Mitochondria and Chloroplast Ultrastructure

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Mitochondria and Chloroplast Ultrastructure
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Mitochondria and Chloroplast Ultrastructure

• The cells of different organs may contain a few
  or large numbers of these organelles depending
  upon the function of the organ.
• We will be looking at chloroplast and
  mitochondria primarily because these organelles
  provide energy to plant and animal cells.

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The Mitochondrion This is the power house of the
cell
Rough Endoplasmic Reticulum
Matrix
Outer Membrane
Inner Membrane
Cristae
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The Mitochondrion This is the power house of the
cell

• There are about 800 mitochondria to each
  hepatocyte
• They are globular occupying about 20 of the
  cytoplasmic volume in liver cells and over 50 in
  heart-muscle cells
• Their outer and inner membranes differ in lipid
  composition and in enzymatic activity.

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The Mitochondrion

• The matrix is rich in enzymes
• Also found in all plant cells and contain
  specific DNA
• They are often located near structures that
  require ATP
• They are also located adjacent to cytoplasmic fat
  droplets
  

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The Mitochondrion Numerous shapes, but
predominantly spherical

• Sometimes they have a very complex irregular
  shape
• Typically about 2 µm in length and 0.5 µm in
  diameter
• They have two membranes
• Outer membrane that is smooth and somewhat
  elastic
• Inner membrane that has inward folds, or
  invaginations, called cristae

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The Mitochondrion Ultrastructure
Cristae vary in number and structure depending
on the cell type Matrix Inside the inner
membrane compartment Gel-like phase containing
about 50 proteins Membranes The outer and
inner mitochondrial membranes differ in
ultrastructure

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The Mitochondrion Ultrastructure
Inner-membrane Sphere regularly spaced
spherical particles (8.0 to 9.0 nm in diameter)
connected to the membrane by a narrow stalk
aka Mitochondrial ATPase
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The Mitochondrion Ultrastructure
Porins Transmembrane proteins with large pores
Make the outer membrane permeable to most small
metabolites Mitochondria evolved from bacteria

The inner membrane is however, selectively
permeable Impermeable to simple sugars such as
glucose or sucrose Cations such as K and Na,
or anions such as Cl- and Br- some coenzymes and
nucleotides
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The Mitochondrion Ultrastructure
The inner membrane allows specific nucleotides
and metabolites to pass quite readily These
include ATP, ADP, inorganic phosphate, pyruvate,
citrate, succinate, a-ketoglutarate, malate, and
amino acids such as glutamate and aspartate.
Mitochondria in Evolution
All Caucasians are from one genetic line
and that Africans are from several different
lineages. This mitochondrial DNA was passed down
from females, Why?
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The Mitochondrion Respiration and the
Mitochondria
C6H12O6 6 O2 6 CO2 6 H2O The aerobic
oxidation of glucose to CO2 and H2O

• This process requires the participation of three
  metabolically interrelated processes
• Tricarboxylic Acid Cycle
• Electron Transport
• Oxidative Phosphorylation
• All of which take place in the mitochondria

