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Energy conversion in mitochondria and chloroplasts
0.5 x 2 um
How can you recognize different
Organelles?
1.5-2 x 1 um
5-10 x 2 um
Chemiosmotic coupling
Stage 1
Stage 2
Oxidative phosphorylation
NADH produced from catabolized
biomolecules transforms to ATP
Use oxygen, release carbon dioxide
Where is NADPH generated for biosynthesis in animal cells?
Photophosphorylation
Solar energy transforms to
NADPH and ATP
Use water, release oxygen
and fix carbon dioxide to
(CH2O)n
Mitochondria tend to be aligned along microtubules.
cell stained with rhodamine 123
(mitochondria specific fluorescent
dye)
cell stained with fluorescent labelled
antibody to microtubules
Mitochondria locate
near sites of high ATP
demand in cardiac
muscle and a sperm tail.
Biochemical fractionation of purified mitochondria into components
Each NADH is transformed
to ~1.5 to 2.5 ATPs.
(Some protons/energy are
used for transporting ions.)
1 glucose gives a net yield
of ~ 30 ATPs.
Outer membrane has porins, forming channels
permeable to molecules <5000 dal.
Inner membrane has cardiolipin (diphosphatidyl
glycerol) and impermeable to ions.
ATPase is at inner membrane
of mitochondria and is at
thylakoid membrane of
chloroplasts.
Leaky to MgCl2,
So no/low potential difference
A Chemiosmotic Process Couples Oxidation Energy to ATP Production
A comparison of biological oxidation with combustion
Energy Derived from Oxidation Is Stored as an Electrochemical Gradient
~150mV
0.5-0.6 pH units
1 pH unit equivalent
~ 60mV
Free energy is a gauge to measure the driving force for reactions.
Negative free energy favors the reaction.
Figure 14-18 (part 4 of 4) Molecular Biology of the Cell (© Garland Science 2008)
Redox potential and free energy
Pigments and electron acceptors in mitochondria
Heme in cytochrome
Iron-sulfur cluster
NADH and NADPH has an absorption peak at 340nm
Conversion between NAD(P)H and NAD(P)+
can be measured by the absorbance change
at 340nm
Figure 14-9 Molecular Biology of the Cell (© Garland Science 2008)
NADH Transfers Its Electrons to Oxygen Through
Three Large Enzyme Complexes Embedded in the Inner Membrane
Ubiquinone is freely
mobile in membrane
FADH2
= complex I
Flavin, FeS
= complex III
Uncoupler collapses the electrochemical
proton gradient, eg. dinitrophenol
= complex IV
Redox potential changes along the mitochondrial electron transport chain
2e-
4
/e-
2
/e-
1
/e-
Structure of NADH dehydrogenase
Directional release and uptake of protons by a quinone
pumps protons across a membrane
2e-
2e-
Quinone picks up protons and e- on the matrix side, then
QH2 diffuses to the crista side of cytochrome C reductase.
Cytochrome C reductase is a dimer
(2 X 11 subunits = total 480 kdal in size)
Two-step mechanism of the cytochrome c reductase Q-cycle
Q cycle : while one of the electrons received from each QH2 molecule is transferred from
ubiquinone through the complex to the carrier protein cytochrome c, the other electron is
recycled back into the quinone pool.
