Endoplasmic Reticulum

Endoplasmic Reticulum

The outer membrane of the nuclear envelope
is continuous with a cytoplasmic membranous system called endoplasmic reticulum (ER) . The largest of the internal membranes is the endoplasmic reticulum (ER). Endoplasmic means “with in the cytoplasm,” and 

reticulum is Latin word for “a little net.” Like the plasma membrane, the 

ER is composed of a phospholipid bilayer embedded with proteins. 

It weaves in sheets through the interior of the cell, creating a series 

of channels between its folds (figure below). Of the many compartments in eukaryotic cells, the two largest are the inner region of the 

ER, called the cisternal space or lumen, and the region exterior to 

it, the cytosol, which is the fluid component of the cytoplasm containing dissolved organic molecules such as proteins and ions.    The space between the membranes of
the nuclear envelope communicates with the space
between the ER membranes (cistetna, pl., cisternae).

The ER membranes may be covered on
their outer surfaces with ribosomes and are thus
designated rough ER, or they may lack ribosomal
covering and be called smooth ER. Ribosomes on
rough ER synthesize polypeptides that enter the ER
cisternae or membrane and are destined for incorporation into the plasma membrane for
export from the cell, or they are bound for the lysosomes. Smooth ER functions in synthesis of lipids
and phospholipids.

Rough Endoplasmic Reticulum

"Rough ER is a site of protein synthesis"

The rough ER (RER) gets its name from its surface appearance, 

which is pebbly due to the presence of ribosomes. The RER is 

not easily visible with a light microscope, but it can be seen 

using  an electron microscope. It appears to be composed of 

flattened sacs, the surfaces of which are bumpy with ribosomes 

(see figure above)

The proteins synthesized on the surface of the RER are destined to be exported from the cell, sent to lysosomes or vacuoles or embedded in the plasma membrane. These proteins enter the cisternal space as a first step in the 

pathway that will sort proteins to their eventual destinations. This 

pathway also involves vesicles and the Golgi apparatus.The sequence of the protein being synthesized 

determines whether the ribosome assembling it will become associated with the ER or remain a cytoplasmic ribosome.

In the ER, newly synthesized proteins can be modified by 

the addition of short-chain carbohydrates to form glycoproteins.

Those proteins destined for secretion are separated from other 

products and later packaged into vesicles. The ER also manufactures membranes by producing membrane proteins and phospholipid molecules. The membrane proteins are inserted into the ER’s 

own membrane, which can then expand and pinch off in the form 

of vesicles to be transferred to other locations.The whole process is discussed in figure below.

Figure:    Protein transport through the endomembrane 

system. Proteins synthesized by ribosomes on the RER are
translocated into the internal compartment of the ER. These
proteins may be used at a distant location within the cell or
secreted from the cell. They are transported within vesicles that
bud off the rough ER. These transport vesicles travel to the cis
face of the Golgi apparatus. There they can be modified and
packaged into vesicles that bud off the trans face of the Golgi
apparatus. Vesicles leaving the trans face transport proteins to
other locations in the cell, or they fuse with the plasma membrane,
releasing their contents to the extracellular environment.

Smooth Endoplasmic Reticulum

Regions of the ER with relatively few bound ribosomes are referred 

to as smooth ER (SER). The SER appears more like a network of 

tubules than the flattened sacs of the RER. The membranes of the 

SER contain many embedded enzymes. Enzymes anchored within 

the ER, for example, catalyze the synthesis of a variety of carbohydrates and lipids. Steroid hormones are synthesized in the SER 

as well. The majority of membrane lipids are assembled in the 

SER and then sent to the parts of the cell that need membrane 

components.

                         The SER is used to store Ca2+ in cells. This keeps the 

cytoplasmic level low, allowing Ca2+ to be used as a signaling 

molecule. In muscle cells, for example, Ca2+ is used to trigger 

muscle contraction. In other cells, Ca2+ release from SER stores is 

involved in diverse signaling pathways.

           The ratio of SER to RER depends on a cell’s function. In 

multicellular animals such as ourselves, this ratio varies greatly. 

Cells that carry out extensive lipid synthesis, such as those in the 

testes, intestine, and brain, have abundant SER. Cells that synthesize proteins that are secreted, such as antibodies, have much more 

extensive RER.

                             Another role of the SER is the modification of foreign substances to make them less toxic. In the liver, the enzymes of the 

SER carry out this detoxification. This action can include neutralizing substances that we have taken for a therapeutic reason, such 

as penicillin. Thus, relatively high doses are prescribed for some 

drugs to offset our body’s efforts to remove them. Liver cells have 

extensive SER as well as enzymes that can process a variety of 

substances by chemically modifying them.