Cell: The Unit of Life MCQs with Answers – Part 2 (Class 11 Biology)

Explanation: Pigment-bearing membrane extensions in a cyanobacterium identify chromatophores, linking P with structural evidence for that photosynthetic system. Vesicular, tubular and lamellar plasma-membrane infoldings identify mesosomes, so Q matches the first description. A reduction in photosynthesis after selective damage supports a chromatophore function, while disruption of wall formation and DNA distribution fits the stated functional associations of mesosomes. The mapping uses both structural and experimental evidence. This prevents the two membrane modifications from being confused merely from both involving extensions or infoldings of cellular membranes. The complete relation is P-2, Q-1, R-3 and S-4. Membrane infoldings, pigment-bearing extensions and surface appendages are identified by both structure and function. Chromatophore damage predicts photosynthetic loss, whereas mesosomal disruption affects the distinct functions assigned to bacterial membrane infoldings.

Explanation: The treatment removes pigment-containing membrane extensions and produces a major decline in photosynthetic output. This links the removed structures with photosynthetic activity. The preservation of mesosomes and nearly normal wall formation helps separate the chromatophore role from mesosome-associated functions. The experiment does not identify flagella or glycocalyx as pigment-bearing photosynthetic structures. Flagella are associated with motility, while glycocalyx forms an external coating. The strongest conclusion follows the manipulated variable and measured result: removal of chromatophores reduces photosynthesis. The near-normal wall formation serves as supporting evidence that the observed effect is not simply a general collapse of every membrane-related cellular process. Chromatophores are recognised by their pigments and photosynthetic role rather than by membrane extension alone. Chromatophores carry the photosynthetic pigments of such prokaryotes, so their removal reduces light-dependent activity without requiring mesosome damage.

Explanation: Structure P is identified by its vesicular, tubular and lamellar form arising from plasma-membrane infolding. These are the characteristic structural descriptions of a mesosome. Structure Q is identified through the presence of photosynthetic pigments, making it a chromatophore. The same prokaryotic cell may contain membrane modifications with different biological roles, so the word membrane extension alone is not sufficient for identification. Pigment content separates the photosynthetic structure from the mesosome, while the vesicle-tubule-lamella pattern separates the mesosome from an external appendage or coating. The correct interpretation preserves both structure and function: P is the mesosome and Q is the chromatophore.

Explanation: Chromatophores contain photosynthetic pigments and provide membrane organisation associated with photosynthesis in certain prokaryotes. Their selective disruption should first reduce the photosynthetic performance of the cell. Mesosomes remain functional, so their associated activities, including wall formation, DNA distribution, respiration and secretion, need not fail immediately. Loss of motility would point toward flagellar damage, while complete arrest of protein synthesis would require disruption of ribosomes or their translation function. The prediction follows from the functional separation of two membrane modifications. Chromatophore damage primarily affects pigment-dependent photosynthesis, whereas intact mesosomes can initially continue their own associated roles. Selective chromatophore damage removes pigment-bearing photosynthetic membrane while leaving the mesosome-associated activities specified as intact. The preserved mesosomes restrict the early defect to the pigment-bearing photosynthetic extensions rather than general membrane failure. The selective treatment therefore separates chromatophore function from the preserved mesosomal functions.

Explanation: A bacterial flagellum has three principal structural parts: filament, hook and basal body. The filament is the longest component and extends outward from the bacterial surface. A shorter curved hook connects this external filament to the basal body, which is anchored within the cell envelope. Fimbriae and pili are separate surface structures and are not components of the bacterial flagellum. Their shapes and principal roles differ from those of the flagellar filament. The blank requires the part identified by both its great length and its external projection. Those two spatial clues uniquely indicate the filament rather than the connecting hook or embedded basal body.

Explanation: The filament is the long portion of a bacterial flagellum that projects farthest from the cell surface. Moving inward from this distal external end, the filament joins the short curved hook. The hook then connects with the basal body embedded in the bacterial envelope. The requested orientation is essential; the reverse journey would begin at the basal body and end at the filament. Following the stated inward direction gives filament, hook and basal body. This sequence also expresses the functional construction of the appendage: an external projecting component is coupled through a connector to an anchored base. Reversing any two parts would place the hook outside the filament or the basal body away from the envelope, neither of which matches the structural organisation of a bacterial flagellum. The positional dependency fixes one order even though the same three parts can be named from either direction.

