Cell Biology

1. ENDOPLASMIC RETICULUM
By
Mrs Sanchita Choubey
(M.Sc., PGDCR, Pursuing Ph. D)
Assistant Professor of Microbiology
Dr. D Y Patil Arts Commerce and Science College Pimpri, Pune

3. INTRODUCTION
⚫In theyear 1945- The lace like membranes of theendoplasmic
reticulum were first seen in the cytoplasm of chick embryo
cells.
⚫Theseare membrane bound channels, seen in the formof a
network of delicate strands and vesicles in thecytoplasm.
⚫Theseare single membrane cell organelles.
⚫These form an interconnected network of tubules, vesicles
and cisternae with in cells.
⚫ER areconsidered as one of thecomponents of cytoskeleton
along with microtubules,microfilaments and intermediate
filaments.
⚫Theseare firstof all observed by Porter, Claude and Full am in
(1945) as a network.
⚫ The term ”Endoplasmic reticulum” was first used by Porter
and Fullman (1952)

4.  Location
⚫Present in almostall eukaryoticcell.
⚫These are found to be absent in mature
erythrocytes,ova,embryonic cells and prokaryotes.
⚫The ER often occupies mostof thecytoplasm.
Amount
• The ER varies in amount from cell tocell. In spermatocytes, it
is represented bya fewvacuolesonly.
• In thecells of adipose tissue, it is quitesimple, having the form
of a few tubules.
• Thecells thatareactivelysynthesizing proteins, such as liver
and pancreatic cells and fibroblast, haveabundant ER.
• Endoplasmic reticulum forms 30-60 % of the total membrane
in acell.

5. Originof endoplasmicreticulum
⚫At present manner of origin of the endoplasmic is not
definitely known. The mostconcrete hypothesis is that
the ER is “budded” off from the nuclear envelope
(wischnitzer, 1974).
⚫The ER appears toarise from theouter membrane of
the nuclearenvelope byout folding , or from the
plasma membrane by in folding.
⚫The smooth ER seem toarise from the rough ER by
detachmentof ribosomes.

6. ⚫Thereare two basic morphological typesof ER namely
RER and SER.
⚫The ER membrane is thinner (50 Ǻ) than thatof
plasma membrane (80-100Ǻ thick)

7. PHYSICAL STRUCTURE-
⚫The ER is 3-dimensional networkof intracellular. It is
formed of three typesof element:
1 Cisternae
2Tubules
3-Vesicles
Cisternae-
⚫Theseare flattened , unbranched, sac-like element.
⚫They lie in stacks parallel tooneanother.
⚫They bear ribosomes on the surface
that, therefore, appears rough.
⚫Itcontain glycoproteins named ribophorin-I &
ribophorin-II that bind the ribosomes.

8. Tubules-
⚫Theseare irregular branching element which form a
network along with otherelement.
⚫Theseareoften free of ribosomes.
Vesicles-
⚫Theseare oval and rounded ,vacuole likeelement.
⚫Thesearealso free of ribosomes.
⚫All the element of ER freely communicates with one
another, and contain a fluid called endoplasmic matrix, in
the ER lumen.
⚫These matrix is different from cytoplasmic matrixoutside
the ER
⚫The ER may pass from onecell toanotherthrough the
plasmodesmata in the form of desmotubules.

11. Molecularstructure
⚫The membrane of ER are composed of two layers of
phospholipid molecules sandwiched by two layers of
proteins molecules likeother membrane in thecell wall.
Types
⚫Theendoplasmic reticulum isof two types:
1-Smooth endoplasmic reticulum (SER)
2-Rough endoplasmic reticulum (RER)

12. Endoplasmic Matrix
⚫The space inside the tubules and vesicles is filled with
awatery medium that isdifferent from the fluid in the
cytosol outside the ER.
⚫Their walls are constructed of lipid bilayers
membranes thatcontains largeamountof proteins
, similar to thecell membrane.

13.  SMOOTH ER
is an arrangement of tubules,vesicles
⚫Smooth ER
and sacs.
⚫The size and structure of the SER varies between
thecells.
⚫The SER can change within a cells lifetime to allow
the cell to adapt to changes in its function and
requirements.
⚫There are no ribosome’s attached to the membrane
surface.
⚫The SER is connected to the nuclearenvelope

14. ⚫The network of the SER allows there to be
enough surfacearea for theaction orstorageof
keyenzymesor the productsof theenzymes.
⚫The SER is less stable.
⚫The SER is characteristic of cells in which
synthesisof non-protein substances takes place.

