ENDOPLASMIC RETICULUM

ENDOPLASMIC RETICULUM

Endoplasmic reticulum is a network of tubular and microsomal vesicular structures which are interconnected with one another. It is covered by a limiting membrane which is formed by proteins and bilayered lipids. The lumen between nucleus and cell membrane by connecting the cell membrane with the nuclear membrane.

Types of Endoplasmic Reticulum

Endoplasmic reticulum is of two types, namely rough endoplasmic reticulum and smooth endoplasmic reticulum. Both the types are interconnected and continuous with one another. Depending upon the activities of the cells, the rough endoplasmic reticulum changes to smooth endoplasmic reticulum and vice versa.

Rough Endoplasmic Reticulum

It is the endoplasmic reticulum with rough, bumpy or bead-like appearance. Rough appearance is due to the attachment of granular ribosomes to its outer surface. Hence, it is also called the granular endoplasmic reticulum. Rough endoplasmic reticulum is vesicular or tubular in structure.

Functions of Rough Endoplasmic Reticulum

1. Synthesis of proteins

Rough endoplasmic reticulum is concerned with the synthesis of proteins in the cell. It is involved with the

synthesis of mainly those proteins which are secreted from the cells such as insulin from βcells

of islets of Langerhans in pancreas and antibodies from B lymphocytes. Ribosomes arrange the amino acids into small units of proteins and transport them into the rough endoplasmic reticulum. Here, the carbohydrates are added to the protein units forming the glycosylated proteins or glycoproteins, which are arranged in the form of reticular vesicles. These vesicles are transported mainly to Golgi apparatus for further modification and processing. Few vesicles are transported to other cytoplasmic

organelles.

2. Degradation of worn-out organelles

Rough endoplasmic reticulum also plays an important role in the degradation of worn-out cytoplasmic organelles like mitochondria. It wraps itself around the wornout organelles and forms a vacuole which is often called the autophagosome. Autophagosome is digested by lysosomal enzymes.

Smooth Endoplasmic Reticulum

It is the endoplasmic reticulum with smooth appearance. It is also called agranular reticulum. It is formed by many interconnected tubules. So, it is also called tubular endoplasmic reticulum.

Functions of Smooth Endoplasmic Reticulum

1. Synthesis of non-protein substance

Smooth endoplasmic reticulum is responsible for synthesis of non-protein substances such as cholesterol

and steroid. This type of endoplasmic reticulum is abundant in cells that are involved in the synthesis of

lipids, phospholipids, lipoprotein substances, steroid hormones, sebum, etc. In most of the other cells, smooth endoplasmic reticulum is less extensive than the rough endoplasmic reticulum.

2. Role in cellular metabolism

Outer surface of smooth endoplasmic reticulum contains many enzymes which are involved in various metabolic processes of the cell.

3. Storage and metabolism of calcium

Smooth endoplasmic reticulum is the major site of storage and metabolism of calcium. In skeletal muscle

fibers, it releases calcium which is necessary to trigger the muscle contraction.

4. Catabolism and detoxification

Smooth endoplasmic reticulum is also concerned with catabolism and detoxification of toxic substances

like some drugs and carcinogens (cancer-producing substances) in the liver