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Cytoplasmic Membrane Systems
Naijil GeorgeAssistant Professor,
Department of Biotechnology,St. Joseph's College,
Irinjalakuda - 680 121,Thrissur Dist., Kerala, India.
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Organelles of endomembrane system
1. Endoplasmic Reticulum,
2. Golgi complex,
3. Endosomes,4. Lysosomes, and
5. Vacuoles.
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Funtions
The organelles of the endomembrane systemare part of a dynamic, integrated network in
which materials are shuttled back and forth
from one part of the cell to another.
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Materials are shuttled between organelles
mostly in small, membrane-bounded
transport vesicles that bud from a donor
membrane compartment
Transport vesicles move through the
cytoplasm in a directed manner on tracksformed by cytoskeleton
When they reach their destination, the
vesicles fuse with the membrane of theacceptor compartment.
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Secretory pathway
Also called Biosynthetic pathway Proteins are synthesized in the endoplasmic
reticulum
Modified during passage through the Golgicomplex, and
Transported from the Golgi complex to
various destinations - such as the plasmamembrane, a lysosome, or the large vacuole
of a plant cell
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Types of Secretory pathway
1. Constitutive secretion, secreation in a
continual manner.
Present in most cells
Role in the formation of the extracellular matrix
and plasma membrane
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2. Regulated secretion
Materials are stored as membrane-bound
packages and discharged only in response to
an appropriate stimulus.
Roles
Endocrine cells that release hormones,
Release of digestive enzymes
Release of neurotransmitters.
In some of these cells, materials to be
secreted are stored in large, densely packed,
membrane-bound secretory granules
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Endocytic pathway
Materials move from the outer surface of the
cell to compartments
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Endoplasmic Reticulum
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Endoplasmic Reticulum (ER)
In 1945 Porter discovered endoplasmicreticulum (ER) in liver cells
ER comprise a system ofcontinuous
membranes that enclose a space, calledlumen, that is separated from the surrounding
cytosol.
The membrane of ER compartment has twofaces
The cytoplasmic face
Luminal face
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MORPHOLOGY
ER consist of 3 forms
1. Lamellar or cisternae
2. Vesicular or vesicles3. Tubular or tubules
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Cisternae
Cisternae-long flattened unbranched sac like
structures of about 40-50m in diameter
Arranged parellely as bundles or stakes
Predominent in cells with synthetic roles
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Vesicles
Vesicles-oval membrane bound vacuolar
structures having diameter of 25-500m
Often seen isolated in the cytoplasm.
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Tubules Tubules-branched structures forming the
reticular system along with vesicles andcisternae
Having diameter of about 50-190m
Occures in almost all the cells
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ORIGIN
Exact process is not known
It is assumed that ER has originated by
evaginations of the nuclear membrane
Seikevitz and Palade have reported that RER
has originated first and latter synthesizes the
SER
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The endoplasmic reticulum (ER) is dividedinto two subcompartments based on the
structural and functional differences,
Rough endoplasmic reticulum (RER) - presence of
ribosomes bound to its cytosolic surface
Smooth endoplasmic reticulum (SER) lacks
associated ribosomes
Two types of ER share many of the sameproteins and engage in certain common
activities, such as the synthesis of certain
lipids and cholesterol.
Types of ER
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COMMON FUNCTIONS OF SER &RER
Provides ultrastructural skeletal framework
Gives mechanical support
Exchange of molecules
Provides increased surface for various enzymatic
reactions
Act as intracellular transport system
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RER
Also called granular ER Ribosomes remain attached on the surface as
polysomes
Commonly seen in cells active in proteinsynthesis
Epical edge is smooth called transitional
element On the surface the ribosomes are attached
with the help of transmembrane glycoproteincalled ribophorins
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The RER is typically composed of a network of
flattened sacs (cisternae). The RER is continuous with the outer
membrane of the nuclear envelope, which
also bears ribosomes on its cytosolic surface. Cells that secrete large amounts of proteins,
such as the cells of the pancreas or salivary
glands, have extensive regions of RER
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FUNCTIONS OF RER
The rough ER is the starting point of the
biosynthetic pathway
It is the site of synthesis of the proteins,
carbohydrate chains, and phospholipids that
journey through the membranous
compartments of the cell.
