How Cells Release Chemical Energy
Chapter 7
7.1 Overview of
Carbohydrate Breakdown Pathways
 All organisms (including photoautotrophs)
convert chemical energy of organic compounds
to chemical energy of ATP
 ATP is a common energy currency that drives
metabolic reactions in cells
Pathways of Carbohydrate Breakdown
 Start with glycolysis in the cytoplasm
• Convert glucose and other sugars to pyruvate
 Fermentation pathways
• End in cytoplasm, do not use oxygen, yield 2 ATP
per molecule of glucose
 Aerobic respiration
• Ends in mitochondria, uses oxygen, yields up to
36 ATP per glucose molecule
Pathways of Carbohydrate Breakdown
a All 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. 7.2b, p.108
Animation: Where pathways start and
finish
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Overview of Aerobic Respiration
 Three main stages of aerobic respiration:
1. Glycolysis
2. Krebs cycle
3. Electron transfer phosphorylation
Summary equation:
C6H12O6 + 6O2 → 6CO2 + 6 H2O
Overview of Aerobic Respiration
Cytoplasm
glucose
2 ATP ATP
GLYCOLYSIS
4 ATP
(2net)
ATP
2 NADH 2 pyruvate
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 atoms,
so 2 NADH form.
Mitochondrion
Krebs
Cycle
6
ATP
CO
2 ATP
2
8 NADH, 2 FADH2
ATP
oxygen
Electron Transfer
Phosphorylation
32 ATP
b The second stage, the Krebs cycle
and a few steps before it, occurs
inside mitochondria. The 2 pyruvates
are broken down to CO2, which leaves
the cell. During the reactions, 8 NAD+
and 2 FAD pick up electrons and
hydrogen atoms, so 8 NADH and 2
FADH2 form. 2 ATP also form.
c The third and final stage, electron
transfer phosphorylation, occurs
inside mitochondria. 10 NADH and 2
FADH2 donate electrons and hydrogen
ions at electron transfer chains.
Electron flow through the chains sets
up H+ gradients that drive ATP
formation. Oxygen
Fig. 7.3, p.109
Animation: Overview of aerobic
respiration
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Key Concepts: ENERGY FROM
CARBOHYDRATE BREAKDOWN
 All organisms produce ATP by degradative
pathways that extract chemical energy from
glucose and other organic compounds
 Aerobic respiration yields the most ATP from
each glucose molecule
 In eukaryotes, aerobic respiration is completed
inside mitochondria
7.2 Glycolysis –
Glucose Breakdown Starts
 Enzymes of glycolysis use two ATP to convert
one molecule of glucose to two molecules of
three-carbon pyruvate
 Reactions transfer electrons and hydrogen
atoms to two NAD+ (reduces to NADH)
 4 ATP form by substrate-level phosphorylation
Products of Glycolysis
 Net yield of glycolysis:
• 2 pyruvate, 2 ATP, and 2 NADH per glucose
 Pyruvate may:
• Enter fermentation pathways in cytoplasm
• Enter mitochondria and be broken down further in
aerobic respiration
Glycolysis
you are here
Fig. 7.4a, p.110
glucose
2 ADP
(net)
Glycolysis
2 pyruvate
to second stage of
aerobic respiration
or another pathway
2 NADH
to third stage of
aerobic respiration
or another pathway
Fig. 7.4b, p.110
Glycolysis
Glycolysis
glucose
ATP
ADP
glucose–6–phosphate
ATP
ADP
P
P
fructose–1,6–bisphosphate
DHAP
Fig. 7.4c1, p.111
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
to second stage
Net 2 ATP + 2 NADH
Fig. 7.4c2, p.111
Key Concepts:
GLYCOLYSIS
 Glycolysis is the first stage of aerobic respiration
and of anaerobic routes (fermentation pathways)
 As enzymes break down glucose to pyruvate,
the coenzyme NAD+ picks up electrons and
hydrogen atoms
 Net energy yield is two ATP
Animation: Glycolysis
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7.3 Second Stage of Aerobic Respiration
 The second stage of aerobic respiration takes
place in the inner compartment of mitochondria
 It starts with acetyl-CoA formation and proceeds
through the Krebs cycle
Second Stage of Aerobic Respiration
outer membrane
(next to cytoplasm)
glucose
(glycolysis)
2 pyruvate
OUTER COMPARTMENT
inner membrane
inner
mitochondrial
compartment
outer mitochondrial
compartment (in
between the two
membranes)
a An inner membrane divides a
mitochondrion’s interior into two
compartments. The second and third
stages of aerobic respiration take
place at this membrane.