15. ROUGH ENDOPLASMIC RETICULIM (RER)
⚫Thesurface of the RER is studded with ribosome, giving ita
roughappearance.
⚫It mainlyconsists of cisternae.
⚫The membrane of the RER forms largedouble membrane
sheets
⚫Which is located nearand continuous with theouter layerof
the nuclear envelope.
⚫RER isvery imp. in the synthesisand packaging of
proteins e:g, Russell’s bodies of plasma, nissel’s granulesof
nervecell
⚫Binding siteof the ribosomeon the RER is the translocon .

16. ⚫The ribosomes bound to the RER atanyone timeare
nota stable partof thisorganelles structure
⚫Because ribosomesare constantly being bound and
released from the membranes.
⚫Ribosomesonly binds to the RER oncea specific
protein-nucleic acid complex forms in thecytosol.
⚫This special complex forms when a free ribosome
begins translating the mRNA of a proteindestined for
thesecretory pathway.
⚫The first 5-30 aminoacid polymerized encodea single
peptide, a molecular message that is recognized and
bound bya single recognition particle (SRP).

17. ⚫The ribosomes that become attached to the
endoplasmic reticulum synthesizeall trans membrane
proteins.
⚫Mostsecreted proteins thatare stored in the Golgi
apparatus, lysosomes, and endosomes.
⚫Translation pausesand the ribosomes complex binds
to the RER translocon

18. Protein Transport
⚫As proteins are formed in the endoplasmic
reticulum, theyare transported through the tubules
toward proteins of the SER that lie nearest to Golgi
apparatus.
⚫At this point, small transport vesicles composed of
small envelopesof smooth ER continually break away
and diffuse to thedeepest layerof Golgi apparatus.
⚫Inside thisvesiclesare the synthesized proteinsand
otherproduct from the ER present.

19. Transportvesicles
⚫Theyare surrounded bycoating proteincalled COP
I, COP II.(Coat Proteincomplex)
⚫COP II targetsvesicles to the Golgi apparatus.
⚫Transparent proteins from the RER to Golgi apparatus.
⚫This process is termed as anterogradetransport.
⚫COP I transports proteins from thecis end of the Golgi
complex back to the RER.
⚫This process is termed as retrogradetransport.

22. ⚫Second method of transportoutof theendoplasmic
reticulum involves areas called membrane contact
sites.
⚫Where membranes of theendoplasmicreticulumand
otherorganellesare held together , allowing the
transferof lipid and othersmall molecules.

23. FUNCTION OF
ER-
ROUGH ENDOPLASMIC RETICULUM (RER)
SMOOTH ENDOPLASMIC RETICULUM (SER)
SACROPLASMIC RETICULUM (SR)

24.  FUNCTION OF RER-
⚫Surface for Ribosomes- The RER provides space and
ribophorins for theattachmentof ribosomes to itself.
⚫Surface forprotein synthesis
⚫Formation of Glycoprotein- Linking of sugars to for
glycoprotein starts in the RER and is completed in
Golgi complex.
⚫ Synthesis of precursors- The RER produce enzyme
precursors for the formation of lysosomes by Golgi
Complex.
⚫ Smooth ER formation- The RER gives rise to the
smooth ER by lossof ribosomes.

25. FUNCTION OF SER
⚫ The smooth endoplasmic
reticulum lacks ribosomes and
functions
in lipid metabolism, carbohydrate
metabolism, and detoxification
and is especially abundant in
mammalian liverand gonad cells.
⚫Italso synthesizes phospholipids.
Cells which secrete these
products, such as those in the
testes, ovaries, and skin oil glands
haveagreat deal of smooth
endoplasmic reticulum.

26. ⚫Detoxification-The SER brings aboutdetoxification in
the liver , i.e., converts harmful
materials(drugs, poisons) into harmlessones for
excretion by thecell.
⚫Formationof organelles- The SER produces Golgi
apparatus , lysosomesand vacuoles.
⚫Italsocarries out theattachmentof receptorson cell
membrane proteinsand steroid metabolism.
⚫ In musclecells, it regulatescalcium ion concentration
⚫The smoothendoplasmic reticulumalsocontains the
enzymeglucose-6-phosphatase, which
convertsglucose-6-phosphate toglucose, a step
in gluconeogenesis.

27. SR
⚫The sarcoplasmic reticulum (SR) is smooth ER found
in smoothand striated muscle.
⚫The only structural difference between this organelle
and the smooth endoplasmic reticulum is the medley
of proteins they have, both bound totheir membranes
and drifting within the confines of their lumens. This
fundamental difference is indicativeof their functions.
⚫ The endoplasmic reticulum synthesizes
molecules, while the sarcoplasmic reticulum stores
and pumpscalcium ions.

28. ⚫The sarcoplasmic reticulum contains large stores of
calcium, which it sequestersand then releaseswhen
the musclecell is stimulated.
⚫ Itplaysa majorrole in excitation-contraction
coupling in musclescells.

29. Sarcoplasmicreticulum in theexcitation-
contraction coupling in musclecells.

30. THANK YOU