Microtubules

The Cytoskeleton Continued: Microtubules

As their name implies, microtubules are small hollow tubes. The walls of the microtubule are made of polymerized dimers of α-tubulin and β-tubulin, two globular proteins (see image below). Tubulin is a globular or spherical protein that is the main constituent of the microtubules of living cells.

With a diameter of about 25 nm, microtubules are the widest components of the cytoskeleton. They help the cell resist compression, provide a track along which vesicles move through the cell. In addition, they pull replicated chromosomes to opposite ends of a dividing cell. Like microfilaments, microtubules can dissolve 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 molecular structure of the tube. Image Attribution: OpenStax Biology

Microtubules are also the structural elements of flagella, cilia, and centrioles (the latter are the two perpendicular bodies of the centrosome). In fact, 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 will discuss below.

Flagella and Cilia

To refresh your memory, flagella (singular = flagellum) are long, hair-like structures that extend from the plasma membrane. Furthermore, they are structures used to move an entire cell (for example, sperm, Euglena). When present, the cell has just one flagellum or a few flagella (see image below).

Cartoon of a Chlamydomonas reinhadrtii cell and one Trypanosoma brucei cell with a flagella in cross-section, only showing microtubules. Image Attribution: Modification of work by Johannahoog via Wikimedia Commons (CC BY-SA 3.0)

How cilia are different from flagella

When cilia (singular = cilium) are present, however, many of them extend along the entire surface of the plasma membrane. Basically, they are short, hair-like structures that are used to move entire cells (such as paramecia) or substances along the outer surface of the cell. 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 which we refer to as 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 (see image below).

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

Summary of Cellular Components in Cells

So far, we have completed a broad survey of the components of prokaryotic and eukaryotic cells. For a summary of cellular components in prokaryotic and eukaryotic cells, see the table below.