Endoplasmic reticulum is a complex membrane lined network of flattened sacs, tubules and vesicles that runs throughout the cytoplasm of eukaryotic cells from plasma membrane to nuclear envelope. It absent in Prokaryotes but present in all the eukaryotes except germinal cells and mature mammalian erythrocytes.
Types of endoplasmic Reticulum.
(a)Smooth Endoplasmic Reticulum
(b) Rough Endoplasmic Reticulum
(a) Smooth Endoplasmic Reticulum.
Smooth ER also plays a large part in detoxifying a number of organic chemicals converting them to safer water-soluble products.
Large amounts of smooth ER are found in liver cells where one of its main functions is to detoxify products of natural metabolism and to endeavour to detoxify overloads of ethanol derived from excess alcoholic drinking and also barbiturates from drug overdose
Function of SER (Smooth Endoplasmic Reticulum)
- Ascorbic acid synthesis occurs over SER.
- Retinal pigments are formed from vitamin A over SER in retinal cells.
- Enzymes located over SER of liver cells take part in breakdown of glycogen.’
- SER take part in fat synthesis inside adipose cells.
- SER take part in synthesis of steroid hormones, e.g., testosterone in testis, estrogens in ovary.
(b) Rough Endoplasmic Reticulum.
This is an extensive organelle composed of a greatly convoluted but flattish sealed sac that is continuous with the nuclear membrane. It is called ‘rough’ endoplasmic reticulum because it is studded on its outer surface (the surface in contact with the cytosol) with ribosomes.
Functions of RER
- Smooth Endoplasmic Reticulum is formed from RER through loss of ribosomes.
- It possess ribophorins for holding ribosomes over it.
- Luminal side of RER possess enzymes for processing polypeptide synthesised by attached ribosome.
- RER has a large surface area which provides proper space to ribosomes for their activity without interference from others.
- Secretory, lysosomal and membrane proteins formed by ribosome attached to RER enter its lumen for intracellular and extracellular transport.
- Zymogens of lysosome enzymes are synthesised by RER.