chapter 8: how cells release stored energy · chapter 8: how cells release stored energy. atp is...
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Chapter 8: How Cells Release Stored Energy
ATP Is Universal Energy Source
• Photosynthesizers get energy from the sun
• Animals get energy second- or third-hand from plants or other organisms
• Regardless, the energy is converted to the chemical bond energy of ATP
Making ATP
• Plants make ATP during photosynthesis
• Cells of all organisms make ATP by breaking
down carbohydrates, fats, and protein
Main Types of Energy-Releasing Pathways
Anaerobic pathways
• Evolved first
• Don’t require oxygen
• Start with glycolysis in cytoplasm
• Completed in cytoplasm
Aerobic pathways
• Evolved later
• Require oxygen
• Start with glycolysis in cytoplasm
• Completed in mitochondria
start (glycolysis) in
cytoplasm
completed in
mitochondrion
start (glycolysis) in
cytoplasm
completed in
cytoplasm
Aerobic
Respiration
Anaerobic Energy-
Releasing Pathways
Fig. 8-2, p.124
Main Types of
Energy-Releasing Pathways
Summary Equation for Aerobic Respiration
C6H1206 + 6O2 6CO2 + 6H20glucose oxygen carbon dioxide water
Overview of Aerobic Respiration
CYTOPLASM
Glycolysis
Electron Transfer
Phosphorylation
Krebs
Cycle 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
Fig. 8-3, p. 135
Glucose
• A simple sugar
(C6H12O6)
• Atoms held together by covalent bonds
In-text figure
Page 126
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
ATP
ATP
2 ATP invested
ENERGY-REQUIRING STEPS
OF GLYCOLYSISglucose
ADP
ADP
P
P
P
P
glucose–6–phosphate
fructose–6–phosphate
fructose–1,6–bisphosphate DHAP
Fig. 8-4b, p.127
Glycolysis
ATPADP
ENERGY-RELEASING STEPS
OF GLYCOLYSIS
NAD+
P
PGAL
1,3–bisphosphoglycerate
substrate-level
phsphorylation
Pi
1,3–bisphosphoglycerate
ATP
NADHNADH
P
PGAL
NAD+
Pi
P PP P
3–phosphoglycerate 3–phosphoglycerate
P P
2 ATP invested
ADP
Fig. 8-4c, p.127
Glycolysis
2 ATP produced
ATPADP
P
substrate-level
phsphorylation
2–phosphoglycerate
ATP
P
pyruvate pyruvate
ADP
P P
2–phosphoglycerate
H2O H2O
PEP PEP
Fig. 8-4d, p.127
Glycolysis
Energy-Requiring Steps
2 ATP invested
Energy-Requiring Steps of Glycolysis
glucose
PGAL PGAL
PP
ADP
P
ATP
glucose-6-phosphate
Pfructose-6-phosphate
ATP
fructose1,6-bisphosphateP P
ADP
Figure 8-4(2)
Page 127
Energy-Releasing
Steps
ADPATP
pyruvate
ADPATP
pyruvate
H2
OP
PEP
H2
OP
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 127
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
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
Fig. 8-5a, p.128
mitochondrion
mitochondrion
Second Stage Reactions
Fig. 8-5b, p.128
Second Stage Reactions
inner
mitochondrial
membrane
outer
mitochondrial
membrane
inner
compartment
outer
compartment
Fig. 8-6a, p.128
Second Stage Reactions
Two pyruvates cross the innermitochondrial membrane.
outer mitochondrialcompartment
NADH
NADH
FADH2
ATP
2
6
2
2
Krebs
Cycle
6 CO2
inner mitochondrialcompartment
Eight NADH, two FADH 2,
and two ATP are the payoff
from the complete break-
down of two pyruvates in the
second-stage reactions.
The six carbon atoms from two pyruvates diffuse out
of the mitochondrion, then out of the cell, in six CO
Fig. 8-6b, p.128
Preparatory Reactions
pyruvate
NAD+
NADH
coenzyme A (CoA)
O O carbon dioxide
CoAacetyl-CoA
glucose
GLYCOLYSIS
pyruvate
KREBS
CYCLE
ELECTRON TRANSFER
PHOSPHORYLATION
Fig. 8-7b, p.129
Preparatory
Reactions
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
a-ketoglutarate
Figure 8-6
Page 129
The Krebs Cycle
Overall Reactants
• Acetyl-CoA
• 3 NAD+
• FAD
• ADP and Pi
Overall Products
• Coenzyme A
• 2 CO2
• 3 NADH
• FADH2
• ATP
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
Electron Transfer Phosphorylation
• 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
Fig. 8-8b, p.130
OUTER COMPARTMENT
INNER COMPARTMENT
Electron Transfer Chain ATP Synthase
ATP
H+
H+H+
H+
H+
H+
H+H+
H+H+
H+
H+H+
H+
H+
H+
H+
H+H+
H+
H+
H+
H+
H+
NADH + H+ NAD+ + 2H+ FAD + 2H+FADH2 2H+ + 1/2 02 H2O ADP + Pi
e-e- e-
Phosphorylation
glucose
glycolysis
e–
KREBS
CYCLE
electrontransfer
phosphorylation
2 PGAL
2 pyruvate
2 NADH
2 CO2
ATP
ATP
2 FADH2
H+
2 NADH
6 NADH
2 FADH2
2 acetyl-CoA
ATP2 Krebs
Cycle
4 CO2
ATP
ATP
ATP
36
ADP
+ Pi
H+
H+
H+
H+
H+
H+
H+
H+
Fig. 8-9, p.131
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
Efficiency ofAerobic Respiration
• 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
Anaerobic Pathways
• Do not use oxygen
• Produce less ATP than aerobic pathways
• Two types
– Fermentation pathways
– Anaerobic electron transport
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+
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
Fig. 8-10d, p.132
Alcoholic
Fermentation
Fig. 8-10a, p.132
Alcoholic
Fermentation
Fig. 8-10b, p.132
Alcoholic
Fermentation
Fig. 8-10c, p.132
Alcoholic Fermentation
C6H12O6
ATP
ATP
NADH
2 lactate
electrons, hydrogen
from NADH
2 NAD+
2
2 ADP
2 pyruvate
2
4
energy output
energy input
glycolysis
lactate
fermentation
2 ATP net
Fig. 8-11, p.133
Lactate
Fermentation
Fig. 8-12, p.133
Lactate Fermentation
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
p.134
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
Fig. 8-13a, p.135
Alternative Energy Sources
FOOD
fats glycogencomplex
carbohydrates proteins
simple sugars(e.g., glucose) amino acids
glucose-6-phosphate
carbon
backbones
NH3
urea
ATP
(2 ATP net)
PGAL
glycolysisATP2
glycerolfatty acids
NADH pyruvate
acetyl-CoA
NADH CO2
Krebs
CycleNADH,
FADH2
CO2
ATP
ATP
ATP
many ATP
waterH+
e– + oxygen
e–
4
ATP2
Fig. 8-13b, p.135
electron transfer
phosphorylation
Alternative
Energy
Sources
Evolution of Metabolic Pathways
• 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
p.136b
Processes Are Linked