Structure and Functions of the Endoplasmic Reticulum

STRUCTURE AND FUNCTIONS OF
ENDOPLASMIC RETICULUM (E R)
By
Prof (Dr.) Ichha Purak
Department of Botany
Ranchi Women’s College,Ranchi
1/12/2020 Endoplasmic Reticulum 1

 DISCOVERY
 INTRODUCTION
 BIOGENESIS OF ER
 ISOLATION OF MICROSOMES FROM E R
 STRUCTURE
 COMPONENTS OF ER
CISTERNAE
VESICLES
TUBULES
 MAIN FUNCTION OF ER
 TYPES OF ENDOPLASMIC RETICULUM
 SMOOTH ENDOPLASMIC RETICULUM (SER)
 FUNCTIONS OF SER
 ROUGH ENDOPLASMIC RETICULUM (RER)
 FUNCTIONS OF RER
 SUMMARY
 REFERENCES
 QUESTIONS
CONTENTS

DISCOVERY
Discovered in 1902 by Italian Scientist Emilio Verrati.
Porter et. al. (1945) showed through electron micrograph that cells contain a lace like network
of strands throughout the cytoplasm.
Palade and Porter (1954) mentioned key characteristics of Endoplasmic Reticulum and
ribosomes.
In 1966 George Palade generated a tramission electron micrograph illustrating that endoplasmic
reticulum is studded with granules (ribosomes).
George Palade (1974) further described structure of endoplasmic reticulum and Golgi complex.
EMILIO VERRATI KEITH ROBERTS PORTER
GEORGE EMIL PALADE

All eukaryotic cells have an endoplasmic reticulum (ER) except red blood cells of
mammals.
In animal cells ER membrane typically constitute more than half of the total membranous
system of the cell.
The endoplasmic reticulum (ER) is absent in the prokaryotic cell.
The entire endoplasmic reticulum is enclosed by a continuous membrane and is the largest
organelle of most eukaryotic cells.
Its membrane may account for about half of all cell membranes, and the space enclosed by
the ER (the lumen or cisternal space) may represent about 10% of the total cell volume.
INTRODUCTION
The endoplasmic reticulum (ER) is an extensive system of a network of membrane enclosed
branching tubules and flattened sacs (cisternae) that extends from the nuclear membrane
throughout the cytoplasm.

BIOGENESIS OF ENDOPLASMIC RETICULUM
According to current concepts, membrane biogenesis is a multi-step process that involves
the synthesis of a basic membrane lipids and intrinsic proteins.
Then the addition of the other components, such as enzymes, sugars or lipids occur in a
sequential manner.
The insertion of protein into ER membrane is independent of that of the lipids.
Some proteins of the ER are formed by the ribosomes in the cytosol which then become
inserted into the membrane.

ISOLATION OF MICROSOMES FROM E R
Figure : 1 Rough and Smooth Regions of ER Can Be Separated by Centrifugation
(From Molecular Biology of cell, 4th Edition)
A)When sedimented to equilibrium through a gradient of sucrose, the two types of microsomes
separate from each other on the basis of their different densities.
B) A thin section electron micrograph of the purified rough ER fraction shows an abundance of
ribosome-studded vesicles. (B, courtesy of George Palade.)

A major contribution to the study of the E R came with a protocol published by Palade
outlining a method to isolate E R derived microsomes.
Isolation of Rough and smooth Microsomes is done by Density Gradient Centrifugation
When cells are disrupted by homogenization, the ER breaks into fragments and give many
small closed vesicles (100-200 nm in diameter) called microsomes , which can easily be
purified.
Microsomes derived from rough ER are studded with ribosomes and are called rough
microsomes.
The ribosomes are always found on the outside surface, so the interior of the microsome is
biochemically equivalent to the lumenal space of the ER.
As rough microsomes can be readily purified in functional form ,they are useful for studying
many processes performed by the rough ER
Continued :

STRUCTURE OF ENDOPLASMIC RETICULUM
Endoplasmic Reticulum extends from nuclear membrane at one end and joins plasma
membrane at other end by ER-plasma membrane junction(Okeke et al, 2016) , it never opens
into cytoplasm. It also continues with other organelles as Golgi apparatus and mitochondria.
All these structures (tubules and sacs) are held in their place by the cytoskeleton. The ER
remains bound by a phospholipid membrane. Inside the membrane, a fluid-filled space
exists, which is known as cisternal space or lumen. The lumen is continuous with the
perinuclear space.
The membranes of ER contain many enzymes, triglycerides, phospholipids and cholesterol.
Most important ER enzymes include stearases, NADH-cytochrome reductase, glucose-6-
phosphatase, glycosyl transferase and Mg++ activated ATPase.
Unit membranes of all the cavities of Endoplasmic Reticulum are inter connected.
In Electron micrograph, the cross section of endoplasmic reticulum appears as two parallel
membranes separated by a narrow light space about 4 nm wide ( called as cisternae) .

Figure 2: Relationship between the cisternal space and the cell sap
Cisternal space, perinuclear space and the external environment are closely related
SER-Smooth Endoplasmic Reticulum RER-Rough Endoplasmic Reticulum
Figure 3: Three dimensional structure of Rough Endoplasmic Reticulum (RER)
showing stacks of flattened cisternae . Ribosomes are bound on cytosolic surface of membrane .

