how cells release stored energy
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How Cells Release Stored Energy. Chapter 8. Aerobic pathways Evolved later Require oxygen Start with glycolysis in cytoplasm Completed in mitochondria. Anaerobic pathways Evolved first Don’t require oxygen Start with glycolysis in cytoplasm Completed in cytoplasm. - PowerPoint PPT PresentationTRANSCRIPT
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How Cells Release Stored Energy
Chapter 8
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8.1 Main Types of Energy-Releasing Pathways
Aerobic pathways
• Evolved later• Require oxygen• Start with glycolysis
in cytoplasm• Completed in
mitochondria
Anaerobic pathways
• Evolved first• Don’t require oxygen• Start with glycolysis in
cytoplasm• Completed in
cytoplasm
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Summary Equation for Aerobic Respiration
C6H1206 + 6O2 6CO2 + 6H20 glucose oxygen carbon water
dioxide
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Overview of Aerobic Respiration
CYTOPLASM
Glycolysis
Electron Transfer
Phosphorylation
KrebsCycle ATP
ATP
2 CO2
4 CO2
2
32
water
2 NADH
8 NADH
2 FADH2
2 NADH 2 pyruvate
e- + H+
e- + oxygen
(2 ATP net)
glucose
Typical Energy Yield: 36 ATP
e-
e- + H+
e- + H+
ATP
H+
e- + H+
ATP2 4
Figure 8.3Page 135
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The Role of Coenzymes
• NAD+ and FAD accept electrons and
hydrogen
• Become NADH and FADH2
• Deliver electrons and hydrogen to the
electron transfer chain
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• A simple sugar
(C6H12O6)
• Atoms held together by covalent bonds
Glucose
In-text figurePage 136
8.2 GLYCOLYSIS
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Glycolysis Occurs in Two Stages
• Energy-requiring steps
– ATP energy activates glucose and its six-carbon
derivatives
• Energy-releasing steps
– The products of the first part are split into three-
carbon pyruvate molecules
– ATP and NADH form
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Energy-Requiring Steps 2 ATP invested
Energy-Requiring Steps of Glycolysis
glucose
PGAL PGALPP
ADP
P
ATP
glucose-6-phosphate
Pfructose-6-phosphate
ATP
fructose1,6-bisphosphateP P
ADP
Figure 8.4(2)Page 137
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Energy-Releasing
Steps
ADPATP
pyruvate
ADPATP
pyruvate
H2OP
PEP
H2OP
PEP
P
2-phosphoglycerate
P
2-phosphoglycerate
ADPATP
P3-phosphoglycerate
ADPATP
P3-phosphoglycerate
NAD+
NADHPi
1,3-bisphosphoglycerateP P
NAD+
NADHPi
1,3-bisphosphoglycerateP P
PGALP
PGALP
Figure 8.4 Page 137
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Glycolysis: Net Energy Yield
Energy requiring steps: 2 ATP invested
Energy releasing steps:2 NADH formed 4 ATP formed
Net yield is 2 ATP and 2 NADH
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8.3 Second Stage Reactions
• Preparatory reactions– Pyruvate is oxidized into two-carbon acetyl
units and carbon dioxide– NAD+ is reduced
• Krebs cycle– The acetyl units are oxidized to carbon
dioxide– NAD+ and FAD are reduced
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Preparatory Reactions
pyruvate
NAD+
NADH
coenzyme A (CoA)
O O carbon dioxide
CoAacetyl-CoA
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Krebs Cycle
NAD+
NADH
=CoAacetyl-CoA
oxaloacetate citrate
CoA
H2O
malate isocitrate
H2O
H2O
FAD
FADH2
fumarate
succinate
ADP + phosphate groupATP
succinyl-CoA
O O
CoANAD+
NADH
O ONAD+
NADH
-ketoglutarate
Figure 8.6Page 139
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The Krebs Cycle
Overall Products
• Coenzyme A
• 2 CO2
• 3 NADH
• FADH2
• ATP
Overall Reactants
• Acetyl-CoA• 3 NAD+
• FAD
• ADP and Pi
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Results of the Second Stage
• All of the carbon molecules in pyruvate end up in carbon dioxide
• Coenzymes are reduced (they pick up electrons and hydrogen)
• One molecule of ATP forms
• Four-carbon oxaloacetate regenerates
