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301. A free \(80S\) ribosome in the cytoplasm of a eukaryotic cell becomes attached to rough endoplasmic reticulum and begins synthesising a polypeptide that enters the ER lumen. Which change is most accurately described?
ⓐ. the ribosome changes to \(70S\) because it is now organelle-associated
ⓑ. the ribosome acquires a surrounding membrane and becomes an ER vesicle
ⓒ. it remains an \(80S\) ribosome, but its location and product route change
ⓓ. its \(60S\) and \(40S\) subunits are replaced by \(50S\) and \(30S\) subunits
Correct Answer: it remains an \(80S\) ribosome, but its location and product route change
Explanation: Free cytoplasmic ribosomes and ribosomes attached to rough endoplasmic reticulum in a eukaryotic cell belong to the \(80S\) class and contain \(60S\) and \(40S\) subunits. Attachment to the ER changes the ribosome's location and directs the growing polypeptide into the endomembrane pathway, but it does not change the sedimentation class or subunit composition of the ribosome. The particle remains on the cytosolic surface of the ER and does not acquire its own enclosing membrane or become an ER vesicle. The decisive distinction is between ribosome identity and functional location. Association with rough ER alters the route followed by the newly synthesised protein, while the translating particle remains a eukaryotic cytoplasmic \(80S\) ribosome. No replacement by \(50S\) and \(30S\) subunits occurs during this transfer.
302. An unknown ribosome is isolated from inside a membrane-bound organelle of a eukaryotic cell. It belongs to the \(70S\) class. The organelle could most reasonably be:
ⓐ. a lysosome or Golgi vesicle
ⓑ. a mitochondrion or chloroplast
ⓒ. a vacuole or peroxisome
ⓓ. a nucleus or smooth ER cisterna
Correct Answer: a mitochondrion or chloroplast
Explanation: Mitochondria and chloroplasts contain internal \(70S\) ribosomes, even though they occur within eukaryotic cells. This location-qualified exception distinguishes organellar ribosomes from the \(80S\) ribosomes of the eukaryotic cytoplasm. Lysosomes, vacuoles, peroxisomes and Golgi vesicles do not contain their own ribosomal populations. The nucleus is involved in ribosomal-subunit formation, but functional ribosomes are not normally retained there as the organelle's internal translation system. Smooth endoplasmic reticulum lacks attached ribosomes. The combination of membrane-bound organelle, eukaryotic cell and \(70S\) ribosome points specifically toward a mitochondrion or chloroplast. The conclusion remains limited to the feature actually described and does not turn a typical pattern into a universal rule. The organellar location is the qualifier that makes the \(70S\) class compatible with a eukaryotic cell.
303. Match each ribosomal particle in Column I with its subunit organisation in Column II. A Column II entry is used once.
| Column I | Column II |
|---|---|
| P. Complete \(80S\) ribosome | 1. \(30S\) |
| Q. Large subunit of \(80S\) ribosome | 2. \(50S+30S\) |
| R. Complete \(70S\) ribosome | 3. \(60S+40S\) |
| S. Small subunit of \(70S\) ribosome | 4. \(60S\) |
ⓐ. P-3, Q-4, R-2, S-1
ⓑ. P-2, Q-3, R-1, S-4
ⓒ. P-4, Q-1, R-3, S-2
ⓓ. P-3, Q-2, R-4, S-1
Correct Answer: P-3, Q-4, R-2, S-1
Explanation: A complete \(80S\) ribosome contains a large \(60S\) subunit and a small \(40S\) subunit. A complete \(70S\) ribosome contains a large \(50S\) subunit and a small \(30S\) subunit. The four entries separately represent a complete \(80S\) ribosome, its large subunit, a complete \(70S\) ribosome and the small \(70S\) subunit. This gives P-3, Q-4, R-2 and S-1. The Svedberg labels are sedimentation coefficients, so the complete-ribosome value is not obtained through ordinary numerical addition. The mappings must be recalled as experimentally defined particle classes rather than treated as simple arithmetic sums. The complete correspondence is P-3, Q-4, R-2, S-1. Each match must keep the complete particle separate from an individual subunit despite the related Svedberg labels.
