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How Cells Release Chemical Energy
Chapter 7
Biology Concepts and Applications, Eight Edition, by Starr, Evers, Starr. Brooks/Cole,
Cengage Learning 2011.
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
 Photoautotrophs  Photosynthetic autotrophs
• Produce sugar
 Fermentation pathways  anaerobic pathway
• End in cytoplasm, do not use oxygen, yield 2 ATP
per molecule of glucose
 Aerobic respiration  oxygen-requiring pathway
that breaks down carbohydrates to produce ATP
• Ends in mitochondria, uses oxygen, yields up to
36 ATP per glucose molecule
• **Occurs in the presence of OXYGEN**
Pathways of Carbohydrate Breakdown
Overview of Aerobic Respiration
 Three main stages of aerobic respiration:
1. Glycolysis in the cytoplasm
• Convert glucose and other sugars to (2) pyruvate
and (2) ATP
• Pyruvate is 3-carbon end product of glycolysis
2. Krebs cycle
3. Electron transfer phosphorylation
Summary equation:
C6H12O6 + 6O2 → 6CO2 + 6 H2O
Overview of Aerobic Respiration
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
7.3 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
• Transfers a phosphate group directly from a
substrate to ADP to form ATP
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
Glycolysis
ENERGY REQUIRING PHASE
Glycolysis
glucose
ATP
ADP
glucose–6–phosphate
ATP
ADP
P
P
fructose–1,6–bisphosphate
DHAP
Fig. 7.4c1, p.111
ENERGY
PRODUCING
2 NAD + 2 P
PHASE
+
2 PGAL
i
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
7.4 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
• Kreb cycle  cyclic pathway that, along with
acetyl-CoA formation, breaks down pyruvate to
carbon dioxide
Second Stage of Aerobic Respiration
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
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
7.5 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
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
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
7.6 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: ATP & Lactate
• Bacteria break down lactose in milk  produce
buttermilk, cheese, and yogurt
• Yeast  preserve pickles, cored beef, and sauerkraut
 Alcoholic fermentation
• End product: ATP & Ethyl alcohol (or ethanol)
• Yeast to make bread  dough rises as CO2 forms
bubbles
 Both pathways have a net yield of 2 ATP per glucose
(from glycolysis) and NAD+
Fig. 7.9a, p.116
Fig. 7.9b, 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
Animal Skeletal Muscle
 Red fibers (legs of chicken)
• A lot of mitochondria and Myoglobin (stores oxygen)
• Produce ATP by aerobic respiration
• Sustain prolonged activity (marathon runs)
 White fibers (wings of chicken)
• Few mitochondria and no myoglobin  can not carry
out a lot of aerobic respiration
• Most ATP produced by lactate fermentation
• ATP produced quick by not for long
• Short strenuous activity (Sprinting and weight lifting)
 Humans  Mixed fibers
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
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
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
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: Alternative energy sources
Animation: Fermentation pathways
Animation: Functional zones in
mitochondria
Animation: Glycolysis
Animation: Overview of aerobic
respiration
Animation: The Krebs Cycle - details
Animation: Third-stage reactions
Animation: Where pathways start and
finish