2 acetyl–CoA
CO2
ATP
NADH
Krebs
Cycle
FADH2
INNER COMPARTMENT
Breakdown of 2 pyruvate to
6CO2 yields 2 ATP. Also, 10
coenzymes are reduced (8
NADH, 2 FADH2). The
coenzymes carry hydrogen
ions and electrons to sites
of the third stage of aerobic
respiration.
b The second stage starts after membrane proteins
transport pyruvate from the cytoplasm, across
both mitochondrial membranes, to the inner
compartment. Six carbon atoms enter these
reactions (in two pyruvate), and six leave (in
six CO2). Many coenzymes form.
Fig. 7.5, p.112
Acetyl-CoA Formation
 Two pyruvates from glycolysis are converted to
two acetyl-CoA
 Two CO2 leave the cell
 Acetyl-CoA enters the Krebs cycle
Krebs Cycle
 Each turn of the Krebs cycle, one acetyl-CoA is
converted to two molecules of CO2
 After two cycles
• Two pyruvates are dismantled
• Glucose molecule that entered glycolysis is fully
broken down
Energy Products
 Reactions transfer electrons and hydrogen
atoms to NAD+ and FAD
• Reduced to NADH and FADH2
 ATP forms by substrate-level phosphorylation
• Direct transfer of a phosphate group from a
reaction intermediate to ADP
Net Results
 Second stage of aerobic respiration results in
• Six CO2, two ATP, eight NADH, and two FADH2
for every two pyruvates
 Adding the yield from glycolysis, the total is
• Twelve reduced coenzymes and four ATP for
each glucose molecule
 Coenzymes deliver electrons and hydrogen to
the third stage of reactions
Second Stage Reactions
Fig. 7.6a, p.113
Acetyl–CoA
Formation
pyruvate
NAD+
coenzyme A
NADH
CO2
acetyl–CoA
coenzyme A
Krebs
Cycle
oxaloacetate
citrate
CO2
NAD+
Krebs
Cycle
NADH
NADH
NAD+
NAD+
FADH2
CO2
NADH
FAD
ADP + Pi
ATP
Fig. 7.6a, p.113
Fig. 7.6b, p.113
glucose
Glycolysis
you
are
here
Krebs
Cycle
Electron Transfer
Phosphorylation
Fig. 7.6b, p.113
Animation: Functional zones in
mitochondria
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Animation: The Krebs Cycle - details
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7.4 Third Stage:
Aerobic Respiration’s Big Energy Payoff
 Coenzymes deliver electrons and hydrogen ions
to electron transfer chains in the inner
mitochondrial membrane
 Energy released by electrons flowing through
the transfer chains moves H+ from the inner to
the outer compartment
Hydrogen Ions and Phosphorylation
 H+ ions accumulate in the outer compartment,
forming a gradient across the inner membrane
 H+ ions flow by concentration gradient back to
the inner compartment through ATP synthases
(transport proteins that drive ATP synthesis)
The Aerobic Part of Aerobic Respiration
 Oxygen combines with electrons and H+ at the
end of the transfer chains, forming water
 Overall, aerobic respiration yields up to 36 ATP
for each glucose molecule
Electron Transfer Phosphorylation
Fig. 7.7a, p.114
glucose
Glycolysis
you
are
here
Krebs
Cycle
Electron Transfer
Phosphorylation
Fig. 7.7a, p.114
Fig. 7.7b, p.114
INNER
COMPARTMENT
H+
NADH
FADH2
H+
H+
H+
H+
H+
H+
H2O
INNER
MITOCHONDRIAL
MEMBRANE
OUTER
COMPARTMENT
H+
1/2 O2
ATP
ADP + Pi
H+
H+
H+
H+
H+
Fig. 7.7b, p.114
Key Concepts:
HOW AEROBIC RESPIRATION ENDS
 In the Krebs cycle (and a few steps before)
• Pyruvate is broken down to carbon dioxide
• Coenzymes pick up electrons and hydrogen atoms
 In electron transfer phosphorylation
• Coenzymes deliver electrons to transfer chains
that set up conditions for ATP formation
 Oxygen accepts electrons at end of chains
Summary: Aerobic Respiration
glucose
2 ATP
Glycolysis
ATP (2 net)
2 NAD+
2 NADH
2 pyruvate
CYTOPLASM
OUTER MITOCHONDRIAL
COMPARTMENT
2 NADH
2 CO2
2 NADH
6 NADH
2 FADH2
INNER MITOCHONDRIAL
COMPARTMENT
2 acetyl-CoA
4 CO2
Krebs
Cycle
2 ATP
ADP + Pi
Electron Transfer
Phosphorylation
H+
water
H+ H+
H+
32 ATP
H+
oxygen
Fig. 7.8, p.115
Animation: Third-stage reactions
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7.5 Anaerobic
Energy-Releasing Pathways
 Different fermentation pathways begin with
