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.