Download Cell and Molecular Biology

Cell and Molecular Biology
Cell Organelles-1
Behrouz Mahmoudi
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 Eukaryotic cells contain well defined cellular
organelles such as:
 Nucleus
 Mitochondria
 Endoplasmic reticulum
 Golgi apparatus
 Peroxisomes
 lysosomes
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MITOCHONDRIA
• Mitochondria has got an inner membrane and an
outer membrane. The space between these two is
called intermembranous space.
• Inner membrane convolutes into cristae and this
increases its surface area.
• Both the membranes have different appearance and
biochemical functions:
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Inner membrane:
• It surrounds the matrix.
• It contains components of electron transport
system.
• It is impermeable to most ions including H,
Na, ATP, GTP, CTP etc and to large molecules.
• For the transport special carriers are present
e.g. adenine nucleotide carrier(ATP –ADP
transport).
• Complex II i.e. Succinate dehydrogenase .
• Complex V i.e. ATP synthase complex.
Outer membrane:
It is permeable to most ions and molecules which can
move from the cytosol to inter membranous space.
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Three Stages of Aerobic Cellular
Respiration:
1-Glycolysis
2-Kreb’s cycle
in cytosol
in mitochondrial
matrix
3-Electron Transport Chain
at
inner membrane of mitochondria
C6H1206 + 6O26CO2 +6H20 + ENERGY
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Glycolysis
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Krebs Cycle
• Each pyruvate (carbohydrate) molecule is
completely oxidized into carbon dioxide
• Energy released from these reactions results in
the formation of 1 ATP molecule but 3 NADH
molecules
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Electron Transport Chain
• During the electron
transport chain, H+
is moved against a
gradient.
• The energy needed
to do this is supplied
by electrons carried
by NADH
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What happens along the inner
membrane of the mitochondria?
• The loss of electrons from
NADH result in the addition
of energy to protein pumps
in the memebrane resulting
in a H+ being moved from the
inside to the outside of the
inner membrane
• This happens many times
creating an imbalance
(gradient) of H+.
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Oxygen pulls electrons to
keep them moving.
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What happens along the inner
membrane of the mitochondria?
Ultimately, aerobic respiration
produces ~36 ATP molecules
from each individual
glucose molecule.
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• ATP is made as H+ ions are
allowed back into the
matrix of the mitochondria
by a different protein (ATP
synthase).
• The energy released by the
“rush” of H+ is used by this
enzyme to make ATP (kind
of like a rush of water in a
stream being used to turn
a water wheel).
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ENDOPLASMIC RETICULUM
• Cytoplasm of eukaryotic cells contain a
network of interconnecting membranes. This
extensive structure is called endoplasmic
reticulum.
• It consists of membranes with smooth
appearance in some areas and rough
appearance in some areasSmooth endoplasmic reticulum and rough
endoplasmic reticulum.
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Rough Endoplasmic Reticulum
• These membranes enclose a lumen.
• In this lumen newly synthesized proteins are
modified.
• Rough appearance is due to the presence of
ribosomes attached on its cytosolic side(outer
side).
• These ribosomes are involved in the biosynthesis
of proteins.
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• These proteins are either incorporated into
the membranes or into the organelles.
• Special proteins are present that are called
CHAPERONES. Theses proteins play a role in
proper folding of proteins.
• Protein glycosylation also occurs in ER i.e. the
carbohydrates are attached to the newly
synthesized proteins.
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Smooth Endoplasmic Reticulum
 Smooth endoplasmic reticulum is involved in lipid
synthesis.
 Cholesterol synthesis
 Steroid hormones synthesis.
 Detoxification of endogenous and exogenous
substances.
 The enzyme system involved in detoxification is
called Microsomal Cytochrome P450
monooxygenase system(xenobiotic metabolism).
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• ER along with Golgi apparatus is involved in
the synthesis of other organelles –lysosomes
& Peroxisomes.
• Elongation of fatty acids e.g. Palmitic acid 16
C- Stearic acid 18 C.
• Desaturation of fatty acids.
• Omega oxidation of fatty acids.
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•A signal sequence of
approximately 20 amino acids and
rich with hydrophobic amino acids
is often located at the N-terminus.
•Since the ribosome masks about 30
amino acids, the signal sequence
isn’t fully exposed until the nascent
polypeptide is about 50 amino acids
long.
•SRP-ribosome attaches to SRP
receptor and then docks on a protein
translocator.
•SRP and receptor dissociate.
•Translation and translocation
proceed in unison - co-translational
transport.
•The energy for transport is provided
by the translation process - as the
polypeptide grows, it is pushed
through the protein translocator.
SRP: signal-recognition particle
SRP receptor
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The signal sequence of secreted proteins is cleaved by a
signal peptidase. In the literature, the signal sequence
of secreted proteins is often called a “leader peptide”.
Translocation of protein across the
ER membrane
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A single-pass membrane protein
In this case, a start transfer sequence
followed by a stop transfer sequence
yields a 2-pass transmembrane protein
with the N and C-terminus in the
cytosol.
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N-linked glycosylation of ER proteins.
Most of the glycosylation associated with proteins outside the cell
begins in the ER.
A large preformed oligosaccharide is transferred
from a lipid called dolichol to the side-chain of
asparagine.
Because the covalent linkage is to the nitrogen of
the asparagine side-chain, this is called N-linked
glycosylation.
In the ER, N-linked oligosaccharides are modified and
the modifications are used as signals to distinguish
properly folded from unfolded proteins.
ABO blood type is
determined by two
glycosyltransferases
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Calnexin is a chaperone, characterized by assisting
protein folding and quality control, ensuring that
only properly folded and assembled proteins
proceed
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If a protein doesn’t achieve a properly folded state, it gets
exported from the ER into the cytosol where it is degraded
by the proteasome.
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Phospholipids are synthesized in the cytoplasmic leaflet of
the ER.
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Phospholipid translocators flip-flop the phospholipids.
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