Chapter 7: Cellular Respiration and Fermentation
Key Concepts
 7.1 Catabolic pathways yield energy by oxidizing organic fuels
 7.2 Glycolysis harvests chemical energy by oxidizing glucose
to pyruvate
 7.3 After pyruvate is oxidized, the citric acid cycle completes
the energy-yielding oxidation of organic molecules
 7.4 During oxidative phosphorylation, chemiosmosis couples
electron transport to ATP synthesis
 7.5 Fermentation and anaerobic respiration enable cells to
produce ATP without oxygen
 7.6 Glycolysis and citric acid cycle connect to many other
metabolic pathways
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Life Is Work
 Living cells require energy from outside sources
 Some animals, like this giraffe, obtain energy by eating plants,
and some animals feed on other organisms that eat plants
How Do These Leaves Power the
Work of Life for This Giraffe?
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Life Is Work, Continued
Concept 7.1: Catabolic pathways yield energy by oxidizing
organic fuels
 Energy flows into an ecosystem
as sunlight and leaves as heat
 Photosynthesis generates
oxygen and organic molecules,
which are used as fuel for
cellular respiration
 Cells use chemical energy
stored in organic molecules to
regenerate ATP, which powers
work
 Catabolic pathways involving electron transfer are central
processes to cellular respiration
Catabolic Pathways and Production of ATP
Energy Flow and Chemical
Recycling in Ecosystems
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Catabolic Pathways and Production of ATP, Continued
 Cellular respiration includes both aerobic and anaerobic
processes but is often used to refer to aerobic respiration
 Although organic compounds like carbohydrates, fats, and
proteins are all consumed as fuel,
Organic compounds + O2
 The breakdown of organic molecules is exergonic
 Fermentation is a partial degradation of sugars that occurs
without oxygen
 Aerobic respiration consumes organic molecules and oxygen
to synthesize ATP
 Anaerobic respiration consumes organic molecules, but not
oxygen to synthesize ATP
CO2 + H2O + Energy (ATP + Heat)
Cellular Respiration:
A Catabolic Pathway and Production of ATP
-Complete breakdown of glucose in the
presence of oxygen.
-Releases energy in the form of ATP
- C6H12O6  6 O2 → 6 CO2  6 H2O  Energy (ATPheat)
It is helpful to trace it with the sugar glucose
C6H12O6 + 6O2
6CO2 + 6H2O + Energy (ATP + Heat)
C6H12O6
Glucose
6
O2
Oxygen
6
CO2
Carbon
dioxide
6
H 2O
ATP
Water
Energy
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Simon/Dickey/Reece Pearson
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Redox Reactions: Oxidation and Reduction
Oxidation of Organic Fuel Molecules During
Cellular Respiration
 In oxidation
Glucose is oxidized and oxygen is reduced
 A substance loses electrons or hydrogen, or is oxidized
 In reduction
Oxidation
 A substance gains electrons or hydrogen, or is reduced
Glucose loses electrons
(and hydrogens)
C6H12O6
Glucose
6
O2
Oxygen
6
CO2
Carbon
dioxide
6 H 2O
 The electron donor is called the reducing agent
 The electron receptor is called the oxidizing agent
 OILRIG
Water
Reduction
Oxygen gains electrons (and hydrogens)
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Chemical energy is stored in reduced molecules of
carbohydrates and lipids
 The shared electrons of C-C and C-H bonds of
carbohydrates and lipids have high potential energy
(chemical energy)
 So oxidation of glucose releases large amount of
energy.
 In oxidation of glucose, electrons always travel with
hydrogen atoms. In biological systems, hydrogen
atoms are used as a marker for electrons to follow
redox reactions.
