4. ENDOPLASMIC RETICULUM

WHAT IS ENDOPLASMIC RETICULUM?
These are irregular branching element which
form a network along with another element
 These are often free of ribosomes
VESICLES
 These are ovel and rounded, vacuole like
element
 These are also free of ribosomes
Endoplasmic – “within the cytoplasm”
Reticulum – latin for a “a little net”
 Extensive network
of
folded
membranes
that
extends from the
nuclear envelope to
which
it
is
connected,
throughout
the
cytoplasm. It is
present in both,
animal and plant cells.
 Divided into two sub-compartments,
1. Rough endoplasmic reticulum
2. Smooth Endoplasmic Reticulum
1. ER Cisternae
- Are flattened, membrane-bound sacs that
make up the bulk of the ER structure. They
provide a large surface area for biochemical
processes
- Function:
o Protein synthesis and processing
o Lipid synthesis
2. ER Vesicles
- Are small, membrane bound sacs that bud
off from the ER and transport proteins and
lipids to various destinations within the cell
or outside it.
- Function:
o Transport
o Secretion
3. ER Tubules
- Are elongated, tubular structures that
extend from the ER and can connect to the
cisternae or vesicles.
- Function:
o Communication
o Membrane extension
DISCOVERY OF ENDOPLASMIC RETICULUM
 discovered by Belgian biologist Albert Claude
(1899-1983)
 used the newly developed electron microscope
to explore the interior of cells and along with his
associate Keith Porter in 1945 observed the
presence of a “lace-work structure.”
STRUCTURE OF ENDOPLASMIC RETICULUM
 PHYSICAL STRUCTURE
o The ER is 3-dimentional network of
intracellular. It is formed of three types of
elements:
 Cisternae
 Tubules
 Vesicles
CISTERNAE
 These are flattened,
unbranched,
sac-like
element
 They lie in stacks parallel
to one another
 They bear ribosomes on
the surface
 MOLECULAR STRUCTURE
o The membrane of the ER are composed of
two layers of phospholipids molecules
sandwiched by two layers of proteins
molecules like other membrane in the cell
wall.
 ENDOPLASMIC MATRIX
o The space inside the tubules and vesicles
is filled with watery medium that is different
from the fluid in the cytosol outside the ER.
LOCATION OF ENDOPLASMIC RETICULUM
 There is no specific base or point in a cell, as
it extends a network of membranes, tubules,
vesicles, and sacs throughout the cell.
TUBULES
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4. ENDOPLASMIC RETICULUM
 Endoplasmic reticulum membranes are
folded and stacked on top of each other and
connected to the nucleus.
SMOOTH ENDOPLASMIC RETICULUM
 Smooth ER is more tubular than rough ER.
 Forms a separate sealed interconnecting
network
 It is found fairly evenly distributed throughout
the cytoplasm
 It is not studded with ribosomes hence
‘smooth ER’
TYPES OF ENDOPLASMIC RETICULUM
 There are two sub-compartments of
Endoplasmic Reticulum based on Ribosome,
their specific functions and their specific
appearance:
o Smooth
Endoplasmic
reticulum:
ribosomes are ABSENT.
o Rough
Endoplasmic
Reticulum:
Ribosomes are PRESENT.
ROUGH ENDOPLASMIC RETICULUM
 Rough ER – is termed rough because it is
studded with ribosomes (the
molecules
where proteins are made)
 This is an extensive organelle composed of a
greatly convoluted but flattish sealed sac
(cisternae) that is continuous with the nuclear
membrane.
 In Rough ER, site of protein synthesis is
destined for secretion.
 FUNCTIONS:
o The network of SER allows increased
surface area for the action or storage of key
enzymes and the products of these
enzymes
o The agranular endoplasmic reticulum is the
site of lipid synthesis (including oils,
phospholipids and steroids)
o It helps in metabolizing of carbohydrates
o It maintains regulation of calcium
concentration.
o In brain cells it synthesizes male and female
hormones
o In skeletal and cardiac muscle, smooth ER
is modified to form sarcoplasmic reticulum,
i.e. in muscle cells SER stores Ca+ ions and
is known as sarcoplasmic reticulum.
o SER also help in the biogenesis of golgi
complex n lysosomes
o Moreover, it helps in detoxification of drugs
and poisons, which is an important function.
o It serves as a transitional area for vesicles
that transport Endoplasmic Reticulum
products to various destinations.
o The network of SER allows for an increased
surface area to be devoted to the action or
storage of key enzymes and the products of
these enzymes.
o It helps in muscle contraction mechanism.
 FUNCTIONS:
o The rough er manufactures membranes
ans secretory proteins.
o In certain leukocytes (white blood cells),
the rough ER produces antibodies.
o In pancreatic cells, the rough ER
produces insulin.
 The membrane of the rough endoplasmic
reticulum forms large double membrane
sheets that are located near, and continuous
with, the outer layer of the nuclear envelope.
 It helps in protein translocation, protein
translocation is the process by which
peptides are transported across a membrane
bilayer.
Difference in Rough & Smooth Endoplasmic
Reticulum
 RER is the starting point of the biosysthesis
pathways of:
o Proteins
o Carbohydrate chains
o Phospholipids
that journey through the membranous
compartments of the cell.
2
4. ENDOPLASMIC RETICULUM
RER: ribosomes are present;
SER: ribosomes are absent; surface is smooth
DISEASES CAUSED BY MALFUNCTIONING OF
ER:
 ENDOPLASMIC RETICULUM STRESS:
o The endoplasmic reticulum (ER) is a
multifunctional organelle required for lipid
biosynthesis, calcium storage, and protein
folding and processing. A number of
physiological and pathological conditions,
as well as variety of pharmacological
agents, are able to disturb proper ER
function and thereby cause ER stress,
which severely impairs protein folding and
therefore poses the risk of proteotoxicity.
 Proteotoxicity in ER can cause following
diseases in organisms:
o Cystic Fibrosis
o Hepatic disorders
o Multiple sclerosis
o Sickle cell disease
o Retinitis pigmentosa
o Alzheimer's disease
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