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The Mitochondrion Respiration and the
Mitochondria
Tricarboxylic Acid Cycle (TCA)
In aerobic oxidation, the pyruvate
produced by glycolysis (glucose degradation) is
not converted into lactate but, rather, into CO2
and acetyl-CoA Acetyl-CoA is then utilized by
the TCA cycle The TCA cycle complexes the
oxidation of the four remaining carbon atoms of a
glucose molecule, i.e., two acetyl-CoA to four
CO2.
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The Mitochondrion Respiration and the
Mitochondria
Electron Transport The subsequent production of
water, to complete the production of six H2O per
oxidized glucose, involves electron transport,
which consists of a series of reactions that
transfer the electron pairs derived from the
oxidation of the sugar to oxygen to form
water. Oxidative Phosphorylation ATP is formed
as electrons are transferred from NADH or FADH2
to O2 by a series of electron carriers This
takes place in the inner mitochondrial membrane
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The Mitochondrion Respiration and the
Mitochondria
Oxidative Phosphorylation Oxidation and
phosphorylation are coupled by a proton gradient
across the inner mitochondrial membrane NADH
½ O2 H rev H2O NAD
?Go' -52.6
kcal/mol This free energy of oxidation is used
to synthesize ATP. ADP Pi H rev ATP
H2O ?Go' 7.3 kcal/mol Mitochondrial ATPase
or H-ATPase enzyme complex synthesizes ATP
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The Mitochondrion Respiration and the
Mitochondria
Three hypotheses have been developed to explain
what happens during oxidative phosphorylation Th
e Chemical-Coupling Hypothesis This postulates
that the energy-yielding electron-transfer
reaction is coupled to the energy-requiring
reaction by which ATP is formed from ADP and
phosphate through a common chemical
intermediate. This intermediate is a high-energy
compound generated by electron transport and then
utilised as the reactant in a second reaction to
form ATP from ADP and phosphate.
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The Mitochondrion Respiration and the
Mitochondria
The Conformation-Coupling Hypothesis This
postulates that the energy yield by electron
transport is conserved in the form of a
conformational change in an electron carrier
protein or in the coupling factor (F1 ATPase)
molecule. The energy inherent in this
energized conformation is used to cause the
formation of ATP from ADP and Pi simultaneously,
the energy-carrying protein undergoes reversion
to its original low-energy conformation.
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The Mitochondrion Respiration and the
Mitochondria
The Chemiosmotic Coupling Hypothesis Postulated
by Peter Mitchell in 1961 It states that a
proton-motive force drives the synthesis of ATP
by the ATPase complex. The primary
energy-conserving event is the movement of
protons across the inner mitochondrial membrane.
This is the most widely accepted or favoured
hypothesis.
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The Mitochondrion Respiration and the
Mitochondria
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The Mitochondrion Respiration and the
Mitochondria
According to this hypothesis, the membrane is an
integral part of the coupling mechanism It is
the function of the electron carriers of the
respiratory chain to serve as an active-transport
system or pump to transport H ions from the
mitochondrial matrix across the inner membrane
This electrochemical gradient drives the
synthesis of ATP by causing the dehydration of
ADP and Pi. In other words, the pH gradient and
membrane potential constitute a proton-motive
force that is used to drive ATP synthesis.
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The Mitochondrion Respiration and the
Mitochondria

• A proton-motive force is generated by three
  electron-transfer complexes as electrons flow
  through the respiratory chain from NADH to O2
• Site 1 is the NADH-Q reductase complex
• Site 2 is cytochrome reductase and
• Site 3 is cytochrome oxidase

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The Mitochondrion Respiration and the
Mitochondria
The proton gradient generated at each site by the
flow of a pair of electrons can be used to
synthesize one molecule of ATP The enzyme
catalyzing this process is ATP synthase.
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The Mitochondrion Respiration and the
Mitochondria
This complex is seen as spherical projections
from the membrane surface These spheres are
referred to as F1 or coupling factor 1
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The Mitochondrion Respiration and the
Mitochondria
F1 consists of five kinds of polypeptide chains
with the stoichiometry a3ß3?de and a mass of 380
kd. The other major unit of ATP synthase is F0
It is the proton channel of the complex,
consisting of four kinds of polypeptide
chains. F0 F1-ATPase or H-ATPase
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The Mitochondrion Respiration and the
Mitochondria
The role of the proton gradient is not to form
ATP but to release it from the enzyme
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The Mitochondrion Respiration and the
Mitochondria
The proton flux through the synthase drives the
inter-conversion of states leading to the release
of tightly bound ATP
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The chloroplast Creates energy in photosynthetic
cells
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The chloroplast Creates energy in photosynthetic
cells
Chloroplasts are relatively large compared to
mitochondria Chloroplasts are receptors of
light energy They convert light energy into the
chemical energy of ATP Oxygen is generated
during plant photosynthesis They are
self-replicating organelles
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The chloroplast Creates energy in photosynthetic
cells
Typically 5 um long and contain an outer and
inner membrane They are usually globular or
discoid but sometimes assume exotic forms, as in
Spirogyra, in which they are ribbon-like spiral
structures In some algae there is only one
chloroplast per cell
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The chloroplast Creates energy in photosynthetic
cells
Higher plant cells may contain as many as 40
chloroplasts
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The chloroplast Creates energy in photosynthetic
cells
Thylakoid membrane
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The chloroplast Ultrastructure
Chloroplasts are surrounded by a single
continuous outer membrane Lamellae The inner
membrane is continuous and arranged in paired
folds called lamellae Stroma Comparable to
the mitochondrial matrix and contains soluble
enzymes Thylakoids Flattened membranous sacs
or vesicles contained in the stroma Grana The
thylakoids in stacked arrangements
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The chloroplast Ultrastructure
The chloroplast Ultrastructure