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Oxidation of Organic Fuel Molecules During
cellular respiration
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Redox Reactions: Oxidation and Reduction
 The transfer of electrons during chemical reactions releases
energy stored in organic molecules
 This released energy is ultimately used to synthesize ATP
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Figure 9.4
NAD+ as an electron carrier
 During cellular respiration, fuel (such as glucose) is oxidized,
and oxygen is reduced
 Organic molecules with an abundance of hydrogen, like
carbohydrates and fats, are excellent fuels
 As hydrogen (with its electron) is transferred to oxygen,
energy is released that can be used in ATP synthesis
2 e− + 2 H+
NAD+
Dehydrogenase
2 e− + H+
NADH
H+
Reduction of NAD+
2[H]
(from food) Oxidation of NADH
Nicotinamide
(oxidized form)
H+
Nicotinamide
(reduced form)
 Enzymes called dehydrogenases
facilitate the transfer of two electrons
and one hydrogen ion to NAD+
 One hydrogen ion is released in this
process
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The Electron Carrier NAD+
empty
loaded
proton
(oxidized)
empty
goes to pick
up more electrons
used in later
stage of respiration
(reduced)
used in later
stage of respiration
Electron or hydrogen carriers: NAD+ and FAD
Dehydrogenase
Two important electron carriers in the cell are:
• NAD+ (Nicotinamide Adenine Dinucleotide)
NAD+
+
2H
NADH
Oxidized form
+
H+
Reduced form
• FAD ( Flavin Adenine Dinucleotide)
FAD
+
2H
FADH2
Oxidized form
Reduced form
Figure 9.UN04
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Stepwise energy Harvest via NAD+ and the Electron
Transport Chain
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Figure 6.5
e e
Electrons from food
e e
NADH
NAD
Stepwise release
of energy used
to make
 Cellular respiration
 Oxidizes glucose in a series of steps
2 H
ATP
2 e
 Electrons from organic compounds
 Are first transferred to NAD+ ( Nicotinamide
Adenine Dinucleotide), a coenzyme
 NADH, the reduced form of NAD+
 Passes the electrons to the electron transport
chain
2
Hydrogen, electrons,
and oxygen combine
to produce water
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Stepwise energy Harvest via NAD+ and the Electron
Transport Chain, Continued-1
e
2 H
1
2
O2
H2 O
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An Overview of cellular respiration
 NADH passes the electrons to the electron transport chain
 Electrons are passed to increasingly electronegative carrier
molecules down the chain through a series of redox reactions
 Electron transfer to oxygen occurs in a series of energyreleasing steps instead of one explosive reaction
 The energy yielded is used to regenerate ATP
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Color Code for Stages of cellular respiration
The Stages of Cellular Respiration
Glycolysis (cytoplasm)
1.
2.
1. Glycolysis (color-coded blue throughout the chapter)
2. PYRUVATE OXIDATION and the CITRIC ACID CYCLE
(color-coded orange)
Pyruvate Oxidation

3.
Citric acid cycle or Krebs cycle

3. OXIDATIVE PHOSPHORYLATION: Electron transport and
chemiosmosis (color-coded purple)
4.
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Complete
breakdown
of glucose
matrix of mitochondria
matrix of mitochondria
Oxidative phosphorylation or Electron Transport
chain (accounts for most ATP synthesis in the inner
membrane of mitochondria)
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Mini-Map, Glycolysis
What is Glycolysis?
