Endoplasmic Reticulum

1. Endoplasmic Reticulum (ER)
The endoplasmic reticulum (ER) is an interconnected network of branching tubules and
flattened sacs that extend throughout the entire cytosol in eukaryotic cells.
Discovery: discovered by K. R. Porter, Claude and Fullman in 1945, but the name ER was given
by Porter (1953).
Occurrence:
• Absent in prokaryotic cells.
• Present in all eukaryotes except ovum, embryonic cells and mature RBCs.
• ER is quite extensive in metabolically active cells, simple in storage cells and reduced in
spermatocytes.
Structure:
The ER exists in three morphological forms:
1. Cisternae
2. Tubules
3. Vesicles

4. 1. Cisternae:
• These are flattened, un-branched sac-like structures having a diameter of 40 to 50 μm.
• They lie parallel to each other.
• They are interconnected and may be covered with ribosomes.
2. Tubules:
• These are irregularly branched tube-like structures which form a network along with the
cisternae and vesicles.
• They usually have a diameter from 50 to 190 μm.
• They do not possess ribosomes on their surface.
3. Vesicles:
• These are round, spherical or ovoid structures.
• Diameter is about 25-500 µm.
• They remain isolated in the cytoplasm.

5. Three components of ER

6. Types of ER:
1. Rough Endoplasmic Reticulum (RER):
• It has rough membranes because a number of ribosomes occur attached to their outer
surfaces.
• Also called granular endoplasmic reticulum.
• RER contains two types of glycoproteins (ribophorin I and ribophorin II) for attachment to
ribosomes.
• The membrane of the endoplasmic reticulum bears a fine pore in the area of attached
ribosome to pass the synthesized polypeptide into the lumen of endoplasmic reticulum for
transport.
• RER is composed of mainly cisternae; tubules are very few.
• Rough ER is prominent in cells where protein synthesis occurs.

7. 2. Smooth Endoplasmic Reticulum (SER):
• It has smooth membranes which do not bear ribosomes.
• It is, therefore, also called agranular endoplasmic reticulum.
• SER is mostly made of tubules.
• Engaged in the synthesis of lipids.
• Also transports the products of the rough ER to other cellular organelles, especially the
Golgi apparatus.

8. Functions of ER:
Functions of SER:
• Synthesis of essential lipids such as phospholipids and cholesterol.
• Production and secretion of steroid hormones.
• In muscle cells, SER modified into sarcoplasmic reticulum which store and release Ca2+ for
muscle contraction.
• SER produce organelles like Golgi apparatus, lysosomes, Sphaerosomes and vacuoles.
• Detoxification of toxins using cytochrome P450.
Functions of RER:
• Protein synthesis.
• Protein folding.
• Protein sorting.