Download Cellular Respiration and Fermentation

Chapter 9
Cellular Respiration and
Fermentation
Dr. Wendy Sera
Houston Community
College
Biology 1406
© 2014 Pearson Education, Inc.
Chapter 9 Concepts
1. Catabolic pathways yield energy by oxidizing
organic fuels.
2. Glycolysis harvests chemical energy by oxidizing
glucose to pyruvate.
3. After pyruvate is oxidized, the citric acid cycle
completes the energy-yielding oxidation of
organic molecules.
4. During oxidative phosphorylation, chemiosmosis
couples electron transport to ATP synthesis.
© 2014 Pearson Education, Inc.
5. Fermentation and anaerobic respiration enable
cells to produce ATP without the use of oxygen.
6. Glycolysis and the citric acid cycle connect to
many other metabolic pathways.
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1
Life Is Work
 Living cells require energy from outside sources
 Some animals, such as the giraffe, obtain energy
by eating plants (herbivores/primary consumers),
and some animals feed on other organisms that
eat plants (carnivories/secondary consumers)
Figure 9.1—How do
these leaves power
the work of life for
this giraffe?
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Energy Flow and Nutrient Cycling in
Ecosystems
 Energy flows into an ecosystem as sunlight and
leaves as heat
 Photosynthesis generates O2 and organic
molecules (e.g., glucose), which are used in
cellular respiration
 Cells use chemical energy stored in organic
molecules to regenerate ATP, which powers
cellular work
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Figure 9.2—
Energy flow
and chemical
recycling in
ecosystems
Light
energy
ECOSYSTEM
CO2 + H2O
Photosynthesis
in chloroplasts
Cellular respiration
in mitochondria
ATP
Organic
O
molecules + 2
ATP powers
most cellular work
Heat
energy
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BioFlix: The Carbon Cycle
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Concept 9.1: Catabolic pathways yield energy
by oxidizing organic fuels
 Catabolic pathways release stored energy by
breaking down complex molecules
 Electron transfers (i.e., redox reactions) play a
major role in these pathways
 These processes are central to cellular respiration
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Catabolic Pathways and Production of ATP
 The breakdown of organic molecules is exergonic
 Fermentation is a partial degradation of sugars
that occurs without O2 and yields very little ATP
 Aerobic respiration consumes organic molecules
and O2 and yields ATP
 Anaerobic respiration is similar to aerobic
respiration, but consumes compounds other
than O2 and yields same amount of ATP as
aerobic respiration
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 Cellular respiration includes both aerobic and
anaerobic respiration, but is often used to refer to
aerobic respiration
 Although carbohydrates, fats, and proteins are all
consumed as fuel, it is helpful to trace cellular
respiration with the sugar glucose:
C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + Energy (ATP + heat)
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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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The Principle of Redox
 Chemical reactions that transfer electrons
between reactants are called oxidation-reduction
reactions, or redox reactions
 In oxidation, a substance loses electrons,
or is oxidized
 In reduction, a substance gains electrons, or is
reduced (the amount of positive charge is
reduced)
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becomes oxidized
(loses electron)
becomes reduced
(gains electron)
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becomes oxidized
becomes reduced
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 The electron donor is called the reducing
agent
 The electron receptor is called the oxidizing
agent
 Some redox reactions do not transfer electrons
but change the electron sharing in covalent
bonds
 An example is the reaction between methane
and O2 (or the burning of any fuel!)
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Figure 9.3—Methane combustion as an energy-yielding redox
reaction
Reactants
Products
becomes oxidized
becomes reduced
Methane
(reducing
agent)
Oxygen
(oxidizing
agent)
Carbon dioxide
Water
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Oxidation of Organic Fuel Molecules During
Cellular Respiration
 During cellular respiration, the fuel (such as
glucose) is oxidized, and O2 is reduced
becomes oxidized
becomes reduced
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Stepwise Energy Harvest via NAD+ and the
Electron Transport Chain
 In cellular respiration, glucose and other organic
molecules are broken down in a series of steps
 Electrons from organic compounds are usually first
transferred to NAD+, a coenzyme
 As an electron acceptor, NAD+ functions as an
oxidizing agent during cellular respiration
 Each NADH (the reduced form of NAD+)
represents stored energy that is tapped to
synthesize ATP
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Figure 9.4—NAD+ as an electron shuttle
2 e− + 2 H+
2 e− + H+
NAD+
Dehydrogenase
NADH
H+
Reduction of NAD+
2[H]
(from food) Oxidation of NADH
Nicotinamide
(oxidized form)
H+
Nicotinamide
(reduced form)
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How does NAD+ trap electrons from glucose and
other organic molecules?
