Chapter 7c - Cytoskeleton

Chapter Overview

  • This chapter explores the dynamic nature of the cytoskeleton within biological cells, focusing on its types, structures, and functions.

Key Concepts

  • Properties of Life: Life's properties emerge from collaboration of internal structures in a cell.

  • Questions Addressed:

    • What are the parts of the cell?

    • How do the parts fit into a whole?

    • What transport mechanisms do chloroplasts and mitochondria use?

    • Nuclear transport as discussed in Section 7.4.

    • Differences between prokaryotic and eukaryotic cell structures.

    • Overview of the endomembrane system in Sections 7.1 to 7.3.

    • Detailed exploration of the dynamic cytoskeleton found in Section 7.6.

The Dynamic Cytoskeleton

  • Definition: The cytoskeleton is a dense and complex network consisting of three distinct cytoskeletal polymers (proteins):

    • Actin filaments (microfilaments)

    • Microtubules

    • Intermediate filaments

  • Functions of the cytoskeleton include:

    • Maintaining cell shape and structure

    • Facilitating organelle movement

    • Enabling whole-cell movement

  • Notably, similar proteins are found in prokaryotic cells, suggesting an evolutionary link.

Cytoskeletal Filaments

Summary Table (Table 7.2)

  • The cytoskeleton is categorized into three types based on size, structure, and protein subunits:

Actin Filaments (Microfilaments)

  • Structure: Composed of two coiled strands, approximately 7 nm in diameter.

  • Functions:

    • Maintain cell shape by resisting tension (pull).

    • Facilitate movement via muscle contraction or cell crawling.

    • Involved in cytokinesis (the division of animal cells).

    • Aid in the transport of organelles and cytoplasm in plants, fungi, and animals.

Intermediate Filaments

  • Structure: Fibers are wound into thicker cables, approximately 10 nm in diameter.

  • Functions:

    • Maintain cell shape by providing tensile strength.

    • Anchor the nucleus and various organelles.

Microtubules

  • Structure: Hollow tubes made of tubulin dimers (α- and β-tubulin), approximately 25 nm in diameter.

  • Functions:

    • Provide structural support by resisting compression (push).

    • Facilitate cell movement via flagella or cilia.

    • Play crucial roles in chromosome movement during cell division.

    • Assist with the formation of the cell plate during plant cell division.

    • Serve as tracks for intracellular transport of organelles.

Actin Filaments

Structure

  • Thinness: Actin filaments are the thinnest type of cytoskeletal polymer.

  • Composition: Comprised of two twisted strands of actin subunits.

  • Polarity: Exhibits structural polarity, where the plus end grows faster than the minus end.

  • Arrangement: Can form bundles or cross-linked networks to support various cellular functions.

Function

  • Cell Shape & Movement: Actin filaments maintain cell shape and are essential in movement.

  • Motor Protein – Myosin: A key motor protein that uses ATP to change shape and perform work, crucial for muscle contraction and cytokinesis.

  • Cytoplasmic Streaming: Facilitated by actin–myosin interactions which move cytoplasm, contributing to cell metabolism.

Intermediate Filaments

Structure

  • Diversity: Comprised of various types, including keratins and nuclear lamins, using different proteins.

  • Configuration: Form coiled-coil dimers that overlap; exhibit no structural polarity.

  • Durability: Known for their toughness and durability; for example, keratin found in hair and nails.

Function

  • Support of Nuclear Envelope: Intermediate filaments, specifically nuclear lamins, form a meshwork providing support and structure to the nuclear envelope.

  • Connection to Disease: Progeria, a genetic condition, arises from mutations in nuclear lamins, leading to symptoms of premature aging.

Microtubules

Structure

  • Largest Cytoskeletal Element: Characterized as hollow tubes made up of tubulin protofilaments.

  • Polarity: Have structural polarity with dynamic growth occurring at the plus ends.

  • Origin: Microtubules originate from the microtubule organizing center (MTOC), with the minus end anchored at centrosomes within animal cells.

Function

  • Support and Stability: Provide structural support for organelles.

  • Role in Cell Division: Critical for the separation of chromosomes during mitosis.

  • Locomotion: Aid cell movement via cilia and flagella, which resemble microtubule-based structures.

  • Vesicle Transport: Motor proteins, such as kinesin and dynein, transport vesicles along microtubules by hydrolyzing ATP.

Cilia and Flagella

Structure

  • Axoneme Arrangement: Composed of a “9 + 2” arrangement of microtubules, which is crucial for functionality.

  • Differences: Eukaryotic cilia and flagella differ significantly from prokaryotic flagella, which are rigid structures made of flagellin rather than microtubules.

Mechanism of Movement

  • Axoneme Bending: Dynein arms provide movement by pulling adjacent microtubule doublets, leading to bending, powered by ATP.

Summary of Cytoskeletal Elements

Summary Table Recap (Review of Table 7.2)

  • Three types of cytoskeletal filaments are detailed based on their structure, size, protein subunits, and functions:

Actin Filaments (Microfilaments)

  • Structure: Two coiled strands of actin, approximately 7 nm in diameter.

  • Functions: Movement, shape, and cell division.

Intermediate Filaments

  • Structure: Fibers wound into thicker cables, approximately 10 nm in diameter.

  • Functions: Mechanical strength and nuclear support.

Microtubules

  • Structure: Hollow tubes made of tubulin dimers, approximately 25 nm in diameter.

  • Functions: Provide tracks for intracellular transport, cell shape maintenance, locomotion, and division processes.