Two traps sit inside one page range. First: ribosome subunits do not add. The 80S eukaryotic ribosome is built from a 60S and a 40S subunit, and the 70S prokaryotic ribosome from a 50S and a 30S subunit — but 60+40 ≠ 80 and 50+30 ≠ 70, because "S" (Svedberg's Unit) is a sedimentation coefficient, an indirect measure of density and shape, not a linear mass unit (NCERT Class 11 Biology, Chapter 8, page 98). Second: two nine-fold tubulin structures on the same page range get their microtubule counts swapped. The axoneme of a cilium or flagellum has nine peripheral doublets of microtubules plus one central pair — the 9+2 array (page 99). The centriole has nine peripheral triplets arranged like a cartwheel around a proteinaceous hub, with no central pair at all (page 100). A centriole forms the basal body from which a cilium's or flagellum's axoneme grows, and also gives rise to the spindle fibres of animal-cell division (page 100) — but the structure it forms has a different microtubule arrangement from the centriole itself.
The cytoskeleton — microtubules, microfilaments and intermediate filaments — gives mechanical support, motility and shape to the cell (page 98); it is a separate structure from both the axoneme and the centriole, though built from the same tubulin/actin family of proteins.
The chapter also names microbodies in a single sentence — membrane-bound vesicles carrying various enzymes, in both plant and animal cells (page 102) — without naming individual types; NEET at this depth does not go further than that one sentence.
Watch-out: when a question pairs "9+2", "doublet", "triplet" and "cartwheel", identify which structure each word belongs to before picking an option — the four words split cleanly between axoneme and centriole, never mix.
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
Ribosomes were first observed under the electron microscope as dense granular particles by which scientist, and in what year?
Answer: C. NCERT Class 11 Biology, Chapter 8, page 98 names George Palade (1953) as the scientist who first observed ribosomes as dense particles under the electron microscope.
Why A is wrong: A is wrong because Robert Hooke's 1665 observation was of cork cells under a compound microscope, leading to the term 'cell' — not ribosomes, which need electron-microscope resolution.
Why B is wrong: B is wrong because Robert Brown described the nucleus in 1831, a different organelle described nearly 120 years before ribosomes were seen.
Why D is wrong: D is wrong because Camillo Golgi is associated with the Golgi apparatus, not with the discovery of ribosomes.
The prokaryotic 70S ribosome is made of which pair of subunits?
Answer: A. NCERT Class 11 Biology, Chapter 8, page 98 states the 70S ribosome's two subunits are 50S (larger) and 30S (smaller).
Why B is wrong: B is wrong because 60S and 40S are the subunits of the eukaryotic 80S ribosome, not the prokaryotic 70S one — a direct subunit swap between the two ribosome types.
Why C is wrong: C is wrong because the two subunits are not equal in size; a ribosome always has one larger and one smaller subunit, named larger/smaller, not two identical halves.
Why D is wrong: D is wrong because this pair sums to 70 by ordinary addition, but S-values are sedimentation coefficients, not additive mass units, and 40S+30S is not the NCERT-named subunit pair for either ribosome.
The cytoskeleton, as described in NCERT Class 11 Biology, is an elaborate network built from which three kinds of filamentous proteins?
Answer: B. NCERT Class 11 Biology, Chapter 8, page 98 defines the cytoskeleton as the network of microtubules, microfilaments and intermediate filaments present in the cytoplasm.
Why A is wrong: A is wrong because collagen is an extracellular structural protein, not one of the three cytoskeletal filament types named in this chapter.
Why C is wrong: C is wrong because ribosomes and centrioles are distinct organelles, not filamentous cytoskeletal proteins.
Why D is wrong: D is wrong because cristae (mitochondrial inner-membrane folds) and thylakoids (chloroplast membrane sacs) are membrane structures of other organelles, not cytoskeletal filaments.