• Enzymes called dehydrogenases remove a pair of
hydrogen atoms (2 electrons & 2 protons) from the
substrate (e.g., glucose), thereby oxidizing it.
• The enzyme delivers the 2 electrons along with 1
proton to its coenzyme, NAD+
• The other proton is released as a hydrogen ion (H+)
dehydrogenase
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 NADH passes the electrons to the electron
transport chain
 Unlike an uncontrolled reaction, the electron
transport chain passes electrons in a series of
steps instead of one explosive reaction
 O2 pulls electrons down the chain in an energyyielding tumble
 The energy yielded is used to regenerate ATP
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Figure 9.5—An introduction to electron transport chains
H2 + ½ O2
2H
Controlled
release of
energy
ATP
Free energy, G
Free energy, G
2 H+ + 2 e−
Explosive
release
½ O2
+
ATP
ATP
2
e−
½ O2
2 H+
H2O
H2O
(a) Uncontrolled reaction
(b) Cellular respiration
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Mitochondria: Chemical Energy Conversion
 Mitochondria are in nearly all eukaryotic cells
 They have a smooth outer membrane and an inner
membrane folded into cristae
 The inner membrane creates two compartments:
intermembrane space and mitochondrial matrix
 Some metabolic steps of cellular respiration are
catalyzed in the mitochondrial matrix
 Cristae present a large surface area for enzymes
that synthesize ATP
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Figure 6.17—The mitochondrion: site of cellular respiration
Intermembrane space
Outer
membrane
Free
ribosomes
in the
mitochondrial
matrix
Inner
membrane
Cristae
Matrix
0.1 µm
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The Stages of Cellular Respiration: A Preview
 Harvesting of energy from glucose has three
stages:
 Glycolysis (breaks down glucose into two
molecules of pyruvate)
 The citric acid cycle (completes the breakdown of
glucose)
 Oxidative phosphorylation (accounts for most of
the ATP synthesis)
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Figure 9.6—An overview of cellular respiration (step 1)
Electrons
via NADH
GLYCOLYSIS
Glucose
Pyruvate
CYTOSOL
MITOCHONDRION
ATP
Substrate-level
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Figure 9.6—An overview of cellular respiration (step 2)
Electrons
via NADH
GLYCOLYSIS
Glucose
Electrons
via NADH
and FADH2
PYRUVATE
OXIDATION
CITRIC
ACID
CYCLE
Pyruvate
Acetyl CoA
CYTOSOL
MITOCHONDRION
ATP
ATP
Substrate-level
Substrate-level
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Figure 9.6—An overview of cellular respiration (step 3)
Electrons
via NADH
GLYCOLYSIS
Glucose
Electrons
via NADH
and FADH2
PYRUVATE
OXIDATION
CITRIC
ACID
CYCLE
Pyruvate
Acetyl CoA
CYTOSOL
MITOCHONDRION
OXIDATIVE
PHOSPHORYLATION
(Electron transport
and chemiosmosis)
ATP
ATP
ATP
Substrate-level
Substrate-level
Oxidative
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BioFlix: Cellular Respiration
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Oxidative vs. Substrate-level Phosphorylation
 The process that generates most of the ATP is
called oxidative phosphorylation because it is
powered by redox reactions
 Oxidative phosphorylation accounts for almost 90%
of the ATP generated by cellular respiration
 A smaller amount of ATP is formed in glycolysis
and the citric acid cycle by substrate-level
phosphorylation
 In total, 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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Figure 9.7—Substrate-level phosphorylation
Enzyme
Enzyme
ADP
P
ATP
Substrate
Product
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Concept 9.2: Glycolysis harvests chemical