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1. PROTEIN SYNTHESIS
Synthesis begins when m RNA binds to
ribosomes
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2. PROTEIN GLYCOSYLATION
The covalent addition of sugars to the secretary
proteins
During this process a single species of oligosaccharideis transferred to proteins in the ER.
Oligosaccharide contains N-acetyl glucosamine,mannose, glucose etc
The precursor oligosacchride is held in the ERmembrane by a special lipid molecule called dolicol
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3. MEMBRANE BIOSYNTHESIS
Newly synthesised proteins and lipids are
inserted into the existing membrane
Membrane forming vesicle move from ER to
PM through GC
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SER
Also called agranular ER
Ribosomes are absent on the surface of ER
Seen in cells which are involved in lipids and glycogen
metabolism
Muscle cells are also rich in SER here it is known as
sarcoplasmic reticulam
In pigmented retinal cells SER is called myeloidbodies.
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The SER is extensively developed in a number
of cell types, including those of
skeletal muscle, kidney tubules, and
steroid-producing endocrine glands
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FUNCTIONS OF SER
1. SYNTHESIS OF LIPIDS
Phospholipid,cholesterol
These lipids ,after synthesis transferred with
the help of lipid exchange proteins
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2.Glycogenolysis And Blood
Glucose Homeostasis
Glycogen granules get attached on the surface
Need of energy is fulfilled by breakdown ofglycogen to glucose
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3. Sterol metabolism
Contains enzymes for the synthesis of
cholesterol and bile acids.
Synthesis of steroid hormones in the
endocrine cells of the gonad and adrenal
cortex.
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4. Detoxification
Detoxification of organic compounds,
including barbiturates and ethanol in liver
Whose chronic use can lead to proliferation of
the SER in liver cells.
Carried out by a system of oxygentransferring
enzymes (oxygenases), including the
cytochrome P450 family
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5. Other functions
Sequestering calcium ions within the
cytoplasm of cells.
The regulated release of Ca2+ from the SER of
skeletal and cardiac muscle cells (known as
the sarcoplasmic reticulum in muscle cells)
triggers contraction.
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Intra cellular impulse conduction
In skeletal and cardiac muscle cells SER is
called sarcoplasmic reticulam
It transmit impulses from the membrane into
deep regions of the muscle fiber and trigger
muscle contraction
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GOLGI COMPLEX
(Golgi complex or Golgi body)
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Discovered by Camillo Golgi in 1898
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Morphology Consisting primarily of flattened, disklike,
membranous cisternae with dilated rims andassociated vesicles and tubules
Cisternae, diameters 0.5 - 1.0m, arranged in
an orderly stack
An individual stack is called dictyosome in
plant cells.
An individual cell contain few to 1000s of GC
GCs are interconnected by membranous
tubules
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Sorting station
cis Golgi network (CGN)- interconnected
network of tubules
trans Golgi network (TGN)- contains a
network of tubules and vesicles
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The membranous elements are supported
mechanically by membrane skeleton
The Golgi scaffold may be physically linked
with motor proteins
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Glycosylation in the Golgi Complex
Glycoproteins and glycolipids
cis and medialcisternae - remove most of the
mannose residues
Other sugars are added sequentially by
various glycosyltransferases.
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Movement of Materials
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Functions
1. Modifying, sorting, and packaging ofmacromolecules
2. Transport of lipids around the cell
3. Creation of lysosomes4. Protein glycosylation and phosphorylation
5. signal sequence incorporation, eg. mannose-6-
phosphate label for lysosomes.
6. Synthesis of proteoglycans
7. role in apoptosis via localised Bcl-2
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Thank you
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