How Cells Release
Chemical Energy
Chapter 8
Impacts, Issues:
When Mitochondria Spin Their Wheels
 More than forty disorders related to defective
mitochondria are known (such as Friedreich’s
ataxia); many of those afflicted die young
8.1 Overview of Carbohydrate
Breakdown Pathways
 Photoautotrophs make ATP during
photosynthesis and use it to synthesize glucose
and other carbohydrates
 Most organisms, including photoautotrophs,
make ATP by breaking down glucose and other
organic compounds
Comparison of the Main Pathways
 Aerobic respiration
• Aerobic metabolic pathways (using oxygen) are
used by most eukaryotic cells
 Fermentation
• Anaerobic metabolic pathways (occur in the
absence of oxygen) are used by prokaryotes and
protists in anaerobic habitats
Comparison of the Main Pathways
 Aerobic respiration and fermentation both begin
with glycolysis, which converts one molecule of
glucose into two molecules of pyruvate
 After glycolysis, the two pathways diverge
• Fermentation is completed in the cytoplasm,
yielding 2 ATP per glucose molecule
• Aerobic respiration is completed in mitochondria,
yielding 36 ATP per glucose molecule
Comparison of the Main Pathways
Fig. 8-2b, p. 124
A Carbohydrate breakdown pathways
start in the cytoplasm, with glycolysis.
B Fermentation pathways
are completed in the semifluid matrix of the cytoplasm.
C In eukaryotes, aerobic
respiration is completed
inside mitochondria.
Fig. 8-2b, p. 124
Animation: Where pathways start and
finish
Overview of Aerobic Respiration
 Three stages
• Glycolysis
• Acetyl-CoA formation and Krebs cycle
• Electron transfer phosphorylation (ATP formation)
C6H12O6 (glucose) + O2 (oxygen)
→
CO2 (carbon dioxide) + H2O (water)
• Coenzymes NADH and FADH2 carry electrons
and hydrogen
Overview of Aerobic Respiration
Cytoplasm
glucose
2 ATP
ATP
Glycolysis
4 ATP
(2 net)
ATP
2 NADH 2 pyruvate
Krebs
Cycle
6 CO2
2 ATP
ATP
8 NADH, 2 FADH2
ATP
oxygen
Electron Transfer
Phosphorylation
32 ATP
A The first stage, glycolysis, occurs in
the cell’s cytoplasm. Enzymes convert
a glucose molecule to 2 pyruvate for
a net yield of 2 ATP. During the
reactions, 2 NAD+ pick up electrons
and hydrogen ions, so 2 NADH form.
Mitochondrion
B The second stage occurs in
mitochondria. The 2 pyruvates are
converted to acetyl–CoA, which enters
the Krebs cycle. CO2 forms and leaves
the cell. 2 ATP form. During the
reactions, 8 NAD+ and 2 FAD pick up
electrons and hydrogen ions, so 8
NADH and 2 FADH2 also form.
C The third and final stage, electron
transfer phosphorylation, occurs
inside mitochondria. 10 NADH and 2
FADH 2 donate electrons and
hydrogen ions to electron transfer
chains. Electron fl ow through the
chains sets up H+ gradients that
drive ATP formation. Oxygen accepts
electrons at the end of the chains.
Fig. 8-3a, p. 125
Cytoplasm
glucose
2 ATP
ATP
Glycolysis
4 ATP
(2 net)
ATP
2 NADH 2 pyruvate
Krebs
Cycle
6 CO2
2 ATP
ATP
8 NADH, 2 FADH2
ATP
oxygen
Electron Transfer
Phosphorylation
32 ATP
A The first stage, glycolysis, occurs in
the cell’s cytoplasm. Enzymes convert
a glucose molecule to 2 pyruvate for
a net yield of 2 ATP. During the
reactions, 2 NAD+ pick up electrons
and hydrogen ions, so 2 NADH form.
Mitochondrion
B The second stage occurs in
mitochondria. The 2 pyruvates are
converted to acetyl–CoA, which enters
the Krebs cycle. CO2 forms and leaves
the cell. 2 ATP form. During the
reactions, 8 NAD+ and 2 FAD pick up
electrons and hydrogen ions, so 8
NADH and 2 FADH2 also form.
C The third and final stage, electron
transfer phosphorylation, occurs
inside mitochondria. 10 NADH and 2
FADH 2 donate electrons and
hydrogen ions to electron transfer
chains. Electron fl ow through the
chains sets up H+ gradients that
drive ATP formation. Oxygen accepts
electrons at the end of the chains.
