Book : Cell Biology
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Posted by: CHELSEA
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Writer CHELSEA

Cytoskeleton

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The cytoskeleton provides and supports in a cell support in a cell. It is a network of protein fibers supporting cell shape and anchoring organelles within the cell. The three main structural components of the cytoskeleton are microtubules (formed by tubulins), microfilaments (formed be actins) and intermediate filaments. All three components interact with each other non-covalently. Eukaryotic cells contain proteins called intermediate filaments, microfilaments, and microtubules that are collectively termed the cytoskeleton. Also, the cytoskeleton proteins are multifunctional and are also involved in whole-cell movements and movements of substances within the cell.

Microtubules:

Microtubules are polymers of tubulin. They help with cell transport. They also help with the cell shape because it resists compression. It also helps facilitate cell motility. There are two motor proteins that assist organelles to move along the microtubules:

  • - Kinesin, which moves things away from the nucleus.
  • - Dynein, which moves things towards the nucleus.

Microtubles have a larger diameter than microfilaments and intermediate filaments. The hollow microtubule structure consists of 13 tubulin dimers. They are one alpha-tubulin protein plus one beta-tubulin protein form one tubulin dimer. Microtubules also help movement of substances within the cell and are also involved in powering whole-cell movement by cilia and flagella. The microtubules provide tracks that can move vesicles from one organelle to the next in an efficient, directed fashion. Microtubules also segregate the duplicated chromosomes during mitosis.

Microfilaments:

Microfilaments are polymers of actin. They help with cell shape also because it bears tension in the cell. It is also involved in cell motility. There are three types of cell shape, which are microvilli, lamellipodia, and filopodia.

  • - Microvilli are projections on surface that increase surface area.
  • - Lamellipodia are membrane ruffles that help sense the environment and direct movement.
  • - Filopodia are like microvilli but are less stable. They also sense the environment. They can turn into lamellipodia.

Microfilaments are formed when individual actin monomers polymerize, in a process fueled by ATP hydrolysis, to form chains of filamentous actin. Microfilaments are dynamic structures, growing and shrinking in a controlled manner. Some microfilaments play a structrual role in the cell to maintain cell shape. These structural microfilaments have protein caps at both ends to prevent changes in microfilament length. Anyways, other microfilaments have functions that require dynamic changes in length. The microfilaments also mediate cytoplasmic streaming, a mixing of the cytoplasm that aids diffusion.

Intermediate filaments:

Intermediate filaments are polymers of keratin. They help with cell shape also because it bears tension in the cell. It is primarily involved in organelle anchorage. The intermediate filaments also consist of various fibrous proteins that have a diameter of about 10 nm. Intermediate filaments often form a meshwork under the cell membrane and, in cells that lack a cell wall, help impart and maintain cell shape. Intermediate filaments are fairly stable and are not thought to undergo acute changes in length the way microfilaments and microtubules do.

Functions of Cytoskeleton:

Cytoskeletons are often synthesized based on the cell's needs; however, some protein fibers are permanent. The changeable nature of the cytoskeleton thus contributes to its five important functions.

  • Cell Shape: The mechanical strength of the cell is due to protein scaffolding in the cytoskeleton. In some cells, protein scaffolding also determines the shape of the cell. Microvilli are supported by cytoskeletal fibers such as microfilaments. Microvilli also increase the surface area of the cell for the absorption of materials.
  • Internal Organization: The cytoskeletal fibers of the cell help stabilize positions of organelles. However, the interior arrangement of the cell varies based on the cell's needs. The organelles are dynamic and change minute to minute.
  • Intracellular Transport: The cytoskeleton has the ability to move materials not only in the cell but also within the cytoplasm, thus aiding in the movement of organelles as well. This function is important especially in the nervous system where materials are often transported over long intracellular distances.
  • Assembly of Cells into Tissues: Cells are connected to one another through the linking of the protein fibers of the cytoskeleton as well as the protein fibers in the extracellular space. In the process, materials outside the cell are stabilized as well. The assembly of cells not only contributes to the mechanical strength of the tissue but also allows information to transfer between cells from one to another.



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