Concept 7.2: Glycolysis harvests chemical energy by oxidizing
glucose to pyruvate
 Glycolysis
 Means “splitting of sugar”
 Breaks down glucose into two molecules of
pyruvate, 2ATP (net) and 2NADH
 Involves 10 enzyme catalyzed reactions
 Occurs in the cytosol of the cell in the presence or absence of
oxygen
 How glycolysis works
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Figure 7.9-1b-s3
Figure 7.9-2a-s3
GLYCOLYSIS: Energy Investment Phase
GLYCOLYSIS: Energy Payoff Phase
Glyceraldehyde
3-phosphate (G3P)
Fructose
ATP
6-phosphate
Glyceraldehyde
3-phosphate (G3P)
Fructose
1,6-bisphosphate
Isomerase
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2 ADP
 2 H
2
2
ADP
Phosphofructokinase
2 ATP
2 NADH
2 NAD
Aldolase
Dihydroxyacetone
phosphate (DHAP)
Isomerase
Aldolase
Dihydroxyacetone
phosphate (DHAP)
Triose
phosphate 2
dehydrogenase
Phosphoglycerokinase
Pi
1,3-Bisphosphoglycerate
3-Phosphoglycerate
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Figure 7.9-2b-s3
The Stages of cellular respiration: A Preview, Continued
GLYCOLYSIS: Energy Payoff Phase
2 H2O
2
2
Phosphoglyceromutase
3-Phosphoglycerate
2
2 ADP
ATP
2
Pyruvate
kinase
Enolase
2-Phosphoglycerate
2
Phosphoenolpyruvate (PEP)
Pyruvate
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Glycolysis: The energy Input and Output Summary
 Oxidative phosphorylation accounts for almost 90% of the
ATP generated by cellular respiration
 This process involves the transfer of inorganic phosphates to
ADP
 A smaller amount of ATP is formed in Glycolysis and the citric
acid cycle by substrate-level phosphorylation
 In this process, an enzyme transfers a phosphate group
directly from a substrate molecule to ADP
 For each molecule of glucose degraded to CO2 and water by
respiration, the cell makes up to 32 molecules of ATP
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Substrate-Level Phosphorylation
Energy investment phase:
ATP provides activation energy by
phosphorylating glucose
-2 ATP per glucose
Energy payoff phase:
ATP is produced by substrate-level
phosphorylation and NAD+ is reduced
to NADH.
+ 4ATP + 2NADH per glucose
_____________________________
Net : 2ATP + 2NADH per glucose
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Which statement about glycolysis is true?
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Mini-Map, Pyruvate Oxidation
A. It splits water.
B. It uses oxygen.
C. It occurs in the
cytoplasm.
D. It makes the most
ATP compared to
the two other steps.
E. It makes lipids.
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An Overview of Pyruvate Oxidation and the Citric Acid Cycle
An Overview of Pyruvate Oxidation
2Pyruvates
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Concept 7.3: Connecting step: Oxidation of
Pyruvate to Acetyl CoA
2CO2 + 2NADH + 2Acetyl CoA
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An Overview of the Citric Acid Cycle
• In the presence of O2, pyruvate enters the
mitochondrion
• One carbon is lost in the form of CO2
• Acetyl group (CH3OC-) combines with
Coenzyme A
• Acetyl Coenzyme A (CoA) links glycolysis to
citric acid cycle
• Products are 2Acetyl CoA, 2CO2 and 2NADH
• Pyruvate Oxidation
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Mini-Map, Citric Acid Cycle
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Concept 7.3 Citric acid cycle or Krebs cycle
• Takes place in the matrix of mitochondria
• 2 molecules of Acetyl CoA enter the cycle
• Acetyl CoA combines with oxaloacetate and
forms citrate
• For each glucose there are 2 cycles
• Products for 2cycles:2 ATP, 4 CO2, 6 NADH,
2FADH2
• Citric acid cycle
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Citric acid cycle contd.
• Citric acid cycle has 8 steps, each
catalyzed by an enzyme
• Acetyl CoA combines with oxaloacetate
and forms citrate
• Citrate then gets converted into
oxaloacetate in the next seven step
• NADH & FADH2 transfer the electrons
extracted from food to the ETC
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Which statement about the citric acid cycle is true?
A Closer Look at the Citric Acid Cycle and Summary
Each cycle produces:
• 1 ATP by substrate-level
phosphorylation
• 3 NADH
• 1 FADH2 (electron
carrier)
• 2 CO2
For each glucose there are
two cycle:
• 2 ATP by substrate-level
phosphorylation
• 6 NADH
• 2 FADH2 (electron carrier)
• 4 CO2
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Mini-Map, Oxidative Phosphorylation
A. It occurs during the movement from the cytosol through
the mitochondrial membranes.