COMPONENTS OF ENDOPLASMIC RETICULUM
On the basis of morphology, endoplasmic reticulum is made up of 3 types of structures
a)Cisternae- These are long, flattened, sac like unbranched tubules, arranged parallel forming
bundles, each tubule is 40-50 µm thick
b) Vesicles-These are usually rounded or ovoid structures, 25-500 mµ thick in diameter,
surrounded by unit membrane, often occur isolated in cytoplasm
c) Tubules-These are of diverse shape and are irregularly branched, 50-100 mµ in diameter.
These are found in cells that are active in synthesis of glycerids. These are also found in big
pigmented epithelial cells of retina, involved in metabolism of Vitamin –A (Retinol)
All the 3 components of ER are bounded by 50-60 A⁰ thick membrane. The membrane
consists of 3 layers as plasma membrane, middle thin and transparent layer of phospholipids
and outer and inner dense layer of proteins.
d) Perinuclear space is the space present between E R and Nucleus. There is close
relationship between perinuclear space, cisternae and cell sap

Figure -4 Components of Endoplasmic Reticulum
Main Function of Endoplasmic Reticulum
 Synthesis and Translocation of secretory proteins across the R ER membrane.
 Integration of proteins into the plasma membrane.
 Folding and modification of proteins in the RER lumen.
 Synthesis of phospholipids and steroids on the cytosolic side of the SER membrane.
 Storage of calcium ion in the SER lumen and their regulated release into the cytosol.
 Carbohydrate metabolism
 Detoxification of drugs

It is basically of two functionally distinct types : Smooth Endoplasmic Reticulum (SER)
and Rough Endoplasmic Reticulum (RER) on absence or presence of ribonucleoprotein
granules (Ribosomes)
Agranular or Smooth Endoplasmic Reticulum (SER) that does not have Ribosomes
attached to its surface
Granular or Rough ER (RER) The endoplasmic reticulum bears ribosomes on its
surface .
SER and RER change into each other as per the need of the cell.
TYPES OF ENDOPLASMIC RETICULUM
Figure: 5 Smooth and Rough Endoplasmic Reticulum

TYPES OF ENDOPLASMIC RETICULUM
Figure 6:The endoplasmic reticulum (ER)
(A) Electron micrograph of rough ER in rat liver cells.
Ribosomes are attached to the cytosolic face of the ER
membrane.
(B) Electron micrograph of smooth ER in Leydig cells of
the testis, which are active in steroid hormone
synthesis.
(A, Richard Rodewald, University of Virginia/Biological
Photo Service; B, Dan Fawcett/Photo Researchers,
Inc.) Copyright © 2000, Geoffrey M Cooper.

SMOOTH ENDOPLASMIC RETICULUM (SER)
It is highly curved and tubular
SER occurs mainly in tubular form and does not have ribosomes.
The tubules measure 500-1000A⁰ long, forming irregular lattices.
Commonly present in cells connected with steroid and lipid biosynthesis and carbohydrate
metabolism.
Different cells may contain different amount of SER, depending on nature and function.
The smooth ER is involved in the synthesis of lipids, including cholesterol and phospholipids,
which are used in the production of new cellular membrane.
In certain cell types, smooth ER plays an important role in the synthesis of steroid hormones
from cholesterol.
In cells of the liver, it contributes to the detoxification of drugs and harmful chemicals.
The sarcoplasmic reticulum is a specialized type of smooth ER that regulates the calcium ion
concentration in the cytoplasm of striated muscle cells and regulate contraction of muscles.

FUNCTIONS OF SMOOTH ENDOPLASMIC RETICULUM
The Smooth Endoplasmic Reticulum plays a crucial role in various life sustaining processes,
like protein transportation, steroid and carbohydrate metabolism ,drug detoxification and
synthesis of phospholipids and cholesterol, which are major components of the plasma and
internal membranes.
Transport of proteins
SER provides route of transport of metabolites as proteins within the cell through formation of
vesicles that transfer proteins to various locations.
SER also transports the proteins formed in Rough ER to Golgi apparatus and other parts of the
cell.
Steroid Metabolism
SER plays a role in steroid metabolism. Many enzymes present in membrane of SER have
a key role in the synthesis of cholesterol, the precursor of steroid hormones and bile acids

Release, uptake and storage of calcium ions
SER attaches receptors to calcium binding proteins of cell membrane and stores calcium
in cytoplasm of striated muscle cells which helps in muscle contraction and relaxation.
In muscle cells specialized ER, called sarcoplasmic reticulum (SR) sequesters Ca 2+ from
the cytosol using Ca 2+-ATPase pumps. It helps in rapid responses to extracellular
signals.
Lipid metabolism Synthesis of fatty acids, glycerids and steroids such as cholesterol
takes place in SER and special cell organelles sphaerosomes ( present in cells of
endosperm, cotyledons of seeds)
Phospholipid biosynthesis is largely confined to the membranes of SER in collaboration
with Golgi complex.
In the liver, the SER is abundant in hepatocytes and is involved in the production of
lipoproteins.

