Cytoplasmic Membrane System

7/31/2019 Cytoplasmic Membrane System

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Cytoplasmic Membrane Systems

Naijil GeorgeAssistant Professor,

Department of Biotechnology,St. Joseph's College,

Irinjalakuda - 680 121,Thrissur Dist., Kerala, India.

[email protected]


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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

Cytoplasmic Membrane System

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