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How Cells Release Stored Energy
Chapter 8
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
Summary Equation for Aerobic Respiration
C6H1206 + 6O2 6CO2 + 6H20 glucose oxygen carbon water
dioxide
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
The Role of Coenzymes
• NAD+ and FAD accept electrons and
hydrogen
• Become NADH and FADH2
• Deliver electrons and hydrogen to the
electron transfer chain
• A simple sugar
(C6H12O6)
• Atoms held together by covalent bonds
Glucose
In-text figurePage 136
8.2 GLYCOLYSIS
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
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
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
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
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
Preparatory Reactions
pyruvate
NAD+
NADH
coenzyme A (CoA)
O O carbon dioxide
CoAacetyl-CoA
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
The Krebs Cycle
Overall Products
• Coenzyme A
• 2 CO2
• 3 NADH
• FADH2
• ATP
Overall Reactants
• Acetyl-CoA• 3 NAD+
• FAD
• ADP and Pi
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
Coenzyme Reductions during First Two Stages
• Glycolysis 2 NADH• Preparatory
reactions 2 NADH• Krebs cycle 2 FADH2 + 6 NADH
• Total 2 FADH2 + 10 NADH
• 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
Creating an H+ Gradient
NADH
OUTER COMPARTMENT
INNER COMPARTMENT
Making ATP: Chemiosmotic Model
ATP
ADP+Pi
INNER COMPARTMENT
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
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
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+
• 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
• Do not use oxygen
• Produce less ATP than aerobic pathways
• Two types
– Fermentation pathways
– Anaerobic electron transport
8.5 Anaerobic Pathways
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+
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
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
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
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
• 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
8.7 Processes Are Linked
sunlight energy
water+
carbondioxide
PHOTOSYNTHESIS
AEROBICRESPIRATION
sugarmolecules oxygen
In-text figurePage 146