• intergranal lamellae
• The paired membranes between the grana
• The membranes of the thylakoids and the
  intergranal lamellae contain the
  energy-transducing machinery
• The light-harvesting proteins, reaction centres
• Electron-transport chains, and ATP synthase
  required for the primary light-dependent
  reactions
• They are functionally comparable to the cristae
  of mitochondria

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The chloroplast Ultrastructure
Thylakoids have nearly equal amounts of lipids
and proteins Like the inner membrane of the
mitochondria, is impermeable to most molecules
and ions. However, the outer membrane of the
chloroplast, like that of mitochondria, is highly
permeable to small molecules and ions. The
thylakoids contain chlorophyll as well as
accessory pigments used to harness light energy
and convert it into chemical energy.
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The chloroplast Photosynthesis
The principal photoreceptor in the chloroplasts
of higher plants is chlorophyll Both
chlorophyll a and b are present, but chlorophyll
a is the principal photoreceptor. Cyanobacteria
and red algae, on the other hand, contain
accessory light-harvesting pigments called
phycobillisomes that enable them to efficiently
utilize light not absorbed strongly by
chlorophyll.
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The chloroplast Photosynthesis
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The chloroplast Photosynthesis
Photosynthesis is a biological process whose
efficiency depends on the coordinated interaction
of several participating protein complexes It
takes place in green plants, algae, cyanobacteria
and photosynthetic bacteria In green-plants
photosynthesis requires, besides light as the
energy source, only two raw materials water and
carbon dioxide from the atmosphere. The organic
compounds ultimately produced by photosynthesis,
directly or indirectly include sugar,
carbohydrates, lipids and proteins
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The chloroplast Photosynthesis
The overall equation for oxygenic
photosynthesis 6 CO2 12 H2O 18 ATP 12
NADPH ? C6H12O6 18 ADP 18 Pi 12 NADP
12 H 6O2? In green plants, algae and
cyanobacteria the hydrogen donor is H2O and in
purple and green photosynthetic bacteria it is
H2S Hydrogen derived from water is used to
reduce CO2, resulting in the evolution of oxygen
as a consequence of the dehydrogenation step.
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The chloroplast Photosynthesis
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The chloroplast Photosynthesis
Electron flow from photosystem II (PSII) to
photosystem I (PSI), as well as electron flow
within each photosystem, generates a
transmembrane proton gradient that drives the
formation of ATP and NADPH. This is called
photophosphorylation of which there are two
types Cyclic Photophosphorylation This
requires only PSI, here electrons move through a
series of electron carriers and return to P700
with ATP being generated.
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The chloroplast Photosynthesis
Non-cyclic Photophosphorylation During the
movement of electrons from PSII to PSI, ATP is
generated. 1 ATP 1 NADPH H are generated
for each pair of electrons that are
transported. 3 ATP 2 NADPH H are required
for each molecule of CO2 fixed. Therefore, the
transport of 2 pairs of electrons 1 ATP from
the cyclic route is enough to fix each molecule
of CO2.