Learn thin filaments, which act like walkways

Learn about the three parts of the cytoskeleton: microfilaments, microtubules, and intermediate filaments. We will cover their structure and the important jobs they perform in the cell.

What is the Cytoskeleton?

Our skeletons give our bodies structure. But cells need structure too, so they have a cytoskeleton.

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‘Cyto-‘ means cell, so like the name implies, the cytoskeleton is the ‘bones’ of the cell. It is composed of materials that give the cell its shape, and can bear loads and stresses, and even help the cell move just like our skeletons do.We have many different bones in our bodies that come in all shapes and sizes.

In the cell, there are three types of ‘bones’: microfilaments, microtubules, and intermediate filaments.However, unlike our bones, the cytoskeleton is always changing. Microfilaments, microtubules, and intermediate filaments are composed of smaller building blocks that can be added or subtracted to make them longer or shorter, whatever the cell needs at the moment. They also act like little highways, and there are special proteins that move along the different filaments carrying cargo around the cell.Let’s go into more detail about each part of the cytoskeleton and its particular role in the cell.

Microfilaments

Microfilaments are the thinnest of three parts of the cytoskeleton, measuring in at 5-9 nm in diameter.

The building block of the microfilament is a protein called actin, so they are sometimes called ‘actin filaments’. The actin molecules stick together in a particular way, similar to the way Legos have to be in a particular position to stick together. This also helps the cell to tell the difference between the two ends of the filament.One end is called the plus end and one end is called the minus end. Because the two ends are different, a microfilament is polar. Typically, growth or shrinkage of the filament occurs by adding or subtracting actin molecules to the plus end.

The microfilament is composed of actin protein subunits that link together in a helical structure.

Microfilament

Actin has many different jobs in the cell. One of the jobs is to support the structure of the cell. If you were to peel back the cell membrane, you would likely find a lot of microfilaments just below it.

This is very important in our tissues because the cells need to stick together, and this requires them to be very strong. There are special proteins in the cell membrane that help attach the cell to surrounding cells; microfilaments inside the cell will then attach to these proteins to reinforce these attachment points.Microfilaments are also involved in cell movement. Some cells will grow the microfilaments in the direction they want to move so that the microfilaments will push the cell membrane forward. This helps the cell to explore its environment or move in the direction of a particular signal. Microfilaments are also an important part of muscle contraction.

They are the backbone of the thin filaments, which act like walkways for myosin proteins. As myosin proteins walk along the thin filaments, the muscle will contract.

Microtubules

Microtubules are the largest structural part of the cytoskeleton.

They are fatter than microfilaments, with a diameter of about 25 nm. They are made up of building blocks called tubulin (you can remember this because they look like tubes!). There are actually two types of tubulin that stick together to form a peanut-shaped dimer.

The tubulin protein dimers then stick together to form long, hollow tubes. As a result, they are much stiffer than microfilaments. Microtubules are also polar, with a plus end and a minus end.

Diagram showing the two types of tubulin, alpha and beta, that form dimers. These dimers are then used to build the hollow microtubule.
Top: Intermediate filament protein monomers (red) can join to make a dimer (red and blue); two of those make a tetramer, which is the building block for the filament.

Bottom: An astrocyte has intermediate filaments (yellow) supporting its long arms.

Intermediate Filaments

The main role of intermediate filaments is to give extra strength to cells. For example, some of our neurons have very long arms, called axons.

Intermediate filaments help to strengthen these long, thin axons. While most cells will have microtubules and microfilaments, there are some cells that don’t have intermediate filaments.

Lesson Summary

Microfilaments, microtubules, and intermediate filaments make up the cytoskeleton. Each type of filament is made up of small protein subunits.

  • Microfilaments are made of actin,
  • microtubules are made of tubulin, and
  • intermediate filaments can be made of a variety of different rope-like proteins.

The cytoskeleton is mainly responsible for giving cells their structure and support.

The components of the cytoskeleton are also important in helping cells move and providing tracks for the movement of cargo around the cell.

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