NCERT grounding
The NCERT Class 11 textbook devotes section 8.5.8 Cilia and Flagella to these two appendages. It defines them as hair-like outgrowths of the cell membrane, then immediately introduces the axoneme, the 9+2 array, the radial spokes, the interdoublet linkers and the basal body. Figure 8.10 of the same section shows both an electron micrograph and a labelled diagrammatic cross-section — exactly the layout NEET examiners draw their match-the-following questions from. The NIOS Senior Secondary Biology Lesson 4 (4.3) supplements the NCERT account with the example pairing of Paramecium (cilia) and Euglena (a flagellum), and adds the comparative measurements that distinguish the two appendages.
"Cilia (sing.: cilium) and flagella (sing.: flagellum) are hair-like outgrowths of the cell membrane. … The axoneme usually has nine doublets of radially arranged peripheral microtubules, and a pair of centrally located microtubules. Such an arrangement of axonemal microtubules is referred to as the 9+2 array."
NCERT, Class 11 Biology, §8.5.8
That single passage is the source of practically every NEET fact tested on this topic. Read it twice: the words "doublets", "pair", "central sheath", "radial spoke", "linkers" and "basal body" are the precise tokens the question setters reuse. The rest of this page expands each token into a self-standing concept block, then closes with the bacterial-versus-eukaryotic comparison that NCERT mentions in one sentence but examiners ask about in full.
Anatomy of a cilium and a flagellum
A cilium and a flagellum share one architecture. From the outside in, three concentric layers must be named in order. First, the entire appendage is covered by the plasma membrane — it is not a separate, naked filament but a sleeve of the cell's own membrane, so anything that crosses it has crossed the cell boundary. Second, inside this membrane sleeve runs a parallel bundle of microtubules called the axoneme, which is the working machinery of bending and beating. Third, below the cell surface, embedded in the cytoplasm, sits the basal body, also known as the kinetosome — a short cylinder from which the axoneme emerges and which keeps it attached to the cell.
The microtubules of the axoneme are tubulin polymers, the same protein that makes the cytoskeletal microtubules, the spindle fibres of cell division and the peripheral fibrils of a centriole. This shared tubulin chemistry is why NEET 2016 grouped "Spindle fibres, Centrioles and Cilia" together as microtubule-based structures (see PYQ snapshot below). Memorise the family: every microtubular structure in a eukaryotic cell — cytoskeleton, spindle, centriole, basal body, cilium, flagellum — is built of tubulin.
Axoneme (above the cell surface)
Nine peripheral microtubule doublets arranged radially around one central pair of singlets, all enclosed by the plasma membrane. This is the working part that bends and beats.
Basal body (below the cell surface)
Nine peripheral microtubule triplets with no central pair — structurally identical to a centriole. This is the anchor and template; it does not project out of the cell.
The reason NEET keeps this contrast on the syllabus is that the same organelle changes its microtubule pattern across the cell surface. Below the membrane it is 9+0 with triplets; above the membrane it becomes 9+2 with doublets and singlets. A clean way to remember: basal body becomes axoneme by losing one tubule per peripheral set (triplet → doublet) and acquiring two new singlets at the centre. The plasma membrane is the dividing line where the bookkeeping changes.
Figure 1. Diagrammatic cross-section of a cilium. Nine peripheral microtubule doublets (A + B subfibres) sit in a ring just inside the plasma membrane; one pair of central singlets, joined by a bridge and enclosed in a central sheath, sits at the axis. Each A-subfibre connects to the sheath by a radial spoke, and adjacent doublets are tied together by faint interdoublet linkers. This is the canonical 9+2 array.
The 9+2 axoneme — piece by piece
NCERT lists six named elements in the axoneme. Each is asked about in NEET as either a match-the-following or a single-line statement, so they are best treated as a fixed checklist rather than as continuous prose. Take the elements one by one.
The axoneme of a cilium or flagellum is the bundle of microtubules running parallel to its long axis. NCERT names six structural elements inside the membrane sleeve; learn each by its position and its partner.
Peripheral doublets ×9
Nine pairs of microtubules arranged radially around the axis. Each doublet has an inner A-subfibre (complete 13-protofilament tubule) and an outer B-subfibre sharing some walls with A.
