Chapter 9: Cellular Respiration

Campbell Biology, 12th Edition

 Life Is Work
• Energy flows into an ecosystem as sunlight
and leaves as heat.

• Photosynthesis generates O2 and organic

molecules, which are used in cellular
respiration.
• Cells use chemical energy stored in organic
molecules to regenerate ATP, which powers
work.
 one way, but chemicals are recycled.
e: Draw a cell
n, labeling
drion. As you
How is the chemical energy stored in food used to
through the generate ATP, the molecule that drives most cellular work?
apter, add key
ctions for each
ge of cellular
piration, Light
king the stages energy
gether. Label the
bon molecule(s)
h the most
used in generates
lecule(s) with
Photosynthesis
can be a simple
Organic
CO2 + H2O + O2
molecules

Biology
generates used in
Study Area)

ar Respiration
Free-Energy
ransport Cellular respiration in mitochondria
in Item Library)
breaks down organic
s
molecules,generating
Respiration:

dule Plant cell powers most Animal cell

ATP
cellular work
n (Concept 9.4)
Heat
Concept 9.1: Catabolic pathways yield
energy by oxidizing organic fuels

• The breakdown of organic molecules is

exergonic.
• Fermentation is a partial degradation of sugars
that occurs without O2.
• Aerobic respiration consumes organic
molecules and O2 and yields ATP.
• Anaerobic respiration is similar to aerobic
respiration but consumes compounds other than
O2.
• 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)
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.
 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).
becomes oxidized
(loses electron)

becomes reduced
(gains electron)
becomes oxidized

becomes reduced
• 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.
dition of . Figure 9.2 Methane combustion as an energy-yielding redox
on. (Note reaction. The reaction releases energy to the surroundings because the
electrons lose potential energy when they end up being shared unequally,
tively
spending more time near electronegative atoms such as oxygen.
positive
Reactants Products
r the becomes oxidized
orine (Cl)
CH4 + 2 O2 CO2 + Energy + 2 H2O

H becomes reduced

H C H O O O C O H O H

H
Methane Oxygen Carbon dioxide Water
(reducing (oxidizing
agent) agent)

VISUAL SKILLS Is the carbon atom oxidized or reduced during this

reaction? Explain.
Mastering Biology Animation: Redox Reactions
n donor,
pts the
 The oxidation of methane by O2 is the main combustion
reaction that occurs
Oxidation at the burner
of Organic Fuel of aMolecules
gas stove. The com-
bustion ofCellular
During gasoline inRespiration
an automobile engine is also a redox
reaction; the energy released pushes the pistons. But the
energy-yielding redox process of greatest interest to biologists
•is During cellular
respiration: respiration,
the oxidation of glucosetheand fuel
other(such
moleculesasin
glucose)
food. Examineisagain
oxidized, andequation
the summary O2 is reduced:
for cellular respi-
ration, but this time think of it as a redox process:
becomes oxidized
C6H12O6 1 6 O2 6 CO2 1 6 H2O 1 Energy
becomes reduced

As in the combustion of methane or gasoline, the fuel (glu-

cose) is oxidized and O2 is reduced. The electrons lose poten-
tial energy along the way, and energy is released.
In general, organic molecules that have an abundance of
hydrogen are excellent fuels because their bonds are a source
 Dehydrogenase

Dehydrogenases remove 2 H+ and 2 e- from the

substrate (fuel molecule) and transfer 1 H+ and 2 e-
to NAD+ (a coenzyme). The other proton is released
as H+ into the surrounding solution.
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.
 2 e– + 2 H+
2 e– + H+
NADH H+
Dehydrogenase
Reduction of NAD+
NAD+ + 2[H] + H+
Oxidation of NADH
Nicotinamide
(reduced form)

Nicotinamide
(oxidized form)
• 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 energy-
yielding tumble.
• The energy yielded is used to regenerate ATP.
 1/
H2 + 1/2 O2 2H + 2 O2
(from food via NADH)
Controlled
release of
+
2H + 2e –
energy for
synthesis of
ATP

