ch09

Chapter 9: The Cytoskeleton and Cell Motility

9.1 Overview of the Cytoskeleton

• Cytoskeleton: The "skeletal system" of a eukaryotic cell, vital for maintaining cell shape and enabling movement.

• Components: Composed of three filamentous structures:

  • Microtubules

  • Actin filaments

  • Intermediate filaments

• Properties of Cytoskeletal Components:

  • Microtubules: Provide structural support, intracellular transport, and cell organization, made of tubulin.

  • Actin Filaments: Involved in motility, contractility, intracellular transport.

  • Intermediate Filaments: Provide mechanical strength and structural support.

9.2 Structure and Function of Microtubules

9.2.1 Microtubule Structure

• Microtubules: Hollow, rigid tubes made of tubulin protein.

• Protofilaments: Composed of linear arrays of tubulin dimers (α-tubulin and β-tubulin).

• Polarity: Microtubules have a plus end (β-tubulin) and a minus end (α-tubulin).

9.2.2 Role of Microtubules

• Support: Resists compression and bending forces, helps determine cell shape.

• Movement: Directs intracellular traffic, particularly in nerve cells, vital for neurotransmitter transport.

9.2.3 Microtubule-Associated Proteins (MAPs)

• Enhance stability and promote assembly, such as tau, which is linked to Alzheimer’s disease.

9.2.4 Effects of Microtubule Arrangement

• Example: Microtubules in animal cells extend radially, giving cells a flattened shape.

9.3 The Role of Microtubules in Plant Growth

• Spiral Growth: Microtubules influence the helical growth patterns observed in climbing plants.

• Organized growth is dependent on microtubule orientation.

9.4 Motor Proteins: Kinesins and Dyneins

9.4.1 Overview of Motor Proteins

• Kinesins: Move towards the plus end of microtubules.

• Dyneins: Move towards the minus end, facilitating vesicle transport and spindle positioning during mitosis.

• Both rely on ATP hydrolysis for movement.

9.4.2 Kinesin Structure and Function

• Consists of two heavy and two light chains with ATPase activity, important for cargo transport.

• Processivity: Kinesins can travel long distances along microtubules without detaching.

9.4.3 Dynein Functionality

• Functions in protein transport, positioning organelles, and chromosome movement during cell division.

9.5 Microtubule Organizing Centers (MTOCs)

9.5.1 Centrosomes as MTOCs

• Function: Nucleate microtubules, controlling their assembly and orientation.

• Composed of centrioles surrounded by pericentriolar material.

9.5.2 Dynamic Properties of Microtubules

• Capable of rapid assembly/disassembly, influenced by various factors like temperature and chemical exposure.

• Dynamic Instability: A phenomenon where microtubules fluctuate between growth and shrinkage phases.

9.6 Structure and Function of Cilia and Flagella

9.6.1 Cilia and Flagella Characteristics

• Cilia: Hairlike structures that often work in coordinated movements to encourage fluid flow.

• Flagella: Typically singular or in pairs, use different motion patterns for propulsion.

9.6.2 Structure

• Axoneme: Core structure made up of a 9 + 2 arrangement of microtubules, enabling movement.

• Basal Bodies: Organize the assembly of cilia and flagella.

9.6.3 Locomotion Mechanism

• Sliding of microtubules driven by dynein motor proteins generates ciliary/flagellar movement.

• Nexin ties adjacent doublets, ensuring coordinated beating.

9.7 Intermediate Filaments

9.7.1 Structure and Functionality

• Intermediate Filaments: Strong, flexible fibers that provide mechanical strength, comprised of various proteins like keratin and vimentin.

• Distinct from microtubules and actin filaments due to their assembly and disassembly process, which is not ATP/GTP dependent.

9.7.2 Assembly Process

• Build-up involves tetrameric subunits without polarity, allowing for different cellular locations and mechanical functions.

9.8 Actin and Myosin

9.8.1 Actin Filament Characteristics

• Structure: Polarized filaments crucial for motility processes, differentiated by barbed and pointed ends.

• Actin filaments undergo cycles of assembly and disassembly influenced by ATP binding and hydrolysis.

9.8.2 Myosin Functionality

• Molecular Motors: Move towards actin's barbed end, involved in muscle contraction and various cellular movements.

• Myosin II facilitates muscle contraction by engaging in power strokes powered by ATP.

9.9 Muscle Organization and Contraction

9.9.1 Sarcomere Structure

• Muscle fibers composed of repeating contractile units (sarcomeres), characterized by Z lines and A, I, and H bands.

9.9.2 Sliding Filament Model

• During muscle contraction, filaments slide past each other; Z lines move closer together.

9.9.3 Energetics of Contraction

• ATP hydrolysis powers the movement of myosin heads along actin, necessary for continuous muscle activity.

• Contractile cycle and excitation-contraction coupling hinge on nerve signals and calcium regulation.