energy by oxidizing glucose to pyruvate
 Glycolysis (“sugar splitting”) breaks down glucose
into two molecules of pyruvate
 Glycolysis occurs in the cytoplasm and has two
major phases
 Energy investment phase
 Energy payoff phase
 Glycolysis occurs whether or not O2 is present
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Figure 9.8—The energy
input and output of
glycolysis
Energy Investment Phase
Glucose
2
ATP used
2 ADP + 2 P
Energy Payoff Phase
4 ADP + 4 P
2 NAD+ + 4 e− + 4 H+
4
ATP
formed
2 NADH + 2 H+
2 Pyruvate + 2 H2O
Net
Glucose
4 ATP formed − 2 ATP used
2 NAD+ + 4 e− + 4 H+
2 Pyruvate + 2 H2O
2 ATP
2 NADH + 2 H+
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Figure 9.9—A closer look at glycolysis
GLYCOLYSIS: Energy Investment Phase
Glucose
6-phosphate
ATP
Glucose
Fructose
6-phosphate
ADP
Phosphoglucoisomerase
Hexokinase
1
2
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Figure 9.9 (cont.)—A closer look at glycolysis
GLYCOLYSIS: Energy Investment Phase
Glyceraldehyde
3-phosphate (G3P)
Fructose ATP
6-phosphate
ADP
Fructose
1,6-bisphosphate
Isomerase
5
Aldolase
Phosphofructokinase
4
3
Dihydroxyacetone
phosphate (DHAP)
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Figure 9.9 (cont.)—A closer look at glycolysis
GLYCOLYSIS: Energy Payoff Phase
Glyceraldehyde
3-phosphate (G3P)
Aldolase
4
2 NAD+
2 NADH
2 H+
2
Isomerase
Triose
phosphate 2
dehydrogenase
5
6
Dihydroxyacetone
phosphate (DHAP)
2 ATP
2 ADP
2
Phosphoglycerokinase
1,3-Bisphosphoglycerate
7
3-Phosphoglycerate
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Figure 9.9 (cont.)—A closer look at glycolysis
GLYCOLYSIS: Energy Payoff Phase
2 H2O
2
2
Phosphoglyceromutase
3-Phosphoglycerate
8
2-Phosphoglycerate
2
Enolase
9
2 ATP
2 ADP
2
Pyruvate
kinase
Phosphoenolpyruvate (PEP)
10
Pyruvate
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Concept 9.3: After pyruvate is oxidized, the
citric acid cycle completes the energy-yielding
oxidation of organic molecules
 In the presence of O2, pyruvate enters the
mitochondrion (in eukaryotic cells) where the
oxidation of glucose is completed
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Oxidation of Pyruvate to Acetyl CoA
 Before the citric acid cycle can begin, however,
pyruvate must first be converted to acetyl
Coenzyme A (acetyl CoA), which links glycolysis
to the citric acid cycle
 This step is carried out by a multi-enzyme complex
that catalyzes three reactions
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GLYCOLYSIS
PYRUVATE
OXIDATION
CITRIC
ACID
CYCLE
OXIDATIVE
PHOSPHORYLATION
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Figure 9.10—Oxidation of pyruvate to acetyl CoA, the step
before the citric acid cycle
MITOCHONDRION
CYTOSOL
CO2
Coenzyme A
3
1
2
Pyruvate
NAD+
NADH + H+
Acetyl CoA
Transport protein
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The Citric Acid Cycle
 The citric acid cycle, also called the Krebs
cycle, completes the break down of pyruvate to
CO2
 The cycle oxidizes organic fuel derived from
pyruvate, generating 1 ATP, 3 NADH, and 1
FADH2 per turn (per pyruvate molecule)
 (or double the numbers above if you are calculating
for 1 glucose molecule!)
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GLYCOLYSIS
CITRIC
ACID
CYCLE
PYRUVATE
OXIDATION
OXIDATIVE
PHOSPHORYLATION
ATP
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PYRUVATE OXIDATION
Pyruvate (from glycolysis,
2 molecules per glucose)
CO2
NAD+
Figure 9.11—An
overview of pyruvate
oxidation and the
citric acid cycle
CoA
NADH
+ H+
Acetyl CoA
CoA
CoA
CITRIC
ACID
CYCLE
FADH2
2 CO2
3 NAD+
CoA
FAD
3 NADH
+ 3 H+
ADP + P i
ATP
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The Citric Acid Cycle, cont.
