sujit - endoplasmic reticulum structure and function

7/30/2019 sujit - endoplasmic reticulum structure and function

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CELLULAR MEMBRANES

Endomembrane Structure % total membrane inHepatocyte % total in PancreaticExocrine Cell

Endoplasmic Reticulum

(smooth) 16


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ORIGIN OF ER

The nucleus and endoplasmic reticulum arose from infoldings of prokaryotic cell

membrane


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Endoplasm is the inner part of a cell's cytoplasm and

Reticulum means a fine network.

Endoplasmic reticulum is an interlinked network of

cisternae (membrane disks), vesicles (intracellulartransport sacs) and tubules inside a eukaryotic cell.

The endoplasmic reticulum was first observed by Ernest

Fullam, Keith Porter and Albert Claude, in the year 1945.


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This network of tubules, vesicles and cisternae, is held together by

the cell's cytoskeleton.

This cytoskeleton is the cell's skeleton, made of proteins and

embedded in cytoplasm.

The cisterns that constitute a major part of endoplasmic reticulum

are sheathed in a phospholipids membrane, that separates it

from the cytosol, which is the inner liquid inside a cell.

The whole network that constitutes the endoplasmic reticulum is

made up of this one continuous phospholipid membrane and the

whole network has a complex internal space or lumen. This space

is called endoplasmic reticulum cisternal space.


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ER membranes are continuous can be proved by using

fluorescently labeled protein and lipid.

Diffusion of these proteins and lipid from one type of ER

into other indicates that their membranes are continuous.

3,3'-dihexyloxacarbocyanine iodide (also known asDiOC6(3)) is a lipophilic dye that vitally stains the ER

network in living cells (such as this 3T3 cell).


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Present in all eukaryotic cell except egg cell, embryoniccell, matured mammalian RBC etc

Absent in prokaryotes All eukaryotic cells have an ER, more than half the total

membrane

ER lumen or ER cisternal: one continuous space10% of

the total cell volume


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The bilipid membrane controls the transport of moleculesbetween cytoplasm and lumen. It's like a molecular pipeline,which connects the nucleus with cytoplasm.

This network transports molecules inside the cell, as well asoutside.

There are two types of proteins that are found in the endoplasmic

reticulum. They are the ones that are transported through it andthe resident proteins, that are required for the functioning of the

reticulum.


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MOLECULAR PIPELINE


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The endoplasmic reticulum is made out of alipid membrane(bimembrane str)

The endoplasmic reticulum is still connectedto the nuclear membrane that is wrap aroundthe cells DNA.

There is a straight connection between thecells nucleus and the endoplasmic reticulum.


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ER consists of following structure

1.Cisternae- Flattened sac like unbranched

membrane bounded structure

3.Tubule-Membrane bounded branched tubularstructure forming reticular system along with

cisternae and vesicles

2.Vesicle- Droplet like membrane bounded structureat the end of tubule


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There are three types of endoplasmic

reticulum, which are:

1.Rough Endoplasmic Reticulum (RER)

2.Smooth Endoplasmic Reticulum (SER)

3.Sarcoplasmic Reticulum (SR)


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One interesting feature of the endoplasmic reticulum isthat the rough and smooth endoplasmic reticulum cantransform from one type to another

Their transformation is subject to the needs of the celland especially the enzymatic needs.

The transformation happens through the embeddingof proteins.


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The RER is dotted with ribosomes. (Which is why it is calledrough.)

The RER is involved with protein production, protein folding,quality control and dispatch.

Consists of network-like tunnels with tubules, vesicles andcisternae which is held together by the cytoskeleton of the cell.

Rough endoplasmic reticulum is connected with the outerenvelope of the cell nucleus.


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Protein Generation Production and processing of specific proteins at ribosomal sites

that are later exported.

The ribosome do their job and create proteins, which are then sent

into the rough endoplasmic reticulum, for advanced processing. The amino acids and complex molecules needed for the job, are

transported from the cytoplasm.

The functioning of the reticulum involves creation of two types ofproteins.

One is the type which fortifies and getsembedded into the reticulum membrane. The other types are water

soluble, which, after creation at ribosomal sites, pass through themembrane and into the lumen.


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Protein Folding The proteins that enter are further processed inside. Just like a two

dimensional cardboard box is folded to make a box, the proteins arefolded into the right three dimensional shapes and carbohydratesmay be added.

Many compounds are connected to the protein chain assembled inthe lumen, according to requirement.

Once the folding is complete, they are ready for delivery. Thefolding is made possible by chaperone proteins present in thelumen.

Molecules like haemoglobin are produced in the rough endoplasmicreticulum.


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Protein Transport

Another RER function is to transport these

ready proteins to the sites where they arerequired.

They may also be sent to the Golgi bodies for

further advanced processing, through vesicles.


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PROTEIN TRANSPORT
http://cellbio.utmb.edu/cellbio/cit1.htm


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Protein Quality Check

After assembly, every protein created in the RER lumen issubjected to a thorough quality evaluation.

The protein is checked for correct ordering and structureand if it doesn't match the exact requirement, it's simplyrejected, stored in the lumen and sent back for recycling.

Many medical conditions like Emphysema and a type of

Cystic Fibrosis is caused due to the rejection of a crucialprotein by this RER quality check system.


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The main difference in the structure of RER and SER is that

the rough endoplasmic reticulum contains ribosomes,while the smooth endoplasmic reticulum does not contain

this cell organelle.

The embedded ribosome in the rough endoplasmic

reticulum give it a rough appearance. On the other hand,

the smooth endoplasmic reticulum looks like a group ofsmooth tubules.