Which of the following is NOT a function of the cytoskeleton as stated in NCERT Class 11 Biology?
Answer: D. NCERT Class 11 Biology, Chapter 8, page 98 lists mechanical support, motility and maintenance of cell shape as cytoskeleton functions; ATP synthesis by oxidative phosphorylation is a mitochondrial function, described elsewhere in the same chapter, not a cytoskeleton function.
Why A is wrong: A is wrong to pick because mechanical support IS a stated cytoskeleton function, so it does not answer a 'NOT' question.
Why B is wrong: B is wrong to pick because motility IS a stated cytoskeleton function.
Why C is wrong: C is wrong to pick because maintenance of cell shape IS a stated cytoskeleton function.
Electron microscopy of a cilium's core shows nine peripheral pairs of microtubules surrounding one central pair, all covered by the plasma membrane. What is this arrangement called, and which structure does it belong to?
Answer: A. NCERT Class 11 Biology, Chapter 8, page 99 names this the 9+2 array: nine doublets of peripheral microtubules plus a central pair, forming the axoneme (the core) of a cilium or flagellum.
Why B is wrong: B is wrong because a description with a central pair present is a 9+2 array, not 9+0; also the centriole (not the axoneme) is the structure with no central pair.
Why C is wrong: C is wrong because the centriole has nine peripheral triplets with no central pair at all — it never carries the 9+2 array described in the stem.
Why D is wrong: D is wrong because the stem describes doublets and a central pair, which is the axoneme's 9+2 array, not the centriole's nine triplets.
A structure is described as nine evenly spaced peripheral fibrils of tubulin protein, each fibril a triplet, arranged like a cartwheel around a proteinaceous hub, with no central pair of microtubules. Which structure is this?
Answer: C. NCERT Class 11 Biology, Chapter 8, page 100 describes the centriole exactly this way: nine peripheral triplet fibrils in a cartwheel organisation around a proteinaceous hub, with no central pair.
Why A is wrong: A is wrong because the axoneme's peripheral microtubules are doublets, not triplets, and it does carry a central pair — the opposite of what the stem describes.
Why B is wrong: B is wrong because a microfilament is a single actin-based cytoskeletal filament, not a nine-fold fibril assembly with a cartwheel organisation.
Why D is wrong: D is wrong because the nuclear pore complex is a channel formed at the fusion of the two nuclear-envelope membranes, unrelated to tubulin triplets or a cartwheel structure.
A centriole organises the basal body from which a flagellum's axoneme grows. Which statement correctly compares the microtubule arrangement of the centriole with that of the axoneme it gives rise to?
Answer: B. Per NCERT Class 11 Biology, Chapter 8, pages 99–100: the centriole (nine triplets, cartwheel, no central pair) forms the basal body of a cilium or flagellum, but the axoneme that grows from it has a different arrangement — nine peripheral doublets plus a central pair, the 9+2 array. Forming the basal body does not mean the axoneme copies the centriole's own microtubule count.
Why A is wrong: A is wrong because it assumes the axoneme inherits the centriole's exact triplet, no-central-pair arrangement, but NCERT names two different arrangements for the two structures.
Why C is wrong: C is wrong because it assigns the axoneme's 9+2 array to the centriole too, when the centriole is specifically described as having no central pair.
Why D is wrong: D is wrong because it swaps the two arrangements entirely — the centriole (not the axoneme) has triplets with no central pair, and the axoneme (not the centriole) has the central pair.
In the axoneme's 9+2 array, a radial spoke connects the central sheath to one tubule of each peripheral doublet. Given that there are nine peripheral doublets, how many radial spokes does one axoneme cross-section show?
Answer: D. NCERT Class 11 Biology, Chapter 8, page 99 states there are nine radial spokes, one connecting the central sheath to a tubule of each of the nine peripheral doublets in turn.
Why A is wrong: A is wrong because two is the count of central microtubules, not radial spokes — a different structure in the same cross-section.
