Biology Class: Non-Membranous Organelles and Filaments

Non-Membranous Organelles

• Ribosomes are non-membranous organelles involved in protein synthesis.
• Ribosomes stain basophilic due to rRNA.
• Ribosomes are composed of ribosomal RNA (rRNA) and proteins.
• Ribosomes are prokaryotic 70S, composed of 50S and 30S subunits.
• E/M: Individual ribosomes appear as electron-dense granules with small and large subunits that may form polyribosomes by messenger RNA.
• Types of ribosomes include free ribosomes, which are scattered and sometimes attached to mRNA forming polyribosomes (polysomes) in the cytoplasm; and attached ribosomes, which are attached to the endoplasmic reticulum.
• Free ribosomes function in synthesizing proteins for the cell.
• Attached ribosomes function in synthesizing proteins to be secreted outside the cell.

Cytoplasmic Filaments

• Cytoplasmic filaments are non-membranous thread-like structures.
• Types of cytoplasmic filaments include thin filaments, thick filaments, and intermediate filaments.
• Thin filaments, also called microfilaments (e.g., actin filaments), are slender rods (6-7 nm in diameter).
• Thin filaments function in microvilli movement, cleavage furrow in cell division, muscle contraction, and blood platelet clot retraction.
• Thick filaments are composed of myosin and are thicker than thin filaments (12-16 nm in diameter).
• Thick filaments function in muscle in association with actin filaments forming myofibrils.
• Intermediate filaments have a variable composition (Desmin filaments, Tonofilament, Vimentin filaments, Neuro-filaments, Glial filaments) and don't contract unlike thin and thick filaments.
• Intermediate filaments function in supporting and maintaining cell shape, providing adhesion between cells in epithelial tissues, and identifying tumor cells.

Microtubules

• Microtubules are pipe-like structures with uniform diameter, present throughout the cytoplasm.
• E/M: They appear as hollow tubules with walls made of 13 protofilaments composed of tubulin dimers. In cross section, they resemble tiny circles.
• Microtubules function in supporting and maintaining the cell shape, forming cilia and flagella, centeriole, and the mitotic spindle.
• Cytotoxic drugs like colchicine and vinblastine prevent microtubule assembly, useful in treating malignant diseases because they arrest mitosis.

Centrosome and Centrioles

• Centrosomes are specialized areas in the cytoplasm containing two centrioles oriented at right angles to each other.
• Centrioles function in cell division.
• Non-dividing cells position centriole pairs near the nucleus, in association with the Golgi apparatus.
• E/M: Centrioles appear as short cylinders with one open and one closed end. The wall consists of 27 microtubules, arranged in 9 bundles. Each bundle contains three microtubules (triplets), surrounded by fibrillar electron-dense material.
• Centrioles play a vital role in cell division (forming the mitotic spindle), and in ciliated epithelium, replicating into basal bodies, each of which gives rise to multiple cilia.
• Centrioles also display varying arrangement: nine triplets of microtubules and two central singles, or nine triplets with no central singles, or nine doublets of microtubules and two central singles, or nine doublets accompanied by no central singles, or nine doublets and one central single.

Cilia

• Cilia are hair-like projections from the free surface of certain epithelial cells (e.g., respiratory system, female genital system).
• Cilia function in moving a thin film of fluid or mucus over the cell surface, as seen in respiratory tracts, and modified cilia on the retina that detect light.
• L/M: Number of cilia can vary; it might range in the hundreds per cell.
• E/M: Cilia consist of a shaft (or axoneme), basal body, and rootlets. The shaft is surrounded by a cell membrane and contains nine peripheral microtubule doublets and two central microtubules. Basal bodies have a similar structure to centrioles (nine triplets of microtubules). Rootlets anchor the basal body to the surrounding cytoplasm.

Flagella

• Flagella are cytoplasmic processes (e.g., tail of a spermatozoon) similar to cilia but much longer. They have a whip-like motion for swimming.
• Flagella have nine peripheral microtubule doublets and one central singlet.
• Number: Usually, one flagellum per cell.

Additional Notes

• Five types of intermediate filaments exist (Desmin filaments, Tonofilament, Vimentin filaments, Neuro-filaments, Glial filaments), identified via immunochemical techniques, are each unique in composition and responsible for cell functions.

This quiz explores the structure and function of non-membranous organelles, specifically ribosomes and cytoplasmic filaments. Learn about the composition of ribosomes, their types, and their role in protein synthesis, as well as the various types of cytoplasmic filaments and their functions in the cell.