Stepped Art
Fig. 8-3a, p. 125
Fig. 8-3b, p. 125
Animation: Overview of aerobic
respiration
8.1 Key Concepts:
Energy From Carbohydrate Breakdown
 Various degradative pathways convert the
chemical energy of glucose and other organic
compounds to the chemical energy of ATP
 Aerobic respiration yields the most ATP from
each glucose molecule; in eukaryotes, it is
completed inside mitochondria
8.2 Glycolysis –
Glucose Breakdown Starts
 Glycolysis starts and ends in the cytoplasm of all
prokaryotic and eukaryotic cells
 An energy investment of ATP starts glycolysis
Glycolysis
 Two ATP are used to split glucose and form 2
PGAL, each with one phosphate group
 Enzymes convert 2 PGAL to 2 PGA, forming 2
NADH
 Four ATP are formed by substrate-level
phosphorylation (net 2 ATP)
Glycolysis
glycolysis occurs
in the cytoplasm
Animal
Cell
(eukaryotic)
Plant Cell
(eukaryotic)
Bacterial Cell (prokaryotic)
Fig. 8-4a, p. 126
Fig. 8-4a (1), p. 126
Fig. 8-4a (2), p. 126
Fig. 8-4b, p. 127
Fig. 8-4b (1), p. 127
ATP-Requiring Steps
Glycolysis
A An enzyme transfers a
phosphate group from ATP
to glucose, forming glucose6-phosphate.
glucose
ATP
ADP
glucose-6-phosphate
ATP
ADP
fructose-1,6-bisphosphate
B A phosphate group
from a second ATP is
transferred to the glucose6- phosphate. The resulting
molecule is unstable, and it
splits into two three- carbon
molecules. The molecules
are interconvertible, so we
will call them both PGAL
(phosphoglyceraldehyde).
So far, two ATP have been
invested in the reactions.
Fig. 8-4b (1), p. 127
Fig. 8-4b (2), p. 127
2 PGAL
2
NAD+
+ 2 Pi
NADH
2 reduced coenzymes
2 PGA
2 ADP
ATP
2 ATP produced
by substrate-level
phosphorylation
2 PEP
2 ADP
ATP
2 pyruvate
2 ATP produced
by substrate-level
phosphorylation
Net 2 ATP + 2 NADH
to second stage
ATP-Generating Steps
C Enzymes attach a
phosphate to the two PGAL,
and transfer two electrons
and a hydrogen ion from each
PGAL to NAD+. Two PGA
(phosphoglycerate) and two
NADH are the result.
D Enzymes transfer a
phosphate group from each
PGA to ADP. Thus, two ATP
have formed by substratelevel phosphorylation.
The original energy
investment of two ATP has
now been recovered.
E Enzymes transfer a
phosphate group from each
of two intermediates to ADP.
Two more ATP have formed
by substrate-level
phosphorylation.
Two molecules of pyruvate
form at this last reaction
step.
F Summing up, glycolysis
yields two NADH, two ATP
(net), and two pyruvate for
each glucose molecule.
Fig. 8-4b (2), p. 127
Animation: Glycolysis
8.2 Key Concepts:
Glycolysis
 Glycolysis is the first stage of aerobic respiration
and of anaerobic routes such as fermentation
pathways
 Enzymes of glycolysis convert glucose to
pyruvate
8.3 Second Stage of Aerobic Respiration
 The second stage of aerobic respiration finishes
breakdown of glucose that began in glycolysis
 Occurs in mitochondria
 Includes two stages: acetyl CoA formation and
the Krebs cycle (each occurs twice in the
breakdown of one glucose molecule)
Acetyl CoA Formation
 In the inner compartment of the mitochondrion,
enzymes split pyruvate, forming acetyl CoA and
CO2 (which diffuses out of the cell)
 NADH is formed
The Krebs Cycle
 Krebs cycle
• A sequence of enzyme-mediated reactions that
break down 1 acetyl CoA into 2 CO2
• Oxaloacetate is used and regenerated
• 3 NADH and 1 FADH2 are formed
• 1 ATP is formed
Inside a Mitochondrion
Fig. 8-5a, p. 128
outer membrane
(next to cytoplasm)
inner membrane
inner mitochondrial
compartment
outer mitochondrial
compartment (in
between the two
membranes)
Fig. 8-5a, p. 128
Fig. 8-5b, p. 128
glucose
(glycolysis)
2 pyruvate
OUTER COMPARTMENT
6
2 acetyl-CoA
Krebs
Cycle
CO2
2
ATP
8
NADH
2
FADH2
INNER COMPARTMENT
Breakdown of 2 pyruvate 6
CO2 yields 2 ATP. Also, 10
coenzymes (8 NAD+, 2 FAD)
are reduced. The coenzymes
carry hydrogen ions and
electrons to the third stage of
aerobic respiration.
