Cell Biology: Microtubules and Centrioles

Microtubule Functions

• Microtubules are important for cell division, forming spindle fibers that help with chromosome separation.
• Microtubules are involved in moving vesicles and organelles within the cell, like Golgi vesicles and those involved in exocytosis.
• They contribute to cell shape and support, often found near the cell surface membrane.
• They are a component of centrioles.
• They contribute to the movement of cilia and flagella.

Centrioles

• Found only in animal cells.
• Occur in pairs, located near the nucleus, at right angles to each other.
• Measured at 0.2 micrometers in diameter and 0.3-0.5 micrometers in length, visible only with an electron microscope.
• Composed of nine triplets of microtubules held together by connecting fibers.
• Tubulin subunits of microtubules are arranged in a helical pattern along the centriole.
• Play a role in cilia production.
• Located at the base of cilia and flagella as basal bodies.
• Act as microtubule organizing centers (MTOCs).

Microvilli

• Finger-like projections of the cell surface membrane.
• Found only in animal cells, not in plant cells due to the cell wall barrier.
• Increase the surface area of the cell membrane.
• Involved in the absorption or secretion of substances.

Cilia and Flagella

• Short and numerous per cell (cilia) or long and one or two per cell (flagella).
• Identical structures, whip-like appendages found in eukaryotic cells.

Lysosomes

• Membrane-bound organelles containing hydrolytic enzymes.
• Involved in:
  • Digestion of worn-out organelles (autophagy): An old organelle is enveloped in a membrane, a lysosome fuses with the vacuole, and the hydrolytic enzymes digest the material.
  • Exocytosis: Release of hydrolytic enzymes outside the cell for extracellular digestion.
  • Autolysis: Self-digestion of the entire cell by the release of lysosome enzymes resulting in cellular breakdown. This is a normal process in cell turnover.

Cytoskeleton

• A network of fibrous protein structures providing support to the cell.
• Two types:
  • Microtubules
  • Microfilaments
• Functions:
  • Cell support
  • Cell shape maintenance
  • Organelle movement within the cell

Microtubules

• Fine, unbranched, hollow tubes, about 25 nm in diameter and several micrometers in length.
• Only visible under an electron microscope.
• Composed of tubulin protein (α-tubulin and β-tubulin).
• Tubulin molecules form dimers, which assemble into long "protofilaments".
• A microtubule contains 13 tubulin subunits in a transverse section, arranged helically.
• Assembly and disassembly of microtubules are controlled by microtubule organizing centers (MTOCs).
• Involved in a range of cellular functions beyond mechanical support.

Smooth Endoplasmic Reticulum (SER)

• A network of interconnected, membrane-bound sacs and tubules.
• Functions:
  • Synthesis of lipids and steroids.
  • Detoxification of drugs and toxins.
  • Calcium storage and release.
  • In the liver, SER is involved in drug metabolism.

Ribosomes

• Tiny organelles, about 20 nm in diameter.
• Visible under an electron microscope.
• Can be attached to the endoplasmic reticulum (forming Rough ER) or free in the cytoplasm.
• Composed of two subunits: a small subunit and a large subunit.
• Subunits are made of rRNA molecules and protein molecules.
• Two types:
  • 70S ribosomes: Found in prokaryotes, smaller and lighter.
  • 80S ribosomes: Found in eukaryotic cells, larger and heavier. Mitochondria and chloroplasts contain 70S ribosomes due to their prokaryotic origins.
• Function: Site of protein synthesis.

Mitochondria

• Present in all eukaryotic cells.
• Function: The site of aerobic respiration (energy production).
• Number of mitochondria varies depending on the cell's energy requirements.
• Structure:
  • Oval, rod-shaped, or spherical.
  • Surrounded by two membranes forming an envelope.
  • Inner membrane is folded into cristae to increase surface area.
  • Cristae contain stalked elementary particles (ATPase, stalk, and base).
  • Matrix contains:
    • Circular DNA molecule.
    • 70S ribosomes.

Explore the functions of microtubules and the structure of centrioles in animal cells. This quiz covers their roles in cell division, vesicle transport, and the production of cilia and flagella. Test your knowledge on these essential components of cellular organization.