glycolysis and end in the cytoplasm
• Do not use oxygen or electron transfer chains
• Final steps do not produce ATP; only regenerate
oxidized NAD+ required for glycolysis to continue
Anaerobic Pathways
 Lactate fermentation
• End product: Lactate
 Alcoholic fermentation
• End product: Ethyl alcohol (or ethanol)
 Both pathways have a net yield of 2 ATP per
glucose (from glycolysis)
Alcoholic and Lactate Fermentation
Fig. 7.9a, p.116
glucose
Glycolysis
you are
here
Fermentation
Pathway
Fig. 7.9a, p.116
Fig. 7.9b, p.116
Glycolysis
glucose
2 NAD+
2
2 NADH
ATP
4
ATP
pyruvate
Alcoholic
Fermentation
2 CO2
acetaldehyde
2 NADH
2 NAD+
ethanol
Fig. 7.9b, p.116
Fig. 7.9c, p.116
Glycolysis
glucose
2 NAD+
2
ATP
2 NADH
4
ATP
pyruvate
Lactate
Fermentation
2 NADH
2 NAD+
lactate
Fig. 7.9c, p.116
Alcoholic Fermentation
Animation: Fermentation pathways
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7.6 The Twitchers
 Slow-twitch and fast-twitch skeletal muscle fibers
can support different activity levels
 Aerobic respiration and lactate fermentation
proceed in different fibers of muscles
Muscles and Lactate Fermentation
Key Concepts:
HOW ANAEROBIC PATHWAYS END
 Fermentation pathways start with glycolysis
 Substances other than oxygen are the final
electron acceptor
 Compared with aerobic respiration, net yield of
ATP is small
7.7 Alternative Energy Sources
in the Body
 In humans and other mammals, foods enter
aerobic respiration at various steps
• Simple sugars from carbohydrates
• Glycerol and fatty acids from fats
• Carbon backbones of amino acids from proteins
Disposition of Organic Compounds
Alternative Energy Sources
FOOD
fats
fatty acids
COMPLEX CARBOHYDRATES
glycerol
glucose, other simple sugars
PROTEINS
amino acids
acetyl-coA
acetyl-coA PGAL
Glycolysis
NADH pyruvate
oxaloacetate
or another
intermediate
of the Krebs
Krebs
Cycle
NADH, FADH2
Electron Transfer
Phosphorylation
Fig. 7.12a, p.119
Fig. 7.12b, p.119
Animation: Alternative energy sources
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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
7.8 Life’s Unity
 Photosynthesis and aerobic respiration are
interconnected on a global scale
 In its organization, diversity, and continuity
through generations, life shows unity at the
bioenergetic and molecular levels
Energy, Photosynthesis, and
Aerobic Respiration
sunlight
energy in
Photosynthesis
Driven by energy input from
the sun, electrons and hydrogen
are used to form ATP. ATP
energy drives the synthesis of
glucose from hydrogen, electrons
(delivered by coenzymes), and
carbon dioxide’s atoms.
glucose
(stored
chemical
energy)
carbon
dioxide,
water
Aerobic Respiration
Energy input from two ATP
initiates three stages of
reactions. Many ATP form
during thecomplete
breakdown of
glucose to carbon dioxide
and water.
chemical energy in
many ATP available to
drive nearly all cellular
tasks
oxygen
energy out (heat)
energy out (heat)
Fig. 7.13, p.120
sunlight
energy in
Photosynthesis
Driven by energy input from
the sun, electrons and hydrogen
are used to form ATP. ATP
energy drives the synthesis of
glucose from hydrogen, electrons
(delivered by coenzymes), and
carbon dioxide’s atoms.
energy out (heat)
glucose
(stored
chemical
energy)
carbon
dioxide,
water
oxygen
Aerobic Respiration
Energy input from two ATP
initiates three stages of
reactions. Many ATP form
during thecomplete
breakdown of
glucose to carbon dioxide
and water.
chemical energy in
many ATP available
to drive nearly all
cellular tasks
energy out (heat)
Stepped Art
Fig. 7-13, p.120
Key Concepts:
PERSPECTIVE AT UNIT’S END
 Life shows unity in its molecular and cellular
organization and in its dependence on a oneway flow of energy
Animation: Links with photosynthesis
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Video: When Mitochondria Spin Their
Wheels
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