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CHAPTER 5
Aerobic Respiration and the
Mitochondrion
Introduction
• The early Earth was populated by anarobes,
which captured and utilized energy by oxygenindependent metabolism.
• Oxygen accumulated in the primitive
atmosphere after cyanobacteria appeared.
• Aerobes evolved to use oxygen to extract more
energy from organic molecules.
• In eukaryotes, aerobic respiration takes place in
the mitochondrion.
5.1 Mitochondrial Structure and
Function (1)
• Mitochondria have characteristic
morphologies despite variable
appearance.
– Typical mitochondria are bean-shaped
organelles but may be round or threadlike.
– The size and number of mitochondria reflect
the energy requirements of the cell.
Mitochondria
Mitochondrial Structure and Function
(2)
• Mitochondria can fuse with one another, or
split in two.
– The balance between fusion and fission is likely a
major determinant of mitochondrial number,
length, and degree of interconnection.
Mitochondrial fusion and fission
Mitochondrial Structure and Function
(3)
• Inner and outer mitochondrial membranes
enclose two spaces: the matrix and
intermembrane space.
– The outer mitochondrial membrane serves as its
outer boundary.
– The inner mitochondrial membrane is subdivided
into two interconnected domains:
• Inner boundary membrane
• Cristae – where the machinery for ATP is located
The structure of a mitochondrion
Mitochondrial Structure and Function
(4)
• Mitochondrial Membranes
– The outer membrane is about 50%; the inner
membrane is more than 75% protein.
– The inner membrane contains cardiolipin but not
cholesterol, both are true of bacterial membranes.
– The outer membrane contains a large poreforming protein called porin.
– The inner membrane is impermeable to even
small molecules; the outer membrane is
permeable to even some proteins.
Porins
Mitochondrial Structure and Function
(5)
• The mitochondrial matrix
– Contains a circular DNA molecule, ribosomes, and
enzymes.
– RNA and proteins can be synthesized in the
matrix.
Overview of carbohydrate metabolism in
eukaryotic cells
5.2 Oxidative Metabolism in the
Mitochondrion (1)
• The first steps in oxidative metabolism are
carried out in glycolysis.
– Glycolysis produces pyruvate, NADH, and two
molecules of ATP.
– Aerobic organisms use O2 to extract more than 30
additional ATPs from pyruvate and NADH.
– Pyruvate is transported across the inner
membrane and decarboxylated to form acetyl
CoA, which enters the next stage.
An overview of glycolysis
Oxidative Metabolism in the
Mitochondrion (2)
• The tricarboxylic acid (TCA) cycle
– It is a stepwise cycle where substrate is oxidized
and its energy conserved.
– The two-carbon acetyl group from acetyl CoA is
condensed with the four-carbon oxaloacetate to
form a six-carbon citrate.
– During the cycle, two carbons are oxidized to CO2,
regenerating the four-carbon oxaloacetate needed
to continue the cycle.
The TCA cycle
Oxidative Metabolism in the
Mitochondrion (3)
• The TCA cycle (continued)
– Four reactions in the cycle transfer a pair of
electrons to NAD+ to form NADH, or to FAD+ to
form FADH2.
– Reaction intermediates in the TCA cycle are
common compounds generated in other catabolic
reactions making the TCA cycle the central
metabolic pathway of the cell.
Catabolic pathways generate compounds that
are fed into the TCA cycle
Oxidative Metabolism in the
Mitochondrion (4)
• The Importance of Reduced Coenzymes in the
Formation of ATP
– The reduced coenzymes FADH2 and NADH are the
primary products of the TCA cycle.
– NADH formed during glycolysis enters the
mitochondria via malate-aspartate or glycerol
phosphate shuttles.
The glycerol phosphate shuttle
Oxidative Metabolism in the
Mitochondrion (5)
• The Importance of Reduced Coenzymes
– As electrons move through the electron-transport
chain, H+ are pumped out across the inner
membrane.
– ATP is formed by the controlled movement of H+
back across the membrane through the ATPsynthesizing enzyme.
Oxidative Metabolism in the
Mitochondrion (6)
• Reduced coenzymes (continued)
– The coupling of H+ translocation to ATP synthesis
is called chemiosmosis.
– Three molecules of ATP are formed from each pair
of electrons donated by NADH; two molecules of
ATP are formed from each pair of electrons
donated by FADH2.
Summary of oxidative phosphorylation
The Human Perspective: The Role of Anaerobic
and Aerobic Metabolism in Exercise (1)
• ATP hydrolysis increases 100-fold during
exercise, quickly exhausting ATP available.
