The Cytoskeleton and Cell Movement
The protein scaffolding that gives cells their shape, moves cargo, and powers everything from crawling immune cells to flexing muscles.
Quick answer: The cytoskeleton is a network of protein filaments that gives a cell its shape, moves cargo and powers movement. Microtubules form transport tracks, actin filaments work with myosin to drive crawling and muscle contraction, and intermediate filaments give mechanical strength.
What you will learn
- Name the three types of cytoskeletal filaments and give one job for each.
- Explain how microtubules act as tracks for transport.
- Describe how actin and myosin shorten a sarcomere.
- Contrast bacterial flagella with eukaryotic cilia and flagella.
A skeleton that rebuilds itself
Your skeleton gives your body shape, but it never changes on the timescale of minutes. The cytoskeletonCytoskeletonA network of protein filaments inside the cell that gives it shape, anchors organelles, and provides tracks and motors for movement. is different. It is a network of protein filaments that gives a cell its shape and holds organelles in place, and it can be taken apart and reassembled in seconds. That is how cells crawl, divide, and change shape.
There are three main types of filament.
| Filament | Width | Built from | Main jobs |
|---|---|---|---|
| Actin filamentActin filamentA thin, flexible protein thread (about 7 nm wide) in the cytoskeleton. Actin filaments shape the cell surface, drive crawling movement, and slide past myosin to make muscles contract. | about 7 nm | actin | Shape of the cell surface, crawling, muscle contraction |
| Intermediate filamentIntermediate filamentA rope-like protein fiber (8 to 12 nm wide) that gives cells mechanical strength. Keratin in skin cells is one example. | 8 to 12 nm | keratin and related proteins | Mechanical strength; anchoring the nucleus |
| MicrotubuleMicrotubuleA hollow protein tube about 25 nm wide. Microtubules act as tracks for transport, form the mitotic spindle, and make up the core of cilia and eukaryotic flagella. | about 25 nm | tubulin | Transport tracks, cell division, cilia and flagella |
Microtubules: highways inside the cell
A microtubule is a hollow tube assembled from tubulin proteins. In most animal cells it radiates out from a region near the nucleus, like the spokes of a wheel.
Motor proteins use these tubes as roads. A motor called kinesin walks toward the cell edge carrying vesicles, while dynein walks the other way. Each step is powered by one ATP. In a neuron, this "axonal transport" carries cargo along an axon that can be a metre long.
During cell division, microtubules also form the spindle that pulls chromosomes apart, which you will see in lesson 8.
Which cytoskeletal filament forms the spindle that separates chromosomes?
Actin and muscle contraction
Actin filaments sit just under the plasma membrane, giving the cell its outline. In crawling cells such as white blood cells, new actin filaments grow at the front edge and push the membrane forward.
Actin does its most dramatic work in muscle. A muscle fiber is packed with myofibrilsMyofibrilA long, thread-like bundle of contractile proteins inside a muscle fiber, made of repeating units called sarcomeres., thread-like bundles that run its entire length. Each myofibril is a chain of repeating units called sarcomeresSarcomereThe repeating contractile unit of a myofibril, in which actin and myosin filaments slide past each other to shorten the muscle.. In a sarcomere, thin actin filaments overlap with thick filaments made of the motor protein myosin.
When a nerve signal arrives, calcium is released inside the fiber. Myosin heads grab the actin filaments, bend, and let go, again and again, using ATP at every cycle. The effect is that the actin filaments slide inward past the myosin ones, the sarcomere shortens, and the muscle contracts. This is the sliding filament model.
What happens to actin and myosin filaments when a sarcomere shortens?
Intermediate filaments: cellular rope
Intermediate filaments are the toughest and least dynamic. They twist together into rope-like fibers that hold up well under tension. Skin cells are filled with keratin, the protein of hair and nails. Epithelial cells also use intermediate filaments to anchor into junctions between neighbors, so that a sheet of cells can resist being stretched or torn.
Swimming and sweeping
Cells can also move by beating extensions of their surface.
- A ciliumCiliumA short, hair-like projection from the cell surface built around microtubules. Cilia beat to move fluid across a surface or to move the cell itself. is a short projection with a core of microtubules. Cilia usually occur in large numbers and beat in waves. The cells lining your airways have cilia that sweep mucus and trapped dust up and out of your lungs.
- A flagellumFlagellumA long, whip-like projection that moves a cell. Bacterial flagella spin like a propeller; eukaryotic flagella bend back and forth. is longer and usually occurs singly or in pairs. A sperm cell is propelled by a eukaryotic flagellum, which bends in a whip-like motion.
Bacterial flagella look similar in a picture but work in a completely different way. A bacterial flagellum is made of a protein called flagellin and spins around like a propeller, driven by a tiny rotary motor in the cell wall. The motor is powered by the flow of ions across the membrane, not by ATP directly.
How does a bacterial flagellum move a cell?
Key takeaways
- The three filament types are microtubules, actin filaments (microfilaments) and intermediate filaments.
- Motor proteins such as kinesin and dynein walk along microtubules carrying vesicles.
- A muscle contracts when myosin pulls actin filaments past each other and shortens the sarcomere.
- Bacterial flagella are rotating protein filaments, while eukaryotic cilia and flagella are built from microtubules and bend to beat.
Sources
- Biology 2e, Chapter 4: Cell Structure (OpenStax) Concepts adapted and rewritten; no text reproduced verbatim.
Last reviewed October 1, 2026