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CHAPTER 6
MITOCHONDRION AND
CHLOROPLAST
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I. Mitochondrion
Structure and distribution:
Usually, mitochondrion is like a particle or rod and others composed of
proteins (65-70%) and lipids (25-30%) at 0.5~1μm diameter and 1.5~3.0μm
length. The mitochondria in the cells of pancreas can be 10~20μm at length
called huge mitochondrion. The number of mitochondrion in a cell can be
hundreds to thousands, and less in plant cell than in animal cell because of
chloroplast. Some unicellular organism contains 500,000 mitochondria inside,
but mammalian erythrocytes contain no any mitochondrion inside.
Mitochondrion can migrate in cell along micro tube, and motorprotein
supplies energy for that.
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1. Outer membrane
Outer membrane contains lipids (40%) and proteins (60%). There are the
hydrophilic tunnels composed of porin that allows the molecules lighter than 5KD
passed through.
2. Inner membrane
Inner membrane contains more than 100 types of polypeptide with a low
permeability. The electron transmission chain of oxidative phosphorylation is located in
inner membrane. Cytochrome C reductase is the marker enzyme for inner membrane.
Inner membrane can be pleated into inside to form cristae. The cristaes enlarge
the area of inner membrane to 5 – 10 folds. Cristae can be two types of shape: lamella
or tube. Elementary particles are located on cristae, and composed of head part (F1
conjugate factor) and elemantary part (F0 conjugate factor). F0 inserts into inner
membrane.
3. Intermembrane space
It is between inner membrane and outer membrane with 6-8nm width. Adenylate
kinase is the marker enzyme for intermembrane space.
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Outer membrane
Intermembrane space
Photo of mitochondrion
Inner membrane
Lamella critae
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Matrix
A model structure of mitochondrion
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4. Matrix
The area surrounded by cristae, intermembrane space and inner membrane.
Excepting glycolysis (in plasma), other bio-oxidation reactions are carried out in
mitochondrion (in matrix). Malic dehydrogenase (MDH) is the marker enzyme for the
matrix of mitochondrion.
The matrix contains a complete system for transcription and translation including
mitochondrion DNA (mtDNA), 70s ribosome, tRNA, rRNA, and DNA polymerase.
Tube cristae
Matrix
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The molecule basis of oxidative phosphorylation
Electronic vectors for the respiratory chain:
1. Nicotinamide adenine dinucleotide (NAD): NAD is a coenzyme for many
dehydrogenases and linked to tricarboxylic acids cycle. NAD present H+ to flavoprotein.
2. Flavoprotein.
3. Cytochromes: Types: a、a3、b、c、c1.
4. Three Copper atoms on the protein located on inner membrane.
5. Ferredoxin.
6. Coenzyme Q.
A model structure for ferredoxin
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Two major respiratory chains
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ATP synthetase:
Molecule weight: 500KD. Two parts: a spherical head (F1), and the
basement (F0) inserted membrane. Each liver cell mitochondrion contains
15,000 molecules of ATP synthetase. Each ATP synthetase can make 100 ATP
molecules per second.
For the detail about ATP synthetase, see your biochemistry text book.
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Model structure
for ATP
synthetase
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The inhibitors for oxidative phosphorylation:
1．Inhibitors for electronic transmission
① Inhibit NADH→CoQ: amytal, rotenone, and piericidin.
② Inhibit Cyt b→Cyt c: actinomycin A.
③ Inhibit cytochrome oxidase→O2: CO, CN, NaN3, and H2S.
2．Inhibitors for phosphorylation
Oligomycin and dicyclohexyl carbodiimide (DCC) can bind to F0 to block the
H+ tunnel and inhibit synthesis of ATP.
3．Uncoupler
Some reagents or medicines can separate the oxidation and phosphorylation,
oxidation is continued but phosphorylation stopped. The uncoupler can cause
body temperature increased. Uncouplers include DNP, FCCP, thermogenin, and
aspirin.
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Mitochondrion is semiautonomous
Mitochondrion contains its whole system for DNA replication, transcription, and
translation. The system includes mtDNA, RNA, DNA polymerase, RNA
polymerase, tRNA, ribosome, and others. That means mitochondrion has the
genetic way for itself. So, we say that mitochondrion is a semiautonomous
organelle.
mtDNA is double strands and circle molecule as the below:
Heavy chain (H)
Light chain (L)
The genes on mtDNA are located very closely without introns. Each mitochondrion
contains several mtDNA molecules, and the length for each is about 16-20kb in
animal. Most of gene are transcripted from H chain. The genes on H chain encode
two rRNAs, 14 tRNAs, and 12 polypeptides. The genes on L Chain encode other
8 tRNAs and 1 peptide. The genes on mtDNA linked together, or overlapped.
Almost each reading frame has no any region that is not translated. Many of them
have no stop codon, and end as T or TA. The stop codon will be added during the
modification after transcription.
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Mitochondrion was originated from a bacterium that parasited in cell probably
because mitochondrion has many features are almost same to bacteria, for
examples, morphology, staining, chemical components, genetic system, and
others.
