Biology Chapter 7C

Biological Science Eighth Edition Chapter 7c: Inside the Cell – The Cytoskeleton

Additional Required Reading

• Becker’s World of the Cell, Pages 352 - 360

Chapter 7: Opening Roadmap

Learning Objectives

• Understand how life’s properties emerge from collaborative internal structures within a cell by exploring the following key concepts:

  • What are the parts of the cell?

  • How do these parts fit into a whole?

  • Transport to chloroplasts and mitochondria

  • Nuclear transport (Section 7.4)

  • Structures of Prokaryotic and Eukaryotic cells

  • Endomembrane system (Sections 7.1, 7.2, 7.3)

  • Dynamic cytoskeleton (Sections 7.5, 7.6)

The Dynamic Cytoskeleton

Overview

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

  • Actin filaments (microfilaments)

  • Microtubules

  • Intermediate filaments

• Functions of the cytoskeleton:

  • Maintains cell shape and structure

  • Facilitates organelle movement

  • Enables whole cell movement

  • Displays dynamic structures

• Similar proteins exist in prokaryotes.

  • Cellular components highlighted in imagery: Red - actin, Green - tubulin, Blue - DNA

  • Example: Mouse melanoma cell in culture

Table 7.2: Cytoskeletal Filaments

• Move cells via muscle contraction or cell crawling

• Divide animal cells in two

• Move organelles and cytoplasm in plants, fungi, and animals |
  | Intermediate filaments | Fibers wound into thicker cables | Keratins, lamins, or others | - Maintain cell shape by resisting tension (pull)

• Anchor nucleus and some other organelles |
  | Microtubules | Hollow tubes | α- and β-tubulin dimers | - Maintain cell shape by resisting compression (push)

• Move cells via flagella or cilia

• Move chromosomes during cell division

• Assist formation of cell plate during plant cell division

• Provide tracks for intracellular transport |

Detailed Analysis of Cytoskeletal Components

Actin Filaments (Microfilaments)

Structure

• Thinnest cytoskeletal polymer consisting of two twisted strands of actin subunits

• Exhibits structural polarity:

  • Plus end: Faster growth

  • Minus end: Slower growth

• Can form bundles or cross-linked networks

Function

• Plays a role in:

  • Maintaining cell shape

  • Cell movement:

  • Myosin: motor protein that utilizes ATP to change shape and perform work

  • Muscle contraction

  • Cytokinesis (cell division)

  • Cytoplasmic streaming

• Example:

  • Actin–myosin interactions cause cytoplasm to move around within the cell. These interactions also facilitate membrane movement, effectively dividing a cell during cytokinesis.

  • Figure 7.23 illustrates how many cellular movements are based on actin–myosin interactions, with examples of cytoplasmic streaming in plants.

Intermediate Filaments

Structure

• Composed of various proteins such as keratin or nuclear lamins

• Structurally characterized by:

  • Coiled coil dimers with partial overlap

  • No structural polarity

  • Tough and durable: Ex: Keratin found in hair, claws, and fingernails

Function

• Nuclear lamins form a meshwork that supports the nuclear envelope:

  • Structural support and mechanical strength provided to the cell nucleus.

• Example of a genetic disease linked to intermediate filaments:

  • Progeria: results from a mutation in a nuclear lamin, leading to symptoms of premature aging.

Microtubules

Structure

• The largest cytoskeletal element

• Made of hollow tubes constructed from tubulin protofilaments (α- and β-tubulin heterodimers)

• Features structural polarity with defined plus and minus ends

• Dynamic in nature, growing faster at their plus ends

• Microtubules emerge from the microtubule organizing center (centrosome in animal cells), where the minus end is anchored while the plus ends grow outward

Function

• Serve multiple roles including:

  • Structural support: Provides stability to organelles

  • Cell division: Essential for the separation of chromosomes

  • Cell locomotion: Involved in structures like cilia and flagella

  • Acts as tracks for movement of organelles within the cell.

• Observe dynamic instability where microtubules rapidly shrink and regrow. An example of this process is captured through time-lapse imaging (See: Inner Life of the Cell ~ 1’ 07”).

• Specific motor proteins, kinesin and dynein, move vesicles along microtubules, using ATP hydrolysis to facilitate their transport.

Cilia and Flagella

• Eukaryotic** cilia and flagella** consist of microtubules organized in an axoneme with a “9 + 2” arrangement.

• The movement of these structures is facilitated by the bending mechanism involving dynein proteins, which walk along adjacent microtubule doublets, generating movement.

• Prokaryotic flagella, on the other hand, are rigid helical rods made from a different protein, flagellin, and are not constructed from microtubules.

Summary of Cytoskeletal Elements

Overview Table 7.2: Cytoskeletal Filaments

• Move cells via muscle contraction or cell crawling

• Divide animal cells in two

• Move organelles and cytoplasm in plants, fungi, and animals |
  | Intermediate filaments | Fibers wound into thicker cables | Keratins, lamins, or others | - Maintain cell shape by resisting tension (pull)

• Anchor nucleus and some other organelles |
  | Microtubules | Hollow tubes | α- and β-tubulin dimers | - Maintain cell shape by resisting compression (push)

• Move cells via flagella or cilia

• Move chromosomes during cell division

• Assist formation of the cell plate during plant cell division

• Provide tracks for intracellular transport |