RIBOSOMES
Ribosomes are the organelles without
limiting membrane. These organelles are granular and small dot-like structures
with a diameter of 15 nm. Ribosomes are made up of 35% of proteins and 65% of
ribonucleic acid (RNA). RNA present in ribosomes is called ribosomal RNA
(rRNA). Ribosomes are concerned with protein
synthesis in the cell.
Types of Ribosomes
Ribosomes are of two types:
i. Ribosomes that are attached to
rough endoplasmic reticulum
ii. Free ribosomes that are
distributed in the cytoplasm.
Functions of
Ribosomes
Ribosomes are called ‘protein factories’ because of their role
in the synthesis of proteins. Messenger RNA (mRNA) carries the genetic code for protein synthesis
from nucleus to the ribosomes. The ribosomes, in turn arrange the amino acids
into small units of proteins. Ribosomes attached to rough endoplasmic reticulum
are involved in the synthesis of proteins such as the enzymatic proteins,
hormonal proteins, lysosomal proteins and the proteins of the cell membrane. Free
ribosomes are responsible for the synthesis of proteins in hemoglobin,
peroxisome and mitochondria.
CYTOSKELETON
Cytoskeleton is the cellular organelle
present throughout the cytoplasm. It determines the shape of the cell and gives
support to the cell. It is a complex network of structures with varying sizes.
In addition to determining the shape of the cell, it is also essential for the
cellular movements and the response of the cell to external stimuli. Cytoskeleton
consists of three major protein components:
1. Microtubule
2. Intermediate
filaments
3. Microfilaments.
1. Microtubules
Microtubules are the straight, hollow
and tubular structures of the cytoskeleton. These organelles without
the limiting membrane are arranged in
different bundles. Each tubule has a diameter of 20 to 30 nm. Length of microtubule
varies and it may be 1000 times more than the thickness. Structurally, the microtubules
are formed by bundles of globular protein called tubulin. Tubulin has two
subunits, namely αsubunit
and βsubunit.
Functions of
microtubules
Microtubules may function
alone or join with other proteins
to form more complex structures like cilia,
flagella or centrioles
and perform various functions.
Microtubules:
i. Determine the shape
of the cell
ii. Give structural
strength to the cell
iii. Act like conveyer belts which
allow the movement of granules, vesicles, protein molecules and
some organelles like mitochondria to
different parts of the cell
iv. Form the spindle
fibers which separate the chromosomes during mitosis
v. Are responsible for the movement of
centrioles and the complex cellular structures like cilia.
2. Intermediate
Filaments
Intermediate filaments
are the structures that form a network around the nucleus and extend to the periphery of
the cell. Diameter of each filament is about 10 nm. The intermediate filaments
are formed by ropelike polymers, which are made up of fibrous
protein.
Subclasses
of intermediate filaments
Intermediate filaments
are divided into five subclasses:
i. Keratins (in epithelial cells)
ii. Glial filaments
(in astrocytes)
iii. Neurofilaments
(in nerve cells)
iv. Vimentin (in many types of cells)
v. Desmin (in muscle
fibers).
Functions
of intermediate filaments
Intermediate filaments
help to maintain the shape of the cell. These filaments also connect the
adjacent cells through
desmosomes.
3. Microfilaments
Microfilaments are
long and fine threadlike structures with a diameter of about 3 to 6 nm. These
filaments are made
up of non-tubular contractile proteins called actin and myosin. Actin is
more abundant than myosin. Microfilaments are present throughout
the cytoplasm. The microfilaments present in ectoplasm contain only actin
molecules and those present in endoplasm contain both actin and myosin molecules.
Functions
of microfilaments
Microfilaments:
i. Give structural
strength to the cell
ii. Provide resistance to the cell
against the pulling forces
iii. Are responsible for cellular
movements like contraction, gliding and cytokinesis (partition of
cytoplasm during cell division).
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