 The citric acid cycle has eight steps, each
catalyzed by a specific enzyme
 The acetyl group of acetyl CoA joins the cycle by
combining with oxaloacetate, forming citrate (i.e.,
citric acid)
 The next seven steps decompose the citrate back
to oxaloacetate, making the process a cycle
 The NADH and FADH2 produced by the cycle
relay electrons extracted from food to the electron
transport chain
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Figure 9.12—A closer
look at the citric acid
cycle
Acetyl CoA
CoA-SH
NADH
+ H+
H2O
1
NAD+
Oxaloacetate
8
2
Malate
H2O
Citrate
Isocitrate
NAD+
CITRIC
ACID
CYCLE
7
Fumarate
CO2
CoA-SH
4
6
NADH
+ H+
3
-Ketoglutarate
CoA-SH
FADH2
5
NAD+
CO2
FAD
Succinate
Pi
GTP GDP
ADP
Succinyl
CoA
NADH
+ H+
ATP
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Concept 9.4: During oxidative phosphorylation,
chemiosmosis couples electron transport to
ATP synthesis
 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
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The Pathway of Electron Transport
 The electron transport chain is in the inner
membrane (cristae) of the mitochondrion
 Most of the chain’s components are proteins,
which exist in multi-protein complexes
 The carriers alternate reduced and oxidized states
as they accept and donate electrons
 Electrons drop in free energy as they go down the
chain and are finally passed to O2, forming H2O
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OXIDATIVE
PHOSPHORYLATION
CITRIC
ACID
CYCLE
PYRUVATE
OXIDATION
GLYCOLYSIS
ATP
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NADH
Free energy (G) relative to O2 (kcal/mol)
50
2 e− NAD+
FADH2
40
2 e−
FAD
Fe•S
II
I
FMN
Fe•S
Multiprotein
complexes
Q
III
Cyt b
30
Figure 9.13—
Free-energy
change during
electron
transport
Fe•S
Cyt c1
IV
Cyt c
Cyt a
20
10
0
Cyt a3
2 e−
(originally from
NADH or FADH2)
2 H+ + ½ O2
H2O
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The Pathway of Electron Transport, cont.
 Electrons are transferred from NADH or FADH2 to
the electron transport chain
 Electrons are passed through a number of
proteins including cytochromes (each with an iron
atom) to O2
 The electron transport chain generates no ATP
directly
 It breaks the large free-energy drop from food to
O2 into smaller steps that release energy in
manageable amounts
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Chemiosmosis: The Energy-Coupling
Mechanism
 Electron transfer in the electron transport chain
causes proteins to pump H+ from the
mitochondrial matrix to the intermembrane space
 H+ then moves back across the membrane,
passing through the protein complex, ATP
synthase
 ATP synthase uses the exergonic flow of H+ to
drive phosphorylation of ATP
 This is an example of chemiosmosis, the use of
energy in a H+ gradient to drive cellular work
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Figure 9.14—
ATP synthase,
a molecular
mill
INTERMEMBRANE
H+
SPACE
Rotor
Stator
Internal rod
Catalytic
knob
ADP
+
Pi
MITOCHONDRIAL
MATRIX
ATP
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Video: ATP Synthase 3-D Structure, Top View
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Video: ATP Synthase 3-D Structure, Side View
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Figure 9.15—Chemiosmosis couples the electron transport
chain to ATP synthesis
Protein
complex
of electron
carriers
ATP
synthase
H+
H+
H+
H+
Cyt c
IV
Q
III
I
II
FADH2 FAD
NADH
(carrying
electrons
from
food)
2 H+ + ½ O2
NAD+
H 2O
ADP + P i
ATP
H+
1 Electron transport chain
2 Chemiosmosis
Oxidative phosphorylation
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Chemiosmosis = A Proto-Motive Force
 The energy stored in a H+ gradient across a
membrane couples the redox reactions of the
electron transport chain to ATP synthesis
 The H+ gradient is referred to as a proton-motive
force, emphasizing its capacity to do work
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An Accounting of ATP Production by Cellular
Respiration
 During cellular respiration, most energy flows in
this sequence:
glucose → NADH → electron transport chain →
proton-motive force → ATP
 About 34% of the energy in a glucose molecule is
transferred to ATP during cellular respiration,
making about 32 ATP
 There are several reasons why the number of ATP
is not known exactly
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Figure 9.16—ATP yield per molecule of
glucose at each stage of cellular respiration
Electron shuttles
span membrane
CYTOSOL
2 NADH
6 NADH
2 NADH
GLYCOLYSIS
Glucose
MITOCHONDRION
2 NADH
or
2 FADH2
2
Pyruvate
PYRUVATE
OXIDATION
2 Acetyl CoA
+ 2 ATP
Maximum per glucose:
2 FADH2
CITRIC
ACID
CYCLE
OXIDATIVE
PHOSPHORYLATION
(Electron transport
and chemiosmosis)
+ 2 ATP
+ about 26 or 28 ATP
About
30 or 32 ATP
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Concept 9.5: Fermentation and anaerobic
respiration enable cells to produce ATP without
the use of oxygen
 Most cellular respiration requires O2 to produce
ATP
 Without O2, the electron transport chain will cease
to operate
 In that case, glycolysis couples with anaerobic
respiration or fermentation to produce ATP