Elec chain
ATP
Free energy, G

Free energy, G

tron
Explosive ATP
release of

trans
heat and light ATP
energy

por
2 e–

t
1/
2 O2
2 H+

H2O H2O

(a) Uncontrolled reaction (b) Cellular respiration

The Stages of Cellular Respiration

• Cellular respiration 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).
ular
glucose
olecules Electrons carried Electrons carried
own via NADH via NADH
ondrion. and FADH2
ch will
c acid
d FADH2 PYRUVATE OXIDATIVE
GLYCOLYSIS
se to OXIDATION CITRIC PHOSPHORYLATION
dative ACID
chains Glucose Pyruvate Acetyl CoA CYCLE (Electron transport
m and chemiosmosis)
called
f cellular CYTOSOL MITOCHONDRION
re
ate-level
processes
32b. ATP ATP ATP

Overview
Substrate-level Substrate-level Oxidative
phosphorylation phosphorylation phosphorylation

enerated during the first two occurs when an enzyme transfers a phosphate group from a
down the chain. At the end of substrate molecule to ADP, rather than adding an inorganic
• 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.
 Enzyme Enzyme

ADP
P
Substrate + ATP
Product
Concept 9.2: Glycolysis harvests
chemical energy by oxidizing glucose
to pyruvate

• Glycolysis (“splitting of sugar”) breaks down

glucose into two molecules of pyruvate

• Glycolysis occurs in the cytoplasm and has two

major phases:
– Energy investment phase
– Energy payoff phase
 . Figure 9.7 The inputs and outputs of glycolysis.

l energy Mastering Biology

Animation: Glycolysis
uvate GLYCOLYSIS
PYRUVATE CITRIC
ACID
OXIDATIVE
PHOSPHORY-
OXIDATION
CYCLE LATION
nd that is
Glucose, a six-
ugars. These GLYCOLYSIS: ATP

maining atoms Energy Investment Phase

e. (Pyruvate is
Glucose

n be divided
e and the energy 2 ATP used 2 ADP + 2 P
phase, the cell
d with inter-
Energy Payoff Phase
P is produced
+
is reduced to
4 ADP + 4 P 4 ATP formed
on of glucose.
ose molecule, is
olytic pathway 2 NAD+ + 4 e– + 4 H+ 2 NADH + 2 H+

cose is 2 Pyruvate + 2 H2O

ate; no carbon is
occurs whether Net Inputs and Outputs
t, the chemi- Glucose 2 Pyruvate + 2 H2O
be extracted
4 ATP formed – 2 ATP used 2 ATP
and oxidative
2 NAD+ + 4 e– + 4 H+ 2 NADH + 2 H+
 . Figure 9.8 The steps of glycolysis. Glycolysis, a source of ATP and NADH, takes place in
PYRUVATE CITRIC OXIDATIVE the cytosol. Two of the enzymes (in steps 1 and 3 ) are shown in Figure 6.32b.
GLYCOLYSIS
OXIDATION ACID
CYCLE
PHOSPHORY-
LATION . Figure 9.8 The steps of glycolysis. Glycolysis, a source of ATP and NADH, t
GLYCOLYSIS: Energy Investment Phase
PYRUVATE CITRIC OXIDATIVE the cytosol. Two of the enzymes (in steps 1 and 3 ) are shown in Figure 6.32b.
ACID PHOSPHORY-
OXIDATION
CYCLE LATION WHAT IF? What would happen if you removed the dihydroxyacetone
ATP
GLYCOLYSIS: Energy
phosphate generated in step 4Investment Phase
as fast as it was produced?

Glyceraldehyde
WHAT IF? What would happen if you removed the dihydroxyacetone
3-phosphate (G3P)
phosphate generated in step 4 as fast as it was produced? HC O
ATP Glucose Fructose ATP Fructose CHOH
Glucose 6-phosphate 6-phosphate 1,6-bisphosphate Glycer
ADP ADP CH2O P
CH2OH CH2O
O
P CH2O
O
P CH2OH P OCH2
O
CH2O P
3-phosp
O H H H H
Isomerase
H H
OH H OH H
H HO H HO 5 HC
H H OH
ATP
HO OH Glucose
Hexokinase HO OH Phosphogluco- Fructose Phospho-
OH
ATP Fructose
Aldolase
Dihydroxyacetone CH
isomerase HO H fructokinase HO H
H OH 6-phosphate
1 H OH 6-phosphate 1,6-bisphosphate
4 phosphate (DHAP)
2 3 CH
ADP CH2O P P CH2OH
CH2O ADP P OCH2 CH2O
CH O
P2
P