Why B is wrong: B is wrong because seven does not match either the peripheral-doublet count (nine) or the central-pair count (two) named on this page.
Why C is wrong: C is wrong because eleven has no basis in the page's description of the axoneme's parts.
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Given
An electron micrograph shows structure X: nine evenly spaced peripheral fibrils of tubulin protein, each fibril a triplet, arranged in a cartwheel pattern around a proteinaceous hub, with radial spokes connecting the hub to the triplets and no central pair of microtubules. Structure X sits at the base of a growing flagellum.
Required
Identify structure X, and state the microtubule arrangement of the axoneme that will grow from it.
Concept
Two nine-fold tubulin structures appear on the same NCERT page range and are easy to swap in an exam: the centriole (nine triplets, cartwheel, no central pair, forms the basal body) and the axoneme (nine doublets plus a central pair, the 9+2 array, grows from the basal body). This is the axoneme-vs-centriole doublet/triplet confusion flagged as this lesson's own in-scope trap.
Formula
Not applicable. This is a structural-identification question; no formula is used, only the definitions on NCERT Class 11 Biology, Chapter 8, pages 99–100.
Substitution
Match the stem's description against the two definitions: "nine triplets, cartwheel, hub, no central pair" is the centriole (page 100). "Grows from a basal body" is the language NCERT uses for the axoneme of a cilium/flagellum (page 100), and the axoneme itself is separately defined as "nine doublets plus a central pair, the 9+2 array" (page 99).
Calculation
Since structure X matches the centriole's description exactly (triplets, cartwheel, no central pair) and sits at the base of a growing flagellum, X is acting as the basal body. The axoneme growing from it does not copy X's own triplet/no-central-pair arrangement — NCERT names a distinct arrangement, 9+2, for the axoneme itself.
Final answer
Structure X is a centriole, functioning as the basal body (nine triplet fibrils, cartwheel, no central pair — page 100). The axoneme growing from it shows the 9+2 array: nine peripheral doublets plus one central pair of microtubules (page 99) — a different arrangement from X.
Common trap
Selecting "9+2, no central pair" or "nine triplets with a central pair" — mixing one feature from the centriole with one from the axoneme — is the exact swap this lesson's in-scope trap and mistake describe.
Similar NEET-style question
A second structure Y, covered by the plasma membrane, shows nine peripheral doublets of microtubules connected by interdoublet linkers, plus a central pair enclosed by a central sheath and linked to each doublet by a radial spoke. Identify Y, and state its total count of peripheral microtubules. (Answer: Y is the axoneme of a cilium/flagellum — NCERT Class 11 Biology, Chapter 8, page 99, the 9+2 array. Nine doublets × 2 microtubules per doublet = 18 peripheral microtubules, plus the 2 central microtubules = 20 microtubules in total.)
An elaborate network of filamentous proteinaceous structures consisting of microtubules, microfilaments and intermediate filaments present in the cytoplasm is collectively referred to as the cytoskeleton. The cytoskeleton in a cell is involved in many functions such as mechanical support, motility and maintenance of the shape of the cell.
-- NCERT Class 11 Biology, Ch. 8, p. 98Many membrane bound minute vesicles called microbodies, that contain various enzymes, are present in both plant and animal cells.
-- NCERT Class 11 Biology, Ch. 8, p. 102Ribosomes are the granular structures first observed under the electron microscope as dense particles by George Palade (1953). They are composed of ribonucleic acid (RNA) and proteins and are not surrounded by any membrane. Eukaryotic ribosomes are 80S while prokaryotic ribosomes are 70S. Each ribosome has two subunits, a larger and a smaller one; the two subunits of 80S ribosomes are 60S and 40S, while those of 70S ribosomes are 50S and 30S. Here 'S' (Svedberg's Unit) stands for the sedimentation coefficient, an indirect measure of density and size. Both 70S and 80S ribosomes are composed of two subunits.