Carbohydrate Metabolism
Glycogen, the reserve food of many cells, is converted first to Glucose-1-Phosphate and
then to Glucose-6-Phosphate by gluconeogenesis. The Glucose-6-Phosphate is then
brought to the smooth endoplasmic reticulum (SER) where it is broken down to Glucose
and Phosphate by the enzyme Glucose-6-Phosphatase present in the SER of animal cell.
The Glucose thus formed is then transported first into the cisternal space and finally to the
extracellular space or to the blood stream for the maintenance of the functions of red blood
cells and nerve tissues
There are other enzymes of carbohydrate
metabolism, such as Amylase and
glucuronidase for the hydrolysis of starch
and mucopolysaccharides

Detoxification of Toxic Chemicals
It detoxifies the toxicity of carcinogens by having necessary enzymes. The toxic chemicals
are secreted out of the body through formation of vesicles by SER
SER helps in detoxification of a wide variety of xenobiotics such as barbiturates, ethanol,
aspirin and petroleum products. Detoxification generally takes place in liver cells.
When large quantities of toxic harmful compounds enter the circulation, detoxification
enzymes are synthesized in large quantities and smooth ER doubles its surface area within a
few days. Once these harmful compound are detoxified and removed, the excess smooth ER
is removed by lysosome dependent autophagocytosis.
Detoxification is carried out by a system of oxygen transferring enzymes (oxygenases) that
catalyze a number of reactions that can make lipid soluble drugs and metabolic wastes into
water soluble, so that they can be easily expelled from the body.

The enzymes present in SER help in converting lipids and other hydrophobic compounds to
water soluble form for easy transport through the membrane. These oxidase enzyme systems
for the detoxification of drugs and other chemicals, consist of an important element, termed
cytochrome P 450.
This cytochrome P450 has an important role in the detoxification of drugs. Cytochrome P450 is
an iron containing compound having a special property of reducing its form when it binds with
the substrate.
This reduced form again can absorb light at 450 nm. When any compound, say drug, is
attached with the cytochrome P450, the iron (Ferric, Fe+++) present is reduced to Ferrous
(Fe++) with the help of NADPH dependent cytochrome P450 reductase.
Now the reduced cytochrome P 450 will bind oxygen. One atom of oxygen is used to oxidize the
substrate to convert the (Fe++) to (Fe+++) and the other atom of oxygen is used to oxidize the
drug with the liberation of one molecule of water.

Other functions of SER
It helps in exchange of materials between nucleoplasm and cytoplasm.
In plant cell SER establishes link between cytoplasm of two cells through pits and
plasmodesmata.
Smooth Endoplasmic Reticulum (SER) is responsible for the synthesis and repair of
membranes.
SER and RER provide surface area for the action and storage of enzymes and their products.
In addition to cytochrome P
450, cytochrome P 448 and
cytochrome b5 are also
present in the endoplasmic
reticulum. Cytochrome b5
has been employed for the
desaturation of fatty acids.
Cytochrome P448 tries to
metabolize polycyclic
hydrocarbons.

ROUGH ENDOPLASMIC RETICULUM
RER (Rough Endoplasmic Reticulum) is characterized by presence of ribosomes over the
surface, connected with production of secretory proteins and is very common in cells
involved in growth and differentiation.
Rough ER is named for its rough appearance, which is due to the ribosomes attached to its
outer (cytoplasmic) surface. Rough ER lies immediately adjacent to the cell nucleus, and its
membrane is continuous with the outer membrane of the nuclear envelope.
RER is found throughout the cell but the density is higher near the nucleus and the golgi
apparatus.
Ribosomes are ribonucleoprotein particles present either free in cytoplasm or attached to
Endoplasmic Reticulum or outer surface of nuclear membrane. Ribosomes are place for
protein synthesis and synthesize structural proteins when present free in cytoplasm and
secretory proteins when attached to E R.

FUNCTIONS OF ROUGH ENDOPLASMIC RETICULUM
Protein synthesis, transport and folding
Proteins that are secreted from the cells, or act as integral membrane proteins or proteins
of certain organelles like Golgi complex, lysosomes, endosomes are assembled on
ribosomes attached to the outer surface of RER membranes.
RER is more prominent in liver cells, as they are concerned with the production of proteins.
So, cells specializing in protein synthesis have more prominent RER, while SER is
prominent in those cells, which are concerned with the production of lipids.
Thus, these proteins, instead of passing into the cytoplasm, appear to pass into the
cisternae of the RER, and are protected from action of cytoplasmic protease enzyme.

