The cytoskeleton is a network of protein fibers that gives eukaryotic cells shape, organization, and movement. This cheat sheet covers the three main cytoskeleton components and the jobs they perform in cells. Students need this reference to connect cell structure with processes such as mitosis, intracellular transport, muscle contraction, and cell crawling.
It also helps compare fibers by size, protein type, polarity, and function.
The three major fiber types are microtubules, microfilaments, and intermediate filaments. Microtubules are made of tubulin, microfilaments are made of actin, and intermediate filaments are made of several rope-like proteins such as keratins. Microtubules and microfilaments are polar, meaning they have plus and minus ends that control growth and motor movement.
Motor proteins use ATP to move cargo, chromosomes, and cell structures along cytoskeletal tracks.
Key Facts
- Microtubules are hollow tubes made of alpha-tubulin and beta-tubulin dimers and are about 25 nm in diameter.
- Microfilaments are solid actin fibers made of actin monomers and are about 7 nm in diameter.
- Intermediate filaments are strong rope-like fibers about 10 nm in diameter that resist stretching and mechanical stress.
- Microtubules have polarity, with a plus end that usually grows faster and a minus end often anchored near the centrosome.
- Actin filaments have polarity, and actin treadmilling occurs when subunits add at the plus end while leaving at the minus end.
- Kinesin usually moves cargo toward the microtubule plus end, while dynein usually moves cargo toward the microtubule minus end.
- The mitotic spindle is built mostly from microtubules and separates chromosomes during cell division.
- ATP hydrolysis powers many cytoskeletal motor proteins, including myosin, kinesin, and dynein.
Vocabulary
- Cytoskeleton
- A dynamic network of protein fibers that supports cell shape, movement, internal organization, and division.
- Microtubule
- A hollow cytoskeletal fiber made of tubulin that supports transport, cilia, flagella, and chromosome movement.
- Microfilament
- A thin actin fiber that supports cell crawling, shape changes, muscle contraction, and cytokinesis.
- Intermediate Filament
- A durable cytoskeletal fiber that gives cells tensile strength and helps anchor structures such as the nucleus.
- Motor Protein
- A protein that uses ATP energy to move along cytoskeletal fibers or move cellular cargo.
- Centrosome
- A microtubule-organizing center in animal cells that helps form and anchor microtubules.
Common Mistakes to Avoid
- Confusing microtubules with microfilaments is wrong because microtubules are tubulin-based hollow tubes, while microfilaments are actin-based solid fibers.
- Saying all cytoskeleton fibers are permanent is wrong because many cytoskeletal structures rapidly assemble and disassemble as cell needs change.
- Assuming intermediate filaments are used as motor protein tracks is wrong because most directed motor transport occurs on microtubules and actin filaments.
- Forgetting fiber polarity is wrong because microtubule and actin plus and minus ends affect growth direction and motor protein movement.
- Mixing up kinesin and dynein direction is wrong because kinesin usually moves toward microtubule plus ends, while dynein usually moves toward minus ends.
Practice Questions
- 1 A cytoskeletal fiber is measured at about 25 nm in diameter and is made of tubulin dimers. Which fiber type is it, and what is one major function?
- 2 A vesicle moves from the cell edge toward the centrosome along a microtubule. Which motor protein is most likely involved, kinesin or dynein?
- 3 An actin filament gains 80 subunits at its plus end and loses 65 subunits at its minus end during the same time interval. What is the net change in filament length in subunits?
- 4 A drug prevents microtubules from shortening during mitosis. Explain why this would disrupt chromosome separation.