The Cytoskeleton

Learning Objectives

By the end of this section, you will be able to do the following:

  • Describe the cytoskeleton
  • Compare the roles of microfilaments, intermediate filaments, and microtubules
  • Compare and contrast cilia and flagella
  • Summarize the differences among the components of prokaryotic cells, animal cells, and plant cells

If you were to remove all the organelles from a cell, would the plasma membrane and the cytoplasm be the only components left? No. Within the cytoplasm, there would still be ions and organic molecules, plus a network of protein fibers that help maintain the cell’s shape, secure some organelles in specific positions, allow cytoplasm and vesicles to move within the cell, and enable cells within multicellular organisms to move. Collectively, scientists call this network of protein fibers the cytoskeleton. There are three types of fibers within the cytoskeleton: microfilaments, intermediate filaments, and microtubules ((Figure)). Here, we will examine each.

Microfilaments thicken the cortex around the cell’s inner edge. Like rubber bands, they resist tension. There are microtubules in the cell’s interior where they maintain their shape by resisting compressive forces. There are intermediate filaments throughout the cell that hold organelles in place.


Microfilaments line the inside of the plasma membrane, whereas microfilaments radiate out from the center of the cell. Intermediate filaments form a network throughout the cell that holds organelles in place.

Microfilaments

Of the three types of protein fibers in the cytoskeleton, microfilaments are the narrowest. They function in cellular movement, have a diameter of about 7 nm, and are comprised of two globular protein intertwined strands, which we call actin ((Figure)). For this reason, we also call microfilaments actin filaments.

Two intertwined actin strands comprise microfilaments.


This illustration shows two actin filaments wound together. Each actin filament is composed of many actin subunits connected together to form a chain.

ATP powers actin to assemble its filamentous form, which serves as a track for the movement of a motor protein we call myosin. This enables actin to engage in cellular events requiring motion, such as cell division in eukaryotic cells and cytoplasmic streaming, which is the cell cytoplasm’s circular movement in plant cells. Actin and myosin are plentiful in muscle cells. When your actin and myosin filaments slide past each other, your muscles contract.

Microfilaments also provide some rigidity and shape to the cell. They can depolymerize (disassemble) and reform quickly, thus enabling a cell to change its shape and move. White blood cells (your body’s infection-fighting cells) make good use of this ability. They can move to an infection site and phagocytize the pathogen.

Link to Learning

To see an example of a white blood cell in action, watch a short time-lapse video of the cell capturing two bacteria. It engulfs one and then moves on to the other.

Intermediate Filaments

Several strands of fibrous proteins that are wound together comprise intermediate filaments ((Figure)). Cytoskeleton elements get their name from the fact that their diameter, 8 to 10 nm, is between those of microfilaments and microtubules.

Intermediate filaments consist of several intertwined strands of fibrous proteins.


This illustration shows 10 intermediate filament fibers bundled together.

Intermediate filaments have no role in cell movement. Their function is purely structural. They bear tension, thus maintaining the cell’s shape, and anchor the nucleus and other organelles in place. (Figure) shows how intermediate filaments create a supportive scaffolding inside the cell.

The intermediate filaments are the most diverse group of cytoskeletal elements. Several fibrous protein types are in the intermediate filaments. You are probably most familiar with keratin, the fibrous protein that strengthens your hair, nails, and the skin’s epidermis.

Microtubules

As their name implies, microtubules are small hollow tubes. Polymerized dimers of α-tubulin and β-tubulin, two globular proteins, comprise the microtubule’s walls ((Figure)). With a diameter of about 25 nm, microtubules are cytoskeletons’ widest components. They help the cell resist compression, provide a track along which vesicles move through the cell, and pull replicated chromosomes to opposite ends of a dividing cell. Like microfilaments, microtubules can disassemble and reform quickly.

Microtubules are hollow. Their walls consist of 13 polymerized dimers of α-tubulin and β-tubulin (right image). The left image shows the tube’s molecular structure.


The left part of this figure is a molecular model of 13 polymerized dimers of alpha- and beta-tubulin joined together to form a hollow tube. The right part of this image shows the tubulin structure as a ring of spheres connected together.

Microtubules are also the structural elements of flagella, cilia, and centrioles (the latter are the centrosome’s two perpendicular bodies). In animal cells, the centrosome is the microtubule-organizing center. In eukaryotic cells, flagella and cilia are quite different structurally from their counterparts in prokaryotes, as we discuss below.

Flagella and Cilia

The flagella (singular = flagellum) are long, hair-like structures that extend from the plasma membrane and enable an entire cell to move (for example, sperm, Euglena, and some prokaryotes). When present, the cell has just one flagellum or a few flagella. However, when cilia (singular = cilium) are present, many of them extend along the plasma membrane’s entire surface. They are short, hair-like structures that move entire cells (such as paramecia) or substances along the cell’s outer surface (for example, the cilia of cells lining the Fallopian tubes that move the ovum toward the uterus, or cilia lining the cells of the respiratory tract that trap particulate matter and move it toward your nostrils.)