The following genetic features are same to a bacterium’s: ① circle DNA without
intron. ② 70S ribosome. ③ RNA polymerase can be inhibited by EB, not by
actinomycin D. ④ Sensitive to chloromycetin that inhibits bacterial protein
synthesis, not to actidione that inhibits the cellular protein synthesis.
The genetic codons of mammalian mtDNA are different from universal genetic
codons: ① UGA is not a stop codon here, it is a codon for Tryptophan. ②
Methionine is encoded by codon AUG, AUA, AUU and AUC. ③ AGA and AGG are
not codons for arginine, they are stop codons here. There are 4 stop codons in
mitochondrion: UAA, UAG, AGA, and AGG.
mtDNA is transferred to new generation from parental generation with a
matrilinear inheritance way, and its mutation rate is higher than nucleus DNA
(nDNA) without efficient repairing function. So, mtDNA is easy to be mutated and
cause mutation genetic diseases, such as Leber optic nerve disease (optic nerve
denaturation and atrophy) and myoclonus epilepsy (convulsive seizure and loss
of consciousness).
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Proliferation of mitochondrion
Mitochondrion is proliferated by the cleavage styles include:
1. Septate division: The mitochondrion membrane forms a ligature ditch
rounding the middle of membrane, then separated to two new mitochondria.
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2. Contracting division: The ligature ditch become slender and is pulled to
longer further, the divided off to new mitochondria.
3. Budding division: Germinated firstly, the small mitochondrion will fall off from
its mother mitochondrion, grow up and develop to a new mitochondrion.
Contracting division of mitochondrion
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II. Chloroplast
We can say that all energy utilized by every life event is originally from sun. But,
how to utilize and transform it? We, human body can not take energy from sun,
but we have to use it by other way. Chloroplast plays key role here! Chloroplast
take and transform the sun energy for plant growth, animals take energy and
nutrition from plants (or other animal). As a energy transformer, chloroplast can
combine carbon dioxide and water to form sugar and release oxygen. So, the
photosynthesis of green plants is the basic energy resource for all bio organs
including human being in the world.
Sunlight
6CO2+6H2O
C6H12O6+6O2
Chloroplast
Structure:
The size of chloroplast is about 5~10um × 2~4um × 2~3um. 50 – 200
chloroplasts are contained in each plant cell usually. The size and shape are
different in different plant species. Chloroplast is composed of envelope,
thylakoid and stroma. Chloroplast contains 3 types of membranes (outer
membrane, inner membrane and thylakoid membrane) and 3 types of cavities
(intermembrane space, stroma cavity and thylakoid space).
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Structure of chloroplast
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Envelope:
Envelope is composed of bilayer of lipid membrane with a 10~20nm
intermembrane space. The outer layer allows almost all large or small molecules
pass through, but the inner layer allows molecules pass through selectively.
Thylakoid:
Thylakoid is a vesicle formed by a monolayer membrane with some
components of photosynthetic pigments and electronic transmission, we call this
layer as photosynthetic membrane.
10 – 100 thylakoids are overlapped together to form a basic particles, so
we call these thylakoids as basic particle thylakoids. Each chloroplast contains
40 – 60 basic particles. The thylakoids that located between basic particles and
did not overlap together are called stroma thylakoids that form stroma lamella.
The basic particles are linked by stroma thylakoids. So, all thylakoids form a
closed system.
The membrane of thylakoid is composed of protein and lipid（60：40）. The
light energy is transformed into chemical energy on thylakoid membrane. The
proteins contained in the thylakoid membrane are the complex of cytochrome b6/f,
flavoprotein, complex of photosystem I, complex of photosystem II, and others.
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Stroma:
Stroma is the area between inner membrane and thylakoid. The components of
stroma include: Enzymes associated with carbon assimilation, The system for
protein synthesis [ chloroplast DNA (ctDNA), RNAs, ribosome], and others.
The mechanism of photosynthesis:
Photosynthesis is a course for the transformation of energy and substance:
Light energy
(Sun energy)
Electronic transmission
Electric
Chemical energy
power
(ATP and NADPH)
Store in
sugars
The course can be divided as two parts: light reaction (the reaction needs light)
and dark reaction (the reaction does not need light).
The photosynthetic pigments and electronic transmission components:
Photosynthetic pigments:
Thylakoid contains two pigments: green chlorophyll and red carotinoid (3:1).
Chlorophyll includes two types: chlorophyll a and b. All chlorophylls and carotinoid
are embedded in thylakoid membrane. So, thylakoid membrane is a very
important place where the photosynthesis and energy exchange are carried out.
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Chlorophyll
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Light harvesting complex
Light harvesting complex is composed of about 200 chlorophylls and some
peptides. Most of chlorophyll a and all chlorophyll b can capture light, they are
called as antenna pigment. Carotinoid and xanthophyll are the helper pigments.
Antenna pigments transfer the light energy to central chlorophyll by a resonance
energy transmission way.
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Actually, there are 3 plant pigments in thylakoid: green chlorophyll,
red carotene plus carotinoid, and yellow xanthophyll. The 3 pigments form 3
basic colors to figure out so colorful and beautiful world to us that we can
not love her enough forever!