B. It makes ATP through substrate-level phosphorylation.
C. It makes the most
ATP compared to
the two other steps.
D. It occurs in the
cytoplasm.
E. It splits glucose.
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Chemiosmosis Couples the Electron Transport Chain to
ATP Synthesis
The electron transport chain (ETC) is located in the mitochondrial
inner membrane. Electrons are passed from electron donors to
acceptors until they reach the final electron acceptor, oxygen.
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Electron Transport Chain (ETC)
• Electron carriers NADH and FADH2 donate electrons to
the electron transport chain
• Electrons are passed through a number of proteins
including cytochromes
• Energy released during electron transfer is used to
pumps H+ ions (protons) from the matrix into the
intermembrane space. Energy is then stored in the H+ ion
gradient.
• H+ ions move back into the matrix through ATP synthase.
The energy of the proton gradient activates the enzyme.
• ATP synthase uses the exergonic flow of H+ ions to drive
the phosphorylation of ADP to make ATP
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Free-energy Change During Electron Transport
• Most of the chain’s
components are proteins,
which exist in multiprotein
complexes
• The carriers alternate
reduced and oxidized
states as they accept and
donate electrons
• At the end of ETC, electrons
are finally accepted by
oxygen, which combines
with H+ ions to form water
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• Following glycolysis and the citric acid cycle,
NADH and FADH2 account for most of the energy
extracted from food
• These two electron carriers donate electrons to
the electron transport chain, which powers ATP
synthesis via oxidative phosphorylation
Electron Transport Chain
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ATP Synthase, A Molecular Mill
ATP synthase is the enzyme
that synthesizes ATP by adding
phosphate to ADP
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The energetic electron, taken from glucose or a breakdown
product of glucose, is stripped of its energy to
A. actively transport H+ into the intermembrane space.
B. actively transport NAD+ into the intermembrane space.
C. actively transport Na+ into the matrix.
D. power facilitated diffusion of H+ into the matrix.
E. actively transport H+ into the matrix.
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During oxidative phosphorylation, chemiosmosis
couples electron transport to ATP synthesis
ATP Synthesis in Electron Transport Chain
• Chemiosmosis: Energy for ATP
synthesis is provided by the diffusion of
H+ ions
• Oxidative phosphorylation: Energy for
ATP synthesis is provided by oxidation of
food
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ATP Yield Per Molecule of glucose at Each Stage of
cellular respiration
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ATP production during cellular respiration
Pyruvate as a Key Juncture in Catabolism
• During cellular respiration, most energy flows in this
sequence:
Glucose  NADH  electron transport chain 
H+ ions  ATP
– 1 NADH → 2.5 ATP (10NADH → 25ATP)
– 1FADH2 → 1.5 ATP (2FADH2 → 3ATP)
– 28ATP in ETC in the mitochondria
• About 34% of the energy in a glucose molecule
– Is transferred to ATP during cellular respiration
– About 32 ATP molecules are formed when
glucose is completely broken down
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Concept 7.5: Fermentation and anaerobic
respiration enable cells to produce ATP without
the use of oxygen
• In the absence of oxygen cells carry out
Fermentation of Glucose which consists of
- Glycolysis and
- Reactions that regenerate NAD+ which can be
used by glycolysis
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Lactic Acid Fermentation
Alcohol Fermentation
• In alcohol fermentation, pyruvate is
converted to ethanol (alcohol) in two
steps,
– The first step releases CO2
– The second step produces ethanol
• Alcohol fermentation by yeast is used in
brewing, winemaking, and baking
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Types of Fermentation
• In lactic acid fermentation
– Pyruvate is converted into lactic acid
• Lactic acid fermentation by some fungi
and bacteria is used to make cheese and
yogurt
• Human muscle cells use lactic acid
fermentation to generate ATP when O2 is
scarce
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Comparing Fermentation with Anaerobic and Aerobic
Respiration
Glycolysis
Cellular
Respiration
Fermentation
Location
Cytoplasm
Mitochondria Cytoplasm
Electron carriers
NAD+
NAD+, FAD
NAD+
Oxygen
Pyruvate or
acetaldehyde
32ATP
2ATP
(glycolysis)
Final electron
acceptor
ATP produced
2ATP
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Comparing Fermentation with Anaerobic and Aerobic
Respiration
 Obligate anaerobes carry out anaerobic respiration or
fermentation and cannot survive in the presence of O2
 Facultative anaerobes carry out anaerobic respiration or
fermentation and can survive in the presence of O2 ,
Example - Yeast and many bacteria, they can survive
using either fermentation or cellular respiration
 In a facultative anaerobe, pyruvate is a fork in the
metabolic road that leads to two alternative catabolic
routes
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What is the purpose of fermentation reactions?
