# 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. Level: High school | Reading time: 11 min | Last reviewed: 2026-10-01 Web page: https://cellnaut.com/learn/cytoskeleton-and-movement **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. ## 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. ## A skeleton that rebuilds itself Your skeleton gives your body shape, but it never changes on the timescale of minutes. The cytoskeleton 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 filament | about 7 nm | actin | Shape of the cell surface, crawling, muscle contraction | | Intermediate filament | 8 to 12 nm | keratin and related proteins | Mechanical strength; anchoring the nucleus | | Microtubule | 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. Interactive 3D model: [Neuron](https://cellnaut.com/studio/neuron/axon) - In neurons, microtubules carry cargo along the whole length of the axon. During cell division, microtubules also form the spindle that pulls chromosomes apart, which you will see in lesson 8. **Check your understanding:** Which cytoskeletal filament forms the spindle that separates chromosomes? - A. Actin filaments - B. Intermediate filaments - C. Microtubules - D. Phospholipids **Answer:** C. Microtubules The mitotic spindle is built from microtubules. ## 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 myofibrils, thread-like bundles that run its entire length. Each myofibril is a chain of repeating units called sarcomeres. 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. Interactive 3D model: [Muscle Cell](https://cellnaut.com/studio/muscle/myofibril) - Striped myofibrils in a muscle fiber. The stripes are the repeating sarcomeres. > **Common misconception** > > **"Muscle filaments get shorter when a muscle contracts."** The filaments keep the same length. They **slide** over each other, so the sarcomere as a whole becomes shorter. **Check your understanding:** What happens to actin and myosin filaments when a sarcomere shortens? - A. Both filaments shrink - B. They slide past each other - C. They break and reform - D. Myosin turns into actin **Answer:** B. They slide past each other In the sliding filament model, the filaments keep their length and slide past each other. ## 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. Interactive 3D model: [Epithelial Cell](https://cellnaut.com/studio/epithelial/junctions) - Epithelial cells link their internal ropes across junctions to act as one sheet. ## Swimming and sweeping Cells can also move by beating extensions of their surface. - A cilium 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 flagellum 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. Interactive 3D model: [Bacteria Cell](https://cellnaut.com/studio/bacteria/flagellum) - A bacterial flagellum is a rotary motor, not a beating whip. Interactive 3D model: [Sperm Cell](https://cellnaut.com/studio/sperm/flagellum) - A sperm tail is a eukaryotic flagellum: a bundle of microtubules that slide against each other to make it bend. Interactive 3D model: [Paramecium](https://cellnaut.com/studio/paramecium/cilia) - A Paramecium is covered in cilia that beat in coordinated waves. **Check your understanding:** How does a bacterial flagellum move a cell? - A. It bends back and forth like a sperm tail - B. It rotates like a propeller - C. It contracts like a muscle - D. It pulls on the nucleus **Answer:** B. It rotates like a propeller Bacterial flagella rotate. This is different from the bending motion of eukaryotic flagella. ## Sources - [Biology 2e, Chapter 4: Cell Structure (OpenStax)](https://openstax.org/books/biology-2e/pages/4-introduction) - CC BY-NC-SA 4.0. Concepts adapted and rewritten; no text reproduced verbatim.