304. Assertion: A \(50S\) subunit and a \(30S\) subunit form a \(70S\) ribosome rather than an \(80S\) ribosome.
Reason: Svedberg values are sedimentation coefficients influenced by particle size, shape and density and are not ordinary additive mass units.
ⓐ. Both Assertion and Reason are true, and Reason correctly explains the Assertion
ⓑ. Both Assertion and Reason are true, but Reason does not explain the Assertion
ⓒ. Assertion is true, but Reason is false; the Reason cannot explain the Assertion
ⓓ. Assertion is false, but Reason is true; the Reason cannot explain the Assertion
Correct Answer: Both Assertion and Reason are true, and Reason correctly explains the Assertion
Explanation: The Assertion is correct because the large \(50S\) and small \(30S\) subunits associate to form the complete \(70S\) ribosome. The apparent numerical mismatch arises because S does not represent an additive mass unit. It is the Svedberg sedimentation coefficient, which reflects how rapidly a particle sediments under centrifugation and depends on features including mass, shape and density. When two subunits associate, their overall sedimentation behaviour changes in a way that cannot be predicted by simple addition. The Reason directly explains why \(50+30\) does not yield an \(80S\) ribosome. The Reason therefore supplies the physical basis for the Assertion: association changes the particle's overall sedimentation behaviour, so the subunit labels cannot be combined as ordinary numbers.
305. Four proposed ribosomal organisations are shown below.
The pair of internally consistent records is:
| Record | Complete ribosome | Large subunit | Small subunit |
|---|---|---|---|
| P | \(80S\) | \(60S\) | \(40S\) |
| Q | \(70S\) | \(50S\) | \(30S\) |
| R | \(80S\) | \(50S\) | \(30S\) |
| S | \(70S\) | \(60S\) | \(40S\) |
ⓐ. P and R
ⓑ. Q and S
ⓒ. P and Q
ⓓ. R and S
Correct Answer: P and Q
Explanation: Record P correctly assigns \(60S\) and \(40S\) subunits to the complete \(80S\) ribosome. Record Q correctly assigns \(50S\) and \(30S\) subunits to the complete \(70S\) ribosome. Records R and S exchange the two subunit combinations and are inconsistent. The sedimentation values should be understood as experimentally defined relationships rather than ordinary arithmetic. the fact that \(60+40\) appears to equal \(100\) does not contradict formation of an \(80S\) ribosome, and \(50S\) plus \(30S\) forms a \(70S\) particle rather than an \(80S\) particle. P and Q preserve the correct biological pairings. Sedimentation classes, subunit composition, cellular location and translation function must be interpreted together. Both records preserve valid subunit sums for their ribosome classes. The two accepted records preserve the non-additive sedimentation relations of their complete ribosomes and subunits. Record P correctly pairs an \(80S\) ribosome with \(60S\) and \(40S\) subunits, while record Q correctly pairs a \(70S\) ribosome with \(50S\) and \(30S\) subunits.
306. The letter S in \(70S\), \(80S\), \(50S\) and \(30S\) refers to:
ⓐ. the sedimentation coefficient of the particle during centrifugation
ⓑ. the number of structural proteins present in the particle
ⓒ. the percentage of ribosomal RNA present in the particle
ⓓ. the molecular mass of the particle measured additively
Correct Answer: the sedimentation coefficient of the particle during centrifugation
Explanation: The symbol S refers to the Svedberg sedimentation coefficient, which describes how a particle sediments during centrifugation. Its value depends on several physical features, including the particle’s size, shape and density. It is therefore not a direct count of proteins, a percentage of ribosomal RNA or a simple measurement of molecular mass. Sedimentation coefficients are also not arithmetically additive. When ribosomal subunits associate, the shape and sedimentation behaviour of the complete particle differ from those of the separated subunits. This explains why \(50S\) and \(30S\) subunits form a \(70S\) ribosome rather than an \(80S\) ribosome, and why \(60S\) and \(40S\) subunits form an \(80S\) ribosome rather than a \(100S\) particle.
