Filaments in the Cytoskeleton

By the end of this lesson and the next few, you should be able to:

• Describe the cytoskeleton
• Compare the roles of microfilaments, intermediate filaments, and microtubules
• Compare and contrast cilia and flagella
• Summarize the differences among the components of prokaryotic cells, animal cells, and plant cells

What is the cytoskeleton?

If you were to remove all the organelles from a cell, would the plasma membrane and the cytoplasm be the only components left? No. Within the cytoplasm, there would still be ions and organic molecules, plus a network of protein fibers that help maintain the shape of the cell, secure some organelles in specific positions, allow cytoplasm and vesicles to move within the cell, and enable cells within multicellular organisms to move.

Collectively, this network of protein fibers is known as the cytoskeleton. There are three types of fibers within the cytoskeleton: microfilaments, intermediate filaments, and microtubules (see image below). Here, we will examine each.

Microfilaments

Of the three types of protein fibers in the cytoskeleton, microfilaments are the narrowest. They function in cellular movement, have a diameter of about 7 nm, and consist of two intertwined strands of a globular protein we refer to as actin (see image below). For this reason, we refer to microfilaments as actin filaments. (Recall that globular proteins are spherical, elliptical or oval in shape and soluble in water, acids and bases.)

ATP powers actin to assemble its filamentous form, which serves as a track for the movement of a motor protein called myosin. This enables actin to engage in cellular events requiring motion, such as cell division in animal cells and cytoplasmic streaming, which is the circular movement of the cell cytoplasm in plant cells. Actin and myosin are plentiful in muscle cells. When your actin and myosin filaments slide past each other, your muscles contract.

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 (see video below). They can move to the site of an infection and phagocytize the pathogen.

Video Animation of White Blood Cell Chasing Bacteria

To see an example of a white blood cell in action, watch a short time-lapse video of the cell capturing two bacteria. It engulfs one and then moves on to the other.

Intermediate Filaments

Intermediate filaments consist of several strands of fibrous proteins wound together (see image below). These elements of the cytoskeleton get their name from the fact that their diameter, 8 to 10 nm, is between those of microfilaments and microtubules. (Recall that fibrous proteins are somewhat rod-like in structure, not soluble in water, except in strong concentrations of acid and alkali.)

Intermediate filaments have no role in cell movement. Their function is purely structural. They bear tension, thus maintaining the shape of the cell, and anchor the nucleus and other organelles in place. The first image in this lesson shows how intermediate filaments create a supportive scaffolding inside the cell.

The intermediate filaments are the most diverse group of cytoskeletal elements. Several types of fibrous proteins are found in the intermediate filaments. You are probably most familiar with keratin, the fibrous protein that strengthens your hair, nails, and the epidermis of the skin.