NCERT 8.5.8 tokenCentral pair ×1
Two singlet microtubules running along the very centre of the axoneme. They are connected by bridges and surrounded by a central sheath. NCERT notes some cilia/flagella lack this pair.
often labelled as "1 pair"Central sheath
A proteinaceous casing that encloses the central pair. It is the inner attachment surface for the radial spokes — without the sheath, the spokes have nowhere to land.
attaches spokesRadial spokes ×9
Each A-subfibre of a peripheral doublet is connected to the central sheath by a radial spoke. Because there are nine doublets, there are nine radial spokes per axoneme — a frequent NCERT-line direct fact.
count = 9Interdoublet linkers
Faint bridges that tie adjacent peripheral doublets together around the ring. They keep the nine-fold symmetry rigid while the axoneme bends.
between adjacent doubletsPlasma membrane (sleeve)
The entire axoneme is wrapped in a continuation of the cell's plasma membrane. This is why cilia and flagella are called outgrowths of the membrane, not separate filaments.
covered by membraneThe functional consequence of this architecture is that the axoneme bends by sliding. Adjacent doublets are walked past each other by dynein arms (small motor projections from each A-subfibre, not shown explicitly in NCERT Figure 8.10 but implicit in the mechanism). The radial spokes and interdoublet linkers convert this sliding into a controlled bend, because they prevent the doublets from sliding too far. Without them, the axoneme would shear apart instead of bending — which is exactly what happens in certain clinical conditions where radial-spoke proteins are missing.
Basal body — the 9+0 anchor
Below the cell surface, in the cytoplasm, sits the basal body (also called the kinetosome). NCERT describes it twice — once at the end of §8.5.8 ("Both the cilium and flagellum emerge from centriole-like structure called the basal bodies") and again in §8.5.9 ("The centrioles form the basal body of cilia or flagella, and spindle fibres that give rise to spindle apparatus during cell division in animal cells"). The point being made is structural identity: a basal body and a centriole have the same architecture.
That architecture, from NCERT 8.5.9, is "nine evenly spaced peripheral fibrils of tubulin protein. Each of the peripheral fibril is a triplet. The adjacent triplets are also linked." There is no central pair. The arrangement is therefore 9+0 — nine triplets, zero centre. The central part of the proximal region carries a proteinaceous hub with radial spokes to the triplets, giving the cartwheel appearance described for centrioles.
Figure 2. Longitudinal view through a cilium and the plasma membrane. The axoneme above the membrane has the canonical 9+2 (nine doublets + central pair). The basal body below the membrane has 9+0 (nine triplets, no central pair) — the same architecture as a centriole. The plasma membrane is the boundary at which the pattern changes.
Cilia versus flagella on the same cell
Up to this point cilia and flagella have been treated as one architecture. They are. The differences are not in structure but in number, length, beat pattern and biological role. NCERT 8.5.8 states the distinction in one sentence: "Cilia are small structures which work like oars, causing the movement of either the cell or the surrounding fluid. Flagella are comparatively longer and responsible for cell movement." NIOS 4.3 adds quantitative ranges and a beat-pattern note. The contrast is best held as a side-by-side card.
Cilia
5 – 10 µm
short outgrowths
- Numerous — often hundreds per cell.
- Beat like oars; strokes are coordinated in waves across the cell.
- Move the cell or move fluid past the cell — e.g. tracheal lining clearing mucus and dust particles.
- Classic example: Paramecium, ciliated epithelium of the trachea.
Flagella
longer
comparatively long
- Few — usually one to a handful per cell.
- Beat with a whiplash motion (NIOS 4.3).
- Responsible for cell locomotion — the cell propels itself.
- Classic example: Euglena, mammalian sperm, antherozoids of bryophytes and pteridophytes.
Note the symmetry of the differences: a cilium is short, plentiful and beats coordinately; a flagellum is long, sparse and beats independently. Both, however, share the 9+2 axoneme, the basal body, the radial spokes and the central sheath. NEET match-the-following questions exploit this by mixing a structural fact (same architecture) with a functional fact (different role) and asking which student statement is wrong. The safe move is to attribute every structural token to both, and split only on number, length, beat and example.