The Cytochrome c Oxidase Complex Pumps Protons and
Reduces O2 Using a Catalytic Iron–Copper Center
The Respiratory Chain Forms a Supercomplex in the Crista Membrane
Protons Can Move Rapidly Through Proteins Along Predefined Pathways
The ATP Synthase Produces ATP by Rotary Catalysis
Generates 100 ATP / second
3 ATP / turn
Mitochondrial Cristae Help to Make ATP Synthesis Efficient
dimer
Special Transport Proteins Exchange ATP and ADP
Through the Inner Membrane
Chemiosmotic Mechanisms First Arose in Bacteria
Synthesis of ATP
Use of ATP
Figure 14-36 Molecular Biology of the Cell (© Garland Science 2008)
Chloroplasts Capture Energy from Sunlight and Use It to Fix Carbon
Carbon Fixation Uses ATP and NADPH to Convert CO2 into Sugars
Sugars Generated by Carbon Fixation Can Be
Stored as Starch or Consumed to Produce ATP
Chlorophyll–Protein Complexes Can Transfer Either Excitation Energy or Electrons
Chlorophyll a, R= CH3
Chlorophyll b, R= CHO
450
470
675
640
A Photosystem Consists of an Antenna Complex and a Reaction Center
Chlorophyll–Protein Complexes Can Transfer
Either Excitation Energy or Electrons
Purple sulfur bacteria use H2S photosynthesis
The Thylakoid Membrane of Chloroplasts Contains
Two Different Photosystems Working in Series
Z scheme
Photosystem II Uses a Manganese Cluster to
Withdraw Electrons From Water
The Cytochrome b6- f Complex Connects Photosystem II to Photosystem I
Photosystem I Carries Out the Second
Charge-Separation Step in the Z Scheme
The Chloroplast ATP Synthase Uses the Proton Gradient
Generated by the Photosynthetic Light Reactions to Produce ATP
plastocyanin (a small copper containing protein)
ferredoxin (a small protein containing an iron-sulfur center)
Figure 14-48 Molecular Biology of the Cell (© Garland Science 2008)
All Photosynthetic Reaction Centers Have Evolved From a Common Ancestor
Cyclic photophosphorylation
Only PS I is involved: the reaction center is P700.
Electrons travels in a cyclic manner: electrons travel back to PS I.
Only ATP is produced without NADPH and oxygen.
This system is predominant in bacteria.
The Proton-Motive Force for ATP Production in
Mitochondria and Chloroplasts Is Essentially the Same
Chemiosmotic Mechanisms Evolved in Stages
By Providing an Inexhaustible
Source of Reducing Power,
Photosynthetic Bacteria Overcame a
Major Evolutionary Obstacle.
The Photosynthetic ElectronTransport Chains of Cyanobacteria
Produced Atmospheric Oxygen and
Permitted New Life-Forms
Electron-transport chains
in bacteria, chloroplasts,
and mitochondria are
similar.
In reconstituted in vitro
systems, the different
complexes can substitute
for one another, and the
structures of their
protein components
reveal that they are
evolutionarily related.
Photosynthetic Electron-Transport Chains of Cyanobacteria
Produced Atmospheric Oxygen and Permitted New Life-Forms
~ 1.5 billion
years ago
~ 2 – 2.7 billion
years ago
~ 3 – 3.5 billion
years ago
An endosymbiotic oxygen-evolving cyanobacterium gave rise to chloroplasts, while
mitochondria arose from an α-proteobacterium.
Over Time, Mitochondria and Chloroplasts Have Exported
Most of Their Genes to the Nucleus by Gene Transfer
Why the last five
mitochondrial genes are
conserved?
Perhaps they need to be cotranslationally inserted into the
inner membrane.
Cytosolic production of
hydrophobic proteins and their
import into the organelle may
present a problem to the
Cell.
Alterations in the genetic code
that made the remaining
mitochondrial genes
nonfunctional if they were
transferred to the nucleus.
Mitochondria Have a Relaxed Codon Usage and Can Have a Variant Genetic Code
Organelle Genes Are Maternally Inherited in Animals and Plants
Mutations in Mitochondrial DNA Can Cause Severe Inherited Diseases
Accumulation of Mitochondrial DNA Mutations Is a Contributor to Aging
The Fission and Fusion of Mitochondria Are Topologically Complex Processes,
which constantly remodel the network.
Mitochondrial division
Mitochondrial fusion
Chloroplasts remain closer to their bacterial origins than mitochondria
Sequences of proteins encoded in
chloroplasts are very similar to
bacterial, and several clusters of genes
are organized in the same way.
Regulatory sequences (promoters and
terminators) of chloroplasts are
virtually identical to bacteria.
The mechanisms by which chloroplasts and bacteria divide are similar.
Both utilize FtsZ proteins, which are self-assembling GTPases to form
a dynamic ring of membrane-attached protofilaments and to
recruitment of other division proteins and generate a contractile force
that results in membrane constriction and eventually in division.
The machinery for chloroplast division acts from the inside, as in
bacteria, while the dynamin-like GTPases divide mitochondria from the
outside