Signal sequence vary in length (13-36AAs, having about 10-15 hydrophobic AAs, one or
two +ve AAs before hydrophobic sequence, a short sequence of polar AAs near C terminus
and Amino Acid with short side chain (Alanine) closest to cleavage site.
These proteins contain a signal sequence of 6 to 20 non-polar amino acid
residues. It targets the nascent polypeptides for the ER membrane and leads
growing polypeptide into the ER lumen.
SIGNAL SEQUENCE
Figure : 7 Structure of Signal Sequence

SRP is a rod shaped complex having single 7S RNA ( 300 nucleotides) and 6 different
proteins. One protein of SRP directly binds to signal sequence, inhibiting elongation by
strictly blocking entry of Amino acyl tRNAs inhibiting peptidyl transferase
Figure: 8 SIGNAL RECOGNITION PARTICLE (SRP)
A signal recognition particle (SRP), recognizes the signal sequence and acts as a tag that
allows the SRP-ribosome-nascent polypeptide to bind to an SRP receptor located on the
cytosolic surface of the ER membrane

As one nascent polypeptide enters the RER cisternae, it acts upon by a variety of enzymes
located either within the membrane or in the lumen of the RER. These enzymes transform the
nascent proteins into their proper functional state.
Integral membrane proteins, such as glycophorin and a fraction of the erythrocyte plasma
membrane are synthesized on membrane bound ribosomes and translocated through the ER
membrane as the polypeptides elongate.
Integral proteins contain one or more stretches of hydrophobic amino acids that serve as
stop transfer sequences, which block further movement of these proteins into the
cisternal chamber, but trigger the opening of the channel laterally and cause the insertion
of the hydrophobic helix into the lipid bilayer

RER plays a central role in the synthesis and export of proteins and glycoproteins in
secretory cells as for example liver cells secreting serum proteins (albumin), endocrine
cells secreting peptide hormones (insulin), pancreatic acinar cells secreting digestive
enzymes, and cartilage cells secreting collagen.
These proteins are glycoproteins, membrane proteins, lysosomal proteins, albumins,
globulins etc. Ribosome become attached to E R when secretory protein is needed by
the cell , for this ribosomes receives specific signals.
The ribosomes on rough ER specialize in the synthesis of proteins that possess a signal
sequence that directs them specifically to the ER for processing. (A number of other
proteins in a cell, including those destined for the nucleus and mitochondria, are targeted
for synthesis on free ribosomes, or those not attached to the ER membrane).

Figure11: Overview of Protein sorting From Chapter 9 : Protein sorting and Transport from
the book : Cell -A Molecular Approach by G M Cooper Page 149

The role of ER in protein processing and sorting was first demonstrated by George
Palade and his colleagues in the 1960s.
He defined the pathway for secretory proteins on the basis of some experiments
Rough ER → G A →Secretory vesicle →Exterior
The entrance of proteins into the ER thus represents a major branch point for the
traffic of proteins within eukaryotic cells, which correspond to the synthesis of proteins
on either free or membrane bound ribosomes.
Proteins synthesized by the rough ER have specific final destinations. Some proteins,
for example, remain within the ER, whereas others are sent to the Golgi apparatus,
which lies next to the ER. Proteins secreted from the Golgi apparatus are directed to
lysosomes or to the cell membrane; still others are destined for secretion to the cell
exterior.

Gunter Blobel David D Sabatini
Gunter Blobel and David D Sabatini ( 1971 ) proposed signal hypothesis.
The mRNA translating secretory proteins on E R membrane bound ribosomes contain a
unique set of codons ( signal codons ) at the 3’ end just after initiation codon which
produces signal sequence at N(Amino) terminus of Growing polypeptide chain which is
recognized by SRP and Places ribosome on ER membrane for completing polypeptide
synthesis in lumen of ER .

Targeting Proteins to the Endoplasmic Reticulum
All protein synthesis initiates on ribosome that are free in the cytosol. Ribosomes engaged in
the synthesis of proteins that are destined for secretion are then targeted to endoplasmic
reticulum due to presence of a Signal Sequence at the amino terminus of the growing
polypeptide chain.
This signal sequence is a short stretch of hydrophobic amino acids. It is translated due to
presence of signal codons on some specified messenger RNAs
The Signal hypothesis was proposed by Gunter Blobel and David D Sabatini in 1971 as
mRNAs to be translated on membrane bound ribosomes contain a unique set of codons at the
3’ end just after initiation codon
Translation of these codons yield a unique sequence at the amino terminus of the growing
polypeptide chain (the signal sequence ) .
This signal sequence triggers attachment of ribosome to the membrane of ER with the help of
SRP ( Signal Receptor Protein ) . The signal sequence is cleaved later on by signal peptidase
after the targeting function is completed

Figure: 10 Schematic diagram showing the mechanism by which signal sequence directs
the destiny of protein through lumen of Endoplasmic Reticulum

Figure: 11 Targeting secretary proteins to the
Endoplasmic Reticulum(E R ) From
Chapter 9 : Protein sorting and Transport from the
book : Cell -A Molecular Approach by G M Cooper
Page 153
Steps
1. As the signal sequence emerges from
the ribosome, it is recognized and bound
by Signal Recognition Particle (SRP )
2. The SRP escorts the complex to the ER
membrane, where it binds SRP receptor
3. The SRP is released, the ribosome
binds to membrane translocation complex
and the signal sequence is inserted into
membrane channel.
4. Translation resumes and the growing
polypeptide chain is translocated across
the membrane
5. Cleavage of the signal sequence by
signal peptidase releases the polypeptide
into the lumen of the ER

STEPS IN SYNTHESIS OF SECRETORY PROTEINS
 Initiation complex formation (mRNA-ribosome ,I tRNA)
 Protein synthesis initiates
 If signal sequence appears at amino terminus of nascent polypeptide
 SRP binds to signal sequence of growing polypeptide
 Ribosome stops further elongation of polypeptide (Translation is blocked) for a while
 SRP is recognized by a Docking protein present on E R membrane
 Ribosome is attached to E R membrane and rest on ribosome receptors
 SRP dissociates and is recycled
 protein synthesis is resumed
 Translocation of polypeptide into the lumen of ER takes place through a channel (pore)
created
 Signal sequence is cleaved by enzyme signal peptidase present in lumen of E R
 Polypeptide is cleaved , ribosome dissociates
 Polypeptide chain after being released into the lumen of ER is further modified , chain folds
due disulphide and other bonds. Many proteins are also glycosylated.

