Motion (Flagellar Axoneme)
Stained glass
31" h x 28" w


ross section of a flagellum showing the classical ""9 + 2"" structure of the axoneme.

Flagella and cilia, from single-celled protozoa to the sperm and respiratory epithelial cells of mammals, have a unique internal structure, the axoneme, which is the basis for their movement. Known as “"9+2,"” the principal components of this structure include 9 outer doublet microtubules and 2 central singlets (red), composed of the protein, tubulin. Remarkably, the “9 + 2” pattern of flagella and cilia is the same from protozoa to human. Never 9+1, 8 +2, or any other arrangement in nature, the universality of this pattern over a billion years of evolution has never been explained and remains one of the fundamental mysteries of cell biology. 

The flagellar axoneme, also includes the motor protein, dynein (yellow) which “walks” on the back of adjacent microtubules to create flagellar movement, using energy obtained from hydrolysis of ATP. Additional components include tektins, nexins, spokes, and the central sheath. One sector, at about 3 o'clock, is intentionally colored differently, as this sector is known to have biochemical differences from the others. 

The circular pattern capturess the axoneme structure as seen in cross section, while the outer stripes represent a longitudinal (lengthwise) view, typically asymmetric.