endoplasmic raticulum the organ of cell and cell cycle

Er is an important organ

1. ~ Endoplasmic
12. Reticulum

Era reticulum isa network of membrane bound cavities, vesicles and tubules,
distributed throughout the cytoplasm}it is ‘concerned with the biosynthesis of proteins and lipid:

Itis more concentrated in the endoplasm than in the ectoplasm. Hence the name.

© Itis the cytoskeleton of the cell.

© Itis a cytoplasmic vesicular system.

+ Itisthe transportation system of the cell

+ Tifinctions as the packaging system.

+ The term endoplasmic reticulum (ER) was introduced by Porter 1948.

+ According to Porter, the endoplasmic reticulum is a complex, finely divided vacuolar
system extending from the nucleus throughout the cytoplasm to the margin of the cell,

+ Since this network is more concentrated in the endoplasm of the cytoplasm, the name

endoplasmic reticulum was proposed.
® De Roberts, Nowinski and Saez have coined another term, the eytoplasmic vacuolar

se for these membrane bound cavities present in the cytoplasm,
+ Itisa cell organelle

Nucke
Rough endoplasmic

reticulum
Ribosomes

‘Smooth endoplasmic
reticulum

Fig. 12.1: A cell showing endoplasmic reticulum.
CB& MB-13

2. Structure *
are cisternae, vesicles and,
Endoplasmic reticulum consists of tree components. They

tubules.

rem
a ee Spon endplsmiraiclum

Cistemae —

Rough endoplasmic reticulum
a Ribosomes
Fig. 12.2 : Endoplasmie reticulum.
1. Cisternae
These are long flattened, unbray
bundles. Their diameter is 40.5
‘normally found in secretory’

ched sac-like structures. They

are arranged in parallel
icron. They have ribosomes on

n their surface. They are

Cistemal space

Ribosome

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3. > Vesicles. Tubules

Fig.12.4 : Components of endoplasmic reticulum.
E „Por makes the vesicle to move to Golgi apparatus. COPII directes vesicles back to the
R]

Fig. 12.5: 3D-View of endoplasmic reticulum.

3. Tubules
These are smooth walled and highly branched tubular spaces having diverse forms. They
ave he diameter 150-100 m. microns. They normally oceurin non-secretory cells ike strated
muscle cells, They arise from the cistemae.
e membrane of endoplasmic reticulum is made up of phospholipids and proteins. It
Lasa luid-mosaic structure. tis 50-60. in thickness. The membranes of endoplasmic reticulum.
onsite more than half of the total membranes ofan animal cell.
+ ‘The membrane of endoplasmic reticulum is continuous with the plasma membrane,
Golgi membrane and nuclear membrane.
Extrinsic
protein
Head Tail

Es
AN I I i] 113 th 1
Inner pid layer { ] Ni Y

Glycolipid wee
pen SR

Extrinsic

proteins
Fig. 12.6: Fluid mosaic model.

4. ll 2
the intracellular transpor,
= Ce lumen: ce reticulumacts as a passage fort
a of the er rice
oo prods acerca seme
These membranes provide increased orale
+ rs an int DS
= Got ‘or rough endoplamie ret =
2 Aeramla or smth endoplasmic rec =
1 EG Raich Endoplasmic Reticulum (| Pree...

Ttoccurs in almost all cells which
xing eo bosoneon be RER cal noes cu inalmos all cls which
aan peek er gov ols a
Plasma cals is inthe Porn ron en
granular or Smooth Endoplasmic Retic cofendoplasmicreticulum. So
TES su mens pe enn eS
a eticulum is smooth. It occurs especially a de
Re er ne a nes
rt preset muscle mm

TS CS re me Why po

zZ)

Chemical Composition
Endoplasmic reticulum is composedoflipoprun fe
ete. Most of the proteins are rproson

ne and other.

thin, cephalin,

nes such as sterases, NADH.
ated ATPase.
tein. About

omprises of phospholip
ted in the form of enzym
Sptochrome C reductase, NADH diaphorase,
The Branules present on the reticulum are «

‘of cytoplasmic RNA js,

glucose 6-phosphat

“Omposed of RNA comb
Associated with endoplasmic reticulum

Microsomes

Microsome is a heterogeneous small par

cerraron feel homo ted by Claude in 195}
icrosomal fraction co endoplasmic reticulum, rough
endoplasmic reticulum, rib, plone retcat,rug
Microsomes constitute 15 to 2094 ofthe total mass ofthe cel
contains a large amount of RN,

jis ide "A Which makes,
Sangliosides. Fee

fraction obtained by high speed

Mpto 50-60% ofthe total RN,

Pt [A of the cell
his inosito, acero

sphatides and

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5. Fig 128: Microsomes
Iteontains a large number of enzymes
NADPH-sytochrome-C reductase gems

as ATPase uridine disphosphatas,strase,
thas enzymes used forthe sy

Elicose--hosphatase, Mg" activated reductase, ec
sis of triglycerides, phospholipids and cholesterol.

