How of the line, the cell membrane

How does a cell move around without arms or legs? Cells have to move, to get to food, or to escape unpleasant places.

This lesson will discuss how a cell can use a process called actin treadmilling to get to where it needs to go.

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Cell Movement

On a popular pre-schoolers’ cartoon show, Grickey Grouse Clubhouse, the train moves around a bit differently from a standard train. This train has arms and hands that it uses to take the piece of track from behind the train and moves it to the front. It keeps using the same few pieces of track to get to where it needs to go.The cytoskeleton has functions similar to your regular skeleton. It’s involved in maintaining structure, providing support, and helping the cell move.

The cytoskeleton in our cells can work in a manner similar to this toy train, moving a piece from the back to the front to push the cell forward.

Structure of the Cytoskeleton

The cytoskeleton can be described as a road system going throughout the cell. In addition to the functions described above, the cytoskeleton also acts as a pathway for molecules to be transported throughout the cell. There are several different fibers, or filaments, that make up the cytoskeleton. Actin filaments are one particular example.

In these cells, the actin filaments are stained in green. The images show how the actin filaments are spread throughout the entire cell
actin

Actin filaments are composed of many individual pieces of actin proteins. These proteins are shaped like balls and join together like beads on a string to make one long fiber.

The individual balls are held together by weak, non-covalent bonds, which makes it easy for them to be joined together and removed. For comparison, the connections between the actin proteins would be similar to clicking together two building bricks. A covalent bond, like we see between DNA pieces, would be more akin to super-gluing the pieces together.

The actin pieces can be moved around more easily than the DNA pieces.

Actin Treadmilling

What is actin treadmilling? Is it a new fad exercise program to help strengthen your cytoskeleton?Not quite. Actin treadmilling is a term that describes how individual pieces of actin are shuffled to move the entire cell around in an environment. Unlike your skeleton, the pieces of the cytoskeleton don’t stay in the same location. While your arm bones will always be connected to each other in the same order, the pieces that make up the cytoskeleton will move around relative to each other.

Actin filaments are polar, meaning they have a direction. This is similar to magnets, which have a north end and a south end. The minus end of the actin filament is considered the back, and the plus end is the front. When a cell needs to move somewhere, the piece of actin from the minus end will detach and move to the front of the plus end. This will happen over and over to help, resulting in the cell being pushed forward.

As new pieces of actin are moved to the front of the line, the cell membrane is forced to move forward. The result is the cell creeping along.Actin treadmilling is a convenient way for cells to move. It requires little energy, and because the cell is constantly recycling actin units, it doesn’t require the cell to have lots of extra proteins.

Lesson Summary

The cytoskeleton provides support, structure, and movement of a cell. It is made of many long threadlike fibers, such as actin filaments. Actin filaments are composed of many ball-shaped pieces of actin joined together in a long chain.

These units can perform actin treadmilling. This means that a piece from the minus end of the actin can move to the plus end and push the cell forward.

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