307. Consider the following statements about the cytoskeleton.
I. It is formed by microtubules, microfilaments and intermediate filaments.
II. It consists of a proteinaceous network within the cytoplasm.
III. It contributes to mechanical support, motility and maintenance of cell shape.
IV. It is a single membrane-bound organelle containing digestive enzymes.
ⓐ. I, III and IV only
ⓑ. I, II and IV only
ⓒ. II, III and IV only
ⓓ. I, II and III only
Correct Answer: I, II and III only
Explanation: The cytoskeleton is an organised proteinaceous network extending through the cytoplasm. Its major filament categories are microtubules, microfilaments and intermediate filaments. Together, these components provide internal support, help maintain or alter cellular shape and contribute to movement of cells or structures within them. The cytoskeleton is not enclosed by one surrounding membrane and is not a digestive organelle. That description would be more appropriate for a lysosomal compartment. Statements I, II and III combine composition, organisation and principal functions, whereas statement IV incorrectly converts a distributed structural network into a membrane-bound vesicle. Within cytoskeletal organisation, related structures cannot be interchanged; each occupies a different place or role. The fourth statement wrongly makes the cytoskeleton membrane-bound.
308. A treatment selectively disrupts one of the three protein filament systems of a eukaryotic cell, while the other two filament systems and all membrane-bound organelles remain intact. The most justified conclusion is:
ⓐ. The cytoskeleton remains fully functional because two filament systems can perform every role
ⓑ. The cytoskeleton remains incomplete, so some support, shape or motility functions may decline
ⓒ. Only membrane-bound organelles are affected, while cytoskeletal functions remain unchanged
ⓓ. All cytoskeletal functions cease immediately, even though two filament systems remain intact
Correct Answer: The cytoskeleton remains incomplete, so some support, shape or motility functions may decline
Explanation: The cytoskeleton is an organised network of microtubules, microfilaments and intermediate filaments that contributes to mechanical support, maintenance of shape and cellular motility. Selective disruption of one filament system does not erase the two systems stated to remain intact, so the cytoskeleton is altered and incomplete rather than wholly absent. The exact effect depends on which component has been damaged and which of its contributions cannot be compensated by the remaining network. Membrane-bound organelles do not arise by conversion of cytoskeletal filaments, and the nuclear envelope is a membrane system rather than a structure formed solely by one filament class. The changed condition supports a limited prediction: some support, shape or motility functions may decline while other filament systems and cellular compartments persist. This distinguishes network integration from an all-or-none view of cellular structure.
309. A drug disrupts much of the cytoskeletal network while leaving the plasma membrane, ribosomes and mitochondria initially intact. Which combination of effects is most likely?
ⓐ. Improved maintenance of shape and increased directed motility
ⓑ. Immediate loss of all ATP synthesis and messenger-RNA translation
ⓒ. Reduced support, altered shape and impaired cellular movement
ⓓ. Formation of a rigid cell wall around the cytoplasm
Correct Answer: Reduced support, altered shape and impaired cellular movement
Explanation: The cytoskeleton contributes directly to cellular architecture and motility. Disrupting its filament network weakens internal mechanical support, makes normal shape harder to maintain and interferes with movements that depend on organised cytoskeletal elements. Preserved mitochondria and ribosomes exclude complete immediate loss of ATP production or translation. A cytoskeletal defect also does not produce an external cell wall. The expected pattern follows from the common functions of microtubules, microfilaments and intermediate filaments as an integrated support and movement system. Several structural and motile defects may appear even though major membrane-bound organelles remain present. Protein filament systems support shape, movement and mechanical organisation without forming one membrane-bound organelle. The combination of altered shape, intracellular organisation and movement follows from loss of filament networks rather than immediate failure of ATP production. An intact plasma membrane cannot substitute for the internal filament network that organises shape and movement.
