Chapter 6 Part 3

Microtubules

• Overview

  • Largest diameter of the three types of filaments in the cytoskeleton.

  • Stained green in visuals, with DNA stained blue; present throughout the cytoplasm during interphase (non-dividing stage).

• Function

  • Maintain cell shape.

  • Facilitate chromosomal movement during cell division and organelle movement.

• Structure

  • Composed of large hollow rigid tubes made from subunits called tubulin (alpha and beta tubulin).

  • Tubulin subunits are not covalently bonded, allowing for rapid assembly and disassembly.

• Centrosome

  • Microtubule organizing center that contains centrioles.

  • Centrioles consist of nine triplets of microtubules arranged in a cylinder, typically found at right angles to one another.

  • Serve as the nucleating site from which microtubules grow out.

• Cilia and Flagella

  • Microtubules form the basic architecture of cilia and flagella.

  • In cilia, microtubules are associated with connecting proteins and motor proteins that enable movement.

• Motor Proteins

  • Kinesin: Move cargo towards the microtubule plus end (grows and disassembles faster).

    • Binds to cargo (like organelles) and transports it along the microtubule tracks in a directed fashion.

  • Dynein: Moves cargo towards the microtubule minus end.

    • Organizes cargo transport directionally along the cytoskeleton.

ATP and Motor Proteins

• Energy Currency:

  • ATP hydrolysis powers motor proteins like kinesin and dynein, causing conformational changes that enable movement.

  • Each movement step involves binding ATP, hydrolysis of ATP to ADP and inorganic phosphate, and conformational changes pushing the motor towards the next binding site.

• Processive Movement

  • Kinesin can move in a coordinated, stepwise fashion along microtubules, similar to "walking" along a track.

Intermediate Filaments

• Definition

  • Comprise a diverse group of proteins that provide structural support and tension resistance within cells.

• Structure and Function

  • Keratin is one example, forming a meshwork that anchors organelles and provides structural integrity.

  • Intermediate filaments form the nuclear lamina, underlying the nuclear envelope and maintaining the shape of the nucleus.

• Cell Connectivity

  • Tight Junctions: Prevent movement between cells, sealing layers tightly against pathogens.

  • Desmosomes: Spot welds that provide strength by anchoring adjacent cells together through their cytoskeletons.

  • Gap Junctions: Channels that allow direct communication and material flow between adjacent cells, facilitating rapid signaling (e.g., in cardiac muscle).

Plant Cell Communication

• Plasmodesmata: Channels in plant cell walls allowing material transport and communication between adjacent plant cells, ensuring continuity similar to that found in animal cells.