Hexokinase transfers
O Glucose 6- OPhosphofructokinase O
Aldolase cleaves C O
H H groupH
a phosphate phosphate is transfers a phosphate the sugar
H HO HO CH2OH
molecule H
H
from ATPOHto glucose,
HOit more
H
OH
converted to H groupOHfrom ATP to the H into OH
Hexokinase
making Phosphogluco-
fructose opposite end of Phospho-
the two different Conversion betweenAldolase
DHAP
HO H investingfructokinase HO H and G3P: This reaction
Dihydr
chemically reactive. isomerase
6-phosphate. sugar, a second three-carbon
H
1 The charged OH
molecule of ATP. This is sugars. 4
never reaches equilibrium; phosph
phosphate also traps 2 3
a key step for regulation G3P is used in the next step CH
the sugar in the cell. of glycolysis. as fast as it forms.
xokinase transfers Glucose 6- Phosphofructokinase Aldolase cleaves C
hosphate group phosphate is transfers a phosphate the sugar CH
m ATP
170 to glucose,
UNIT TWO The Cell converted to group from ATP to the molecule into
king it more fructose opposite end of the two different Conversion
mically reactive. 6-phosphate. sugar, investing a second three-carbon and G3P: Th
e charged molecule of ATP. This is sugars. never reach
osphate also traps a key step for regulation G3P is used
sugar in the cell. of glycolysis. as fast as it
the citric acid cycle, is carried out by a multienzyme complex
that catalyzes three reactions: 1 Pyruvate’s carboxyl group Mastering Biology BioFlix® Animation: Acetyl CoA

The energy payoff phase occurs after glucose is split into two three-carbon
sugars. Thus, the coefficient 2 precedes all molecules in this phase.

GLYCOLYSIS: Energy Payoff Phase

2 ATP 2 ATP
2 NADH 2 H 2O
2 ADP
2 NAD + + 2 H+ 2 ADP 2 2 2 2
O– O– O– O–
2
P OC O C O C O C O C O

CHOH CHOH H CO P CO P C O
Triose Phospho- Phospho- Enolase Pyruvate
phosphate 2 Pi CH2O P glycerokinase CH2 O P glyceromutase CH2OH CH2 kinase CH3
dehydrogenase 9
1,3-Bisphospho- 7 3-Phospho- 8 2-Phospho- Phosphoenol- 10 Pyruvate
6 glycerate glycerate glycerate pyruvate (PEP)

Two sequential reactions: The phosphate group is This enzyme Enolase causes a The phosphate
(1) G3P is oxidized by the transferred to ADP relocates the double bond to form group is transferred
transfer of electrons to (substrate-level remaining in the substrate by from PEP to ADP
NAD+, forming NADH. phosphorylation) in an phosphate extracting a water (a second example
(2) Using energy from this exergonic reaction. The group. molecule, yielding of substrate-level
exergonic redox reaction, carbonyl group of G3P phosphoenolpyruvate phosphorylation),
a phosphate group is has been oxidized to (PEP), a compound forming pyruvate.
attached to the oxidized the carboxyl group with a very high
substrate, making a (—COO–) of an organic potential energy.
high-energy product. acid (3-phosphoglycerate).
Mastering Biology BioFlix®
Animation: Glycolysis

CHAPTER 9 Cellular Respiration and Fermentation 171

Concept 9.3: The citric acid cycle
completes the energy-yielding oxidation
of organic molecules

• In the presence of O2, pyruvate enters the

mitochondrion.

• Before the citric acid cycle can begin, pyruvate

must be converted to acetyl CoA, which links
the cycle to glycolysis.
 S-CoA when it is attached to a molecule, emphasizing its sulfur
atom, S.)

