Flagella and cilia are microtubule-based structures that either help cells to move themselves forward as in the case of flagella or substances along the outer surface of the cell as by cilia.

Flagella (singular = flagellum) are long, wavy structures that extend from the plasma membrane and are used to move an entire cell. A well known example of a cell with a flagellum is sperm. When present, the cell has just one flagellum or a few flagella. Worthy to note here, that bacterial flagella has a different structure than eukaryotic flagella.

When cilia (singular = cilium) are present, however, many of them extend along the entire surface of the plasma membrane. They are short, hair-like structures that are used to move entire cells or substances along the outer surface of the cells. An example of cilia in the human body is the cilia lining of the respiratory tract that trap particulate matter and move it toward your nostrils for exit from the body.

Despite their differences in length and number, flagella and cilia share a common structural arrangement of microtubules called a “9 + 2 array.” This name denotes a ring of nine microtubule doublets surrounding two central microtubules. The motor proteins called dyneins are attached to each of the nine doublets. Dyneins make possible the bending movement of flagella and cilia.

As it extends from plasma membrane, the “9+2 array” structure is covered (or “sheathed”) by the membrane. At its base, also named as basal body, central microtubules disappear and “9+2” turns to a “9+0” structure. Also, instead of nine doublets, nine triplets exist in the basal body, similar to centrioles.