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Cilia and flagella take up only a paragraph or two in NCERT, but they carry a lot of exam questions: what the 9 + 2 refers to, how a basal body differs from the axoneme, and why a bacterial flagellum is a different thing with a similar name. This post builds the structure from the outside in, explains how it bends, and uses a real medical finding to show why the details matter.
Where is this in the syllabus?
The NEET (UG) 2026 syllabus lists "Cytoskeleton, cilia, flagella, centrioles (ultra structure and function)" in the cell unit, and ciliary and flagellar movement under Locomotion and Movement. The text is sections 8.5.8 and 8.5.9 of NCERT Class 11 Biology Chapter 8, Cell: The Unit of Life, with functions in Chapter 17, Locomotion and Movement (both 2026-27 reprints). Check your own year's syllabus.
What is inside a cilium?
NCERT describes cilia and flagella as hair-like outgrowths of the cell membrane. Cilia are small and work like oars, moving either the cell or the fluid around it. Flagella are longer and move the cell. From the outside in:
Plasma membrane covering the whole structure.
Axoneme, the core, with microtubules running along its length.
Nine peripheral doublets, arranged radially and joined to each other by linkers.
A central pair of single microtubules, connected by bridges and enclosed in a central sheath.
Nine radial spokes, each joining the central sheath to one tubule of a peripheral doublet.
This arrangement is the 9 + 2 array: nine doublets plus two central tubules.
What is the basal body?
Both cilia and flagella emerge from a basal body, which NCERT calls a centriole-like structure. A centriole has a cartwheel organisation: nine evenly spaced peripheral fibrils of tubulin, each a triplet, with a proteinaceous hub at the centre joined to the triplets by radial spokes. NCERT says the centrioles form the basal bodies of cilia and flagella. So the axoneme is 9 doublets plus 2, and the basal body beneath it is 9 triplets with no central pair.
How is a bacterial flagellum different?
NCERT says bacterial flagella are "structurally different" from eukaryotic ones. A bacterial flagellum is a thin filamentous extension from the cell wall with three parts: a filament, a hook and a basal body. There is no 9 + 2 axoneme. The phrase "basal body" appears in both, which is exactly why questions test it.
How does a cilium bend?
NCERT describes the structure but not the motor. Research filled that in. Gibbons and Rowe (1965) isolated a protein with ATP-splitting (ATPase) activity from the cilia of *Tetrahymena* and named it dynein. Summers and Gibbons (1971) briefly digested sea-urchin sperm axonemes with trypsin; adding ATP then made the outer doublets actively slide apart. They proposed that the bending waves of a live sperm tail come from ATP-driven sliding between outer doublets that the rest of the structure resists, so the sliding turns into a bend. That is why the linkers and radial spokes matter: without them the doublets would just slide past each other.
What happens when cilia stop working?
In 1976, Björn Afzelius studied four subjects whose sperm were immotile. Three of them also had frequent bronchitis and sinusitis, and tests showed no mucociliary transport in their airways. Electron microscopy showed that their cilia lacked dynein arms.
NCERT explains why one defect gives both problems. Chapter 17 says the coordinated movement of cilia in the trachea removes dust and inhaled foreign particles, and that flagellar movement helps spermatozoa swim. Sperm tails and airway cilia share the same 9 + 2 machinery, so one missing part stops both.
Worked example: identify the section
A question describes three cross-sections. Name each.
Section P: a ring of nine doublets around two central microtubules, surrounded by a membrane. This is the axoneme of a cilium or eukaryotic flagellum.
Section Q: nine triplets in a cartwheel with a central hub, no central pair. This is a basal body or centriole.
Section R: no microtubule ring; the structure is a filament joined by a hook to a basal body, extending from a cell wall. This is a bacterial flagellum.
A quick count separates P from Q: P has 9 × 2 = 18 peripheral microtubules plus 2 central ones; Q has 9 × 3 = 27 peripheral ones and none in the centre. Our post on the cell as the unit of life compares all the other organelles the same way.
Which mistakes cost the most marks?
Calling the basal body 9 + 2. It is nine triplets, like a centriole.
Giving bacterial flagella a 9 + 2 axoneme. They have a filament, hook and basal body.
Saying cilia are not membrane-bound. NCERT says they are covered with plasma membrane.
Counting radial spokes wrongly. There are nine, one per doublet.
Swapping functions. Cilia move fluid or the cell; flagella, being longer, move the cell.
A practice routine
Draw the cross-section of an axoneme from memory and label all six parts. Check against NCERT Figure 8.10.
Draw a centriole beside it and write three differences.
Make one card for each function in Chapter 17 (trachea, oviduct, sperm, sponges, *Euglena*) and name cilia or flagella for each.
Two days later, redo steps 1 and 2 without notes.
Some students find the sliding idea hard to picture from a still diagram. The Fermi Simulation Lab has interactive simulations for ideas that are easier to see than read.
Tonight, draw a 9 + 2 cross-section from memory and label the radial spokes and central sheath.