What is the purpose of fermentation reactions?
 to regenerate NAD+ so glycolysis can continue
 to make alcohol or lactic acid that cells can metabolize for
energy under anaerobic conditions
 to make additional ATP when respiration can’t make ATP fast
enough
 to slow down cellular oxygen consumption when oxygen is
scarce
 to make organic molecules that cells can store until oxygen
becomes available
 to regenerate NAD+ so glycolysis can continue
 to make alcohol or lactic acid that cells can metabolize for
energy under anaerobic conditions
 to make additional ATP when respiration can’t make ATP fast
enough
 to slow down cellular oxygen consumption when oxygen is
scarce
 to make organic molecules that cells can store until oxygen
becomes available
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The Evolutionary Significance of Glycolysis
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The Catabolism of Various Molecules From Food
 Glycolysis is the most common metabolic pathway among
organisms on Earth, indicating that it evolved early in the
history of life
 Early prokaryotes may have generated ATP exclusively
through Glycolysis due to the low oxygen content in the
atmosphere
 The location of Glycolysis in the cytosol also indicates its
ancient origins
 Eukaryotic cells with mitochondria evolved much later than
prokaryotic cells
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Concept 7.6: Glycolysis and the citric acid cycle connect
to many other metabolic pathways
The Versatility of Catabolism
 Catabolic pathways break down proteins, fats and
carbohydrates.
 Their monomers can be metabolized by glycolysis or another
stage of cellular respiration
 Proteins are digested into amino acids and can then enter
glycolysis or the citric acid cycle.
 Fats are digested to glycerol (used in glycolysis) and fatty
acids (used in generating acetyl CoA)
 Fatty acids are broken down by beta oxidation and yield
acetyl CoA
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Test Your Understanding, Question 9 (Regulation of Phosphofructokinase)
Phosphofructokinase is an enzyme that is part of glycolysis. Under which
condition is the enzyme active and why?
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Summary of Key Concepts: Citric Acid Cycle
Biosynthesis (Anabolic Pathways)
 The body uses small molecules to build other substances
 Some of these small molecules come directly from food; others
can be produced during Glycolysis or the citric acid cycle
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Summary of Key Concepts: Glycolysis
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You should now be able to:
Explain in general terms how redox reactions are involved in energy exchanges.
What reactant is oxidized and what reactant is reduced in cellular respiration
3. Name the three(four) stages of cellular respiration; for each, state the part of the
eukaryotic cell where it occurs, the reactants and the products
4. What is substrate level phosphorylation?
5. In general terms, explain the role of the electron transport chain in cellular
respiration
6. What is chemiosmosis?
7. Why is the process in the electron transport chain called oxidative phosphorylation?
8. Explain where and how the respiratory electron transport chain creates a proton
gradient
9. What is the role of oxygen in cellular respiration?
10. What is the total number of ATP molecules produced when one glucose molecule is
broken down?
11. Distinguish between fermentation and anaerobic respiration
12. Distinguish between obligate and facultative anaerobes
1.
2.
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