2.7: Cytoskeleton and Cell Extensions

The cytoskeleton is a protein fiber network that supports cell shape, movement, and internal transport.

Master this section and you'll be able to

• Explain the three components of the cytoskeleton, including their composition and functions

The Cytoskeleton: The Cell's Support and Transport Network

If all the organelles were removed from a cell, the plasma membrane and cytoplasm would not be the only components left. Within the cytoplasm there would still be ions and organic molecules, plus a network of protein fibers called the cytoskeleton.  The cytoskeleton runs through the cell and the fibers help the cell to

• Keep its shape
• Move (like crawling or changing form)
• Reproduce
• Transport materials

Just like your bony skeleton holds your body up and gives it shape, the cytoskeleton helps a cell keep its structure and stay stable. But it does a lot more than just provide support!

There are three main types of protein fibers within the cytoskeleton:
 

1) Microtubules

As their name implies, microtubules are small hollow tubes made of a globular (round) protein called tubulin. Even though they only have a diameter of about 25 nm (nanometers), microtubules are the widest components of the three types of cytoskeletal fibers. They help the cell keep its shape and stay sturdy, especially by resisting compression, so the cell does not get squished or collapse under pressure. They also hold organelles in place and act as "tracks" along which vesicles move through the cell.  Finally, during cell division, microtubules pull replicated chromosomes to opposite ends of a dividing cell. Microtubules can dissolve and reform quickly.               ,

Figure \(\PageIndex{1}\): Microtubule Structure. In this image, you can see that the microtubules are hollow with walls that are made of stacks of 13 polymerized dimers of α-tubulin and β-tubulin (right image). The left image shows the molecular structure of the tube.

 

Microtubules are also the structural elements of flagella, cilia, and centrioles (the latter are the two perpendicular bodies of the centrosome which is used during cell division). In animal cells, the centrosome is the microtubule-organizing center.  

• Cilia are tiny, hair-like structures that beat rhythmically in waves. In the respiratory tract, they are found on top of the epithelial cells and sweep out mucus, dust, and bacteria to keep the lungs clean. Cilia in the fallopian tubes help move egg cells from the ovary toward the uterus. A flagellum (plural = flagella) is an appendage larger than a cilium and specialized for cell locomotion. The only flagellated cell in humans is the sperm cell that must propel itself towards female egg cells.
   
• Flagella (singular = flagellum), are longer, whip-like structures that allow the cells to swim. In humans, the only cell with one flagellum is the sperm cell, which uses it to swim toward an egg.

 

2) Intermediate Filaments

Intermediate filaments are named such, because their thickness falls between that of microfilaments and microtubules. They are made of strong, fibrous proteins like keratin, which are twisted together like the strands of a rope. This rope-like structure gives the cell strength and helps it hold its shape.

Figure \(\PageIndex{2}\): Stained Keratin Intermediate filaments. Keratin cytoskeletal intermediate filaments are concentrated around the edge of the cells and merge into the surface membrane. This network of intermediate filaments from cell to cell holds together tissues like skin.

While microtubules resist compression (squeezing forces), intermediate filaments resist tension, the pulling or stretching forces that might try to tear the cell apart. For example, skin cells are constantly being pushed, pulled, and stretched, and intermediate filaments help them stay intact.

However, intermediate filaments do more than just strengthen the cell:

• they anchor organelles in place inside the cell,
• and they connect neighboring cells by forming special junctions, which helps tissues stay strong and connected.

 

3) Microfilaments

Microfilaments are the thinnest type of cytoskeletal fibers — much thinner than microtubules, with a diameter of about 7 nm. They are made mostly of a globular (round) protein called actin, which forms long, twisted chains (see image on the right). For this reason, microfilaments are also known as actin filaments. Microfilaments function in cellular movement. 

In muscle cells, actin filaments team up with another protein called myosin to make muscles contract. The actin filaments (called thin filaments) get pulled by thicker myosin filaments, causing the muscle cell to shorten or contract. We will study the mechanics of the muscle movement in more detail in a future module.

Microfilaments also provide some rigidity and shape to the cell. They can depolymerize (disassemble) and reform quickly, thus enabling a cell to change its shape and move. White blood cells (your body’s infection-fighting cells) make good use of this ability. They can move to the site of an infection and engulf the pathogen.

 

 

     

Figure \(\PageIndex{3}\): The Three Components of the Cytoskeleton. The cytoskeleton consists of (a) microtubules, (b) microfilaments, and
(c) intermediate filaments. The cytoskeleton plays an important role in maintaining cell shape and structure, promoting cellular movement, and aiding cell division.