Elements of the Cytoskeleton

All eukaryotic cells have at least two cytoskeletal elements: microtubules and microfilaments.
Animal cells also additionally possess intermediate filaments.
Cytoskeletal elements are composed of long chains made up of protein subunits.
The functions of cytoskeletal elements include:
- Providing structural support to the cell
- Enabling movement of substances within the cell.

Cell movement can be categorized into three types:
- General cell movement itself
- Change in cell shape
- Movement of molecules and organelles within the cell.

There are three main types of cytoskeletal filaments distinguished by:
- Size
- Structure
- Type of protein subunit

Structure: Two coiled strands
Subunits: Actin
Diameter: ~7 nm
Functions:
- Maintain cell shape by resisting tension (pull).
- Facilitate cell movement via muscle contraction or cell crawling.
- Divide animal cells in two during cytokinesis.
- Transport organelles and cytoplasm in plants, fungi, and animals.

Structure: Fibres wound into thicker cables
Subunits: Keratins, lamins, or others
Diameter: ~10 nm
Functions:
- Maintain cell shape by resisting tension (pull).
- Anchor the nucleus and some other organelles in place.

Structure: Hollow tube
Subunits: α- and β-tubulin dimers
Diameter: ~25 nm
Functions:
- Maintain cell shape by resisting compression (push).
- Enable cell movement via flagella or cilia.
- Separate chromosomes during cell division.
- Assist in the formation of the cell plate during plant cell division.
- Provide tracks for intracellular transport.

Microtubules are hollow, tube-like polymers measuring approximately 25 nm in diameter, composed of tubulin proteins.
Dimer Subunits: Composed of an α-tubulin and a β-tubulin.

Microtubules rapidly assemble and disassemble.
The faster-growing end is referred to as the plus end; the slower-growing end is the minus end.
They originate from a microtubule organizing center (MTOC).
- The plus end grows outward, radiating throughout the cell.
- This arrangement helps maintain cell shape and resist compressive forces.
In animal cells, the MTOC is called the centrosome, which contains two bundles of microtubules known as centrioles.
Plant cells possess microtubules but lack a centrosome; they organize their microtubule network differently.

Microtubules act as tracks for transport proteins that carry cellular cargo, such as transport vesicles, to various destinations.
- Kinesin proteins move towards the plus end of the microtubule.
- Dynein proteins move towards the minus end of the microtubule.
This movement requires energy, supplied by the hydrolysis of ATP.

Cilia and flagella are both locomotor appendages found on some eukaryotic cells.
- They both have identical structure but exhibit distinct movement.
- Each typically consists of a (9+2) arrangement of microtubules covered by the plasma membrane.
- The core of cilia and flagella is anchored to the rest of the cell by the basal body, which shares the same structure as a centriole.

Dynein proteins form sets of arms between microtubule doublets, 'walking' up one microtubule towards the minus end while pulling the adjacent doublet with it.
When dynein proteins on one side of the appendage move while those on the other side do not, the appendage bends.
Spokes anchor peripheral doublets to the central microtubules, preventing them from sliding completely past each other.

The summary of the three types of cytoskeletal filaments includes the structure, subunits, and functions, as detailed in Table 7.2:

Filament	Structure	Subunits	Functions
Actin Filaments	Two coiled strands	Actin	- Maintain cell shape by resisting tension (pull)

Move cells via muscle contraction or cell crawling
Divide animal cells in two
Move organelles and cytoplasm in plants, fungi, and animals |
| Intermediate Filaments | Fibres wound into thicker cables | Keratins, lamins, others | - Maintain cell shape by resisting tension (pull)
Anchor nucleus and some other organelles |
| Microtubules | Hollow tube | 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 |

Microfilaments are solid rod-like polymers of approximately 7 nm in diameter, made of actin proteins.
They readily polymerize and depolymerize, showing both a plus end and a minus end.
Two chains of actin subunits form a twisted double chain.
The structural role of microfilaments is to bear tension and resist pulling forces within the cell.
They are grouped into long bundles or dense networks.

Microfilaments form a 3-D network known as the cortex, located just inside the plasma membrane to help support the cell's shape.
They make up the core of microvilli in intestinal cells, enhancing absorption.

Microfilaments:
- Myosin uses microfilaments as tracks to move cellular cargo, such as transport vesicles, throughout the cell.
- Myosin moves towards the plus end of microfilaments.
- The energy used for this movement comes from the hydrolysis of ATP.

Microfilaments involved in cellular motility interact with the motor protein myosin.
In muscle cells, bundles of actin filaments are arranged parallel to each other.
Thicker myosin filaments pull on the thinner actin fibers, generating movement.

This type of movement relies heavily on actin filaments.
Actin assembles at the front of the cell and disassembles elsewhere, allowing the cell to change shape and crawl forward.

Examples of Movement Caused by Actin-Myosin Interactions:
- Cytokinesis in Animals: Actin-myosin interactions draw the membrane in, dividing a cell into two.
- Cytoplasmic Streaming in Plants: Actin-myosin interactions facilitate the movement of cytoplasm around a cell.

Intermediate filaments are intermediate in size between microfilaments and microtubules and do not readily polymerize or depolymerize.
Composed of polymers of specific intermediate filament proteins.
Combine to form strong, cable-like structures providing mechanical strength to cells.
There are over 100 different types of intermediate filaments with various protein types, such as:
- In epithelial cells, intermediate filaments are made from keratin subunits.
- Nuclear lamins are a type of intermediate filament found inside the nucleus, providing support to the nuclear envelope.