310. Two animal cells are observed before and after a treatment.
The treatment most directly affected the:
| Observation | Before treatment | After treatment |
|---|---|---|
| Cell outline | Stable | Irregular and unstable |
| Movement of cellular projections | Coordinated | Greatly reduced |
| ATP production | Normal | Initially normal |
ⓐ. cytoskeletal network
ⓑ. lysosomal hydrolases
ⓒ. nuclear chromatin
ⓓ. mitochondrial matrix
Correct Answer: cytoskeletal network
Explanation: The treatment changes two functions strongly associated with the cytoskeleton: maintenance of cell form and coordinated movement. Initial preservation of ATP production argues against a primary mitochondrial defect. Damage to lysosomal hydrolases would principally affect intracellular digestion, while chromatin disruption would alter genetic functions rather than produce this immediate paired mechanical pattern. The cytoskeleton forms a protein network that reinforces the cell and participates in movement of the cell and its projections. The combination of unstable outline and impaired movement identifies a structural-network defect more directly than failure of a membrane-bound organelle. Protein filament systems support shape, movement and mechanical organisation without forming one membrane-bound organelle. Loss of internal support and movement with preserved membrane and organelles identifies disruption of the cytoskeletal framework. A weakened cell outline together with reduced movement of cellular projections, despite normal ATP supply, points to disruption of the cytoskeleton, which supports shape and participates in motility.
311. An unknown intracellular structure extends throughout the cytoplasm, is composed of several protein filament systems and has no single surrounding membrane. It is best classified as:
ⓐ. the nucleoplasm
ⓑ. the endomembrane system
ⓒ. a giant lysosome
ⓓ. the cytoskeleton
Correct Answer: the cytoskeleton
Explanation: A distributed network of protein filaments extending through the cytoplasm is the cytoskeleton. It includes microtubules, microfilaments and intermediate filaments and contributes to support, shape and motility. The endomembrane system consists of membrane-bound organelles such as ER, Golgi apparatus, lysosomes and vacuoles rather than one continuous non-membranous filament network. A lysosome is a discrete single-membrane digestive vesicle, and nucleoplasm is the material inside the nucleus. The description combines network distribution, protein composition and absence of one enclosing membrane, all of which support cytoskeletal identity. The defining property is linked directly with its biological role rather than treated as an isolated name. A network distributed through the cytoplasm is distinguished from a discrete organelle by both its extent and its lack of one enclosing boundary.
312. Cilia are generally distinguished from flagella in eukaryotic cells because cilia are:
ⓐ. internal protein fibres that never project from the cell
ⓑ. bacterial appendages composed of filament, hook and basal body
ⓒ. shorter projections that move the cell or surrounding fluid
ⓓ. longer projections used only for attachment
Correct Answer: shorter projections that move the cell or surrounding fluid
Explanation: Eukaryotic cilia are relatively short, often numerous projections that perform oar-like movements. Their beating may propel a cell or move fluid and suspended material across a cellular surface. Eukaryotic flagella are generally longer and are used chiefly to move the cell. Both contain a membrane-covered axoneme and arise from basal bodies, so the distinction is based mainly on relative length, number and characteristic movement rather than on complete structural unrelatedness. The bacterial filament-hook-basal-body arrangement belongs to a fundamentally different flagellar system. Cilia are correctly described as shorter projections capable of moving either the cell or the surrounding medium.
313. Two eukaryotic cells possess membrane-covered projections. Cell P has many short projections that sweep fluid across its surface. Cell Q has one long projection that propels the entire cell. P and Q possess respectively:
ⓐ. pili and fimbriae
ⓑ. cilia and a flagellum
ⓒ. bacterial flagella and cilia
ⓓ. microvilli and pili
Correct Answer: cilia and a flagellum
Explanation: Numerous short projections that move fluid over a cellular surface are cilia. A longer projection whose main role is propulsion of the whole cell is a flagellum. Both structures are eukaryotic membrane-covered appendages containing axonemes and arising from basal bodies. Pili and fimbriae are bacterial surface structures and do not fit the described internal organisation or movement pattern. The identification depends on combining relative length, number and functional outcome. P uses many short coordinated projections to move surrounding fluid, while Q uses a long appendage chiefly for cellular locomotion. Numerous short projections that sweep fluid are cilia, whereas a single long projection that propels the whole cell is a flagellum.