Annulat
Annulate lamellae are membranous leo

nel ranous flattened sacs containing annuli and pores. They
scendoplasmic reticulum containing anu poes sima ofnucler membrane

Annulate lamellae

|
CH#OROFOFO

Theannulate lamellae were first described by Me Cullock in 1952.

"They are found in the cytoplasm of oocytes, spermatocytes, embryonic cells, tumour cells
and invertebrates.

The annulate lamellae frequently contain ribosomes.

Hruban (1965) suggested that annulate lamellae may represent an intermediate stage in
the formation of the endoplasmic reticulum. In some instances there is continuity between
endoplasmic reticulum and annulate lamellae. So the annulate lamella are transitory cytoplasmic
organelles.

The annulate lamellae originate from the nuclear membrane. The outer nuclear membrane
forms finger like processes. They are pinched off into the cytoplasm to form vesicles. Rows of
vesicles fuse together to form cisternae. Matrix materials become associated with the pores to
form annuli. This results in the formation of annulate lamellae

The annulate lamella has the following functions;

1 Ithelps in the formation of endoplasmic reticulum.

2 Itinereases the cytoplasmic membranes in times of increased metabolic needs asin
embryonic cells.

Origin of Endoplasmic Reticulum
Several theoieshave been forvardedto expan he ri of eopasniereutum. Bu
he ekact nature of origin of the endoplasmic reticulum has not yet been clearly known
1.Ithas been suggested thatthe endoplasmic reticulum formed fiom the ground substance
A hyaloplas I
rem

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6. poser
as te
orc
1° \Wesicles Bullded off from
Su clear membrane
ACTAS Nuclear membrane
ig. 12.10: Origin of endoplasmic reticulum from annulate lamellae.

2. Itmay originate as the infoldings of plasma membrane (Palade),
3-Endoplasmic reticulum may be formed from the evagination of nuclear membrane (Gay
Rebhun 1956) through the formation of annulate lamellae,

Functions of Endoplasmic Reticulum
Endoplasmic reticulum performs the following functions
1. Mechanical support
The endopl
by which it giv
2. Fransport
Endoplasmic reticulum acts as a kind!
ellular circulation of v
are transported to the various

A lasmic reticulum divides the fluid content ofthe cel into diffe

rent compartments
mechanical supportto the cell. Henc

itis known as the eytoskeleton ofthe

of circulatory system, involved in te import, export
Sus substances. By this process, proteins, lipids, enzymes et.
of the cell

Pasma membrane

Ir

Potosi

=

o
N rein = Golgi complex
ee Endopl
Po ibósome
À A Ritmo

Nucleus,
lasmic reticulum,

Fig.12.11 : Transport and, ‘membrane flow,

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7. Polypeptide chain

Zu Endoplasmicreticulum Protein
8.12.12 : Endoplasmic reticulum collects and transports the protein
= synthesized on ribosomes.
= doplasmic reticulum provides pace forthe tachment of ribosomes.
„ [he thesized proteins ae collected by the endoplasmic reticulum. Th rocessed
andnsporid o otherparsofthecellby heendoplasmic rola
4,F6rmation of Microbodies

Microbodies are small granular bodies filled

i ular bodies filled with an electron dense granule rich in

peroxidase. They include peroxisomes and glyoxysomes. They are formed in protozoa,
ver, kidney and higher plants =

Microbodiesr association with endoplasmicreticulum. Endoplasmic reticulum
buds off mi o s, microbodies show connections with endoplasmic
ren.

Endoplasmic reticulum

Sad

Microbodies

Fig. 12.13 : Formation of microbodies from endoplasmic reticulum.

mthesis of Cholesterol and Steroid Hormones

Endoplasmic reticulum isthe major site forthe synthesis of cholesterol the precursor fo
stoidhormones

In the testis, ovary and adrena
Tole inthe synthesis of steroid hormones.

Glycosylation
Glycosylation is the ad
to the formation of glycoprot@

al cortex, the smooth endoplasmic reticulum plays the major

{dition of carbohydrate units to other cellular macromolecules. It
"mucopolysaccharides, glycolipids, glyeogen, et.