Explanation: The long projecting strand is the filament, which forms the major external portion of the bacterial flagellum. The short curved region joining it to the cell is the hook. The component embedded in the envelope is the basal body, which anchors the flagellum. Identification requires using all three positional clues together. The hook cannot be the longest projecting region, and the basal body cannot occupy the position of the external connector. Pili and fimbriae are separate surface structures and do not form this three-part flagellar arrangement. The spatial pattern P outside, Q connecting and R embedded corresponds to filament, hook and basal body. Within prokaryotic membrane specialisations and appendages, the evidence is meaningful only when membrane infoldings, pigment-bearing extensions and surface appendages are identified by both structure and function. The labelled arrangement identifies a complete bacterial flagellum: the filament, hook and basal body occupy successive external-to-internal positions.

Explanation: Records P, Q and R correctly describe the three major parts of a bacterial flagellum. The filament is the longest external component, the hook connects it to the base, and the basal body anchors the assembly within the bacterial envelope. Record S is the one requiring correction; the \(9+2\) axoneme is the microtubular core of eukaryotic cilia and flagella, not of bacterial flagella. Bacterial flagella have a different construction and are described through filament, hook and basal body. The error is therefore not a minor naming issue; it confuses locomotory structures from two different cellular organisations. Keeping the positions and connections of P, Q and R intact while removing the eukaryotic axoneme claim from S restores a biologically consistent set. The consistent records also show that structural position and cellular domain must agree before a flagellar description is accepted.

Explanation: The hook lies between the external filament and the basal body and provides the structural connection linking these two regions. Damage to the hook can leave the filament visible and the basal body anchored, yet the complete flagellar assembly no longer transmits movement effectively through an intact connection. Reduced motility is expected even though neither terminal component has disappeared. Fimbrial attachment is controlled by different surface fibres and is not directly abolished by hook damage. A flagellar filament also does not transform into a pilus when its connector fails. The prediction depends on the hook's position within the three-part arrangement and the requirement for structural continuity during locomotion. In prokaryotic membrane specialisations and appendages, membrane infoldings, pigment-bearing extensions and surface appendages are identified by both structure and function. The damaged hook interrupts the mechanical link between basal body and filament while leaving both structures present.

Explanation: Records Q and R both show directional movement despite differing in the number and placement of flagella. This supports variation in flagellar arrangement among motile bacteria. Records P and S lack flagella and show no directional movement under the recorded conditions, while S possesses numerous fimbriae. The S record helps distinguish attachment-related surface fibres from locomotory flagella. The evidence does not show that every external structure produces movement, nor does it restrict motility to cells with only one flagellum. The table supports two connected conclusions: flagella are associated with bacterial motility, and motile bacteria need not share one fixed number or surface arrangement of flagella. Combining the movement records with the observed appendages identifies flagella as locomotory structures while leaving fimbriae associated with surface attachment. The absence of movement in records without flagella provides the complementary evidence needed for that association.

Explanation: The experiment changes flagellar activity while preserving growth, viability and fimbrial attachment. The sharp decline in directional movement tracks the one deliberately altered variable: functional flagella. This supports a locomotory role for the flagellar system under the tested conditions. The preserved viability separates movement from general cell survival, while continued fimbrial attachment shows that non-flagellar surface fibres remain present and functional. The result cannot establish that the cells lack all external structures. It also does not assign motility to fimbriae, since attachment remains intact while movement declines. The control-treatment comparison links active flagella specifically with the observed directional locomotion. Structural position and biological role together separate bacterial membrane specialisations from external appendages. Preserved growth and attachment rule out a general loss of viability or fimbrial function, isolating motility as the affected process.

Explanation: Bacterial cells differ in locomotory ability, so both motile and non-motile forms occur. Among motile bacteria, flagella may vary in number and position rather than following one universal arrangement. Pili and fimbriae are distinct from flagella and are not treated as the principal locomotory appendages. Fimbriae may remain abundant on a cell that shows no directional movement, especially when they are serving attachment functions. This makes statement IV valid: non-motility does not demonstrate complete absence of surface structures. The correct combination joins the motile-non-motile distinction, flagellar variation and the separation of attachment fibres from locomotory flagella. The valid combination recognises variation in bacterial motility and flagellar arrangement while rejecting the claim that pili and fimbriae are the main locomotory structures.

Explanation: All three cells are stated to be motile, yet their flagellar patterns differ. Cell P has a single flagellum at one end, Q has several grouped in one region, and R has flagella distributed across several sites. The shared locomotory outcome shows that movement is compatible with more than one arrangement. The evidence does not support naming one pattern as the only functional design, nor does it convert flagella into attachment fibres. The biological conclusion concerns variation: bacterial motility may be achieved with different numbers and surface distributions of flagella. The descriptions provide enough spatial information for this inference without requiring additional arrangement terminology.