314. A mutation prevents coordinated beating of numerous short surface projections but leaves one long locomotory projection in another cell type unaffected. The function most directly reduced in the first cell type is:
ⓐ. propulsion of a cell by a single long eukaryotic flagellum
ⓑ. bacterial attachment to surfaces through pili or fimbriae
ⓒ. rotation-driven locomotion produced by a bacterial flagellum
ⓓ. movement of fluid across the surface by coordinated cilia
Correct Answer: movement of fluid across the surface by coordinated cilia
Explanation: Numerous short surface projections that beat in a coordinated manner are cilia. Their repeated oar-like movements can move fluid, mucus or suspended particles across a cellular surface. Preventing coordinated ciliary beating therefore reduces this surface-transport function. The unaffected long locomotory projection in the other cell type is consistent with a eukaryotic flagellum, which is generally longer and chiefly propels the entire cell. Pili and fimbriae are bacterial attachment structures rather than coordinated motile projections. A bacterial flagellum also differs from a eukaryotic cilium in both organisation and movement. The altered condition specifically separates the collective action of numerous short cilia from propulsion by a long flagellum and from bacterial surface appendages used for attachment or rotation-driven motility.
315. Consider the following statements about eukaryotic cilia and flagella.
I. Cilia are usually shorter than flagella.
II. Cilia may move a cell or surrounding fluid.
III. Flagella chiefly propel the cell.
IV. Cilia and flagella have completely unrelated internal organisation.
ⓐ. I, II and III only
ⓑ. I, III and IV only
ⓒ. I, II and IV only
ⓓ. II, III and IV only
Correct Answer: I, II and III only
Explanation: Cilia and flagella differ mainly in their relative length, abundance and usual movement pattern. Cilia are typically shorter and often numerous, and their beating may propel a cell or move fluid across its surface. Flagella are generally longer and chiefly propel the cell. Statement IV is incorrect because the two eukaryotic appendages share the same fundamental internal plan: both are covered by plasma membrane, contain an axoneme and arise from centriole-like basal bodies. Their functional distinction should not be exaggerated into a claim of complete structural unrelatedness. The first three statements accurately describe their characteristic differences. Cilia and eukaryotic flagella share the membrane-bound \(9+2\) axonemal plan and differ chiefly in length, number and movement pattern; they are not built from unrelated internal arrangements.
316. A motile appendage is covered by plasma membrane and contains a \(9+2\) microtubular axoneme. It must be:
ⓐ. a bacterial flagellum composed of filament, hook and basal body
ⓑ. a pilus used for bacterial attachment
ⓒ. a eukaryotic cilium or flagellum
ⓓ. a prokaryotic fimbria
Correct Answer: a eukaryotic cilium or flagellum
Explanation: A plasma-membrane covering and a \(9+2\) axoneme are defining features of eukaryotic cilia and flagella. Their internal structure is built from microtubules arranged as nine peripheral doublets around two central microtubules. A bacterial flagellum has a fundamentally different construction involving filament, hook and basal body and does not contain a membrane-covered \(9+2\) axoneme. Pili and fimbriae are bacterial surface fibres used mainly in attachment or related interactions and also lack this microtubular plan. The internal architecture provides stronger classification evidence than the general observation that the appendage is involved in movement.
317. Two flagella are compared.
P and Q are respectively:
| Feature | Flagellum P | Flagellum Q |
|---|---|---|
| Membrane covering | Present | Absent |
| Internal organisation | \(9+2\) microtubular axoneme | Filament, hook and basal body |
ⓐ. bacterial and eukaryotic flagella
ⓑ. eukaryotic and bacterial flagella
ⓒ. two structurally identical bacterial flagella
ⓓ. a cilium and a centriole
Correct Answer: eukaryotic and bacterial flagella
Explanation: Flagellum P is enclosed by the plasma membrane and contains a \(9+2\) axoneme, the characteristic organisation of a eukaryotic flagellum. Flagellum Q lacks a membrane-covered microtubular axoneme and instead has a filament connected through a hook to a basal body, which identifies a bacterial flagellum. Both appendages can propel cells, but a shared locomotory function does not make their structures identical. The eukaryotic appendage is built from microtubules continuous with a basal-body system, whereas the bacterial appendage follows a distinct filament-hook-basal-body plan. Classification must therefore rest on membrane covering and internal architecture rather than on the common name or common function. P is eukaryotic and Q is bacterial, representing structurally different solutions to cellular movement. This distinction also shows why a shared locomotory label does not imply structural equivalence between the two appendages.
