
The endoplasmic reticulum (ER) in the cells of eukaryotic organisms is
an interconnected network of flattened, membrane-enclosed sacs or
tubes known as cisternae (latin “reservoir for liquid”)

Endoplasmic means “within the cytoplasm,” and reticulum latin “little
net”

The membranes of the ER are continuous with the outer membrane of
the nuclear envelope. Membrane of ER separates the lumen of ER
from cytoplasm but continuous with perinuclear space.

ER occurs in most eukaryotic cells except RBCs and spermatoza.

There are two types of ER, rough endoplasmic
(RER) and smooth endoplasmic reticulum (SER).

The outer (cytosolic) face of the RER is studded with ribosomes the
sites of protein synthesis. RER is prominent in hepatocytes where
active protein synthesis occurs

The SER lacks ribosomes and functions in lipid metabolism,
carbohydrate metabolism, and detoxification. SER is abundant in
mammalian liver and gonad cells.

The ER were first seen in 1945 by Keith R, Porter, Albert Claude,
Brody Meskers and Ernest F. Fullam
reticulum
1 Nucleus 2 Nuclear pore 3 Rough endoplasmic reticulum (RER) 4 Smooth
endoplasmic reticulum (SER) 5 Ribosomes on the rough ER 6 Proteins
that are transported 7 Transport vesicle 8 golgi apparatus 9 Cis face of
the Golgi apparatus 10 Trans face of the Golgi apparatus 11 Cisternae of
the Golgi apparatus
3-D MODEL OF ENDOPLASMIC
RETICULUM
The ER is a membranous network of cisternae
(sac-like structures) held together by the
cytoskeleton.
 The phospholipid membrane encloses a space,
the cisternal space (lumen), which is continuous
with the perinuclear space but separated from
the cytosol.
 The functions of the ER are the synthesis and
export of proteins and membrane lipids.
 The quantity of RER and SER in a cell can slowly
interchange, depending on the changing
metabolic activities of the cell.
 Transformation can include embedding of new
proteins in membrane as well as structural
changes. Changes in protein content may occur
without noticeable structural changes




The surface of RER is studded with ribosomes giving it a
"rough" appearance. The binding site of the ribosome to
RER is the transcolon.
Ribosomes are constantly being bound and released from
the RER membrane. A ribosome only binds to the RER once
a specific protein-nucleic acid complex forms in the
cytosol. This special complex forms when a free ribosome
begins translating the mRNA of a protein destined for the
secretary pathway.
The first 5-3 AAs polymerized encode a signal peptide, a
molecular message that is recognized and bound by
a signal recognition particle (SRP). Translation pauses and
the ribosome complex binds to the RER transcolon where
translation continues with the nascent protein forming into
the RER lumen. The protein is processed in the ER lumen
by an enzyme (signal peptidase), which removes the signal
peptide. Ribosomes at this point may be released back into
the cytosol; however, non-translating ribosomes stay
associated with transcolons.









There is no continuous membrane between the ER and golgi
apparatus, membrane-bound vesicles shuttle proteins between
these two compartments
Vesicles are surrounded by coating proteins COPI and COPII. COPII
directs vesicles to golgi and COPI brings them back to RER.
The RER works in concert with the golgi complex to target new
proteins in vesicles to their proper destinations.
A second method of transport out of the ER involves areas called
membrane contact sites, where the membranes of the ER and
other organelles are held closely together, allowing the transfer
of lipids and other small molecules
The RER manufactures lysosomal enzymes with a mannose-6phosphate marker added in the cis-Golgi network
RER manufactures secreted proteins (glycoproteins) glycosylation
occurs in lumen of RER.
RER is involved in synthesis of signal peptides, integral membrane
proteins, Rab proteins the key in targeting the membranes and
SNAP proteins involved in fusion event and glycosylation.
RER makes its own phospholipids and enzymes of ER membrane.
RER is component of endomembrane system.








SER is membranous network without ribosomes in the cytoplasm
near the periphery of cells.
SER is involved in several metabolic processes. It synthesizes
lipids, oils, phospholipids and steroids. Cells which secrete these
products, such as those in the testes, ovaries, and skin oil glands
have large amount of SER.
SER also carries out the metabolism of carbohydrates
SER detoxify drugs and poison in liver cells by adding hydroxyl
groups to drug molecules making them soluble and easier to flush
out from body. Drugs like phenobarbital and barbiturates
metabolized by SER by proliferation to increase the
detoxification enzymes. It may increase the tolerance towards
other drugs i.e. Barbiturates abuse decrease the effectiveness of
certain antibiotics.
SER involves in the attachment of receptors on cell membrane
proteins, and steroid metabolism.
In muscle cells, it regulates calcium ion concentration.
SER contains the enzyme glucose-6-phosphatase,
which
converts Glu-6-phosphate to glucose, a step in glucogenesis.
SER produces sex hormones in vertebrates and steroid hormones
that are secreted by adrenal glands.
 The
sarcoplasmic reticulum (SR), from the
Greek sarx, ("flesh"), is smooth ER found in
smooth and striated muscles. The only
structural difference between SR and SER is
the medley of proteins both bound to their
membranes and drifting within their lumens.
 This fundamental difference indicates their
functions: The SER synthesizes molecules,
while SR stores and pumps calcium ions. The
SR contains large stores of calcium, which
releases when the muscle cell is stimulated.
It plays a major role in excitation contraction
coupling.
 ER
is involved in the folding of protein
molecules by several ER proteins
in
cisternae and transport them in vesicles to
Golgi apparatus. Only properly folded
proteins are transported from the RER to the
Golgi apparatus.
 Disturbances in redox regulation, calcium
regulation, glucose deprivation, and viral
infection
or
the
over-expression
of
proteins can lead to ER stress, a state in
which the folding of proteins slows, leading
to an increase in unfolded proteins.
Protein transport
 Secretory proteins, glycoproteins, are moved
across the ER membrane. Proteins that are
transported by the ER called a signal sequence.
The N-terminus of a polypeptide chain contains a
few amino acids that work as signal sequence
which are removed when the polypeptide
reaches its destination. Proteins are packed into
transport vesicles and moved along cytoskeleton
toward their destination.
 The ER is also part of a protein sorting pathway.
It is, the transportation system of the eukaryotic
cell. The majority of its resident proteins are
retained within it through a retention motif. This
motif is composed of four amino acids at the end
of the protein sequence.