Fig. 8-5b, p. 128
Animation: Functional zones in
mitochondria
Acetyl CoA Formation
and the Krebs Cycle
A An enzyme splits a
pyruvate molecule into a
two-carbon acetyl group
and CO2. Coenzyme A
binds the acetyl group
(forming acetyl–CoA). NAD+
combines with released
hydrogen ions and
electrons, forming NADH.
Acetyl–CoA
Formation
pyruvate
NAD+
coenzyme A
NADH
CO2
B The Krebs cycle starts as
one carbon atom is
transferred from acetyl–
CoA to oxaloacetate. Citrate
forms, and coenzyme A is
regenerated.
acetyl–CoA
coenzyme A
H The final steps of the
Krebs cycle regenerate
oxaloacetate.
citrate
C A carbon atom is removed
from an intermediate and
leaves the cell as CO2. NAD+
combines with released
hydrogen ions and
electrons, forming NADH.
D A carbon atom is
removed from another
intermediate and leaves the
cell as CO2, and another
NADH forms.
CO2
oxaloacetate
Krebs
Cycle
NAD+
G NAD+ combines with
hydrogen ions and
electrons, forming NADH.
NADH
NADH
NAD+
CO2
NAD+
FADH2
FAD
NADH
Pyruvate’s three carbon atoms have
now exited the cell, in CO2.
ADP + Pi
ATP
F The coenzyme FAD combines with hydrogen ions
and electrons, forming
FADH2.
E One ATP forms by
substrate-level
phosphorylation.
Stepped Art
Fig. 8-6, p. 129
Animation: The Krebs Cycle - details
8.4 Aerobic Respiration’s
Big Energy Payoff
 Many ATP are formed during the third and final
stage of aerobic respiration
 Electron transfer phosphorylation
• Occurs in mitochondria
• Results in attachment of phosphate to ADP to
form ATP
Electron Transfer Phosphorylation
 Coenzymes NADH and FADH2 donate electrons
and H+ to electron transfer chains
 Active transport forms a H+ concentration
gradient in the outer mitochondrial compartment
 H+ follows its gradient through ATP synthase,
which attaches a phosphate to ADP
 Finally, oxygen accepts electrons and combines
with H+, forming water
Electron Transfer Phosphorylation
Summary: The Energy Harvest
 Typically, the breakdown of one glucose
molecule yields 36 ATP
• Glycolysis: 2 ATP
• Acetyl CoA formation and Krebs cycle: 2 ATP
• Electron transfer phosphorylation: 32 ATP
Summary: Aerobic Respiration
Animation: Third-stage reactions
8.3-8.4 Key Concepts:
How Aerobic Respiration Ends
 The final stages of aerobic respiration break
down pyruvate to CO2
 Many coenzymes that become reduced deliver
electrons and hydrogen ions to electron transfer
chains; energy released by electrons flowing
through the chains is captured in ATP
 Oxygen accepts electrons at ends of the chains
8.5 Anaerobic
Energy-Releasing Pathways
 Fermentation pathways break down
carbohydrates without using oxygen
 The final steps in these pathways regenerate
NAD+ but do not produce ATP
Fermentation Pathways
 Glycolysis is the first stage of fermentation
• Forms 2 pyruvate, 2 NADH, and 2 ATP
 Pyruvate is converted to other molecules, but is
not fully broken down to CO2 and water
• Regenerates NAD+ but doesn’t produce ATP
 Provides enough energy for some single-celled
anaerobic species
Two Pathways of Fermentation
 Alcoholic fermentation
• Pyruvate is split into acetaldehyde and CO2
• Acetaldehyde receives electrons and hydrogen
from NADH, forming NAD+ and ethanol
 Lactate fermentation
• Pyruvate receives electrons and hydrogen from
NADH, forming NAD+ and lactate
Two Pathways of Fermentation
Fig. 8-9a, p. 132
Glycolysis
glucose
2 NAD+
2
ATP
2
NADH
4
ATP
pyruvate
Alcoholic
Fermentation
2 CO2
acetaldehyde
2 NADH
2 NAD+
ethanol
Fig. 8-9a, p. 132
Fig. 8-9b, p. 132
Glycolysis
glucose
2 NAD+
2
ATP
2 NADH
4 ATP
pyruvate
Lactate
Fermentation
2 NADH
2 NAD+
lactate
Fig. 8-9b, p. 132
Glycolysis
2
ATP
glucose
2 NAD+
2 NADH
Glycolysis
2
ATP
2 NADH
4 ATP
4 ATP
pyruvate
pyruvate
Alcoholic
Fermentation
glucose
2 NAD+
2 CO2
acetaldehyde
Lactate
Fermentation
2 NADH
2 NAD+
2 NADH
2 NAD+
