Cilia And Flagella

Cilia and flagella are hair-like projections of the cell surface whose movement depends on an internal skeleton of microtubules known as the axoneme. As described in NCERT Class 11, Chapter 8 (Cell: The Unit of Life), the axoneme of a typical motile cilium or flagellum contains nine pairs, or doublets, of microtubules arranged around the periphery and one additional pair of singlet microtubules at the centre, producing the familiar 9+2 organisation. The 9+0 option is incorrect for cilia because that pattern, lacking central tubules, characterises the basal body and the centriole rather than the motile shaft. The arrangement of eleven peripheral doublets around three central tubules does not correspond to any real cytological structure and misstates both numbers. The choice reversing the pattern, with two peripheral doublets around nine central tubules, inverts the true geometry entirely. The peripheral doublets are connected to one another by dynein arms, motor proteins that use ATP to make neighbouring doublets slide past each other, and this sliding is converted into the bending stroke of the cilium. The central pair and radial spokes coordinate the timing of that bending so the cilium beats rhythmically rather than chaotically. A structural consistency check confirms the answer: coordinated bending of a cilium requires the sliding of nine outer doublets relative to a central pair, which is possible only in the 9+2 configuration.