Cell organelles: Endoplasmic reticulum

1. Cell organelles

2. Endoplasmic reticulum:

3. Endoplasmic reticulum:
 The endoplasmic reticulum (ER) is a network of membrane
enclosed tubules and sacs (cisternae) that extends from the nuclear
membrane throughout the cytoplasm.
 The entire endoplasmic reticulum is largest organelle of most
eukaryotic cells.
 The endoplasmic reticulum (ER) is divided into two sub
compartments, rough endoplasmic reticulum (RER) and the
smooth endoplasmic reticulum (SER).

4.  The rough ER, which is covered by ribosomes on its outer surface
involved in protein metabolism.
 The smooth ER is not associated with ribosomes and is involved in
lipid, rather than protein, metabolism.
 Both types of ER comprise a system of membranes that enclose a
space, or lumen, that is separated from the surrounding cytosol.

5.  When cells are homogenised and centrifuged, the ER breaks up into
small vesicles called microsomes.
 The vesicles derived from the rough ER are covered with
ribosomes, they can be separated from similar vesicles derived
from the smooth ER.
 Due to the presence of large amounts of RNA and ribosomes, the
rough microsomes are denser than smooth microsomes.

6. The Smooth Endoplasmic Reticulum:
 The SER is extensively developed in a number of cell types,
including those of liver, kidney, skeletal muscle, and steroid-
producing endocrine glands.
SER functions include:
 SER involved in lipid metabolism.
 Synthesis of steroid hormones in the endocrine cells.
 Detoxification of a wide variety of organic compounds in the liver,
like ethanol and Drugs.

7.  The membranes of eukaryotic cells are composed of three
main types of lipids:
 phospholipids,
 glycolipids,
 cholesterol are synthesized on the cytosolic side of the SER.

8. Functions of the Rough Endoplasmic Reticulum:
 RER extensively developed in the cells that secrete large quantities
of proteins especially pancreatic cells, CD4+ cells, antibody
producing cells and mucus secreting cells.
 The role of the endoplasmic reticulum in protein processing and
sorting.
 Secretory pathway: Rough ER Golgi Secretory vesicles Cell
exterior.

9.  Proteins are synthesized on ribosomes contain a signal sequence at their N-
terminus which contain 6 to 15 hydrophobic amino acids.
 Signal sequence bound by a signal recognition particle (SRP), SRP made up of
of six polypeptides and a small cytoplasmic RNA.
 The SRP binds the ribosome as well as the signal sequence and form SRP-
ribosome protein complex

11. As translocation proceeds, the signal sequence is cleaved by signal
peptidase and the polypeptide is released into the lumen of the ER.
signal sequence is
cleaved by signal
peptidase

12.  The SRP-ribosome protein complex binds to an SRP receptor (a signal
recognition particle receptor) present on the the ER membrane.
 Attachment of this complex to the SRP receptor is followed by release
of the SRP and the association of the ribosome to ER and
translocation of growing peptide in to ER lumen.
 As translocation proceeds, the signal sequence is cleaved by signal
peptidase and the polypeptide is released into the lumen of the ER.

13. As translocation proceeds, the signal sequence is cleaved by signal
peptidase and the polypeptide is released into the lumen of the ER.
signal sequence is
cleaved by signal
peptidase

14. Processing of Newly Synthesized Proteins in the Endoplasmic Reticulum:
 The N-terminal portion containing the signal peptide is removed
from most nascent polypeptides by a proteolytic enzyme i.e the
signal peptidase.
 Carbohydrates are added to the nascent protein by the enzyme
oligosaccharyl-transferase.
 The ER lumen also contains a number of protein-processing
enzymes, such as protein disulfide isomerase (PDI ).
 The formation of disulfide bonds is catalyzed by PDI.

15. Glycosylation in the Rough Endoplasmic:
 Carbohydrate groups have key roles in the function of many
glycoproteins.
 Oligosaccharide units consisting of 14 sugar residues are added to
asparagine residues of polypeptide chains these process catalyzed by
enzymes called glycosyltransferases (oligosaccharyl-transferase.).
 These enzymes transfers a specific 2 molecules of N-acetylglucosamine,
3 molecules of glucose and 9 molecules of Mannose (Totally 14 sugars)
are added to asparagine amino acid of polypeptide (Asn).


16. Glycosylation in the Rough Endoplasmic:
9 Mannose
N-acetylglucosamine
2 glucose
14 sugar residues are added to asparagine
residues of polypeptide chains By the enzyme
oligosaccharyl-transferase

17. Export of Proteins and Lipids from the ER:
 Both proteins and phospholipids transported to Golgi complex
through the transport vesicles by secretory pathway.
 Vesicles that bud from the transitional ER, all vesicles fuse to
form ER-Golgi intermediate compartment (ERGIC) and then to the
Golgi apparatus.