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.