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.
Understanding Cytoskeleton Components and Functions
Cytoskeletal fibers are constantly assembled, taken apart, and rearranged. This makes the cell responsive rather than rigid. Microtubules can switch rapidly between growth and shrinkage, a behavior called dynamic instability.
Tubulin carrying GTP tends to support growth at a microtubule end. If that protective GTP cap is lost, the fiber can rapidly shorten. A cell uses this behavior to search through space.
During division, many microtubules grow in different directions until some attach to chromosome structures. Others interact with the cell edge and help position the spindle. Centrosomes organize many of these fibers, but the cell can alter their arrangement when its needs change.
Actin is especially important near the cell surface, where it forms a dense layer called the cell cortex. This layer supports the cell membrane and helps control changes in cell shape. Growing actin filaments can push the membrane forward.
This produces broad extensions in crawling cells. Myosin proteins pull on actin fibers, creating tension. In muscle cells, organized actin and myosin produce contraction.
In many animal cells, actin and myosin form a tightening ring near the end of cell division. The ring pinches the cell into two daughter cells. Actin treadmilling helps maintain a moving edge because older subunits leave while new subunits join.
Intermediate filaments behave differently from actin and microtubules. They are less suited to rapid growth, directed motor travel, or pushing a membrane. Their main value is toughness.
Their rope-like structure lets cells tolerate pulling and stretching without tearing. Keratin filaments help protect skin cells from everyday stress. Other intermediate filaments support nerve cells, muscle cells, and the nucleus.
Nuclear lamins form a supportive layer beneath the nuclear envelope. Defects in these proteins can have serious effects.
For example, damaged keratin networks can make skin unusually fragile. This shows that cell structure is closely connected to the strength of whole tissues.
Transport inside a cell depends on direction, timing, and energy use. Vesicles carrying proteins, mitochondria needing new locations, and chromosomes being moved during division cannot simply drift efficiently through crowded cytoplasm. Motor proteins make repeated shape changes as they use ATP.
Each change helps a motor take a small step along a fiber. Kinesin and dynein usually travel in opposite directions on microtubules, while myosin moves on actin.
When studying these systems, pay close attention to the fiber type, its location, its polarity, and the job being done. A useful comparison is whether the structure mainly resists force, pushes a surface, provides a transport route, or pulls cell parts into position.