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:

• Function to prevent the movement of materials between adjacent cells.

• Seal the layers tightly, creating a barrier that protects against pathogens and regulates the paracellular flow (flow between cells).

• Important in maintaining the integrity of epithelial tissues, such as in the intestines and blood-brain barrier.

• Desmosomes:

• Function as spot welds that provide mechanical strength to tissues by anchoring adjacent cells together.

• Composed of protein complexes that link the cytoskeletons of adjacent cells, allowing them to resist mechanical stress.

• Commonly found in tissues subject to stretching, such as skin and cardiac muscle.

• Gap Junctions:

• Composed of connexin proteins that form channels allowing direct communication between adjacent cells.

• Enable the passage of ions and small molecules, facilitating rapid signaling and coordination, especially in excitable tissues such as cardiac and smooth muscle.

• Allow for metabolic coupling and synchronization of cellular activities, enhancing tissue function and response to stimuli.

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.