Endoplasmic Reticulum

Electron microscopic study of sectioned cells has revealed the presence of a three dimensional network of sac-like and tubular cavities called cisternae bounded by a unit membrane inside the cell. Since these structuresare concentrated in the endoplasmic portion of the cytoplasm, the entire organisation is called the endoplasmic reticulum. This name was coined by Porter in 1953. The occurrence of ER varies from cell to cell. They are absent in erythrocytes, egg cells and embryonic cells. The ER is the site of specific enzyme controlled biochemical reactions. Its outer surface carries numerous ribosomes. The presence of ribosomes gives a granular appearance. In this condition ER is described as rough endoplasmic reticulum (RER). RER is the site of synthesis of proteins. Ribosomes are absent on smooth endoplasmic reticulum (SER). SER is concerned with lipid metabolism.

Lamellar form or Cisternae :
    These are long, flat, sac like tubules. Their diameter is about 40-50 µm. The RER has a synthetic role. It is mostly seen in cells of pancreas, notochord and brain.

Vesicles :
    These are oval, vacuolar structures. Their diameter is about 25-500 µm. They occur in most of the cells.

Tubules :
    These are branched structures forming the reticular system along with the cisternae and vesicles. They have a diameter of 50-190 µm. They occur in almost all cells.

Functions :
1. It provides skeletal framework to the cell.
2. It facilitates exchange of molecules by the process of osmosis, diffusion and active transport.
3. Enzymes of ER control several metabolic activities.
4. They serve as intracellular transporting system.
5. It conducts intra-cellular impulses.
6. It helps to form nuclear membrane after cell division.
7. SER synthesises lipids.

Golgi apparatus :
   The Golgi apparatus was discoverd by an Italian neurologist, Camillo Golgi in 1873.The Golgi apparatus occurs in almost all animal cells except red blood cells. Animal cells usually have a single Golgi apparatus. Some cells have more of Golgi apparatus. In most of the ectodermal and endodermal cells it occurs in between the nucleus and the periphery. In nerve cells it occupies a circum-nuclear position.
The simplest unit of the Golgi apparatus is the cisterna. A cisterna is about 1 µm in diameter. It has a membrane bound space. This space accumulates secretions. Numerous such cisternae are associated with each other and appear in a lamellar arrangement. In the lamellar arrangement the space between each cisterna is 20-30 nm. A group of these cisternae is called the dictyosome. A group of dictyosomes constitute the Golgi apparatus.Typically a Golgi apparatus appears as a complex arrangement of interconnecting tubules, vesicles and cisternae.
The Golgi apparatus is the site of synthesis of biochemicals.They also collect proteins and lipids made in the ER and add additional substances.