lactate
ethanol
Stepped Art
Fig. 8-9, p. 132
Animation: Fermentation pathways
Alcoholic Fermentation
Fig. 8-10a, p. 133
Fig. 8-10b, p. 133
Fig. 8-10c, p. 133
8.6 The Twitchers
 Slow-twitch muscle fibers (“red” muscles) make
ATP by aerobic respiration
• Have many mitochondria
• Dominate in prolonged activity
 Fast-twitch muscle fibers (“white” muscles) make
ATP by lactate fermentation
• Have few mitochondria and no myoglobin
• Sustain short bursts of activity
Sprinters and Lactate Fermentation
Fig. 8-11b, p. 133
8.5-8.6 Key Concepts:
How Anaerobic Pathways End
 Fermentation pathways start with glycolysis
 Substances other than oxygen accept electrons
at the end of the pathways
 Compared with aerobic respiration, the net yield
of ATP from fermentation is small
8.7 Alternative
Energy Sources in the Body
 Pathways that break down molecules other than
carbohydrates also keep organisms alive
 In humans and other mammals, the entrance of
glucose and other organic compounds into an
energy-releasing pathway depends on the kinds
and proportions of carbohydrates, fats and
proteins in the diet
The Fate of Glucose at Mealtime
and Between Meals
 When blood glucose concentration rises, the
pancreas increases insulin secretion
• Cells take up glucose faster, more ATP is formed,
glycogen and fatty-acid production increases
 When blood glucose concentration falls, the
pancreas increases glucagon secretion
• Stored glycogen is converted to glucose
Energy From Fats
 About 78% of an adult’s energy reserves is
stored in fat (mostly triglycerides)
 Enzymes cleave fats into glycerol and fatty acids
• Glycerol products enter glycolysis
• Fatty acid products enter the Krebs cycle
 Compared to carbohydrates, fatty acid
breakdown yields more ATP per carbon atom
Energy from Proteins
 Enzymes split dietary proteins into amino acid
subunits, which enter the bloodstream
• Used to build proteins or other molecules
 Excess amino acids are broken down into
ammonia (NH3) and various products that can
enter the Krebs cycle
Alternative Energy Sources
in the Human Body
FOOD
FATS
fatty acids
glycerol
acetyl–CoA
PGAL
COMPLEX CARBOHYDRATES
PROTEINS
glucose, other simple sugars
amino acids
acetyl–CoA
Glycolysis
NADH
pyruvate
Krebs
Cycle
oxaloacetate or
another intermediate
of the Krebs
NADH, FADH2
Electron Transfer
Phosphorylation
Fig. 8-12, p. 135
FOOD
FATS
fatty acids
acetyl–CoA
glycerol
COMPLEX CARBOHYDRATES
PROTEINS
glucose, other simple sugars
amino acids
acetyl–CoA
PGAL
Glycolysis
NADH
pyruvate
Krebs
Cycle
oxaloacetate or
another intermediate
of the Krebs
NADH, FADH2
Electron Transfer
Phosphorylation
Stepped Art
Fig. 8-12, p. 135
Animation: Alternative energy sources
8.7 Key Concepts:
Other Metabolic Pathways
 Molecules other than glucose are common
energy sources
 Different pathways convert lipids and proteins to
substances that may enter glycolysis or the
Krebs cycle
8.8 Reflections on Life’s Unity
 Life’s diversity and continuity arise from unity at
the level of molecules and energy
• Energy inputs drive the organization of molecules
into cells (one-way flow of energy)
 Energy from the sun sustains life’s organization
• Photosynthesizers use energy from the sun to
feed themselves and other forms of life
• Aerobic respiration balances photosynthesis
Links Between Photosynthesis
and Aerobic Respiration
energy in
(mainly from
sunlight)
Photosynthesis
glucose,
oxygen
Aerobic Respiration
carbon
dioxide,
water
energy out (ATP, heat)
energy out (ATP, heat)
Fig. 8-13, p. 136
Animation: Electron transfer
phosphorylation
Animation: Electron transfer system and
oxidative phosphorylation
Animation: Krebs cycle overview
Animation: Links with photosynthesis
Animation: Recreating the reactions of
glycolysis
Video: When mitochondria spin their
wheels