Synthesis of Membrane Lipids
The enzymes involved in the synthesis of phospholipids are themselves integral proteins
of the RER membrane with their active sites facing the cytosol.
.
Vesicular Transport The cisternae of the RER are typically large with flattened cavities that
can act as transport channels for membrane and luminal proteins to be moved from their site of
synthesis to the apical tips of the RER.
The apical edges of the RER cisternae are typically smooth, devoid of ribosomes and are
referred to as transitional elements, which serve as sites of formation of the first set of transport
vesicles in the biosynthetic pathway leading towards Golgi complex.
Formation of Tertiary Structure of Protein ( Protein folding)
If a polypepetide is translocated into the ER from a membrane bound ribosome it is known as
co-translational translocation.
If a polypeptide is incorporated into the ER from a free ribosome it is known as post-tranlational
translocation
Protein folding involves disulphide bond formation by the enzyme disulphide isomerase of E R
between two cysteine molecules present distantly on polypeptide chain .

.
This requires an oxidising environment of ER , whereas cytosol provides a
reducing environment
The folding of a polypeptide into its correct three dimensional protein structure is
mediated by molecular chaperones.
Chaperones bind to unfloded polypeptides, stabilising them to prevent incorrect
folding.
One of the primary chaperone proteins is Binding immunoglobulin Protein (BiP).
BiP belongs to a heat sensitive protein family (hsp70) and has both a protein
binding and an ATPase domain.
BiP is also used to seal the translocon pore when not occupied by a ribosome.

Nearly all the proteins produced on the membrane bound ribosomes are glycoproteins.
The addition of sugars is catalyzed by a group of membrane bound enzymes called glycosyl
transferase in ER or Golgi apparatus.
Glycoproteins often are linked to oligosaccharides through Asn (Aspargin ) residues.
through the calnexin / calreticulin cycle
In almost all glycoproteins a 14 residue core (tree) oligosaccharide consisting of 2
acetyl glucoseamine, 9 mannose, 3 glucose is added to protein.
The presence of one or more mannose 6P residue in their linked oligosaccharide is the structural
signal that targets these proteins into lysosome
There are two types of protein glycosylation for polypeptides imported into ER lumen .First type
is N-linked glycosylation which actually begins in the ER by a co-translational mechanism
whereas second type is O-linked glycosylation occurs only when after polypeptide is
transported into Golgi apparatus by a post translational mechanism
N-linked Protein Glycosylation Begins in the ER

Figure: 12 B Proteins move from the
ER to the golgi apparatus and then ,
within secretory vesicles to the plasma
membrane and cell exterior
Figure: 12A Interrelated position of golgi
complex, endoplasmic reticulum and
nucleus in an animal cell
A receptor protein in the membrane of Golgi complex recognizes this mannose 6P signal.
Proteins are moved from ER to Golgi Complex in transport vehicles.
Glycosylation plays a key role in protein targeting

The proximity of the rough ER to the cell nucleus gives the ER unique control over protein
processing.
The rough ER is able to rapidly send signals to the nucleus when problems in protein
synthesis and folding occur and thereby influences the overall rate of protein translation.
When misfolded or unfolded proteins accumulate in the ER lumen, a signaling mechanism
known as the unfolded protein response (UPR) is activated.
The response is adaptive, such that UPR activation triggers reductions in protein synthesis
and enhancements in ER protein-folding capacity and ER-associated protein degradation. If
the adaptive response fails, cells are directed to undergo apoptosis (programmed cell
death).
Unfolded Protein Response (UPR)

The ubiquitin (Ub)-proteasome pathway (UPP) of misfolded proteins degradation.
Ubiquitin is conjugated to misfolded proteins that are destined for degradation by an
ATP-dependent process that involves three enzymes.
A chain of five Ub(ubiquitin) molecules provides a recognition signal for the 26S
proteasome ,where receptors initiate degradation of proteins into smaller peptides and
finally into amino acids by peptidases/proteases

RIBOSOME AND POLYSOMES /POLYRIBOSOMES
Ribosomes are Ribonucleoprotein non membranous particles .
Ribosomes are the site of Protein Synthesis
Ribosomes are present in cytoplasm either free or attached to ER or nuclear
membrane.
In eukaryotic cells ribosomes are also present in mitochondria and plastids.
Prokaryotic ribosomes are 70S particles having 2 subunits 30 S & 50S
Eukaryotic cytoplasmic ribosomes are 80S particles having 2 subunits 40S & 60 S
The two sub units are dissociated at low Mg++ concentrations and associate at high
Mg++ concentrations.
The smaller subunit fits on larger subunit leaving a tunnel between two,which can
accommodate the mRNA , aminoacyl tRNAs and other factors during protein synthesis.
40Appendix1/12/2020

