What Is Endoplasmic Reticulum (ER)?

The largest internal membrane system in a cell is the endoplasmic reticulum. Endoplasmic means “within the cytoplasm” and reticulum in Latin means “a little net”.

This endo membrane system packs the interior of eukaryotic cells and is not visible under light microscope. This was discovered independently by Porter and Thompson in 1945 and the name was given to it by Porter.

This is a compartment comprising of a network of interconnected, closed, membrane bound vesicles dividing the cells into compartments, channeling the passage of molecules through the interior of the cells and providing surfaces for the synthesis of lipids and some proteins.

Often it constitutes more than half of the total membrane in a cell, although the extent of endoplasmic reticulum varies from cell to cell. Metabolically active cells like cells of pancreas, liver and those responsible for production of hormones and antibodies have extensive network of ER.

The storage cells like the adipose cells have simple ER only in the form of tubules. ER is absent in eggs, mature erythrocytes, embryonic cells and prokaryotic cells. In spermatocytes ER is reduced and present in the form of a few vesicles. Of the many compartments in eukaryotic cells the larger two are the inner region of the ER called cisternal space, and the region exterior to it is the cytosol.

The endoplasmic reticulum is composed of regions with ribosome and regions without ribosome. The regions with ribosome are called Rough Endoplasmic Reticulum or RER and those without ribosome are called Smooth Endoplasmic Reticulum or SER. The RER and SER may be continuous with one another, with plasma membrane and with nuclear envelope. SER is also called granular endoplasm reticulum as it lacks ribosome on its membranes.

The synthesis of fatty acids and phospholipids occurs in SER. Although many cells have very little SER it is abundant in hepatocytes, interstitial cells, adreno- cortical cells, muscle cells, retinal cells etc. Enzymes in the smooth ER of hepatocytes modify or detoxify hydrophobic chemicals such as pesticides and some carcinogens by chemically converting them into more water soluble conjugated products that can be excreted from the body.

In muscle cells smooth ER known as Sarcoplasmic reticulum stores Ca++ to be released during muscle contraction. In brain cells smooth ER is associated with the synthesis of male and female hormone. It is also involved in production and storage of steroids.

It also stores ions and is involved in the synthesis of phospholipids, cholesterol, glycogen, ascorbic acid and visual pigments from vitamin A. The SER is typically a network of tubules and serves as a transition zone from where synthetic products of RER are passed on to Golgi after being packed in transport vesicles.

The RER is responsible for manufacturing and transport of membrane bound a secretary proteins. In leukocytes the RER produces antibodies. In pancreatic cells RER is involved with Insulin production.

The RER is composed of a series of flatten sacs. This flattened sac like appearance is due to the presence of transmembrane proteins called Ribophorin I and Ribophorin II. Ribophorins assist the attachment o ribosome to the RER membrane.

A protein translocon is present near the site of attachment of ribosome. This is a passage for the new protein synthesized at the attached ribosome to enter into the lumen of RER.

The proteins synthesized by attached ribosome on the surface of RER are destined to be exported from the cell or to be sent to the lysosomes or vacuoles or to be embedded in the membrane. All these proteins contain a special amino acid sequences called signal sequences or signal peptide.

At first the free ribosome synthesizes the signal sequences of these proteins in the cytosol. Then, a multi subunit Ribonucleoprotein called signal receptor protein (SRP) present in cytosol recognizes the signal.

The SRP then binds to ribosome and stops further protein synthesis. The SRP – Ribosome complex is then guided to the surface of RER. The ribosome is then bound to Rough ER membrane through interaction with Ribophorin I and II present in the membrane of RER.

The SRP is then removed through the hydrolysis of GTP. Now further protein synthesis begins and the protein enters into the ER lumen through the passage or translocon present near the Ribophorin. These proteins are then added with oligosaccharides and sent to Goligi for further modifications.

Functions

1. Formation of a skeletal frame work inside the cytoplasm.

2. RER is responsible for manufacturing and transport of membrane bound and secretary proteins.

3. In leukocytes RER produces antibodies and in pancreas it is involved with insulin production.

4. Glycosylation of proteins begins in RER.

5. Synthesis of fatty acids, phospholipids occur in SER

6. Enzymes present in SER of hepatocytes detoxify hydrophobic chemicals.

7. In muscle cells, SER known as ‘Sarcoplasmic reticulum’ sequesters Ca++ for muscle contraction.

Structure: The Golgi is composed of membrane bound sacs known as cisternae or saccules, tubules arising from the periphery of cisternae and vesicles surrounding the main cisternae. About 5 to 8 sacs form a cisterna, although, up to sixty sacs have been reported.

The tubules form a highly branched anastomosing network. The vesicles bud off from the cisternae is of two types: (i) Smooth vesicles or secretory vesicles and (ii) coated vesicles.

A Golgi apparatus has a distinct polarity with a front and a back having distinct membrane compositions at the two ends. The front or receiving end or formation is convex and is called the cis face and the back or maturing end is concave and called the Trans face.

The cis face is located towards the Endoplasmic reticulum Materials from ER move to cis face of Golgi packed in transport vesicles that bud from ER. These ER-synthesized molecules then move from cisterna to cisterna Golgi until they reach the Trans face where they are discharged outside in secreto vesicles.

As they pass through the Golgi apparatus the materials are variously modified. Mostly addition of short sugar chains (oligosaccharides) to proteins occur resulting in the formation of glycoprotein. When sugar chains are added to lipids or sugar chains of lipids are modified it results in the formation of glycol lipids.

Inside the Golgi apparatus glycol lipids and glycol proteins made in the ER are modified by enzymatic cleavage of one or more sugars. The finished products are finally packed in membrane bound vesicles that pinch off from the Trans cisternae and move to other locations in the cell or out of the cell. For this reason Golgi apparatus is also known as Delivery system of the cell.

Function

Some of the major functions of Golgi apparatus are:

1. Modifications of proteins and lipids to Glycoproteins and Glycolipids.

2. Synthesis of Proteoglycans molecules present in the extra cellular matrix of animals, and are also a major site of carbohydrate synthesis.

3. Polysaccharides like glycosaminoglycans (GAGS) are synthesized in Golgi.

4. Phosphorylate certain molecules with Golgi resident kinases like Casein kinase. One molecule that is phosphorylated in Golgi is Apolipoprotein found in blood serum as VLDL (Very low density lipid).

5. Synthesis and secretion of cell wall materials like pectin and other carbohydrates.

6. Secretion of gum and mucilage.

7. Formation of lysosomes.

8. Transformation of one type of membrane to another.

9. Secretes antibodies, neuro transmitters.

10. Sorting and transporting proteins to different intra cellular destination.