Cytoskeleton Overview

Non-membranous Cell Organelles (Cytoskeleton)

• The cytoskeleton is a complex, dynamic network of interlinking protein filaments found in the cytoplasm of all cells (including bacteria and archaea).
• In 1903, Nikolai Koltsov proposed the cytoskeleton determines cell shape.
• It extends from the cell nucleus to the cell membrane and consists of similar proteins in many organisms.
• Its primary function is to give the cell its shape and mechanical resistance, preventing deformation.
• It aids in intracellular transport of vesicles and organelles.
• The cytoskeleton can act as a template for the construction of a cell wall and specialized structures such as cilia and flagella.

Objectives

• Students should be able to list the components of the cytoskeleton.
• Students should be able to define the structure and function of microtubules.
• Students should be able to define the structure and function of microfilaments.
• Students should be able to define the structure and function of intermediate filaments.
• Students should be able to define the structure and function of centrioles.

Microtubules

• Microtubules are long, hollow, cylindrical and filamentous structures found in the cytoplasm of eukaryotic cells, absent in prokaryotes.
• Microtubules are found in thrombocytes (blood platelets) of humans and rats.
• They are 25 nm in diameter and 200 nm to 25 μm in length.
• Microtubules are major components of the cytoskeleton, and are involved in mitosis, cell motility, intracellular transport, and maintaining cell shape.
• They are composed of alpha- and beta-tubulin subunits assembled into linear protofilaments.
• These two units are arranged alternately in the protofilament.
• 13 protofilaments fold to form a cylindrical microtubule.
• Microtubules function as a supporting framework, giving shape to the cell, maintaining shape of long processes like cilia, flagella, and axons in nerve cells. They also create motion in cilia and flagella. Centrioles are identical in structure to basal bodies of cilia or flagella. Microtubules change cell shape during differentiation and help elongate certain cells.

Microfilaments

• Microfilaments, or actin filaments, are solid protein rods.
• Their diameters are about 7 nm, making them the smallest of cytoskeletal filaments.
• Actin is the protein that composes these filaments.
• Actin filaments serve as a track for the movement of myosin, a motor protein.
• Actin and myosin are abundant in muscle cells, and when actin and myosin filaments slide past each other, muscle contraction occurs.
• Microfilaments provide rigidity and shape to the cell.
• These filaments can rapidly assemble and disassemble, allowing cells to change shape and move.
• White blood cells use this ability for phagocytosis and movement.

Intermediate Filaments

• Intermediate filaments (IF) have a diameter of about 10 nm.
• IFs are composed of a variety of proteins expressed in different cell types.
• Over 50 different intermediate filament proteins are identified and classified into six groups based on similarities in amino acid sequences.
• Types I and II keratins form structural elements like hair, nails, and horns.
• Other types I and II keratins, or soft keratins, are found in the cytoplasm of epithelial cells.
• Type III IFs include vimentin, found in fibroblasts, smooth muscle cells, and white blood cells.
• Type IV intermediate filaments include neurofilament (NF) protein.
• Type V intermediate filaments are nuclear lamins.

Centrioles

• The centrosome is the barrel-shaped organelle in the cytoplasm near the nucleus.
• Two centrioles reside within the centrosome.
• Centrioles are crucial for cell division.
• During cell division, they move to opposite ends of the nucleus.
• As chromosomes condense for mitosis, centrioles form the areas from which mitotic spindles are made.
• Mitotic spindles attach to chromosomes and pull them to opposite ends of the cell, enabling cytokinesis.
• The centrosome's function is maintaining an equal distribution of chromosomes in daughter cells.
• Centrosomes are made of microtubule components arranged in sets of nine, each comprising three microtubules (9+3 arrangement).
• Centrioles are oriented at 90-degree angles to each other.
• Centrosomes are present in animal cells but not in plant cells.
• Centrioles are also known as basal bodies, as they are located at the base of cilia and flagella.

Explore the fascinating world of non-membranous cell organelles, focusing on the cytoskeleton. Learn about its structure, function, and importance in maintaining cell shape and facilitating intracellular transport. Perfect for biology students looking to deepen their understanding of cell biology.