Eukaryotic Cell Structures: Walls, Glycocalyx, Cytoskeleton

Eukaryotic Cell Structures

Plant Cell Wall

Functions:

  • Provides rigidity and contributes to the maintenance of cell shape.
  • Links adjacent cells.
  • Enables the exchange of fluids and intercellular communication.
  • Allows the plant cell to live in a hypotonic medium, preventing bursting.
  • Waterproofs plant surface tissue.
  • Serves as a protective barrier against pathogens.

Structure and Composition:

Composed of cellulose and polysaccharides (pectin and hemicellulose). Layers are:

  • Lamella Media (outermost layer): Consists of pectin and binds Ca2+ ions.
  • Primary Wall: Thick layer formed by fibers of cellulose, hemicellulose, pectins, and glycoproteins.
  • Secondary Wall (innermost layer): Formed by layers of fibrillar cellulose and lacking in pectin.
  • Plasmodesmata: Channels that connect the cytoplasm of adjacent cells.

Glycocalyx of Animal Cells

Composed of polysaccharides, glycolipids, and glycoproteins.

Functions:

  • Support.
  • Substance exchange between adjacent cells.
  • Cell recognition and adhesion.
  • Enzyme placement.
  • Cell movement and diffusion.

Cytoskeleton

A set of protein filaments that are interconnected in complex networks, responsible for maintaining cell shape.

Microtubules

Fibers with a diameter of 25nm, composed of tubulin (a globular protein). Alpha and beta-tubulin dimers form protofilaments that polymerize. Gamma-tubulin forms a ring in microtubule-organizing centers, which are the basis for the polymerization of new microtubules. They have regions called microtubule-organizing centers or centrosomes. A centrosome is a pair of centrioles.

Functions:

  • Maintenance of cell shape.
  • Transportation.
  • Form the mitotic spindle and centrioles.
  • Form a cytoskeletal axis in eukaryotic cilia and flagella and the basal corpuscles.

Cilia and Flagella

External appendages of eukaryotic cells involved in motility.

  • Axoneme: An internal cytoskeletal axis consisting of 9 pairs of peripheral microtubules and a pair of central microtubules, associated with dynein and nexin.
  • Basal Corpuscle: Located at the base of the axoneme and is composed of 9 peripheral triplets of microtubules, associated with ciliary roots.
  • Transition Zone: Located between the axoneme and the basal corpuscle; it shows the basal plate.

The difference between cilia and flagella is in the mechanism of movement; flagella are usually longer and less numerous than cilia.

Actin Microfilaments

Actin is a globular protein that forms filaments consisting of two twisted strands. They are dynamic and have polarity.

Functions:

  • Muscle Contraction: Actin, associated with myosin in myofibrils, is responsible for muscle contraction.
  • Cyclosis movement.
  • Formation of pseudopods and protrusions.
  • Structural Functions: Actin can form support networks and the contractile ring.

Intermediate Filaments

Very stable cytoskeletal components.

Function: Structural.

Composition: Neurofilaments, keratin filaments, and desmin filaments.