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Coenzyme Reductions during First Two Stages
• Glycolysis 2 NADH• Preparatory
reactions 2 NADH• Krebs cycle 2 FADH2 + 6 NADH
• Total 2 FADH2 + 10 NADH
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• Occurs in the mitochondria
• Coenzymes deliver electrons to electron transfer chains
• Electron transfer sets up H+ ion gradients
• Flow of H+ down gradients powers ATP formation
8.4 Electron Transfer Phosphorylation
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Creating an H+ Gradient
NADH
OUTER COMPARTMENT
INNER COMPARTMENT
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Making ATP: Chemiosmotic Model
ATP
ADP+Pi
INNER COMPARTMENT
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Importance of Oxygen
• Electron transport phosphorylation requires the presence of oxygen
• Oxygen withdraws spent electrons from the electron transfer chain, then combines with H+ to form water
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Summary of Energy Harvest(per molecule of glucose)
• Glycolysis– 2 ATP formed by substrate-level phosphorylation
• Krebs cycle and preparatory reactions– 2 ATP formed by substrate-level phosphorylation
• Electron transport phosphorylation– 32 ATP formed
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Energy Harvest Varies
• NADH formed in cytoplasm cannot enter mitochondrion
• It delivers electrons to mitochondrial membrane
• Membrane proteins shuttle electrons to NAD+ or FAD inside mitochondrion
• Electrons given to FAD yield less ATP than those given to NAD+
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• 686 kcal of energy are released
• 7.5 kcal are conserved in each ATP
• When 36 ATP form, 270 kcal (36 X 7.5) are
captured in ATP
• Efficiency is 270 / 686 X 100 = 39 percent
• Most energy is lost as heat
Efficiency of Aerobic Respiration
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• Do not use oxygen
• Produce less ATP than aerobic pathways
• Two types
– Fermentation pathways
– Anaerobic electron transport
8.5 Anaerobic Pathways
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Fermentation Pathways
• Begin with glycolysis
• Do not break glucose down completely to
carbon dioxide and water
• Yield only the 2 ATP from glycolysis
• Steps that follow glycolysis serve only to
regenerate NAD+
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Lactate Fermentation
C6H12O6
ATP
ATPNADH
2 lactate
electrons, hydrogen from NADH
2 NAD+
2
2 ADP
2 pyruvate
2
4
energy output
energy input
GLYCOLYSIS
LACTATE FORMATION
2 ATP net
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Alcoholic Fermentation
C6H12O6
ATP
ATPNADH
2 acetaldehyde
electrons, hydrogen from NADH
2 NAD+
2
2 ADP
2 pyruvate
2
4
energy output
energy input
GLYCOLYSIS
ETHANOL FORMATION
2 ATP net
2 ethanol
2 H2O
2 CO2
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Anaerobic Electron Transport
• Carried out by certain bacteria
• Electron transfer chain is in bacterial plasma membrane
• Final electron acceptor is compound from environment (such as nitrate), not oxygen
• ATP yield is low
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FOOD
complex carbohydrates
simple sugars
pyruvate
acetyl-CoA
glycogenfats proteins
amino acids
carbon backbones
fatty acids
glycerol
NH3
PGAL
glucose-6-phosphate
GLYCOLYSIS
KREBS CYCLE
urea
Figure 8.11Page 145
8.6 ALTERNATIVE
ENERGY SOURCES
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• When life originated, atmosphere had little
oxygen
• Earliest organisms used anaerobic pathways
• Later, noncyclic pathway of photosynthesis
increased atmospheric oxygen
• Cells arose that used oxygen as final
acceptor in electron transport
Evolution of Metabolic Pathways
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8.7 Processes Are Linked
sunlight energy
water+
carbondioxide
PHOTOSYNTHESIS
AEROBICRESPIRATION
sugarmolecules oxygen
In-text figurePage 146