CITRIC OXIDATIVE
PYRUVATE
GLYCOLYSIS ACID PHOSPHORY-
OXIDATION
CYCLE LATION

n 10 nm
ins Pyruvate dehydrogenase
nt,
e MITOCHONDRION
CYTOSOL
Coenzyme A
CO2
e
O– 1 3
S-CoA
O2.) C O
Pyruvate dehydrogenase C O
C O
2 CH3
CH3
Acetyl CoA
Pyruvate NAD + NADH + H +
u-
me Transport protein
and
ex
Mastering Biology BioFlix® Animation: Acetyl CoA
• The citric acid cycle, also called the Krebs
cycle, takes place within the mitochondrial
matrix.
• The cycle oxidizes organic fuel derived from
pyruvate, generating 1 ATP, 3 NADH, and 1
FADH2 per turn.
a per-glucose basis, multiply by 2 because each glucose molecule is oxaloacetate, forming citrate
split during glycolysis into two pyruvate molecules.
ized form of citric acid, for w
CYTOSOL next seven steps decompose
etate. It is this regeneration o
Pyruvate
(from glycolysis, process a cycle.
2 molecules per glucose) GLYCOLYSIS
PYRUVATE
OXIDATION
CITRIC
ACID
OXIDATIVE
PHOSPHORY- Referring to Figure 9.11, w
CYCLE LATION
C C C molecules produced by the c
tyl group entering the cycle,
ATP (steps 3 , 4 , and 8 ). In step
not to NAD + , but to FAD, wh
C C C PYRUVATE OXIDATION 2 protons to become FADH2.
the reaction in step 5 produ
C CO2 (GTP) molecule by substrate
NAD+
CoA a molecule similar to ATP in
NADH tion. This GTP may be used t
+ H+ shown) or directly power wo
Acetyl CoA
C C plants, bacteria, and some an
CoA
ATP molecule directly by sub
NADH
CoA The output from step 5 repr
+ H+
during the citric acid cycle. R
rise to two molecules of acet
NAD +
Because the numbers noted
CITRIC C C gle acetyl group entering the
ACID
CYCLE 2 CO2 glucose from the citric acid c
or 6 NADH, 2 FADH2, and th
FADH2 2 NAD+ Most of the ATP produced
2 NADH from oxidative phosphorylati
FAD produced by the citric acid cy
+ 2 H+
ADP + P i electrons extracted from food
In the process, they supply th
ATP
MITOCHONDRION phorylation of ADP to ATP. W
the next section.
• 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.
• 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.
 CITRIC OXIDATIVE
PYRUVATE
GLYCOLYSIS ACID PHOSPHORY-
OXIDATION
CYCLE LATION

ATP
S-CoA 1 Acetyl CoA (from
oxidation of pyruvate)
C O
adds its two-carbon acetyl 2 Citrate is
CH3 group to oxaloacetate, converted to
producing citrate. its isomer,
Acetyl CoA
isocitrate, by
8 The substrate removal of
is oxidized, CoA-SH one water
reducing NAD+ to molecule and
NADH and addition of
regenerating NADH O C COO– another.
oxaloacetate. + H+ CH2 1 COO– H2O

+ COO– CH2 COO–

NAD
8 Oxaloacetate HO C COO– CH2
2
–
CH2 HC COO–
COO
COO– HO CH
HO CH
Malate Citrate COO–
CH2 3 Isocitrate
Isocitrate is oxidized,
COO–
7 Addition of NAD + reducing
a water NAD+ to
CITRIC NADH NADH. Then
molecule 3
ACID + H+ the resulting
rearranges 7 CYCLE
bonds in the H2O compound
CO2 loses a CO2
substrate. COO–
COO – molecule.
CH
Fumarate CoA-SH

c-Ketoglutarate
CH2
HC
–
CH2
COO
4 C O
6 CoA-SH COO–
COO– COO–

CH2 CH2 4 Another CO2

FADH 2 5 CO2
CH2 CH2 NAD + is lost, and the
FAD resulting
COO– C O compound is
6 Two
Succinate S-CoA NADH oxidized,
hydrogens are Pi
transferred to + H+ reducing NAD+
GTP GDP Succinyl to NADH.
FAD, forming CoA
FADH2 and The remain-
ing molecule is
oxidizing ADP
then attached
succinate.
to coenzyme A
ATP by an unstable
5 CoA is displaced by a bond.
phosphate group, which is
transferred to GDP, forming GTP,
a molecule with functions
similar to ATP. GTP can also be
Mastering Biology BioFlix® Animation: The Citric Acid Cycle used, as shown, to generate ATP.
 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.
The Pathway of Electron Transport

