3.4 Cytoskeleton and Cell Movement

Cytoskeleton and Cell Movement

The cytoskeleton is a network of protein filaments that gives cells their shape, provides structural support, and enables movement. Understanding it is key to grasping how cells divide, how tissues heal, and even how cancer spreads.

Three types of filaments make up the cytoskeleton: microfilaments, intermediate filaments, and microtubules. Cells can also have specialized appendages (cilia and flagella) built from cytoskeletal components that drive movement at the cellular level.

more resources to help you study

practice questions

Cytoskeletal Components

Microfilaments

Microfilaments are the thinnest cytoskeletal fibers at about 7 nm in diameter. They're built from actin proteins twisted into a double-helix structure.

Their main jobs include:

• Cell shape and mechanical support — actin filaments form a dense mesh just beneath the plasma membrane, giving the cell its contour
• Cell movement — actin polymerization pushes the membrane outward to form pseudopodia (temporary cytoplasmic projections that let cells crawl)
• Muscle contraction — actin interacts with myosin motor proteins to generate the sliding force behind muscle movement
• Cytokinesis — during cell division, actin filaments assemble into a contractile ring that pinches the cell in two

Intermediate Filaments

Intermediate filaments are the toughest of the three, measuring 8–12 nm in diameter. Unlike the other two types, they're built from a variety of different proteins depending on the cell type, including keratin (in epithelial cells), vimentin (in connective tissue cells), and lamin (lining the nucleus).

• Provide mechanical strength and resistance to shear stress, which is why they're especially abundant in cells that experience physical force, like skin epithelial cells
• Anchor organelles in place within the cytoplasm
• Form the nuclear lamina, a meshwork of lamin filaments that lines the inner nuclear membrane and helps maintain nuclear shape

A useful way to remember intermediate filaments: they're the "ropes" of the cytoskeleton. They don't actively drive movement like actin or microtubules, but they hold everything together under stress.

Microtubules

Microtubules are the largest cytoskeletal components at 25 nm in diameter. They're hollow cylinders built from repeating dimers of $\alpha$-tubulin and $\beta$-tubulin.

• Dynamic instability — microtubules constantly grow and shrink by adding or removing tubulin dimers at their ends. This lets the cell rapidly reorganize its internal structure.
• Intracellular transport — motor proteins like kinesin and dynein walk along microtubules carrying organelles and vesicles to specific destinations
• Mitotic spindle — during cell division, microtubules form the spindle apparatus that separates chromosomes
• Organelle positioning — microtubules help determine where organelles like the Golgi apparatus sit within the cell

Microtubules radiate outward from the centrosome (the main microtubule-organizing center, or MTOC) near the nucleus, creating a transport network that spans the entire cell.

Cellular Appendages

Cilia

Cilia are short, hair-like projections that extend from the cell surface, often in large numbers. Their internal structure follows a 9+2 arrangement: 9 outer doublets of microtubules surrounding 2 central singlets. Each cilium is anchored to the cell by a basal body and covered by the plasma membrane.

Cilia beat in a coordinated, wave-like rhythm to move fluids or particles across the cell surface. A classic example: the epithelial cells lining your respiratory tract use cilia to sweep mucus (and trapped debris) up and out of the lungs. Some single-celled organisms like Paramecium use cilia for locomotion.

Flagella

Flagella share the same 9+2 microtubule structure as cilia, but they're much longer and typically present in small numbers (often just one or two per cell). Instead of the back-and-forth beating of cilia, flagella move in an undulating, whip-like motion that propels the cell through liquid.

The best-known example in humans is the sperm cell, which uses a single flagellum to swim. Certain protists like Euglena also use flagella for locomotion.

Cell Movement

Cell Motility

Cell motility refers to a cell's ability to move from one location to another. This process depends heavily on microfilaments and involves the formation of cytoplasmic protrusions at the cell's leading edge:

• Pseudopodia — broad, blunt extensions (think of an amoeba)
• Lamellipodia — flat, sheet-like extensions at the front of a crawling cell
• Filopodia — thin, finger-like projections that sense the environment

Here's how a cell crawls, step by step:

1. Actin polymerizes at the leading edge, pushing the membrane forward to form a protrusion
2. The protrusion attaches to the surface using cell adhesion molecules (like integrins) that grip the extracellular matrix
3. The rest of the cell contracts and pulls itself forward
4. Adhesions at the trailing edge release, and the rear of the cell retracts

Cell motility is critical during embryonic development, wound healing, and immune responses (white blood cells migrate toward infection sites). It also plays a role in cancer metastasis, where tumor cells use these same mechanisms to invade surrounding tissues and spread to distant organs.

Cytoplasmic Streaming

Cytoplasmic streaming is the directed flow of cytoplasm within a cell, typically in a circular pattern. It's driven by myosin motor proteins walking along actin microfilaments, dragging the cytoplasm along with them.

This process helps distribute nutrients, organelles, and other materials throughout the cell, which is especially important in large cells where diffusion alone would be too slow. It's particularly prominent in large plant cells like Chara (a freshwater algae), where streaming keeps chloroplasts circulating to maximize light exposure for photosynthesis.