Functions of Ribosomes

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 ╬▒subunit and ╬▓subunit.

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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