Cytoskeleton, Cell Movement, and Cell Junctions Notes

The Cytoskeleton, Cell Movement, and Cell Junctions

Cytoskeleton

  • The cytoskeleton is a network of protein fibers crisscrossing the cytoplasm of a cell.
  • It maintains cell shape and anchors or assists in the movement of organelles.
  • Three major types of protein fibers:
    • Microtubules
    • Actin filaments
    • Intermediate filaments
Microtubules
  • Cylinders containing rows of a protein called tubulin.
  • Regulation of assembly is controlled by the centrosome (microtubule organizing center).
  • Functions:
    • Help maintain cell shape.
    • Act as tracks along which organelles move.
    • Form spindle fibers during cell division to assist in chromosome movement.
Actin Filaments
  • Long, thin fibers made of the protein actin.
  • Usually occur in bundles or groups.
  • Involved in cell movement, such as the shortening and extending of microvilli.
Intermediate Filaments
  • Intermediate in size between microtubules and actin filaments.
  • Structure and function vary depending on the cell type.

Cilia and Flagella

  • Involved in the movement of materials along the plasma membrane or in cellular movement.
  • Ciliated cells in the respiratory tract sweep debris trapped in mucus back up the throat, keeping the lungs clean.
  • Ciliated cells move an egg along the uterine tube.
  • Flagellated sperm cells can fertilize the egg.
  • Motor molecules, powered by ATP, allow microtubules in cilia and flagella to bend, facilitating movement.
  • Ciliary dyskinesia: A genetic disorder where a protein in microtubules of cilia and flagella is not formed correctly, resulting in non-bending cilia and flagella.
    • Leads to recurrent respiratory infections due to the failure of ciliated cells to clean the lungs.
    • Can cause infertility due to lack of ciliary action to move the egg or flagellar action by sperm.

Extracellular Matrix (ECM)

  • A meshwork of proteins and polysaccharides closely associated with cells.
  • Collagen and elastin fibers:
    • Collagen resists stretching.
    • Elastin provides resilience.
  • Fibronectin: An adhesive protein that binds to integrin (a protein in the plasma membrane).
  • Integrins:
    • Integral membrane proteins connecting fibronectin externally and the actin cytoskeleton internally.
    • Play a role in cell signaling, allowing the ECM to influence the cytoskeleton and, therefore, cell shape and activities.
  • Proteoglycans:
    • Interact with polysaccharides in the ECM to resist compression.
    • Influence cell signaling by regulating molecule passage through the ECM to the plasma membrane receptors.
  • The extracellular matrix varies in quantity and consistency in connective tissue, being flexible (loose connective tissue), semiflexible (cartilage), or solid (bone).
  • Bone ECM is hard due to the deposition of mineral salts, such as calcium salts.

Junctions Between Cells

  • Allow human tissues to function in a coordinated manner.
  • Three main types:
    • Adhesion junctions
    • Tight junctions
    • Gap junctions
Adhesion Junctions
  • Mechanically attach adjacent cells by interconnecting their cytoskeletons.
  • Common between skin cells.
Tight Junctions
  • Connections between plasma membrane proteins of neighboring cells create a zipper-like barrier.
  • Common in the digestive system and kidney to contain fluids.
Gap Junctions
  • Serve as communication portals between cells.
  • Channel proteins in the plasma membrane fuse, allowing easy movement between adjacent cells.