• The electron transport chain is in the cristae of

the mitochondrion.
• 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.
• Electrons drop in free energy as they go down
the chain and are finally passed to O2, forming
H2O.
• 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.
• The chain’s function is to break the large free-
energy drop from food to O2 into smaller steps that
release energy in manageable amounts.
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 channels in 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.
cytochrome is similar (See Figure 6.32b and c.)
otein of red blood
Inner
carries oxygen, not Intermembrane space mitochondrial
has several types of Mitochondrial matrix membrane
tter and number to
slightly different
ytochrome of the 1 H+ ions flowing
INTERMEMBRANE SPACE
n (in O2), which is down their gradient
a pair of hydrogen enter a channel in
a stator, which is
, neutralizing the anchored in the
ming water. membrane.
ectron transport 2 H+ ions enter binding
uct of the citric acid H+ ions Stator sites within a rotor,
changing the shape of
adds its electrons Rotor each subunit so that
y level than NADH the rotor spins within
nd FADH2 each donate the membrane.
r oxygen reduction, 3 Each H+ ion makes one
out one-third less complete turn before
leaving the rotor and
ron donor is FADH2 passing through a second
next section. channel in the stator
into the mitochondrial
o ATP directly. Internal matrix.
food to oxygen, rod
4 Spinning of the
eries of smaller steps rotor causes an internal
nts, step by step. How Catalytic rod to spin as well. This
membrane in prokary- knob rod extends like a stalk
into the knob below it,
nergy release to ATP which is held stationary
lled chemiosmosis. ADP by part of the stator.
+ 5 Turning of the rod
Pi ATP activates catalytic sites
in the knob that
produce ATP from ADP
nism MITOCHONDRIAL MATRIX
and P i .
mitochondrion or the Mastering Biology BioFlix® Animation: ATP Synthase
• 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.
an electron transport chain built into the large complexes. As the complexes shuttle their gradient via ATP synthase, which is built
inner mitochondrial membrane. (See Figure electrons, they pump protons from the into the membrane nearby. The ATP synthase
6.32b.) The gold arrows trace the transport mitochondrial matrix into the intermembrane harnesses the proton-motive force to
of electrons, which are finally passed to a space. FADH2 deposits its electrons via phosphorylate ADP, forming ATP. Together,
terminal acceptor (O2, in the case of aerobic complex II and so results in fewer protons electron transport and chemiosmosis make
respiration) at the “downhill” end of the being pumped into the intermembrane space up oxidative phosphorylation.

Intermembrane space Inner

Mitochondrial matrix mitochondrial
membrane
CITRIC OXIDATIVE
PYRUVATE
GLYCOLYSIS ACID PHOSPHORY-
OXIDATION
CYCLE LATION

ATP

H+
H+ H+ H+
H+ H+ ATP
H+ H+ synthase
H+
H+ H+ H+ H+
Protein complex Cyt c
Intermembrane H+
space of electron
carriers

IV
Q
I III

Inner II
2 H+ + 1 2 O2 H2O
mitochondrial FADH2 FAD
membrane
NADH NAD+
ADP + P i ATP
(carrying electrons
from food)
H+

Mitochondrial 1 Electron transport chain 2 Chemiosmosis

matrix Electron transport and pumping of protons (H+), ATP synthesis powered by the flow
which create an H+ gradient across the membrane of H+ back across the membrane

Oxidative phosphorylation

WHAT IF? If complex IV were nonfunctional, could chemiosmosis Mastering Biology Animation: Electron Transport
produce any ATP, and if so, how would the rate of synthesis differ? BioFlix® Animation: Electron Transport

176 UNIT TWO The Cell

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.
formed. But chemiosmosis also occurs elsewhere and in other pyruvate oxidation and the citric acid cycle, and the electron
variations. Chloroplasts use chemiosmosis to generate ATP transport chain, which drives oxidative phosphorylation.
during photosynthesis; in these organelles, light (rather than Figure 9.15 gives a detailed accounting of the ATP yield for

. Figure 9.15 ATP yield per molecule of glucose at each stage of cellular respiration.