Bacterial flagellum versus eukaryotic flagellum
NCERT 8.4.1 introduces the bacterial flagellum separately, then 8.5.8 explicitly warns that "The prokaryotic bacteria also possess flagella but these are structurally different from that of the eukaryotic flagella." This single line is one of the most reliable NEET prompts on the topic — examiners pick a "which of the following statements about flagella is incorrect" stem and slip in a sentence such as "Bacterial flagella also have a 9+2 axoneme". They do not.
Bacterial flagellum (prokaryote)
- A single helical filament made of the protein flagellin.
- No microtubules. No axoneme. No 9+2.
- Not covered by plasma membrane — it is an external surface structure.
- Three parts: filament (longest, extends outward), hook, and a basal body embedded in the cell envelope.
- Driven by a rotary motor in the basal body that spins the filament.
- Found in motile bacteria; alongside non-motile pili and fimbriae (which are surface structures but do not move the cell).
Eukaryotic flagellum
- A bundle of microtubules — the 9+2 axoneme — built from the protein tubulin.
- Always covered by the plasma membrane as a sleeve.
- Anchored below the surface by a basal body (9+0, identical to a centriole).
- Bends by sliding of microtubule doublets, not by rotation.
- Beat pattern: whiplash; few per cell.
- Found in Euglena, sperm of animals, motile gametes of mosses, ferns and a few gymnosperms (Cycas, Ginkgo).
The word "basal body" appears on both sides of this comparison, but it means different things. In the bacterial flagellum the basal body is a motor assembly embedded in the cell envelope that rotates the filament. In the eukaryotic flagellum the basal body is a microtubular cylinder (9+0 triplets) that templates the axoneme. They share a name and a position, nothing else. NEET examiners have used this exact ambiguity to construct distractor options; the only defence is to attach the protein name (flagellin vs tubulin) to the architecture in your head.
Worked examples
1. In a typical eukaryotic flagellum, how many radial spokes are present in one transverse section of the axoneme, and what do they connect?
Solution. Each peripheral doublet has one radial spoke that runs from its A-subfibre inward to the central sheath. Because the axoneme has nine peripheral doublets, there are nine radial spokes per cross-section. The radial spokes connect each A-subfibre to the central sheath (which encloses the central pair). This is a direct NCERT 8.5.8 statement.
2. The microtubule arrangement in the basal body and in the axoneme of the same cilium are, respectively:
Solution. The basal body lies below the plasma membrane and shows the centriole pattern: nine peripheral triplets of tubulin with no central pair, written as 9+0. The axoneme lies above the plasma membrane and shows nine peripheral doublets plus one central pair, written as 9+2. Answer: basal body = 9+0; axoneme = 9+2. (The plasma membrane is the dividing surface where the bookkeeping changes from triplets to doublets and where the central pair is gained.)
3. A student writes: "Bacterial flagellum has a 9+2 axoneme like the eukaryotic flagellum but is shorter." Identify the errors.
Solution. Two errors. (i) Bacterial flagella have no 9+2 axoneme; they are made of a single helical filament of the protein flagellin, with no microtubules. (ii) Length comparisons between the two are not meaningful because the structures are unrelated; the eukaryotic flagellum is membrane-bound and microtubular, the bacterial flagellum is an external flagellin filament. The only correct half of the student's claim is that both are called "flagella". (Source: NCERT 8.4.1 and 8.5.8.)
4. Match the structure with its tubulin organisation: (A) Spindle fibre, (B) Centriole, (C) Cilium axoneme, (D) Basal body.
Solution. All four are microtubular (built of tubulin). (A) Spindle fibre — bundles of microtubules running between centrosomes/poles; no fixed 9+2 or 9+0 layout. (B) Centriole — nine peripheral triplets, no central pair → 9+0. (C) Cilium axoneme — nine peripheral doublets + central pair → 9+2. (D) Basal body — same as a centriole → 9+0 with triplets. Therefore B and D share the 9+0 pattern; C is the only 9+2 structure in this list.