Role and function of cell organelle

1. Structure and function of endoplasmic reticulum
and golgi bodies
Presented By,
S.Divya,
2017600805

2. INTRODUCTION - Er
• Light microscope –the structure of endoplasmic reticulum is described as
filamentous which was basophilic in staining property.
• This basophilic material was termed as “ergastoplasm”.
• Electron microscope -structure of endoplasmic reticulum is an extensive network
of membrane enclosed channels present throughout the cell.

3. Origin of Endoplasmic reticulum
The origin of endoplasmic reticulum is not definitely known.
According to Dallmer (1966), endoplasmic reticulum originated from the
plasma membrane by the process of invagination.
According to De Robertis 1970, endoplasmic reticulum originates from the
evagination of nuclear envelope

6. FUNCTIONS OF ER
• Mechanical support
• Membrane fluidity , flow and circulation
• Transport across ER Membrane and nuclear membrane
• Membrane biogenesis,Assembly and Recycling.
• Endoplasmic reticulum is involved in formation of cell plate and
plasmodesmata. (P.K.Hepler)
RER Functions:
• Site of protein synthesis and processing.
• Post translation

7. SER Functions:
• Detoxification
• Lipid synthesis
• Synthesis of steroids
• Glycogenolysis
• Ionic gradients
• Photoreception

9. RER AND SYNTHESIS OF EXPORTABLE
PROTEINS and protein segregation

10. Non secretary pathway
Free Ribosomes Mitochondria, Nucleus, Peroxisome

11. Secretary pathway
Rough Endoplasmic Reticulum Golgi apparatus
Cell Exterior Secretary vesicles

13. Cotranslational
translocation
SECRETARY PROTEINS

14. •A signal sequence of approximately 20
amino acids and rich with hydrophobic
amino acids is often located at the N-
terminus.
•Since the ribosome masks about 30
amino acids, the signal sequence isn’t
fully exposed until the nascent
polypeptide is about 50 amino acids
long.
•SRP-ribosome attaches to SRP receptor
and then docks on a protein
translocator.
•SRP and receptor dissociate.
•Translation and translocation proceed
in unison - co-translational transport.
•The energy for transport is provided by
the translation process - as the
polypeptide grows, it is pushed through
the protein translocator.
SRP: signal-recognition particle
SRP receptor

15. The signal sequence of secreted proteins is cleaved by a signal peptidase. In the
literature, the signal sequence of secreted proteins is often called a “leader
peptide”.

16. cotranslational translocation
Transmembrane PROTEINS

17. Translocation of Double Pass TM Protein

18. Translocation of Single Pass TM Protein

19. Translocation of Secretory Protein
Translocation of Single Pass TM Protein Translocation of Double Pass TM Protein

20. Transport from the ER through the Golgi Apparatus - continuation of the biosynthetic-
secretory pathway.
Properly folded proteins are
loaded into COPII transport
vesicles, while unfolded
proteins remain associated
with chaperones in the ER
until folding is complete.

21. If a protein doesn’t achieve a properly folded state, it gets exported from the ER into
the cytosol where it is degraded by the proteosome.

22. When the protein is properly folded, COPII coated vesicles transport the proteins via the
vesicular tubular cluster (vtc) to the cis-Golgi network.
•The COPII coating is removed
(Sar1 hydolyzes GTP) and the
vesicles fuse with each other to
form the vtc.
•The vtc is motored along
microtubules that function like
railroad tracks.
•The vtc fuses with the cis-Golgi
network.

23. Some proteins exiting the ER are returned to the ER by COPI coated vesicles.
These proteins are identified by the presence of specific signal sequences that
interact with the COPI vesicles or associate with specific receptors.
Examples of retrieved
proteins:
•ER chaperones like BiP
that are mistakenly
transported.

24. It is uncertain how proteins move through the Golgi apparatus.
Stationary compartments
with vesicles transporting
between compartments.
Large moving compartments
that mature into the TGN, and
return enzymes to trailing
compartments by retrieval
vesicles.

25. GOLGI BODIES

26. Structure
 Located near the end of the ER close to the nucleus
 Composed of several layers of cisternae (fluid-filled sacs)
 Has 3 different parts: cis-Golgi, medial-Golgi, and trans-Golgi

27. Function
 Is responsible for modifying, sorting, and packaging macromolecules (lipids
proteins, and carbs) coming from the ER
 Then sends the modified macro-molecules to different parts of the cell or
outside of the cell
 Can be thought of as a post office for a cell

28. • Transport vesicles are used to move back
and forth between the ER and Golgi
bodies
• One side of the Golgi body receives
transport vesicles produced by the ER
• The Golgi body marks and sorts the
molecules into different groups to be
sent in secretory vesicles inside or
outside of the cell
Transport & Secretory Vesicles

29. Protein Modification
Proteins going to organelles or
outside of the cell must be
modified so that they are
delivered
Modifications happen when
specific sugar molecules are
added to a core oligosaccharide
that is attached to a protein.
 These sugar molecules are
required for proper delivery.

30. • One example is Mannose 6-phosphate which is important in glycolysis
• Enzymes synthesized in the ER are delivered to the Golgi body via a transport
vesicle, and in the cis-face specific sugars are added to the protein
• Eventually, once the enzyme has arrived at the trans-face, it has the required
Mannose 6-phosphate
• Receptors receive the enzyme, it is transported via a vesicle, and when it arrives
at the endosome, the enzyme is released and the phosphate removed

31. Adaptins bridge the
M6P receptor to
clathrin.
Hydrolases are transported to
the late endosome which
later matures into a
lysosome.
Acidic pH causes hydrolase
to dissociate from the
receptor.

34. References
• Gupta, Jangir, CELL BIOLOGY Fundamentals and Applications,AGROBIOS
Publishers .
• E.D.P,De Robertis, Cell and Molecular Biology, Eight Edition -Wolters
Kluwer publishers Pvt.Ltd