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 Anaerobic respiration uses an electron transport
chain with a final electron acceptor other than O2,
for example sulfate
 Fermentation uses substrate-level phosphorylation
instead of an electron transport chain to generate
ATP
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Types of Fermentation
 Fermentation consists of glycolysis plus reactions
that regenerate NAD+, which can be reused by
glycolysis
 Two common types are:
 Alcohol fermentation
 Lactic acid fermentation
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Alcohol Fermentation
 In alcohol fermentation, pyruvate is converted to
ethanol in two steps:
1. The first step releases CO2
2. The second step produces ethanol
 Alcohol fermentation by yeast is used in brewing,
winemaking, and baking
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Figure 9.17—Fermentation
2 ADP + 2 P i
ATP
2
GLYCOLYSIS
Glucose
2 Pyruvate
2 NAD+
2 NADH
+ 2 H+
2 Ethanol
2 CO2
2 Acetaldehyde
(a) Alcohol fermentation
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Lactic Acid Fermentation
 In lactic acid fermentation, pyruvate is reduced
by NADH, forming lactate as an end product, with
no release of CO2
 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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Figure 9.17—Fermentation
2 ADP + 2 P i
Glucose
2
ATP
GLYCOLYSIS
2 NAD+
2 NADH
+ 2 H+
2 Pyruvate
2 Lactate
(b) Lactic acid fermentation
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Animation: Fermentation Overview
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Comparing Fermentation with Anaerobic and
Aerobic Respiration
 All use glycolysis (net ATP = 2) to oxidize glucose and
harvest chemical energy of food
 In all three, NAD+ is the oxidizing agent that accepts
electrons during glycolysis
 However, the processes have different final electron
acceptors: an organic molecule (such as pyruvate or
acetaldehyde) in fermentation, O2 in aerobic
respiration, or something else in anaerobic respiration
 Cellular respiration produces 30-32 ATP per glucose
molecule; fermentation produces 2 ATP per glucose
molecule
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 Obligate anaerobes carry out fermentation or
anaerobic respiration and cannot survive in the
presence of O2
 Yeast and many bacteria are facultative
anaerobes, meaning that 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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Glucose
CYTOSOL
Glycolysis
Pyruvate
No O2 present:
Fermentation
O2 present:
Aerobic cellular
respiration
MITOCHONDRION
Ethanol,
lactate, or
other products
Figure 9.18—
Pyruvate as a key
juncture in catabolism
Acetyl CoA
CITRIC
ACID
CYCLE
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The Evolutionary Significance of Glycolysis
 Ancient prokaryotes are thought to have used
glycolysis long before there was oxygen in the
atmosphere
 Very little O2 was available in the atmosphere until
about 2.7 billion years ago, so early prokaryotes
likely used only glycolysis to generate ATP
 Glycolysis is a very ancient process
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Concept 9.6: Glycolysis and the citric acid cycle
connect to many other metabolic pathways
 Glycolysis and the citric acid cycle are major
intersections to various catabolic and anabolic
pathways
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The Versatility of Catabolism
 Catabolic pathways funnel electrons from many
kinds of organic molecules into cellular respiration
 Glycolysis accepts a wide range of carbohydrates
 Proteins must be digested to amino acids; amino
groups can feed 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
 An oxidized gram of fat produces more than twice as
much ATP as an oxidized gram of carbohydrate
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Proteins
Carbohydrates
Amino
acids
Sugars
Fats
Glycerol
Fatty
acids
GLYCOLYSIS
Glucose
Glyceraldehyde 3- P
NH3
Pyruvate
Acetyl CoA
Figure 9.19—The
catabolism of various
molecules from food
CITRIC
ACID
CYCLE
OXIDATIVE
PHOSPHORYLATION
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Biosynthesis (Anabolic Pathways)
 The body uses small molecules to build other
substances
 These small molecules may come:
1. directly from food,
2. from glycolysis,
3. or from the citric acid cycle.
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Regulation of Cellular Respiration via Feedback
Mechanisms
 Feedback inhibition is the most common
mechanism for metabolic control
 If ATP concentration begins to drop, respiration
speeds up; when there is plenty of ATP,
respiration slows down
 Control of catabolism is based mainly on
regulating the activity of enzymes at strategic
points in the catabolic pathway
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Glucose
GLYCOLYSIS
Fructose 6-phosphate
AMP
Stimulates
Phosphofructokinase
Fructose 1,6-bisphosphate
Inhibits
Inhibits
Pyruvate
Citrate
ATP
Acetyl CoA
Figure 9.20—The
control of cellular
respiration
CITRIC
ACID
CYCLE
Oxidative
phosphorylation
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