CYTOSOL Electron shuttles MITOCHONDRION

span membrane 2 NADH
or
2 FADH2

2 NADH 2 NADH 6 NADH 2 FADH2

GLYCOLYSIS PYRUVATE OXIDATION OXIDATIVE

CITRIC PHOSPHORYLATION
ACID
Glucose 2 Pyruvate 2 Acetyl CoA CYCLE (Electron transport
and chemiosmosis)

+ 2 ATP + 2 ATP + about 26 or 28 ATP

by substrate-level by substrate-level by oxidative phosphorylation, depending
phosphorylation phosphorylation on which shuttle transports electrons
from NADH in cytosol

Maximum per glucose: About

30 or 32 ATP

VISUAL SKILLS After reading the discussion in the text, explain exactly Mastering Biology Animation: ATP Yield from
how the total of 26 or 28 ATP from oxidative phosphorylation was Cellular Respiration
calculated (see the yellow bar).

CHAPTER 9 Cellular Respiration and Fermentation 177

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.

• Glycolysis can produce ATP with or without O2

(in aerobic or anaerobic conditions).
• In the absence of O2, glycolysis couples with
fermentation or anaerobic respiration to
produce ATP.
• Anaerobic respiration uses an electron
transport chain with an electron acceptor other
than O2, for example sulfate.

• Fermentation uses phosphorylation instead of

an electron transport chain to generate ATP.
 Types of Fermentation

• Fermentation consists of glycolysis plus

reactions that regenerate NAD+, which can be
reused by glycolysis.

• Two common types are alcohol fermentation

and lactic acid fermentation.
• In alcohol fermentation, pyruvate is converted
to ethanol in two steps, with the first releasing
CO2.
• Alcohol fermentation by yeast is used in brewing,
winemaking, and baking.
 ransport chain—in other are (a) ethanol and (b) lactate, the ionized form of lactic acid.
How can food be oxidized
mber, oxidation simply
2 ADP + 2 P i 2 ATP O–
ectron acceptor, so it does
xidizes glucose to two mol- C O
ent of glycolysis is NAD +, O
C
sfer chain is involved. Glucose GLYCOLYSIS
CH3
some of the energy made
et) by substrate-level 2 Pyruvate
en additional ATP is made
2 NAD+ 2 NADH 2 CO2
NADH passes electrons + 2 H+
n transport chain. But gly- H H
gen is present or not—that
H C OH C O
anaerobic. NAD+ REGENERATION
CH3 CH3
idation of organic nutri-
glycolysis that allows con- 2 Ethanol 2 Acetaldehyde
bstrate-level phosphoryla-
here must be a sufficient (a) Alcohol fermentation
uring the oxidation step
ism to recycle NAD + from 2 ADP + 2 P i 2 ATP
e the cell’s pool of NAD +
d shut itself down for
obic conditions, NAD + is
Glucose GLYCOLYSIS
of electrons to the electron O–
ative is to transfer electrons C O
duct of glycolysis.
2 NAD+ 2 NADH C O
O– + 2 H+ CH3
C O
2 Pyruvate
us reactions that regenerate H C OH
NAD+ REGENERATION
NADH to pyruvate or deriv- CH3
n be reused to oxidize sugar
2 Lactate
les of ATP by substrate-level
es of fermentation, differ- (b) Lactic acid fermentation
pyruvate. Two types are
• In lactic acid fermentation, pyruvate is
reduced to 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.
Fermentation and Aerobic Respiration
Compared
• Both processes use glycolysis to oxidize
glucose and other organic fuels to pyruvate.
• The processes have different final electron
acceptors: an organic molecule (such as
pyruvate or acetaldehyde) in fermentation and
O2 in cellular respiration.
• Cellular respiration produces 38 ATP or 32
ATP? per glucose molecule; fermentation
produces 2 ATP per glucose molecule.
• 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.
on catabolic pathways of glucose oxidation. In a facultative anaerobe,
capable of both aerobic cellular respiration and fermentation,
bic respiration
pyruvate is committed to one of those two pathways, usually
ducing ATP by depending on whether or not oxygen is present.
ree use glycoly-
Glucose
o pyruvate,
vel phosphory-
Glycolysis
oxidizing agent CYTOSOL
ysis.
Pyruvate
sms for oxi-
to sustain No O2 present: O2 present:
Fermentation Aerobic cellular
acceptor is an respiration
fermentation)
lular respira-
re transferred MITOCHONDRION
ates the NAD + Ethanol, Acetyl CoA
lactate, or
ATP produced. other products
CITRIC
duced by ACID
e of an electron CYCLE
is unavailable.
pletely oxidized
rgy from this