Both the sub unit of ribosome contain rRNAs and proteins
Ribosomes are dumb bell shaped or spherical having a diameter of 140A-160Aᵒ
Number of ribosomes/cell is directly related to RNA content and protein synthesis rates.
Ribosomal RNA are large molecules ,larger than diameter of ribosome,remains coiled
like a spring with helical and non helical regions.
rRNA of smaller unit helps in correct orientation of mRNA on ribosome during protein
synthesis
2 ribosomal units after organisation in nucleolous pass to cytosol and aggregate to form
complete functional ribosomal complex in cytosol.
Many ribosomes can bind same messenger RNA one after one for polypeptide
synthesis of same nature forming polysome
41Appendix1/12/2020

42Appendix1/12/2020
en13666482isch1
Figure: 13 proteins and RNAs of Prokaryotic and Eukaryotic ribosomes

Both in case of free and membrane bound ribosomes, when the ribosomes moves some
distance on mRNA, a second ribosome may bind with the help of initiation codons, so
many ribosomes may use same mRNA for polypeptide synthesis of same nature differing
only in length of PPC (Number of amino acids polymerized). This assemblage is referred
as polysome or polyribosome
POLYRIBOSOME
Figure : 14 Formation of Polyribosomes

All of cell’s lipids are produced in ER.
Synthess of a major portion of cell’s proteins occurs on the cytosolic surface of the ER.
These proteins are destined for secretion through ER to Golgi apparatus, Lysosomes
,endosomes and plasma membrane.
In the ER lumen, the proteins fold and oligomerize, disulfide bonds are formed, and N-
linked oligosaccharides are added.
N-linked glycosylation is used to indicate the extent of protein folding, so that proteins
leave the ER only when they are properly folded.
Proteins that do not fold or oligomerize correctly are translocated back into the cytosol,
where they are deglycosylated, ubiquitylated, and degraded in proteasomes.
Only proteins that carry a special ER signal sequence are imported into the ER.
The signal sequence is recognized by a signal recognition particle (SRP), which binds both
the growing polypeptide chain and ribosome and directs them to a receptor protein on the
cytosolic surface of the rough ER membrane.
SUMMARY

This binding to the ER membrane initiates the translocation process by threading a loop of
polypeptide chain across the ER membrane through the hydrophilic pore in a
transmembrane protein translocator.
Soluble proteins—destined for the ER lumen, for secretion, or for transfer to the lumen of
other organelles—pass completely into the ER lumen.
Transmembrane proteins destined for the ER or for other cell membranes are translocated
partway across the ER membrane and remain anchored there by one or more membrane-
spanning α-helical regions in their polypeptide chains.
These hydrophobic portions of the protein can act either as start-transfer or stop-transfer
signals during the translocation process. When a polypeptide contains multiple, alternating
start-transfer and stop-transfer signals, it will pass back and forth across the bilayer multiple
times as a multipass transmembrane protein.

Blobel G and Sabatini D D (1971). Ribosome-membrane interaction in eukaryotic cells.
Biomembranes, 2, 193–195
Blobel G and Dobberstein B ( 1975). "Transfer of proteins across membranes. I. Presence of
proteolytically processed and unprocessed nascent immunoglobulin light chains on membrane-
bound ribosomes of murine myeloma". Journal of Cell Biology. 67 (3): 835–851
Okeke et al (2016) Endoplasmic reticulum–plasma membrane junctions: structure, function and
dynamics. J Physiol.594(11): 2837-2847
Palade G E and Porter KR. (1954) Studies on the endoplasmic reticulum. I. Its identification
in cells in situ. J Exp Med. ;100(6):641–656.
Palade G E (1974) Intracellular aspects of the process of protein secretion –Nobel Prize
lecture,published in 1992 by the Nobel Prize Foundation
Porter KR, Claude A, Fullam EF (1945). "A Study of Tissue Culture Cells by Electron
Microscopy". J Exp Med. 81 (3): 233–246.
Porter KR ( 1953). "Observations on a submicroscopic basophilic component of cytoplasm".
The Journal of Experimental Medicine. 97 (5): 727–750.
Veratti E., 1902. Ricerche sulla fine struttura della fibra muscolare striata. Mem. R. Ist.
Lombardo, Cl. Sc. Mat. Nat., 19: 87-133.
REFERENCES