An interesting fact about rough endoplasmic reticulum

(RER) is that the ribosomes are not attached to it

permanently. Instead, they attach to the surface of RER at

the time of protein synthesis.

Another type of endoplasmic reticulum is sarcoplasmic reticulum, which is atype of smooth endoplasmic reticulum and can be found in smooth, as well as

striated muscles.


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SER


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Electron micrograph of smooth ER of

Leydig cell of testis

(site of steroid hormone synthesis)


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Vary in different cell types

More commonly, it is concerned with certain metabolic

processes

It takes part in several metabolic processes, besides

providing surface area for the action of the enzymes.

It also serves as storage space for several importantenzymes.

Among the various metabolic processes, it plays an

important role in carbohydrate metabolism and synthesis of

lipids and steroids.


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Drug detoxification, attachment of receptors on cell

membrane proteins and packaging proteins for

transportation are some other important functions of SER.

Drug detoxification takes place in SER, which are found in

liver cells.

It regulates the transportation of proteins to their proper

locations.

The small vesicles of SER are responsible for transporting

proteins to the new locations.

The enzyme, glucose-6-phosphate found in it takes part in

glucogenesis. It converts glucose-6-phosphate to glucose.


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Another significant function of SER is, to storecalcium in the muscle cells, and this crucialfunction is carried out by the sarcoplasmic

reticulum.

This type of SER is also concerned with the

release of calcium at the time of need such as,during muscle contraction. (sarcoplasmicreticulum)


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RER is abundant in- Pancreatic

cell,Fibrioblast,Cells of endocrine gland,

Goblet cell, Liver cell etc

SER is abundant in- Adipocyte of adipose

tissue,Adrenocortical cell, Intestinal

absorptive cell, Liver cell etc


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Polypeptides produced at RER includes -

Secreted protein

Integral membrane protein

Soluble protein (for endomembrane system)


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Cytosolic (like enzyme of glycolysis and proteinfor cytoskeleton)

Peripheral protein of cytosolic surface of

membrane

Protein that are transported to nucleus

Protein to be incorporate into the peroxisome ,

chloroplast and mitochondria(in last two protein is transported after completion

in cytosol)


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SIGNAL HYPOTHESIS

(Nobel Prize for Medicine - 1999)

Nascent polypeptides possess N-terminal address code

targets ribosome

complex to ER

membrane

Synthesis of SECRETED PROTEIN on ER


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Synthesis of SECRETED PROTEIN on ER

membrane-bound ribosome


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Binding immunoglobulin protein (BiP) also

known as 78 kDa glucose-regulated

protein (GRP-78) or heat shock 70 kDa protein

5(HSPA5) is a HSP70 molecular chaperone

It is located in the lumen of the endoplasmic

reticulum that binds newly-synthesized proteinsas they are translocated into the ER, and

maintains them in a state competent for

subsequent folding and oligomerization.

Synthesis of INTEGRAL MEMBRANE


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Synthesis of INTEGRAL MEMBRANE

PROTEIN on ER-bound ribosome


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Considering the various rough and smooth

endoplasmic reticulum roles, it can be concluded

that endoplasmic reticulum is one of the crucial

cell organelle.

Their existence is so important for vital life

processes like, protein synthesis, carbohydratemetabolism, detoxification and lipid synthesis.


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Golgi Apparatus


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Introduction

The name comes from Italian anatomist

Camillo Golgi, who identified it in 1898.

Factory for packaging proteins and consists of

stacks of flattened membranous sacs that may

be distended in certain regions because they

are filled with cell products.


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Structure

The Golgi apparatus has a structure that is

made up of cisternae, which are flattened

stacks of membrane usually found in a series

of five to eight.

These cisternae help proteins travel from

different points in the cell using enzyme

The face of golgibody towards the nucleus is

called as cis face and towards the plasma

membrane is called as trans face


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Structure.

The Golgi apparatus consists of stacks of sacs with vesicles pinching off from

the edges of the sacs.


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Once the proteins are produced by the rough E.R.

they pass into the sack like cisternae that are the

main part of the golgi body. These proteins are then

squeezed off into the little blebs which drift off intothe cytoplasm.


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Function

The golgi complex processes, sorts, modifies,and packages proteins. Proteins that pass

through the golgi are:

secreted from the cell

plasma membrane proteins

and proteins routed to other intracellular

organelles pass through the golgi.


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Storage and packaging of materials that will

be exported from the cell.

The Golgi apparatus is often called the"shipping department" of the cell.


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The vesicles that pinch off from the Golgiapparatus move to the cell membrane and thematerial in the vesicle is released to the

outside of the cell. Some of these pinched off vesicles also

become lysosomes

Along with protein modification, Golgiapparatus is involved in the transport of lipidsaround the cell,


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Protein transport

From the ER they are transported to the Golgi complex in

small membrane-bound vesicles formed from the ER

membrane.

These vesicles then fuse with the membranes of the Golgi

complex. The proteins then pass through the separate layersof the Golgi complex moving by way of membrane transport

vesicles.

During their passage- proteins are modified in different ways

Carbohydrates are often added here or previously addedcarbohydrates are modified.

In some cases, the carbohydrates and other modifications act

as "sorting signals" allowing the GOLGI to route the protein to

different parts of the cell.


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Portion of the golgi closest to

the plasma membrane andserves as the site where

membrane-bound vesicles

bud and then exit the golgi

complex.

Is the site where

membrane-boundvesicles from the ER

first fuse.


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Transport from the Golgi apparatus


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sujit - endoplasmic reticulum structure and function

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