8. 186

SELON
Ribosome mRNA

sferase
Glycol tan
war

12)

Jema Norglydoprotein Carbohydrate
ERcislama je
Giycosylran:
echa | Se

RS >> Su N
( y 74)

rotein Protein Glycoproteins

npletion of glycosylation in
Fig. 12.14 : Diagrams showin ins. Compl 8
he ER cisterna and release of ghic
Almost all secretory proteins ar inthe form of
Proteins and carbohydrate.

Proteins are synthesized on

glycosylation of prot

Iycoproteins. Glycoprotein is fomed of

ibosomes attached tothe
lumen ofendoplasmic reticulum. In he endoplasmic 1

iculum, The ro
lum, carbohydrate
enya ankedtothe polypeptide chain by glyeaplation. The glycosylation is cata Cy the
enzyme glycosyl transferase.

m conve proc Hace en when protein synthesis is going on. Thus endoplasmig
reticulum converts proteins into glycoproteins by glycosylation
7-Detoxification

Detoxification refers to the reductio drugs
a ants. Detoxification occurs in the,

Detoxificat al reactions by which harmful mate

ninvolves biochemic
less substances suitable for excretion by the cell 1
oxidefons, reductions hydrolysis or conjugation to soluble molecu

Metabolic waste products such as fatty acids a, salts, steroids a
Setoxfiedby the smooth endoplasmic eticulign

8, Lipid Synthesis

ER synthesizes triglyceride.
A Glycogenolysis

The conversion ofglyen;
ER. The ER contains
into glucose which is

into

m ate also

*and phospholipids, als stores lipids,

cose called,

an enzyme called glucose-6-phoy
ransported tothe blood

19-Storage of Calcium ions (Ca)

im ions. Ins

shycogenolysis, 1

takes place
sphatase,Itcony

inside the

ertsglucose-6-phosp

is stimulated, ER releases
cle contraction,
from the cytosol

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9. endoplasmic re ibosames scale
The surface of SER smooth a a
[Us agranular as the surface has no ribosomes,
. ju is. uniformly ‘distributed in the er
{Hf occurs in inactive el. di
+ His abundantin cel ingsteroi
+ is more tubular has ER ere hormonas
ee not basophilic.
The SER of muscle cells is smooth and it is called sarcoplasmic reticulum.

SER of retinal cells i yeloic ie
+ Itis made up of tubules. a

+ It consists of a membrane and lumen.
+ The membrane has a fluid-mosaie structure
funcions
1. Itsynthesizes steroid hormones.
It synthesizes carbohydrates, li
It synthesizes plasma membrane.
The SER of testis and ovary cells synthesizes mate and female hormones.
In liver cells, SER detoxify drugs and harmful substances.
In the muscle cells, it assists in the contraction of muscle cells.
7. The SER of muscle cells stores calcium ions.

Highlights
Rough Endoplasmic Reticulum (RER)

The endoplasmic reticulum studded with ribosomes are called RER.
+ They are granular as the surface has granular ribosomes.

+ The site of attachment of ribosome is called translocon.

+ The RER of cell contains 13 million ribosomes.

+ RER is basophilic

+ It is made up of flattened sacs.

+ It consists of a membrane, a lumen and ribosomes.

+ It is the cisternae of ER

+ The membrane has a fluid-mosaic structure.

+ It is more concentrated near the nucleus.

+ It is directly connected with the nuclear membrane.

+ The lumen of RER opens into the peri nuclear space.

À The membrane of RER has a fluid mosaic structure like a plasma membrane.
À [tis abundant in cells synthesizing proteins. Bg. Plasma cells, pancreatic cells,

liver cells, goblet cells, etc,
Functions
NE | 1. Ribosome of RER is the sit

te of protein synthesis.

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10. deals Desa à
§ The RER of panereai els secrets insulin, ing

7. Inthe lumen of RER, haemos

polypeptide chains.

i in folding uni
8. The lumen of RER functions as a protein fold
teins functional. (Just ike, a flat card board is folded
fo kit as N
9. the Tees ore functions as a quality control section a bps c
proteins are subjected to a quality control check; Incorrectly

folded proteins are rejected.

it for making the pro.
Ito make a car board

10. In the lumen, proteins are linked with sugars to form glycoproteins. This

process is called glycosylation.

Table 12.1: Difference between RER and SER.

RER

SER

Contains ribosomes

2.| Appears granular

3.| Rough surface

4.| Flattened sacs

5.| Cisternas

6.| Concentrated near the nucleus

7. | Directly connected with the nuclear
membrane.

8. Involved in protein synthesis,

Noribosomes
Appears non-granular

Smooth surface
Tubular

Tubules

Uniformly distributed

Not directly connected with the
‘nuclear membrane,

Involved in steroid and carbohydrate
synthesis,

a |