Cytoskeleton Structure: Microtubules

Cytoskeleton Structure

• Comprises three types of fibers: microfilaments, microtubules, and intermediate filaments.

Microtubules

• Small, hollow, round tubes with a diameter of about 24 nanometers.
• Composed of tubulin, a protein.
• Thirteen tubulins link together to form a single microtubule.
• Highly dynamic, capable of rapid changes.
• Can continuously grow or shrink.
• Transporting cellular materials.
• Facilitating the division of chromosomes during cell division.

Microfilaments

• Thread-like protein fibers, 3-6 nm in diameter.
• Composed of the protein actin.
• Responsible for muscle contraction, found in muscle cells.
• Play a role in various cellular movements, including cytokinesis, contraction, and gliding.

Intermediate Filaments

• Diameter: About 10 nm.
• Provide tensile strength to the cell.
• Facilitate the formation of keratins and neurofilaments.

Motor Proteins

• Include kinesin, dyneins, and myosin.
• Kinesin: move along the microtubules carrying the cellular components.
• Dyneins: pull the cell organelles towards the nucleus.
• Myosin: interact with actin protein, responsible for muscle contractions, and perform cytokinesis, exocytosis, and endocytosis.

Centrosome

• Consists of four phases: G1, G2, mitotic, and late mitotic phase.
• Helps in cell division.
• Maintains the chromosome number during cell division.
• Stimulates changes in the shape of the cell membrane by phagocytosis.
• In mitosis, helps in organizing the microtubules ensuring that the centrosomes are distributed to each daughter cell.

Cytoskeleton Function

• Maintains the shape and internal organization of the cell.
• Provides mechanical support.
• Present in eukaryotic cells, prokaryotic cells, and archaeans.
• Composed of a network of fibrous structures.
• Provides structural shape and support to the cell.
• Organizes organelles within the cell.
• Facilitates the intracellular transport of molecules.
• Essential for cell division processes.
• Participates in cell signaling pathways.

Plasma Membrane

• Outermost envelope-like membrane surrounding the cell and its organelles.
• Also known as the phospholipid bilayer.
• Present in both prokaryotic and eukaryotic cells.
• Functions as a boundary.
• Selectively permeable, regulating the entry and exit of specific substances.
• Acts as a connecting system, facilitating communication between the cell and its environment.

Plasma Membrane Structure

• Composed of carbohydrates, phospholipids, proteins, and conjugated molecules.
• Thickness ranges from 5 to 8 nanometers.
• Structure: flexible, lipid bilayer surrounding and containing the cell's cytoplasm.
• Described as the fluid mosaic model based on molecular arrangement.

Fluid Mosaic Model

• Proposed in 1972 by biologists Garth L. Nicolson and Seymour Jonathan Singer.
• Explains the detailed structure of the plasma membrane in eukaryotic cells.
• Components such as phospholipids, proteins, carbohydrates, and cholesterol create a fluid appearance in the plasma membrane.

Plasma Membrane Function

• Functions as a physical barrier between the external environment and the inner cell organelles.