Explanation: Removal of flagella is followed by a major reduction in directional movement, directly linking those appendages with locomotion. Attachment persists while pili and fimbriae remain present, showing that a surface-interaction function can continue after flagellar loss. The result separates two biological tasks that are easily confused: movement through the environment and adhesion to a surface. If fimbriae or pili supplied equivalent locomotion, directional movement would have remained near its original level. Instead, preserved attachment with reduced swimming demonstrates functional specialisation among bacterial surface structures rather than interchangeability of all external fibres. The comparison does not prove that every pilus or fimbria has an identical adhesive role, but it supports continued surface attachment by the remaining fibres in this population. This treatment–response pattern ties each surviving or lost behaviour to the appendage class that changed or remained.

Explanation: The flagellar filament is the long external component involved in bacterial locomotion. It joins the basal body through the hook, making the hook the connector within the three-part flagellar structure. Pili are elongated tubular structures composed of specialised protein, whereas fimbriae are shorter, bristle-like fibres projecting from the bacterial surface. Fimbriae may assist attachment to surfaces or host tissues and are not equivalent to the flagellar filament. The mapping separates components of one locomotory appendage from two independent surface structures. Their external positions may appear similar in a simple diagram, but differences in form, structural relation and function establish P-1, Q-2, R-3 and S-4. Filament length, hook position, pilus form and fimbrial attachment function independently fix P-1, Q-2, R-3 and S-4.

Explanation: The fibres are described as numerous, short and bristle-like, which identifies them as fimbriae. Their presence on non-motile cells and the observed adhesion to host tissue fit the stated attachment role of fimbriae. A basal body is part of a flagellum and would not appear as numerous free bristles over the cell surface. Mesosomes are internal plasma-membrane infoldings, while hooks are connectors within flagellar assemblies. Morphology and function point to the same structure: short bristles indicate fimbriae, and maintained adhesion without flagella separates attachment from locomotion. The surface fibres allow interaction with host tissue without acting as propulsive appendages.

Explanation: Pili and fimbriae are bacterial surface structures, but their forms and principal relations differ from those of flagella. Pili are elongated proteinaceous tubules, while fimbriae are shorter bristle-like fibres. Fimbriae may assist adhesion to surfaces such as rocks or to host tissues. Neither structure should be identified as the long locomotory flagellum composed of filament, hook and basal body. All four statements preserve these distinctions. The first two describe morphology, the third establishes the non-locomotory qualifier, and the fourth gives a biologically relevant attachment role. Together they prevent the common error of treating every external bacterial fibre as a flagellum. The complete set keeps the elongated pilus and short bristle-like fimbria distinct from the filament–hook–basal-body apparatus used for bacterial motility.

Explanation: All four statements describe complementary features of a prokaryotic ribosome. Its approximate dimensions are \(15\,\text{nm} \times 20\,\text{nm}\), showing that it is a very small cytoplasmic particle. Structurally, the complete ribosome contains a large \(50S\) subunit and a small \(30S\) subunit and is designated \(70S\). The letter S refers to sedimentation behaviour rather than an ordinary unit of mass, so the complete value is not obtained through simple numerical addition of subunit labels. Functionally, ribosomes translate the information carried by messenger RNA into polypeptides. The dimensions, subunit organisation, sedimentation class and protein-synthetic role together identify the prokaryotic ribosome accurately. The evaluated statements support the set I, II, III and IV.

Explanation: A typical prokaryotic ribosome is an approximately \(15\,\text{nm} \times 20\,\text{nm}\) \(70S\) particle formed from \(50S\) and \(30S\) subunits, and its function is protein synthesis. Record S is the only row in which dimensions, subunit composition and activity agree simultaneously. Record P has the approximate size and correct activity but assigns the \(60S\) and \(40S\) subunits of a eukaryotic cytoplasmic ribosome. Record Q has the prokaryotic subunits but assigns lipid synthesis, which is not a ribosomal function. Record R combines the correct subunit labels with unsupported dimensions and DNA storage. Reliable identification requires all diagnostic features to form one coherent profile; matching only size, only subunits or only function is insufficient. Thus the complete evidence identifies S as the typical bacterial ribosome. Its protein-synthetic role also distinguishes a ribosome from membrane systems involved in lipid production.

Explanation: A functional prokaryotic ribosome is the complete \(70S\) particle formed by association of a \(50S\) large subunit with a \(30S\) small subunit. The sedimentation values are not added arithmetically, but the two components must still assemble into the complete ribosomal unit for normal translation. If association is blocked, the cell may retain intact subunits, yet it cannot form the usual population of functional \(70S\) ribosomes. Protein synthesis will therefore decline since isolated subunits do not independently perform the complete translation process. The treatment does not convert prokaryotic subunits into an \(80S\) eukaryotic ribosome, nor does it act primarily on storage granules. The changed condition separates structural preservation of the subunits from functional assembly of the whole ribosome, showing why both subunit identity and association are required.