-- NCERT Class 11 Biology, Ch. 8, p. 98Cilia and flagella are hair-like outgrowths of the cell membrane. Cilia are small structures that work like oars, causing movement of either the cell or the surrounding fluid; flagella are comparatively longer and responsible for cell movement. Prokaryotic bacteria also possess flagella, structurally different from eukaryotic flagella. Electron microscopy shows both are covered with plasma membrane, with a core called the axoneme possessing microtubules running parallel to the long axis. The axoneme usually has nine doublets of radially arranged peripheral microtubules and a pair of centrally located microtubules, called the 9+2 array. The central tubules are connected by bridges and enclosed by a central sheath, connected to one tubule of each peripheral doublet by a radial spoke (nine radial spokes total); peripheral doublets are also interconnected by linkers. Both cilium and flagellum emerge from centriole-like structures called basal bodies.
-- NCERT Class 11 Biology, Ch. 8, p. 99A centrosome is an organelle usually containing two cylindrical structures called centrioles. The centrioles are surrounded by amorphous pericentriolar materials. Both centrioles in a centrosome lie perpendicular to each other, and each has an organisation like a cartwheel.
-- NCERT Class 11 Biology, Ch. 8, p. 99Centrioles are made up of nine evenly spaced peripheral fibrils of tubulin protein; each peripheral fibril is a triplet, and adjacent triplets are also linked. The central part of the proximal region of the centriole is proteinaceous, called the hub, which is connected with tubules of the peripheral triplets by radial spokes made of protein. The centrioles form the basal body of cilia or flagella, and spindle fibres that give rise to the spindle apparatus during cell division in animal cells.
-- NCERT Class 11 Biology, Ch. 8, p. 100These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.
Category: Similar Terms
Students expect the subunits of an 80S ribosome to sum to 80 (and pick 60S+20S or 40S+40S), or swap the 70S and 80S subunit pairs.
An option lists the subunits of 80S as 60S and 20S, or of 70S as 50S and 20S, or gives prokaryotes 80S ribosomes.
Per C11_BIO_08_FACT_17: 80S (eukaryotic) = 60S + 40S; 70S (prokaryotic) = 50S + 30S. S is a sedimentation coefficient, an indirect measure of density and size, so the values are not additive.
Category: Similar Terms
Students give the centriole the axoneme's 9+2 doublet arrangement, or give the axoneme triplets, because both are nine-fold tubulin structures linked to the basal body.
A match or statement question pairs 'axoneme', 'cartwheel', 'triplet' and '9+2' and swaps one pair.
Per C11_BIO_08_FACT_19 and C11_BIO_08_FACT_21: the axoneme has nine peripheral doublets plus a central pair (9+2); a centriole has nine peripheral triplet fibrils around a proteinaceous hub, in a cartwheel organisation, and no central pair.
Root cause: concept gap
Per C11_BIO_08_FACT_17, 'S' (Svedberg's Unit) is the sedimentation coefficient, an indirect measure of density and size, so 60S+40S=80S and 50S+30S=70S do not hold arithmetically even though the ribosome named 80S/70S is built from those named subunits.
treats Svedberg units as additive mass units
Root cause: term confusion
Per C11_BIO_08_FACT_19, the ciliary/flagellar axoneme has nine doublets of peripheral microtubules plus a central pair (9+2 array); per C11_BIO_08_FACT_21, the centriole is made of nine peripheral fibrils that are triplets, with no central pair.
states the centriole has a 9+2 arrangement or that the axoneme has triplets
More in Cell: Structure and Function: 10 exam traps and mistakes · 1 formula · 1 question pattern from its other lessons.
5 questions from NEET 2021, 2023, 2024, 2025. Answers verified against NTA official keys.
How many different proteins does the ribosome consist of?
Which of the following functions is carried out by cytoskeleton in a cell?
The centriole undergoes duplication during:
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