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
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