The Endoplasmic Reticulum Molecular Biology of the Cell. 4th edition. Alberts B, Johnson A,
Lewis J, et al.New York: Garland Science; 2002.
The Endoplasmic Reticulum The Cell: A Molecular Approach. 2nd edition. Cooper
GM.Sunderland (MA): Sinauer Associates; 2000.
The endoplasmic reticulum: structure, function and response to cellular signaling Dianne S.
Schwarz and Michael D. Blower Cell Mol Life Sci. 2016; 73: 79–94.
Cell and molecular biology Concepts and Experiments Gerald Karp 5th Edition 2007 John
Willey and Sons, Inc
Cell and Molecular biology By P K Gupta: Rastogi Publications
Cell Biology (Cytology,Biomolecules and Molecular Biology) By Verma PS and Agarwal V K )
S Chand & Company Pvt Ltd.
Biocyclopaedia Is online for students and researchers in Biology Biocyclopedia.com
Cell Biology by Dr S P Singh and Dr B S Tomar Rastogi Publication,Meerut,U P ,India
Cell Biology 1997 Dr SC Roy and Dr K K De New Central Book Agency (P ) Ltd.
Kolkatta , India
Molecular Cell Biology. 4th edition.2000 H Lodish, A Berk and S L Zipursky W. H. Freeman;
New York
BOOKS CONSULTED

QUESTIONS ON ENDOPLASMIC RETICULUM
MCQs on ER
1) Which of the following is the largest single membrane-bound intracellular compartment?
a) Ribosome b) Golgi apparatus c) Nucleus d) Endoplasmic reticulum
2) Endoplasmic reticulum membrane which is associated with ribosomes is called_______
a) ER lumen b) Smooth endoplasmic reticulum c) Rough endoplasmic reticulum
d) Endosome
3) Which of the following is not the function of Glycosylation?
a) Helps in proper folding of the protein b) Confer stability in proteins
c) Helps in cell-cell adhesion d) Synthesis of membrane lipid
4) Name the site where detoxification of xenobiotic compounds takes place?
a) Cytosol b) RER c) SER d) Ribosome
5) Name the sequence which allows the resident protein to retain in ER lumen?
a) KDEL b) KKXX c) KLDE d) KXXK
6) Which of the following coated vesicle transport protein from ER to Golgi?
a) Clathrin b) COP II c) COP I d) COP III
7) Protein folding is a process in which a polypeptide folds in to ___________
a) 2-D structure b) Globular form c) 3-D structure d) Linear form

8) Name the coated vesicle which is used to transfer protein from plasma membrane to endosome?
a) Clathrin b) COP I c) COP II d) COP III
9) Smooth form of endoplasmic reticulum is without
a) Golgi complex b) Ribosome c) Nucleus d) Mitochondria
10) Rough endoplasmic reticulum assists in the synthesis of
a) Carbohydrates b) Proteins c) fats, glucose, starch d) Lipids
11) Secretions produced by ribosome are passed via endoplasmic reticulum and
a) Golgi complex b) Mitochondria c) Nucleus d) Vacuole
12) The term Endoplasmic Reticulum was coined by
a) Camillo Golgi b) Porter c) Robert Brown d) Benda
13) Which of the following organelle has a continuous connection with nuclear membrane
a) Golgi apparatus b) Lysosome c) RER d) SER
14) In RER, ribosomes are located on
a) the cytoplasmic side b) on the luminal side c) both (a) and (b) d) all throughout
15) Which of the following statements were true regarding ER
a) ER provides structural framework to the cell b) ER acts as intra cellular transporting system
c) SER is involved in the synthesis of lipid d) All of the above

16) The rough ER is specially well developed in cells actively engaged in
a) Protein synthesis b) Nucleotide synthesis c) Lipid synthesis d) Secretory functions
17) The term Endoplasmic Reticulum was coined by
a) Reinert b) Johnson c) Pomaret d) Porter
18) Which of the following statements are incorrect regarding Endoplasmic Reticulum ?
a) The adipose tissue has both SER and RER b) Plasma cells has RER only.
c) RBC lacks both RER and SER d) Hepatocytes has both RER and SER
19) SER is involved in
a) Phospholipid biosynthesis and detoxification reaction b) Protein synthesis
c) Phospholipid biosynthesis and Protein synthesis d) Phospholipid biosynthesis
20) The functions of RER include
a)Protein synthesis and detoxification
b)Protein synthesis and post translational modification
c) Protein synthesis and phospholipid biosynthesis d) Protein synthesis only
21) Which of the following organelle is involved in xenobiotic detoxification
a) Golgi body b) Lysosome c) SER d) RER
22) Protein glycosylation occurs in
a)Lumen of mitochondria b) Lumen of RER c)Lumen of SER d) Lumen of Lysosome

23) Ribophorins are
a) Transmembrane glycophorin on RER b) Transmembrane Glycophorin on SER
c) Luminal proteins on RER d) Luminal proteins on SER
24) RER is involved in the synthesis of
a) Membrane proteins and Secretory proteins b) Different proteins of the cell
c) Membrane proteins, secretory proteins and lysosomal proteins
d) Membrane proteins and secretory proteins and nuclear proteins
25) Rough ER is called ‘rough’ because
a) Rough texture of the surface b) Surface is studded with membrane proteins
c) Surface is studded with ribosomes d) All of these
26) SER produces
a)Proteins b)Carbohydrate c) Lipid d) Nucleic acid
27) The main organelle involved in modification and routing of newly synthesized proteins to their
destination is
a) Chloroplast b) Mitochondria c) Lysosome d) Endoplasmic Reticulum
28) The transfer vesicle from RER fuse with which region of golgi complex
a)Cis b) Medial c) Trans d) Protein arms

