endoplasmic reticulum function

1. Structure,
Types and
Function
of
Endoplasmic
Reticulum
Dr Akanksha Jain

2. The electron microscope reveals an extensive membrane
system in the cytoplasm called Endoplasmic reticulum (ER).
It was first reported by Porter in 1945. This continuous
membrane system connects the nuclear membrane on one
end and the cell membrane on the other.
All cells do not have Endoplasmic reticulum. It is generally
absent in egg and embryonic cells.
Endoplasmic Reticulum contains three different types of
structure.
These are cisternae, vesicles and tubeless:

3. (a)Cisternae:
These are long, flat and un-branched plates or lamellae arranged
in parallel rows.
(b) Vesicles:
They are usually round or ovoid sacs. They often occur isolated in
the cytoplasm.
(c) Tubules:
They are irregularly branched tube-like structures having a
diameter of 50-100n. These are surrounded by this unit
membrane of 50-60 thickness and their lumen is filled with the
secretary products of the cell.

4. There are two types of ER, such as smooth walled and
rough walled.
They may be present in the same or different types of
cells namely:
(i) Smooth Endoplasmic Reticulum (SEE):
The surface of this type of reticulum is smooth as
ribosome’s not attached to it. Smooth ER is actively
engaged in steroid synthesis, carbohydrate metabolism,
pigment production etc. in cells, (u) Rough
Endoplasmic Reticulum: The rough ER have ribosome
attached throughout the surface. These are present in
cells, which are active in protein synthesis.
The major functions these are the following:

5. (a)Common to both Endoplasmic Reticulum:
(i) Forms the skeletal framework.
(ii) Active transport of cellular materials.
(iii) Metabolic activities due to presence of different enzymes.
(iv) Provides increased surface area for cellular reactions.
(v) Formation of nuclear membrane during cell division.
(b) Function of Smooth Endoplasmic Reticulum:
1. SER takes part in synthesis of lipids, steroid hormones ascorbic
acid etc.
2. SER helps in detoxification of toxins using cytochrome P-450.
3. In liver cells, SER possesses enzyme bodis called glycosomes for
glycogen metabolism (glycogenesis and glycogenolysis.
4. In retinal cells. SER produce visual pigments from vitamin A.
5. In muscle cells, SER modified into sarcoplasmic reticulum ‘SR)
which store and release Ca2+ for muscle contraction.
6. SER produce organelles like Golgi apparatus, lysosomes,
Sphaerosomes and vacuoles.

6. (c) Function of Rough Endoplasmic Reticulum:
(i) It provides site for protein synthesis.
(ii) Protein translocation, folding and transport of
protein.
(iii) Glycosylation (this is the relation of a saccharides
group with a hydroxyl or amino functional group to form
a glucoside).
(iv) Disulfide bond formation (disulfide bonds stabilize
the tertiary and quaternary structures of many proteins).
(v) Membrane synthesis.
(vi)RER synthesizes proteins destined for secretions,
lysosomes or the plasma membrane.
(vii)RER forms nuclear envelope, plasma membrane and
SER.

7. Modifications of ER:
1. Annulate ER or Annulate lamellae (Mc Culloch
1952):
These are the ER having pores like nuclear pores. They
formed from blebing nuclear envelope. They can be
smooth of rough.
2. Sarcoplasmic Reticulum (SR):
A modified from of SER is striated muscle cells (both
skeletal and cardiac), supply Ca2+ to muscle cytoplasm.
Help in intra cellular impulse transmission and
contraction.
3. Myeloid body:
Modified SER in pigmented epithelial cells of retina of
frog. Helps in photoreception.
4. Nissl granules (tigroid bodies):
Masses of RER in cyton of a neuron.