
NADH
FADH2
2 H + 1/2 O2
2 e
2 e
2 e
H2O
NAD
Multiproteincomplexes
(originally from NADH or FADH2)
III
III
IV
50
40
30
20
10
0
Fre
e e
ner
gy
(G)
rela
tiv
e to
O2 (
kcal
/mo
l)
FMN
FeS FeS
FAD
Q
Cyt b
Cyt c1
Cyt c
Cyt a
Cyt a3
FeS
Chemiosmosis: ATP production through H+
• Electron transfer causes proteins to pump H+ from the mitochondrial matrix to the intermembrane space
• H+ then moves back across the membrane, passing through the proton, ATP synthase
• ATP synthase uses the exergonic flow of H+ to drive phosphorylation of ATP
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Figure 9.14INTERMEMBRANE SPACE
Rotor
StatorH
Internalrod
Catalyticknob
ADP+P i ATP
MITOCHONDRIAL MATRIX
Figure 9.15
Proteincomplexof electroncarriers
(carrying electronsfrom food)
Electron transport chain
Oxidative phosphorylation
Chemiosmosis
ATPsynth-ase
I
II
III
IVQ
Cyt c
FADFADH2
NADH ADP P i
NAD
H
2 H + 1/2O2
H
HH
21
H
H2O
ATP
An Accounting of ATP Production by Cellular Respiration
• glucose NADH electron transport chain ATP
About 30-32 total ATP are made (26/28 via ETC and 4 via substrate level)
• Takes energy to move ATP into cytosol after it is made, takes E to move pyruvate in
• Depends on shuttle systems that bring the NADH electrons into the mitochondria
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Figure 9.16
Electron shuttlesspan membrane
MITOCHONDRION2 NADH
2 NADH 2 NADH 6 NADH
2 FADH2
2 FADH2
or
2 ATP 2 ATP about 26 or 28 ATP
Glycolysis
Glucose 2 Pyruvate
Pyruvate oxidation
2 Acetyl CoACitricacidcycle
Oxidativephosphorylation:electron transport
andchemiosmosis
CYTOSOL
Maximum per glucose:About
30 or 32 ATP
Anaerobic respiration - Fermentation
• Most cellular respiration requires O2 to produce ATP
• Without O2, no electron transport chain
• Fermentation uses substrate-level 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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Animation: Fermentation Overview Right-click slide / select “Play”
Figure 9.17
2 ADP 2 ATP
Glucose Glycolysis
2 Pyruvate
2 CO22
2 NADH
2 Ethanol 2 Acetaldehyde
(a) Alcohol fermentation (b) Lactic acid fermentation
2 Lactate
2 Pyruvate
2 NADH
Glucose Glycolysis
2 ATP2 ADP 2 Pi
NAD
2 H
2 Pi
2 NAD
2 H
• 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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Comparing Fermentation with Anaerobic and Aerobic Respiration
• All use glycolysis (net ATP = 2) to oxidize glucose and harvest chemical energy of food
• In all three, NAD+ is the oxidizing agent that accepts electrons during glycolysis
• 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 32 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
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Figure 9.18Glucose
CYTOSOLGlycolysis
Pyruvate
No O2 present:Fermentation
O2 present: Aerobic cellular respiration
Ethanol,lactate, or
other products
Acetyl CoA
MITOCHONDRION
Citricacidcycle
The Evolutionary Significance of Glycolysis
• Ancient prokaryotes are thought to have used glycolysis long before there was oxygen in the atmosphere
• Very little O2 was available in the atmosphere until about 2.7 billion years ago, so early prokaryotes likely used only glycolysis to generate ATP
• Glycolysis is a very ancient process
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Getting E from other sources
• electrons from many kinds of organic molecules funnel into cellular respiration
• 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
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Figure 9.19CarbohydratesProteins
Fattyacids
Aminoacids
Sugars
Fats
Glycerol
Glycolysis
Glucose
Glyceraldehyde 3- P
NH3 Pyruvate
Acetyl CoA
Citricacidcycle
Oxidativephosphorylation
Regulation of Cellular Respiration via Feedback Mechanisms
• If ATP concentration begins to drop, respiration speeds up; when there is plenty of ATP, respiration slows down
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Figure 9.20
Phosphofructokinase
Glucose
GlycolysisAMP
Stimulates
Fructose 6-phosphate
Fructose 1,6-bisphosphate
Pyruvate
Inhibits Inhibits
ATP Citrate
Citricacidcycle
Oxidativephosphorylation
Acetyl CoA
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