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How Cells Release Stored
Energy
Chapter 7
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 Pathways Start
with Glycolysis
• Glycolysis occurs in cytoplasm
• Reactions are catalyzed by enzymes
Glucose
(six carbons)
2 Pyruvate
(three carbons)
Overview of Aerobic
Respiration
C6H1206 + 6O2
6CO2 + 6H20
glucose
carbon
oxygen
dioxide
water
Overview of Aerobic Respiration
CYTOPLASM
glucose
ATP
GLYCOLYSIS
energy input to
start reactions
e- + H+
(2 ATP net)
2 pyruvate
2 NADH
MITOCHONDRION
2 NADH
8 NADH
2 FADH2
e-
e- + H+
2
CO2
e- + H+
KREBS
CYCLE
e- + H+
ELECTRON
TRANSPORT
PHOSPHORYLATION
H+
4
CO2
2
32
ATP
ATP
water
e- + oxygen
TYPICAL ENERGY YIELD: 36 ATP
The Role of Coenzymes
• NAD+ and FAD accept electrons and
hydrogen from intermediates during the
first two stages
• When reduced, they are NADH and
FADH2
• In the third stage, these coenzymes
deliver the electrons and hydrogen to
the transport system
Glucose
• A simple sugar
(C6H12O6)
• Atoms held
together by
covalent bonds
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
glucose
ATP
ADP
P
glucose-6-phosphate
P
fructose-6-phosphate
ATP
ADP
P
fructose-1,6-bisphosphate
2 ATP invested
EnergyReleasing
Steps
PGAL
PGAL
NAD+
NADH
Pi
P
P
NADH
Pi
P
1,3-bisphosphoglycerate
ADP
NAD+
ATP
P
1,3-bisphosphoglycerate
ADP
ATP
substrate-level
phosphorylation
2 ATP invested
P
P
3-phosphoglycerate
3-phosphoglycerate
P
P
2-phosphoglycerate
H2
O
2-phosphoglycerate
H2
O
PEP
PEP
P
ADP
ATP
P
ADP
ATP
substrate-level
phosphorylation
2 ATP invested
pyruvate
pyruvate
Net Energy Yield
from Glycolysis
• Energy requiring steps:
2 ATP invested
• Energy releasing steps:
2 NADH formed
4 ATP formed
• Net yield is 2 ATP and 2 NADH
PREPARATORY
STEPS
pyruvate
Second-Stage
Reactions
coenzyme A (CoA)
NAD+
(CO2)
NADH
CoA
Acetyl–CoA
KREBS CYCLE
• Occur in the
mitochondria
• Pyruvate is
broken down to
carbon dioxide
• More ATP is
formed
• More coenzymes
are reduced
CoA
oxaloacetate
citrate H O
2
NADH
H2O
NAD+
malate
NAD+
H2O
isocitrate
NADH
fumarate
FADH2
FAD
a-ketogluterate
CoA
NAD+
NADH
succinate
CoA
succinyl–CoA
ATP
ADP + phosphate
group (from GTP)
Two Parts of Second Stage
• 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 + coenzyme A + NAD+
acetyl-CoA + NADH + CO2
• One of the carbons from pyruvate is released
in CO2
• Two carbons are attached to coenzyme A and
continue on to the Krebs cycle
What is Acetyl-CoA?
• A two-carbon acetyl group linked to
coenzyme A
CH3
Acetyl group
C=O
Coenzyme A
The Krebs Cycle
(for each pyruvate)
Overall Reactants
Overall Products
•
•
•
•
•
•
•
•
•
Acetyl-CoA
3 NAD+
FAD
ADP and Pi
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 is formed
• Four-carbon oxaloacetate is
regenerated
Coenzyme Reductions During
First Two Stages
• Glycolysis
• Preparatory
reactions
• Krebs cycle
2 NADH
2 NADH
2 FADH2 + 6 NADH
• Total
2 FADH2 + 10 NADH
For each glucose = 2 pyruvates
Electron Transport
Phosphorylation
• Occurs in the mitochondria
• Coenzymes deliver electrons to electron
transport systems
• Electron transport sets up H+ ion
gradients
• Flow of H+ down gradients powers ATP
formation
Electron Transport
• Electron transport systems are embedded in
inner mitochondrial compartment
• NADH and FADH2 give up electrons that they
picked up in earlier stages to electron
transport system
• Electrons are transported through the system
• The final electron acceptor is oxygen
Creating an H+ Gradient
OUTER COMPARTMENT
NADH
INNER COMPARTMENT
Making ATP:
Chemiosmotic Model
ATP
INNER
COMPARTMENT
ADP
+
Pi
Importance of Oxygen
• Electron transport phosphorylation
requires the presence of oxygen
• Oxygen withdraws spent electrons
from the electron transport system,
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 from
Coenzyme Reductions
What are the sources of electrons used to
generate the 32 ATP in the final stage?
