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The Cytoskeleton: Microtubules and Microfilaments

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  • 0:05 The Cytoskeleton
  • 1:26 Microtubules
  • 3:14 Intermediate Filaments
  • 4:05 Microfilaments
  • 5:06 Lesson Summary
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Lesson Transcript
Instructor: Kristin Klucevsek

Kristin has taught college Biology courses and has her doctorate in Biology.

In this lesson, we'll learn about the cytoskeleton of your cells. This network of microtubules, intermediate filaments, and microfilaments helps different types of cells maintain a unique set of characteristics, including shape and movement.

The Cytoskeleton

Together with your muscles, your nerves, and your skin, your skeleton gives your body that characteristic human shape. It allows you to stand on two feet, to leap, and to play a round of flag football. Did you know that all of the cells in your body also have a skeleton all their own? Many different types of cells have a complex called the cytoskeleton, or network of thin fibers, that can help these cells do a range of activities. Maybe a cell can't pass around a pigskin, but it can do some equally cell-like, impressive things with the help of its very own skeleton.

A cytoskeleton can provide support and shape for a cell like your skeleton supports and shapes your body. Your skeleton helps you stand, and it also helps you run. In a cell, a cytoskeleton can also anchor a cell in one place or allow a cell to move. The bones in your arm help you catch a football independent from what's going on in your legs. The parts of your cells' cytoskeletons can also help structures move independently within your cells. A cytoskeleton can position cell structures in specific places within the cell, or it can move cell structures from one end of the cell to the other.

A cell's cytoskeleton is comprised of three different types of fibers: microtubules, intermediate filaments, and microfilaments.

Microtubules

Microtubules are the largest fibers of the cytoskeleton, with a 25-nanometer diameter. They are composed of two different proteins called alpha and beta tubulin. One dimer of tubulin contains one alpha subunit and one beta subunit. A microtubule is made up of many, many tubulin dimers. These dimers form long chains of alpha and beta tubulin. Thirteen chains of tubulin dimers make up one microtubule. The chains form a hollow tube, which is where microtubules get their name. They look like a paper towel roll. Each end of a microtubule is referred to as the plus end or the minus end, and each microtubule is dynamic, meaning it can grow and it can shrink by adding or subtracting dimers, usually to the plus end.

Thirteen chains of dimers make up a microtubule
Microtubule

Microtubules function to support the cell shape, but they also help cell division. Microtubules can be used like train tracks in the cell along which vesicles and structures can be transported by riding across them. A fourth important function of microtubules is in cell movement. Some cells have flagella and cilia made of microtubules. Flagella are long, snake-like whips that drive cell movement. Sperm cells are an example of cells that have flagella, which allows them to swim. Cilia are multiple short, hair-like structures that beat to move liquid around a cell. Cilia are found in the cells that line your respiratory track, where they rhythmically beat to help you move mucous when you're sick. Both flagella and cilia are made with microtubule structures surrounded by a plasma membrane. If you looked at a cross section of flagella or cilia, you would see what's referred to as a '9 plus 2' arrangement of microtubules. This is nine pairs of microtubules in a circle with two single microtubules in the center.

Intermediate Filaments

The next components of a cytoskeleton are the intermediate filaments. These fibers are called intermediate because they are the mid-sized fibers of the cytoskeleton, measuring 8-12 nanometers in diameter. They can be made from any of several fibrous proteins twisted together like twine to make a rope.

Intermediate filaments make up the nuclear lamina
Nuclear Lamina

Intermediate filaments have a diverse role in structure and support within the cell. One specialized group of intermediate filaments has an important role in the nucleus, where they can make up the nuclear lamina. Remember that here they are essentially the cytoskeleton of the nucleus, specifically supporting a structure of your cells like the bones supporting your hands. Here, intermediate filaments provide support to the nuclear lamina and aid in regulating nuclear processes. They also extend from the nuclear envelope, helping the nucleus keep its position within the cell.

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