Endoplasmic Reticulam

First observed by Porter, Claude and Fallum (1945) under EM.

Garnier earlier named it ergastoplasm.

Porter and Kallman (1952) coined the term ‘Endoplasmic reticulum’.

An elaborate network of membrane bound tubules highly concentrated in the endoplasm hence called endoplasmic reticulum.

Fig. Components of endoplasmic reticulum

Structure :

Fine branched vacuolar system extending from the nucleus through the cytoplasm to the margins of the cell.

In young meristematic cells, it form a continuous system extending from the nuclear envelope to the cell membrane and even to the cell wall and may even extend to the neighbouring cells.

In old cells it may be less prominent and is represented by discontinuous vesicles.

It occurs in the form of cisternae, tubules and vesicles.

The morphology of the ER depends upon the physiological and developmental stage of a cell. It is not a stable structure rather capable of being broken down and reconstructed.

The structure of the ER membrane is similar to that of the cell membrane in the sense that there is a lipid bilayer with peripheral and integral proteins. The membrane of the ER is thinner (50-60 Å) than the plasma membrane.

The main components of the endomembrane system are the ER with its three portions (The nuclear envelope, the RER and the SER) and the Golgi complex.

The RER has ribosomes attached to its outer surface and is particularly well developed in the basophilic regions of the cytoplasm (i.e., the ergastoplasm). The tubular and cisternal cavities of the ER may be closed, but more often contain material that has been synthesized on the ribosomes (e.g., protein) or by the enzymes present in the membranes (e.g., lipids, oligosaccharides).

The ribosomes are bounded by their 60S (large) subunits. There are two proteins (ribophorins I and II) in RER that are absent in SER. These are transmembrane glycoproteins that may correspond to the ribosomal binding sites.

The SER is devoid of ribosomes. Frequently, it forms a tubular network, and in the liver it is related to glycogen deposits (glycosomes) and peroxisomes.

Microsomes are small spherical vesicles formed from disrupted endoplasmic reticulum when a cell is homogenized and is subjected to differential centrifugation.

Functions :

Intracellular transport of substances.

Storage of glycogen, lipid, calcium.

Glycosidation or synthesis of glycoprotein, glycolipids etc.

Detoxification of various toxic, waste materials like drugs, pollutants, bile salts, degraded amino acids and fatty acids.

Provides skeletal framework to the cell and gives rise to membrane organelles like Golgi bodies, spherosomes.

Plays important role in cycling of membranes.