– 4 ATP - generated using electrons released
during glycolysis and carried by NADH
– 28 ATP - generated using electrons formed
during second-stage reactions and carried
by NADH and FADH2
Efficiency of
Aerobic 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
Overview of Aerobic Respiration
CYTOPLASM
glucose
ATP
GLYCOLYSIS
energy input to
start reactions
e- + H+
(2 ATP net)
2 pyruvate
2 NADH
MITOCHONDRION
2 NADH
8 NADH
2 FADH2
e-
e- + H+
2
CO2
e- + H+
KREBS
CYCLE
e- + H+
ELECTRON
TRANSPORT
PHOSPHORYLATION
H+
4
CO2
2
32
ATP
ATP
water
e- + oxygen
TYPICAL ENERGY YIELD: 36 ATP
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+
Lactate Fermentation
GLYCOLYSIS
C6H12O6
2
ATP
energy input
2 NAD+
2 ADP
2
4
NADH
ATP
energy output
2 pyruvate
2 ATP net
LACTATE
FORMATION
electrons, hydrogen
from NADH
2 lactate
GLYCOLYSIS
Alcoholic
Fermentation
C6H12O6
2
ATP
energy input
2 NAD+
2 ADP
2
4
NADH
ATP
2 pyruvate
energy output
2 ATP net
ETHANOL
FORMATION
2 H2O
2 CO2
2 acetaldehyde
electrons, hydrogen
from NADH
2 ethanol
Carbohydrate Breakdown
and Storage
• Glucose is absorbed into blood
• Pancreas releases insulin
• Insulin stimulates glucose uptake by cells
• Cells convert glucose to glucose-6-phosphate
• This traps glucose in cytoplasm where it can
be used for glycolysis
Making Glycogen
• If glucose intake is high, ATP-making
machinery goes into high gear
• When ATP levels rise high enough, glucose6-phosphate is diverted into glycogen
synthesis (mainly in liver and muscle)
• Glycogen is the main storage polysaccharide
in animals
Using Glycogen
• When blood levels of glucose decline,
pancreas releases glucagon
• Glucagon stimulates liver cells to convert
glycogen back to glucose and to release it to
the blood
• (Muscle cells do not release their stored
glycogen)
Energy Reserves
• Glycogen makes up only about 1 percent of the
body’s energy reserves
• Proteins make up 21 percent of energy reserves
• Fat makes up the bulk of reserves (78 percent)
Energy from Proteins
• Proteins are broken down to amino acids
• Amino acids are broken apart
• Amino group is removed, ammonia forms, is
converted to urea and excreted
• Carbon backbones can enter the Krebs cycle
or its preparatory reactions
Energy from Fats
• Most stored fats are triglycerides
• Triglycerides are broken down to glycerol and
fatty acids
• Glycerol is converted to PGAL, an
intermediate of glycolysis
• Fatty acids are broken down and converted to
acetyl-CoA, which enters Krebs cycle
GLYCOLYSIS
outer mitochondrial compartment
glucose
inner mitochondrial compartment
ATP
2NAD+
2 PGAL
ATP
2 NADH
ATP
a In glycolysis, 2 ATP used;
4 ATP form by substratelevel phosphorylation.
So net yield is 2 ATP.
2 NADH
for in the
cytoplasm
2
2 pyruvate
ATP
b In Krebs cycle of
second stage, 2 ATP
form by substrate-level
phosphorylation.
cytoplasm
2 CO2
2
FADH2
2 acetyl-CoA
2 NADH
ATP
2
KREBS
CYCLE
Electrons and hydrogen from cytoplasmic
NADH are shuttled into inner compartment.
Two coenzymes already inside transfer the
electrons to a transport system.
4
c In third stage, NADH
from glycolysis used to
form 4 ATP by electron
transport phosphorylation.
Coenzymes (8 NAD+, 2 FAD
total) transfer electrons and
hydrogen from remnants of
pyruvate to a transfer system.
6 NADH
ATP
2
FADH2
ATP
ATP
ADP + Pi
ATP
28
ATP
Electrons
flow through
transport
system
Transport
system
pumps H+
to the outer
compartment
ELECTRON TRANSPORT
PHOSPHORYLATION
d In third stage, NADH
and FADH2 from second
stage used to make 28 ATP
by electron transport
Phosphorylation.
36 ATP
At ATP synthases H+ flowing
back in drives ATP formation
Fig. 7.8, p. 139
FOOD
fats
fatty
acids
glycogen
glycerol
complex
carbohydrates
proteins
simple sugars,
e.g., glucose
amino acids
NH
glucose-6-phosphate
3
urea
carbon
backbones
PGAL
2 ATP
4 ATP
GLYCOLYSIS
pyruvate
NADH
acetyl-CoA
NADH
CO2
NADH
FADH2
KREBS
CYCLE
e-
2 ATP
CO2
ATP
ELECTRON
TRANSPORT
PHOSPHORYLATION
H+
e- oxygen
ATP
ATP
many ATP
water
Fig. 7.12, p. 121