Cell Biology PDF Notes

# Cytoskeleton - **Microtubules** - Non-branching and rigid hollow fine tubes formed of a protein called tubulin. - Difficult to see by light microscopy except by using special stains. - Diameter: 20-25 nm, appear as fine tubules. - Length: can be changed by adding or removing tubulin molecu...

# Cytoskeleton - **Microtubules** - Non-branching and rigid hollow fine tubes formed of a protein called tubulin. - Difficult to see by light microscopy except by using special stains. - Diameter: 20-25 nm, appear as fine tubules. - Length: can be changed by adding or removing tubulin molecules at their end. - Functions: - Maintenance of cell shape, e.g. platelets. - Intracellular vesicular transport. - Formation of the mitotic spindle during cell division. - Elongation and movement of the cell. - Formation of centrioles, cilia, and flagella. - **Centrioles** - Derived from the microtubules. - Responsible for cell division. - Absent in non-dividing cells, e.g. red blood cells and nerve cells. - Visible by light microscopy as dark paired short rods or dots after staining with iron hematoxylin. - Appear as two short hollow cylinders (0.5 x 0.2 µm) perpendicular to each other. - The wall of each centriole is formed of 9 bundles. - Each bundle consists of 3 microtubules (triplet). - Site: Usually present near the nucleus in an area called the centrosome. - Functions: - Share in the formation of the mitotic spindle during cell division. - Responsible for the formation of cilia and flagella. # Cilia & Flagella - **Cilia:** - Motile hair-like processes projecting over the cell surface, capable of moving fluids and particles along the surface. - Appear as short, fine, hair-like structures arising from the free surface of the cell, giving it a brush border. - Each cilium consists of three parts: - A finger-like projection over the cell surface covered by cell membrane. - Contains nine doublets and two single microtubules (9 x 2 + 2 = 20). - A basal body which is similar to a centriole in structure, formed of nine triplets (9 x 3 = 27). - Formed of nine single microtubules (9 x 1 = 9), which are present below the basal body. - Functions: - Move in wave-like manner to move secretions or particles over the tissue surface, e.g. in the respiratory system and female genital system. - Can modify and act as receptors for stimuli, e.g. in rods and cones in the retina they receive light. - **Flagella:** - A long, whip-like filament extending from the cell, forming a tail. - Example: Sperm tail. # Development Of Cilia - Cilia develop from a centriole, first by replication of it to give “baby centrioles”, one for each cilium. Each baby centriole moves toward the surface of the cell to become the basal body from which a cilium can grow. - The basal body is similar to a centriole in structure: nine triplets of microtubules. - Two inner microtubules (doublet) grow upwards from each triplet, pushing in front of them the free surface of the cell membrane. - At the same time, two single microtubules grow in the center (9 x 2 + 2 = 20), this part is called the shaft of the cilium. - The third outer microtubule of each triplet of the basal body goes downwards forming the rootlets of the cilium, which fix it (9 x 1 = 9). # The Structure Of Centrioles - They have nine triplets of microtubules. - They have radial connecting fibers. # Filaments - Minute threads that act as part of the cytoskeleton, which maintains the shape of the cell. - **Microfilaments** - Also known as thin filaments. - Formed of the protein called **actin**. - Diameter: 6 nm – Found in: Muscles and microvilli. - Function: Form a supporting network within the cell that helps to maintain its shape. - **Intermediate filaments:** - Diameter: about 8 - 10 nm. - Found in: There are about 50 different types of intermediate filaments in humans. - **Cytokeratin:** Present in epithelial cells. - **Vimentin:** Present in connective tissue and muscles. - **Desmin:** Present in muscles. - **Neurofilaments:** Present in nerve cells. - **Glial filaments:** Present in glial cells. - **Thick filaments:** - Diameter: about 15 nm. - Formed of the protein called **myosin**. - Found in: Skeletal muscle. # Cell Inclusions - Non-living, temporary structures not essential for the vitality of the cell. - They are no more than stored substances within some cells, e.g. glycogen, fat, and pigments. - **Stored Food** - Carbohydrates: - Stored in the form of glycogen. - Found in liver and muscles. - Cannot be demonstrated by hematoxylin and eosin (H&E) as it’s water-soluble. - Can be seen if stained by special stains such as Best’s Carmine (red) or Periodic Acid Schiff (PAS) (purple). - Lipids: - Stored in the form of small droplets or large globules. - Found in fat cells (the main site for storage of lipids), other cells like liver cells may contain fat. - Cannot be demonstrated by hematoxylin and eosin (H&E) as it dissolves in xylol (a solvent). - Can be stained orange by Sudan III and black by Sudan Black. - **Pigments** - **Endogenous:** Produced by the cell: - **Hemoglobin:** Present in red blood cells to carry gases. - **Melanin:** Gives the skin and hair their color. - **Lipofuscin:** Accumulated residual bodies in long-lived cells like nerve cells and cardiac muscles. - **Exogenous:** Enter the cell from outside: - **Carotene:** Found in carrots and gives fats in cells their color. - **Dust and carbon particles:** Blacken the lungs.