Despite their differences in length and number, flagella and cilia share a common structural arrangement of microtubules called a “9 + 2 array.” This is an appropriate name because a single flagellum or cilium is made of a ring of nine microtubule doublets, surrounding a single microtubule doublet in the center ((Figure)).

This transmission electron micrograph of two flagella shows the microtubules’ 9 + 2 array: nine microtubule doublets surround a single microtubule doublet. (credit: modification of work by Dartmouth Electron Microscope Facility, Dartmouth College; scale-bar data from Matt Russell)


This transmission electron micrograph shows a cross section of nine microtubule doublets that form a hollow tube. Another microtubule doublet sits in the center of the tube.

You have now completed a broad survey of prokaryotic and eukaryotic cell components. For a summary of cellular components in prokaryotic and eukaryotic cells, see (Figure).

Components of Prokaryotic and Eukaryotic Cells
Cell Component Function Present in Prokaryotes? Present in Animal Cells? Present in Plant Cells?
Plasma membrane Separates cell from external environment; controls passage of organic molecules, ions, water, oxygen, and wastes into and out of cell Yes Yes Yes
Cytoplasm Provides turgor pressure to plant cells as fluid inside the central vacuole; site of many metabolic reactions; medium in which organelles are found Yes Yes Yes
Nucleolus Darkened area within the nucleus where ribosomal subunits are synthesized. No Yes Yes
Nucleus Cell organelle that houses DNA and directs synthesis of ribosomes and proteins No Yes Yes
Ribosomes Protein synthesis Yes Yes Yes
Mitochondria ATP production/cellular respiration No Yes Yes
Peroxisomes Oxidize and thus break down fatty acids and amino acids, and detoxify poisons No Yes Yes
Vesicles and vacuoles Storage and transport; digestive function in plant cells No Yes Yes
Centrosome Unspecified role in cell division in animal cells; microtubule source in animal cells No Yes No
Lysosomes Digestion of macromolecules; recycling of worn-out organelles No Yes Some
Cell wall Protection, structural support, and maintenance of cell shape Yes, primarily peptidoglycan No Yes, primarily cellulose
Chloroplasts Photosynthesis No No Yes
Endoplasmic reticulum Modifies proteins and synthesizes lipids No Yes Yes
Golgi apparatus Modifies, sorts, tags, packages, and distributes lipids and proteins No Yes Yes
Cytoskeleton Maintains cell’s shape, secures organelles in specific positions, allows cytoplasm and vesicles to move within cell, and enables unicellular organisms to move independently Yes Yes Yes
Flagella Cellular locomotion Some Some No, except for some plant sperm cells
Cilia Cellular locomotion, movement of particles along plasma membrane’s extracellular surface, and filtration Some Some No

Section Summary

The cytoskeleton has three different protein element types. From narrowest to widest, they are the microfilaments (actin filaments), intermediate filaments, and microtubules. Biologists often associate microfilaments with myosin. They provide rigidity and shape to the cell and facilitate cellular movements. Intermediate filaments bear tension and anchor the nucleus and other organelles in place. Microtubules help the cell resist compression, serve as tracks for motor proteins that move vesicles through the cell, and pull replicated chromosomes to opposite ends of a dividing cell. They are also the structural element of centrioles, flagella, and cilia.

Review Questions

Which of the following have the ability to disassemble and reform quickly?

  1. microfilaments and intermediate filaments
  2. microfilaments and microtubules
  3. intermediate filaments and microtubules
  4. only intermediate filaments

Which of the following do not play a role in intracellular movement?

  1. microfilaments and intermediate filaments
  2. microfilaments and microtubules
  3. intermediate filaments and microtubules
  4. only intermediate filaments

In humans, _____ are used to move a cell within its environment while _____ are used to move the environment relative to the cell.

  1. cilia, pseudopodia
  2. flagella; cilia
  3. microtubules; flagella
  4. microfilaments; microtubules

Free Response

What are the similarities and differences between the structures of centrioles and flagella?

How do cilia and flagella differ?

Describe how microfilaments and microtubules are involved in the phagocytosis and destruction of a pathogen by a macrophage.

Compare and contrast the boundaries that plant, animal, and bacteria cells use to separate themselves from their surrounding environment.

Glossary

cilium
(plural = cilia) short, hair-like structure that extends from the plasma membrane in large numbers and functions to move an entire cell or move substances along the cell’s outer surface
cytoskeleton
protein fiber network that collectively maintains the cell’s shape, secures some organelles in specific positions, allows cytoplasm and vesicles to move within the cell, and enables unicellular organisms to move independently
flagellum
(plural = flagella) long, hair-like structure that extends from the plasma membrane and moves the cell
intermediate filament
cytoskeletal component, comprised of several fibrous protein intertwined strands, that bears tension, supports cell-cell junctions, and anchors cells to extracellular structures
microfilament
the cytoskeleton system’s narrowest element; it provides rigidity and shape to the cell and enables cellular movements
microtubule
the cytoskeleton system’s widest element; it helps the cell resist compression, provides a track along which vesicles move through the cell, pulls replicated chromosomes to opposite ends of a dividing cell, and is the structural element of centrioles, flagella, and cilia