Photosystem II (PSⅡ):
PS II is called as P680 also. PS II includes 12 peptide chains at least. PS II is
located in thylakoid membrane, and contains one light-hawesting comnplex Ⅱ
(LHC Ⅱ), one central chlorophyll, and one oxygen evolving complex. D1 and D2
are the key peptide chains combined to P680、pheophytin and plastoquinone.
Cytochrome b6/f complex (cyt b6/f complex):
Cytochrome b6/f complex exists in dimer style. Each monomer contains four
subunits: cytochrome b6 (b563), cytochrome f, ferredoxin, and subunit Ⅳ.
Photosystem I (PS I):
PS I is called as P700 also. PS I is located in stroma and thylakoid membrane.
PS I is composed of light harvesting complex I and a reaction center formed by
some special chlorophylls, electron vectors.
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Light capturing reaction and electron transmission:
Light capture:
Light energy
P680 (ground state)
Pheophytin
QA on D2
of cytochrome b6/f
4Fe-4S
Electron trnasfer:
P680e+(excited state)
QB on D1
Plastoquinone of PS II
Cu2+ on plastocyanin
Ferredoxin
NADP+
P700 of PS I
A0
Complex
A1
NADPH
The “Z” way of electron transmission above is called as non-cycle
photosynthetic phosphorylation. Both ATP and NADPH are released out here.
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Non-cycle photosynthetic phosphorylation
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If the NADP+ is inefficient in plant, electron will cycle in PS I to form ATP only, no
NADPH synthesized at this time. We call this transmission as cycle
photosynthetic phosphorylation shown as the fig below.
Cycle photosynthetic phosphorylation
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The cooperation
of PS I and PS II
(The fig just show
you the “Z” way)
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Photosynthetic phosphorylation:
Just like the described as above, a pair of electrons is transferred from P680 to
NADP+, this transmission will put 4 H+ ions into the cavity of thylakoid making the
pH here low (about 5) and forming H+ force. Under the ATP synthetase
enhancing, the H+ force will push ADP combined by Pi to form ATP.
Oxidative phosphorylation and photosynthetic phosphorylation
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Semiautonomy of chloroplast
Like mitochondrion, chloroplast is the energy station in plant cell. Chloroplast is a
semiautonomous organelle also.
Chloroplast DNA (ctDNA) is cycle DNA with 200Kb-2500Kb length usually. Like
mitochondrion, chloroplast synthesizes the some of proteins that chloroplast
needs. Other needed proteins must be synthesized in cell plasma. So, we say that
chloroplast is a semiautonomous organelle also.
The proliferation of chloroplast
The proliferation of chloroplast is very similar to the septate division of
mitochondrion. The chloroplast membrane forms a ligature ditch rounding the
middle of membrane, then separated two new chloroplasts.
Chloroplast cleavage is easier to be found in baby plant or leaf than in adult plant
or leaf usually. A adult chloroplast will not start division again almost.
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Directive transportation of proteins in mitochondrion and
chloroplast
Transportation of mitochondrion proteins
Most of mitochondrion proteins are synthesized in plasma and transported into
mitochondrion directionally. The signal sequence (leader sequence, presequence
or transit-peptide) at N terminal of protein will lead the transportation specifically.
After the transportation, the signal sequence will be cut off by signal peptidase.
We call this splicing as posttranslation modification.
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The protein transportation signal sequence is not specific to target protein.
Any protein with the signal sequence can be transported into mitochondrion.
Many protein complexes are associated with the protein transportation:
These complexes are the translocators actually including the follows::
①TOM complex: Protein can enter intermembrane space by this complex.
The important human TOMs are TOM34, TOM40, TOM22, TOM7, TOM6, and
Tom5. The tunnel of TOM complex is called as general import pore (GIP).
②TIM complex: TIM23 can transport protein to stroma or insert some
proteins into inner membrane. TIM22 can insert the transportation proteins for
metabolism needed substances into inner membrane.
③OXA complex: OXA can insert both the proteins synthesized by
mitochondrion and the proteins passed through TOM/TIM into inner membrane.
The proteins entered outer membrane contain a N terminal signal that will
not be cut off and will be inserted into outer membrane by TOM complex. For
stroma proteins, they have to be transported into intermembrane space by TOM
complex firstly, then inserted into stroma by TIM complex. As another choice,
they can enter stroma by the cooperation of TOM and TIM at the site where the
mitochondrion inner and outer membranes are touched each other.
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The complex of TOM and TIM
(Proteins can pass through outer
membrane and inner membrane
here with one step only)
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The touched sites of the
outer membrane and inner
membrane of
mitochondrion
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Protein enter inner membrane and intermembrane space: A. The protein contains
two signal sequences enter stroma by first signal for TOM／TIM23, then, inserted
into inner membrane by second signal for OXA complex. B. The proteins entered
intermembrane space can be inserted inner membrane by the stop-transfer
sequence for TIM23. C. The inserted proteins can be modified by the protease as
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soluble proteins. D. The transporter for metabolized substance can be
inserted into inner membrane byTIM22.