• Muscles used stored creatine phosphate (CrP)
to rapidly generate but must rely on aerobic or
anaerobic synthesis of new ATP for sustained
activity.
CrP + ADP  Cr + ATP
The Human Perspective: The Role of Anaerobic
and Aerobic Metabolism in Exercise (2)
• Fast-twitch muscle fibers contract rapidly,
have few mitochondria ad produce ATP
anaerobically.
– Anaerobic metabolism produces fewer ATPs per
glucose but produces them very fast.
– Anaerobic metabolism rapidly depletes available
glucose and builds up lactic acid which reduces
cellular pH.
Skeletal muscles
The Human Perspective: The Role of Anaerobic
and Aerobic Metabolism in Exercise (3)
• Slow-twitch fibers contract slowly, have many
mitochondria and produce most of their ATP by
aerobic metabolism.
– Aerobic metabolism initially uses glucose as a
substrate.
– Free fatty acids are oxidized during prolonged
exercise.
• Ratio of fast- to slow-twitch fibers is variable and
depends on the normal function of the muscle.
5.3 The Role of Mitochondria in the
Formation of ATP (1)
• ATP can be formed by
substrate-level
phosphorylation or
oxidative
phosphorylation.
– Accounts for > 160 kg of
ATP in our bodies per
day
The Role of Mitochondria in the
Formation of ATP (2)
• Oxidation-Reduction (Redox) Potentials
– Strong oxidizing agents have a high affinity for
electrons; strong reducing agents have a weak
affinity for electrons
– Redox reactions are accompanied by a decrease in
free energy.
– The transfer of electrons causes charge separation
that can be measured as a redox potential.
Redox potential of some reaction couples
The Role of Mitochondria in the Formation of
ATP (3)
• Electron Transport
– Electrons move through the inner membrane via a
series of carriers of decreasing redox potential.
– Electrons associated with either NADH or FADH2
are transferred through specific electron carriers
that make up the electron transport chain.
The Role of Mitochondria in the Formation of
ATP (4)
• Types of Electron Carriers
– Flavoproteins are polypeptides bound to either
FAD or FMN.
– Cytochromes contain heme groups bearing Fe or
Cu metal ions.
– Three cooper atoms are located within a single
protein complex and alternate between Cu2+/Cu3+
– Ubiquinone (coenzyme Q) is a lipid-soluble
molecule made of five-carbon isoprenoid units.
Structures of three electron carriers
The Role of Mitochondria in the Formation of
ATP (5)
• Types of Electron
Carriers (continued)
– Iron-sulfur proteins
contain Fe in association
with inorganic sulfur.
– These carriers are
arranged in order of
increasingly positive
redox potential.
– Sequence of carriers
determined by use if
inhibitors.
Sequence of electron carriers
The Role of Mitochondria in the Formation of
ATP (6)
• Electron-Transport Complexes
– Complex I (NADH dehydrogenase) catalyzes
transfer of electrons from NADH to ubiquinone
and transports four H+ per pair.
– Complex II (succinate dehydrogenase) catalyzes
transfer of electrons from succinate to FAD to
ubiquinone without transport of H+.
– Complex III (cytochrome bc1) catalyzes the transfer
of electrons from ubiquinone to cytochrome c and
transports four H+ per pair.
The electron-transport chain of the inner
mitochondrial membrane
The electron-transport chain of the inner
mitochondrial membrane
The Role of Mitochondria in the Formation of
ATP (7)
• Electron-Transport Complexes (continued)
– Complex IV (cytochrome c oxidase) catalyzes
transfer of electrons to O2 and transports H+
across the inner membrane.
– Cytochrome oxidase is a large complex that adds
four electrons to O2 to form two molecules of H2O.
– The metabolic poisons CO, N3–, and CN– bind
catalytic sites in Complex IV.
Cytochrome oxidase
The Role of Mitochondria in the
Formation of ATP (8)
• Cytochrome oxidase
– Electrons are transferred one at a time.
– Energy released by O2 reduction is presumably
used to drive conformational changes.
– These changes would promote the movement of
H+ ions and through the protein.
5.4 Translocation of Protons and the
Establishment of a Proton-Motive Force (1)
• There are two components of the proton
gradient:
– The concentration gradient between the matrix
and intermembrane space creates a pH gradient
(ΔpH).
– The separation of charge across the membrane
creates an electric potential (Ψ).
– The energy present in both components of the
gradients is proton-motive force (Δp).
Visualizing the proton-motive force
Translocation of Protons and the Establishment
of a Proton-Motive Force (2)
• Dinitrophenol (DNP) uncouples glucose
oxidation and ATP formation by increasing the
permeability of the inner membrane to H+,
thus eliminating the proton gradient.