318. Assertion: The presence of a flagellum does not by itself establish whether a cell is prokaryotic or eukaryotic.
Reason: Prokaryotic and eukaryotic cells may both possess flagella, but their flagella differ fundamentally in structure.
ⓐ. Both Assertion and Reason are true, and Reason correctly explains the Assertion
ⓑ. Both Assertion and Reason are true, but Reason does not explain the Assertion
ⓒ. Assertion is true, but Reason is false; the Reason cannot explain the Assertion
ⓓ. Assertion is false, but Reason is true; the Reason cannot explain the Assertion
Correct Answer: Both Assertion and Reason are true, and Reason correctly explains the Assertion
Explanation: Both prokaryotic and eukaryotic cells may possess structures called flagella, so simple observation of a locomotory appendage cannot determine the cellular category. The internal organisation supplies the necessary distinction. A bacterial flagellum contains filament, hook and basal body, whereas a eukaryotic flagellum is membrane-covered and contains a microtubular axoneme. The Reason directly explains the Assertion: flagella occur in both cellular types, but they are not structurally equivalent. Classification must use architecture rather than the shared name or general locomotory function. The prokaryotic cell organisation relation remains valid only under the exact condition described, not as an unrestricted rule. The explanatory link is complete: the same functional label occurs in both groups, while ultrastructure supplies the criterion needed to classify the cell.
319. In a transverse section of a typical eukaryotic cilium, the axoneme contains:
ⓐ. nine peripheral microtubule triplets surrounding no central pair
ⓑ. eight peripheral microtubule doublets surrounding two central microtubules
ⓒ. nine peripheral microtubule singlets surrounding one central pair
ⓓ. nine peripheral microtubule doublets surrounding two central microtubules
Correct Answer: nine peripheral microtubule doublets surrounding two central microtubules
Explanation: The characteristic axonemal arrangement is described as \(9+2\). Nine microtubule doublets form a ring around the periphery, while two individual microtubules occupy the centre. The peripheral elements are doublets rather than triplets; triplets characterise centrioles and basal bodies. The notation also does not mean eight doublets plus a central pair or nine single microtubules around one doublet. Correct interpretation requires translating the symbolic expression into a cross-sectional arrangement. This membrane-covered axoneme forms the structural core of eukaryotic cilia and flagella. The distinction is useful when similarly named structures occur in different cellular compartments or organisational plans.
320. A complete transverse section of a \(9+2\) axoneme contains how many individual microtubules if each peripheral doublet contributes two and the central region contains two singlets?
ⓐ. \(18\)
ⓑ. \(20\)
ⓒ. \(11\)
ⓓ. \(27\)
Correct Answer: \(20\)
Explanation: The notation \(9+2\) describes nine peripheral doublets surrounding two central singlet microtubules. Each doublet contains two individual microtubules, so the peripheral ring contributes \[ 9 \times 2=18. \] Adding the two central singlets gives \[ 18+2=20. \] The complete idealised transverse section therefore contains \(20\) individual microtubules. The value \(11\) would arise from counting each doublet as one unit rather than counting its two constituent microtubules. The value \(27\) belongs to nine triplets, the arrangement found in centrioles and basal bodies, not to a \(9+2\) axoneme. The calculation links the symbolic ultrastructural description to the actual component count and preserves the distinction between structural units and individual microtubules. This distinction is biologically important when comparing the motile axoneme with the cartwheel organisation of a centriole: both use nine peripheral groups, but doublets and triplets produce different totals and belong to different structures.