microfilaments:
Microfilaments are cytoplasmic fibrillar structures made up of actin.
They are found in the cytoplasm of all eukaryotic cells.
They are found in abundance in muscle cells, microvilli, etc.
They have a diameter from 5-7 nm.
They form a cytoskeleton. They do a variety of functions such as giving shape to cells, muscle contraction, amoeboid movement, streaming movement, etc.
The microfilaments are made up of protein sub units called actin. Actin is a globular protein.
Actin exists in two forms, namely a monomeric form called G-actin(G-Globular) and Filamentous form called F-actin(F-filament).
The G-actin has a well defined polarity. It has two ends like an arrow. One end is used for the assembly of actin molecules and it is the fast growing end. Hence it is called plus end.
The barbed end is used for the assembly of actin molecules and it is the slow growing end. Hence it is called minus end.
The G-actin molecules aggregate together in a linear fashion to form F-actin. The process of aggregation of G-actin is called polymerization.
The F-actin is the component of microfilament. A microfilament is made up of two F-actin filaments, helically coiled. Hence a microfilament is a double helix.
Many double helices are packed together to form a bundles. Each double helix is associated with two other proteins called troponin and tropomyosin. Tropomyosins are filaments located at the grooves of the helix. Troponin are oval in shape and are located at the ends of tropomyosin. Troponin and tropomyosin are found in the microfilaments of muscles.
Myosin is another microfilament found in muscle cells. It has a diameter of 10nm. It is a protein. It is in the form of filaments. Each myosin filament has two heads and a tail.
The myosin filaments are packed together to form a bundle. Many such bundles are arranged heads to trails to form a long fibre. The heads are directed towards on the myosin bundle.
Functions of Microfilaments
- Shape: Microfilaments function as a cytoskeleton. They give shape to the cells.
- Muscle contraction: Actin and myosin filaments bring about muscle contraction.
- Amoeboid Movement: Amoeboid movement is brought about by actin and myosin filaments.
- Cytoplasmic streaming: The streaming movement of cytoplasm in Chara is brought about by actin filaments.
- Cytokinesis: During cell division, cell constriction is made possible by actin and myosin filaments. They form a belt-like bundle called contractile ring.
- Acrosomal filament: During fertilization, the acrosome of sperm produces a filament called acrosomal filament. It is produced by actin molecules.
- RBC: The biconcave shape of RBC is given by actin filaments.
- Endocytosis and Exocytosis: Microfilaments involve in cell functions like endocytosis and exocytosis.