• Differences in uncoupling proteins (UCPs)
account for differences in metabolic rate.
5.5 The Machinery for ATP
Formation (1)
• Isolation of coupling
factor 1, or F1, showed
that it hydrolyzed ATP.
– Under experimental
conditions, it behaves as
an ATP synthase.
– Led to conclusion that an
ionic gradient
establishes a protonmotive force to
phosphorylate ADP.
An experiment to drive ATP formation in
membrane vesicles reconstituted with the
Na+/K+-ATPase
The Machinery for ATP Formation
(2)
• The structure of the ATP synthase:
– The F1 particle is the catalytic subunit, and
contains three catalytic sites for ATP synthesis.
– The F0 particle attaches to the F1 and is embedded
in the inner membrane.
– The F0 base contains a channel through which
protons are conducted from the intermembrane
space to the matrix–demonstrated in experiments
with submitochondrial particles.
The structure of the ATP synthase
ATP formation in experiments with
submitochondrial particles
The Machinery for ATP Formation
(3)
• The Basis of ATP Formation According to the
Binding Change Mechanism
– The binding change mechanism states the
following:
• Movement of protons through ATP synthase alters the
binding affinity of the active site.
• Each active site goes through distinct conformations
that have different affinities for substrates and product.
The structural basis of catalytic site
conformation
The binding change mechanism for ATP
synthesis
The Machinery for ATP Formation
(4)
• Binding change mechanism
– Binding sites on the catalytic subunit can be tight,
loose, or open.
– ATP is synthesized through rotational catalysis
where the stalk of ATP synthase rotates relative to
the head.
– There is structural and experimental evidence to
support this mechanism
Direct observation of rotational catalysis
The Machinery for ATP Formation
(5)
• Using the Proton Gradient to Drive the
Catalytic Machinery: The Role of the F0
Portion of ATP Synthase
– The c subunits of the F0 base form a ring.
– The c ring is bound to  subunit of the stalk.
– Protons moving through membrane rotate the
ring.
– Rotation of the ring provides twisting force that
drives ATP synthesis.
A model of the proton diffusion coupled to
rotation of c ring in the F0 complex
The Machinery for ATP Formation
(6)
• Other Roles for the Proton-Motive Force in
Addition to ATP Synthesis
– The H+ gradient drives transport of ADP into and
ATP out of the mitochondrion.
– ADP is the most important factor controlling the
respiration rate.
– Many factors influence the rate of respiration, but
the pathways are poorly understood.
Summary of the major activities during aerobic
respiration in the mitochondrion
5.6 Peroxisomes (1)
• Peroxisomes are membrane-bound vesicles
that contain oxidative enzymes.
• They oxidize very-long-chain fatty acids, and
synthesize plasmalogens (a class of
phospholipids).
• They form by splitting from preexisting
organelles, import preformed proteins, and
engage in oxidative metabolism.
The structure and function of peroxisomes
Peroxisomes (2)
• Hydrogen peroxide (H2O2), a reactive and
toxic compound, is formed in peroxisomes
and is broken down by the enzyme catalase.
• Plants contain a special peroxisome called
glyoxysome, which can convert fatty acids to
glucose by germinating seedlings.
Glyoxysome localization within plant seedlings
The Human Perspective: Diseases that Result
from Abnormal Mitochondrial or Peroxisomal
Function (1)
• Mitochondria
– A variety of disorders are known that result from
abnormalities in mitochondria structure/function.
– Majority of mutations linked to mitochondrial
diseases are traced to mutations in mtDNA.
– Mitochondrial disorders are inherited maternally.
Mitochondrial abnormalities in skeletal muscle
The Human Perspective: Diseases that Result
from Abnormal Mitochondrial or Peroxisomal
Function (2)
• It is speculated that accumulations of
mutations in mtDNA is a major cause of aging.
• In mice encoding a mutation in their mtDNA,
signs of premature aging develop.
• Additional findings suggest that mutations in
mtDNA may cause premature aging but are
not sufficient for the normal aging process.
A premature aging phenotype caused by
mutations in mtDNA
The Human Perspective: Diseases that Result
from Abnormal Mitochondrial or Peroxisomal
Function (3)
• Peroxisomes
– Patients with Zellweger syndrome lack
peroxisomal enzymes due to defects in
translocation of proteins from the cytoplasm into
the peroxisome.
– Adrenoleukodydstrophy is caused by lack of a
peroxisomal enzyme, leading to fatty acid
accumulation in the brain and destruction of the
myelin sheath of nerve cells.