The Cytoskeleton ● Network of Protein Filaments ● Used in: – Structural Support – Cell Movement – Movement of Vesicles within Cells ● Composed of three types of Cytoskeletal fibers – Actin Filaments – Microtubules – Intermediate Filaments

The Cytoskeleton ● Actin Filaments – Smallest of the fibers, 7nm in diameter – Two protein chains, loosely twined – Each subunit is the globular protein actin – Polarity designates direction of growth – Turn on polymerization when appropriate – Responsible for cellular movements

The Cytoskeleton Actin Filaments

The Cytoskeleton ● Microtubules – Ring-shaped tubes, 25nm in diameter – Consist of 13 protofilaments, arranged side by side around a central core – Facilitate cellular movement and move materials within the cell – Form nucleation centers near center of cell ● + end of microtubule radiates away from nucleation center ● - end of microtubule radiates towards nucleation center

The Cytoskeleton Microtubules

The Cytoskeleton ● Intermediate Filaments – Most durable of filaments – Tough, fibrous protein molecules twined together in overlapping arrangement – Stable – Constitute mixed group of cytoskeletal fibers ● Vimentin (most common) ● Keratin (epithelial cells)

The Cytoskeleton Intermediate Filaments

The Cytoskeleton ● Microtubule Organizing Centers – Centrioles – Centrosome – Pericentriolar Material

The Cytoskeleton ● Centrioles – Barrel-shaped organelles – Found in cells of animals and protists – Occur in pairs – Located in right angles to each other outside the nuclear membranes

The Cytoskeleton ● Centrosome – Region surrounding centrioles – Responsible for reorganization of microtubules that occurs during cell division – Centrosomes of plants and fungi lack centrioles

The Cytoskeleton

● Pericentriolar Material – Contains ring-shaped structures composed of tubulin – Nucleates the assembly of microtubules in animal cells – Called microtubule-organizing centers

The Cytoskeleton Pericentriolar Material

The Cytoskeleton ● Movement of Materials within Cells – Use of Actin Filaments in cells allows for movement – Cytoskeleton provides scaffold that holds certain enzymes and other macromolecules in defined areas of cytoplasm – By moving and anchoring particular enzymes near one another, responsible for organizing cell's activities

The Cytoskeleton ● Molecular Motors – A vesicle or organelle needs to be transported – A motor protein that provies the energy-driven motion – A connector molecule that connects the vesicle to the motor molecule – Microtubules on which the vesicle will ride like a train on a rail