29) Which of the following is related to glycosylation of proteins
a) Endoplasmic Reticulum b) Peroxisome c) Lysosome d) Mitochondria
30) Sarcoplasmic reticulum is related with
a)Protein synthesis b) Hormone synthesis c) Release of Ca++ ions and contraction of muscles d) None of
these
31) During ultracentrifugation the Endoplasmic Reticulum and bodies associated with it are separated as
a fraction known as
a)Microsomes b) Polysome c) Quantosome d) Episome
32) The most important function of ER is
a) Protein synthesis b) Nourishing the nucleus c) Secretion of materials d) To give shape to the cell
33) In rapidly dividing cells, ER is
a) Highly developed b) Poorly developed c) Absent d) Non-functional
34) In endoplasmic Reticulum the following process takes place
a)Lipid synthesis b) Channeling of biosynthetic process c) Steroid synthesis d) All of the above
35) Single unit membrane structure present in the cytoplasm in the form of a net is
a)Golgi Complex b) Peroxisome c) Lysosomes d) Endoplasmic Reticulum

36) The Endoplasmic Reticulum often bears
a) Lysosomes b) Centrioles c) Peroxisomes d) Ribosomes
37) When the region of endoplasmic reticulum are studded by ribosome on the outer surface of the
cisternae, it is called
a)Sarcoplasmic Reticulum b) SER c) Granular Endoplasmic Reticulum d) None of the above
38) Which of the following is responsible for mechanical support, enzyme transport and protein
synthesis
a)Dictyosome b) Cell membrane c) Mitochondria d) Endoplasmic reticulum
39) Less stable Endoplasmic Reticulum is
a) RER b) SER c) Cisternae d) Tubules
40) Endoplasmic Reticulum is in continuation with
a) Golgi body b) nuclear wall c)Mitochondria d) Cell wall
41) A eukaryotic cell without endoplasmic reticulum is
a) Egg cell b) RBC of mammals c) WBC d) None of the above
42) Ribosomes are attached to endoplasmic reticulum through
a) Ribophorins b) r-RNA c) t -RNA d) hydrostatic reactions

43) Polyribosomes are aggregation of
a) Ribosome and r-RNA b) only r-RNA
b) c) peroxisomes d) Several ribosomes held together by a string of m-RNA
44) Which of the following organelles has the function of storing intracellular calcium?
a) Lysosome b) Endoplasmic reticulum c) Nucleolus d) Golgi complex
45) Which component of the Golgi apparatus is primarily responsible for "receiving" proteins to be packaged
and modified?
a) Cis Golgi b) Medial Golgi c) Trans Golgi d) Vacuoles
46) Following are the membrane bound cell organelles except
a) Endoplasmic reticulum b)Lysosome c) Ribosomes d) Peroxisome
47) ---------------- is the organelle which produces transition vesicles to transport molecules produced
in ER to the Golgi.
a) Lysosome b) Mitochondria c) Smooth ER d) Nucleus
48) 1) Which is the site of protein synthesis in a cell?
a) Lysosome b) Golgi body c) Smooth endoplasmic reticulum d)Ribosomes
49) ---------------- is the organelle responsible for the origin of other membranes in the cell.
a) Mitochondria b) Smooth endoplasmic reticulum c)Golgi body d) Lysosome
50) ) -------------- is an important element of oxidase enzyme present in the ER which is involved in the
modification of drugs.
a) Cytochrome P450 b) CYP3A4 c) Aromatase d)M21- Hydroxylase
Answers : Note : Options in red colour are correct choice for MCQs

Short Answer Questions
Write short notes on the following
a) Granular Endoplasmic reticulum
b) Agranular Endoplasmic Reticulum
c) Role of RER in synthesis of secretory proteins
d) Structure of Signal Recognition Particle (SRP)
e) Receptor Protein on membrane of E R
f) Signal Hypothesis
g) Structure of ribosome
h) Polyribosome
i) Diagram of Endoplasmic Reticulum
j) Components of E R
Explain the following in brief
a) Role of SER in detoxification
b) Signal sequence
c) Structure of Signal Recognition Particle (SPR)
d) Role of Signal Recognition Particle in protein targeting
e) Functions of SER
f) Structure of Signal peptide

Long Answer Questions
1)Give an account of ultrastructure of endoplasmic reticulum and describe its
function.
2) What do you mean by membrane system ? Describe the same with reference to
Endoplasmic Reticulum.
3) Describe the structure and function of Endoplasmic Reticulum
4) Explain the types of Endoplasmic Reticulum and their function.
5) Discuss the functions of smooth endoplasmic reticulum.
6) Discuss the functions of Rough endoplasmic Reticulum.
7) Give the structure of endoplasmic reticulum and point out the significance of ribosomes found
attached to it .

Structure and Functions of the Endoplasmic Reticulum

Recomendados

Recomendados

Más contenido relacionado

La actualidad más candente

La actualidad más candente (20)

Similar a Structure and Functions of the Endoplasmic Reticulum

Similar a Structure and Functions of the Endoplasmic Reticulum (20)

Más de ICHHA PURAK

Más de ICHHA PURAK (20)

Último

Último (20)

Structure and Functions of the Endoplasmic Reticulum

Notas del editor