CHAPTER 9 Cellular Respiration and Fermentation 181

Concept 9.6: Glycolysis and the citric
acid cycle connect to many other
metabolic pathways

Gycolysis and the citric acid cycle are major

intersections to various catabolic and anabolic
pathways.
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.
 cycle are catabolic funnels through which electrons from all kinds of
e of Glycolysis organic molecules flow on their exergonic fall to oxygen.

both fermentation Proteins Carbohydrates Fats

asis. Ancient prokary-
is to make ATP long
tmosphere. The old- Amino Sugars Glycerol Fatty
acids acids
3.5 billion years, but
ably did not begin
GLYCOLYSIS
about 2.7 billion
s O2 as a by-product Glucose
okaryotes may have
ysis. The fact that Glyceraldehyde 3- P
d metabolic pathway
t it evolved very early NH3 Pyruvate
ation of glycolysis
way does not require
lles of the eukaryotic Acetyl CoA
illion years after the
etabolic heirloom
tion in fermentation
CITRIC
of organic molecules ACID
CYCLE

olysis. What is the

ermentation? During
OXIDATIVE PHOSPHORYLATION
espiration?
oved from an aerobic
would its rate of
re to be generated at
animals. We obtain most of our calories in the form of fats,
Biosynthesis (Anabolic Pathways)

• The body uses small molecules to build other

substances.
• These small molecules may come directly from
food, from glycolysis, or from the citric acid
cycle.
Regulation of Cellular Respiration via
Feedback Mechanisms

• Feedback inhibition is the most common

mechanism for 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.
 as energy. Not all the organic mol- enzyme. It is stimulated by AMP (derived from ADP) but is inhibited
be oxidized as fuel to make ATP. In by ATP and by citrate. This feedback regulation adjusts the rate of
ust also provide the carbon skeletons respiration as the cell’s catabolic and anabolic demands change.
eir own molecules. Some organic Glucose
gestion can be used directly. For
ioned, amino acids from the hydro- AMP
GLYCOLYSIS
be incorporated into the organism’s Fructose 6-phosphate Stimulates
er, the body needs specific molecules +
n food. Compounds formed as inter- Phosphofructokinase
–
e citric acid cycle can be diverted –
ecursors from which the cell can Fructose 1,6-bisphosphate
Inhibits Inhibits
equires. For example, humans can
mino acids in proteins by modifying
from the citric acid cycle; the rest are
must be obtained in the diet. Also,
yruvate, and fatty acids can be syn-
course, these anabolic, or biosyn-
erate ATP, but instead consume it. Pyruvate
nd the citric acid cycle function as
t enable our cells to convert some
ATP Citrate
s as we need them. For example, Acetyl CoA
d generated during glycolysis,
ate (see Figure 9.8, step 5), can
major precursors of fats. If we eat
e store fat even if our diet is fat- CITRIC
ably versatile and adaptable. ACID
CYCLE

ar Respiration
anisms
nd demand regulate the metabolic
waste energy making more of a par- Oxidative
phosphorylation
ds. If there is a surplus of a certain

CHAPTER 9 Cellular Respiration and Fermentation 183