Cell Theory and Prokaryotic Cell Structure (2B) - Bio & Biochem

1. A research group studied chemotaxis in a motile bacterium. When the gene encoding flagellin was deleted, cells remained viable and continued to metabolize glucose but no longer migrated toward an attractant gradient in soft agar. The organism lacked a nucleus and endomembrane system. Core cell theory idea: cells perform coordinated functions using specialized structures. Based on the vignette, which structure is most critical for the observed function?
   Answer: Flagellum that provides propulsion enabling movement along chemical gradients The skill being tested is identifying prokaryotic motility structures per cell theory. Cell theory posits cells use specialized structures for coordinated functions like movement. In the vignette, flagellin deletion abolishes chemotaxis in the bacterium. Choice D is correct because flagella enable propulsion along gradients in prokaryotes. Choice B fails as cilia are eukaryotic, misconstruing bacterial motility which uses rotary flagella. To verify understanding, differentiate prokaryotic and eukaryotic motility. A transferable check is to confirm structures supporting directed movement without cytoskeletal organelles.
2. A researcher cultured a Gram-negative bacterium in the presence of a hydrophobic antibiotic. A mutant strain with a disrupted gene for lipopolysaccharide (LPS) core synthesis showed increased antibiotic sensitivity but unchanged cytosolic enzyme activity. Complementation restored resistance. No changes were observed in peptidoglycan thickness. Based on the findings, which prokaryotic structure is most critical for the observed change in antibiotic sensitivity?
   Answer: Outer membrane containing LPS that limits entry of certain hydrophobic compounds This question tests knowledge of Gram-negative bacterial cell envelope structure and antibiotic resistance. Gram-negative bacteria possess an outer membrane containing lipopolysaccharide (LPS) that serves as a permeability barrier, particularly against hydrophobic compounds including many antibiotics. The vignette describes increased antibiotic sensitivity in a mutant with disrupted LPS core synthesis, which compromises outer membrane integrity and allows greater antibiotic penetration without affecting the peptidoglycan layer. The correct answer (D) identifies the outer membrane containing LPS as the critical structure for limiting antibiotic entry. Answer choice B is incorrect because bacteria lack mitochondria—the outer membrane referenced in choice D is a prokaryotic structure unique to Gram-negative bacteria, not a mitochondrial membrane. To verify understanding, remember that Gram-negative bacteria have two membranes (inner plasma membrane and outer membrane with LPS), while Gram-positive bacteria have only one membrane.
3. A team examined an acidophilic bacterium living at pH 2.0. Wild-type cells maintained near-neutral cytosolic pH, while mutants lacking a specific membrane ATPase showed cytosolic acidification and reduced growth. The bacterium lacked mitochondria and other membrane-bound organelles. Defined term: ATPase = enzyme that hydrolyzes ATP. Core cell theory idea: membranes regulate internal conditions. Which structure is most critical for survival in this extreme environment?
   Answer: Plasma membrane proton pump that expels $\mathrm{H^+}$ to maintain cytosolic pH The skill being tested is applying cell theory's membrane role to prokaryotic pH regulation. Cell theory emphasizes membranes in maintaining internal conditions. In the vignette, the ATPase pump prevents acidification in the acidophile. Choice D is correct because proton pumps maintain cytosolic pH in prokaryotes. Choice B fails as no nuclear membrane exists, misconstruing buffering mechanisms. To verify understanding, examine extremophile membrane adaptations. A transferable check is to identify ion transport's role in prokaryotic homeostasis without organelles.
4. Investigators explored why a bacterium can express a newly acquired gene rapidly after horizontal gene transfer. They introduced a plasmid carrying a fluorescent reporter under a bacterial promoter. Fluorescence appeared within minutes, and microscopy showed no separation between DNA-containing region and ribosome-rich cytosol. Core cell theory idea: cells execute gene expression to produce functional molecules. Which prokaryotic structural feature best explains the rapid onset of fluorescence?
   Answer: Lack of a nuclear envelope, allowing transcription and translation to be coupled in the cytosol The skill being tested is understanding prokaryotic gene expression in cell theory. Cell theory states cells express genes to produce functional molecules using domain-specific structures. In the vignette, rapid fluorescence occurs due to coupled processes in the bacterium. Choice A is correct because lacking a nucleus allows immediate translation in the cytosol. Choice B fails by implying a nucleus accelerates expression, misconstruing prokaryotic efficiency without separation. To verify understanding, compare transcription-translation in domains. A transferable check is to note how nuclear absence speeds prokaryotic responses.
5. A lab evolved E. coli in the presence of a lytic bacteriophage that adsorbs to a specific outer-membrane porin. After 200 generations, a dominant bacterial lineage carried mutations that eliminated expression of that porin. The lineage showed reduced phage adsorption but also slower growth in minimal medium where that porin normally facilitates nutrient uptake. (Terms: porin = outer-membrane channel protein in Gram-negative bacteria; adsorption = virus binding to cell surface.) Which evolutionary adaptation would be expected given the described conditions?
   Answer: Loss or modification of the porin receptor, reducing phage attachment despite a tradeoff in nutrient transport This question tests evolutionary adaptations in prokaryotic surface structures against phage pressure. Prokaryotes like E. coli have outer-membrane porins that serve as channels but can be phage receptors, leading to selection for modifications under viral pressure. In the vignette, evolved E. coli mutants lose porin expression, reducing phage adsorption but slowing growth in minimal medium due to impaired nutrient uptake. Loss or modification of the porin is expected as it reduces phage attachment with a nutrient tradeoff, as in choice B. Choice C fails because prokaryotes lack nuclear envelopes; this misconstrues eukaryotic compartmentalization as a prokaryotic defense. To verify understanding, note that porin mutations balance survival against efficiency in prokaryotes. A transferable check is to assess tradeoffs in adaptations: if surface changes reduce infection but impair function, it exemplifies prokaryotic evolution without organelles.
6. Investigators compared a bacterium (strain P) and a yeast cell exposed to a cell wall–targeting enzyme. Strain P remained osmotically stable in isotonic medium but lysed rapidly in hypotonic medium after treatment. Yeast cells showed minimal lysis in either medium. The team confirmed that strain P lacks internal membrane-bound compartments but contains cytosolic ribosomes and a circular chromosome. Cell theory states that the cell is the fundamental unit of life and that hereditary information is passed from cell to cell. Based on these observations, which feature best exemplifies cell theory as applied to strain P? Defined term: isotonic = equal solute concentration inside and outside the cell.
   Answer: Containment of hereditary information in a chromosome that is replicated and transmitted during division The skill being tested is applying cell theory to prokaryotic cells, emphasizing the transmission of hereditary information. Cell theory posits that cells are the basic units of life, arise from pre-existing cells, and pass hereditary information during division. In the vignette, strain P is a prokaryotic bacterium lacking membrane-bound organelles but possessing a circular chromosome, which is replicated and transmitted. Choice C is correct because it directly illustrates the core principle of hereditary information containment and transmission in prokaryotes via chromosome replication during binary fission. Choice A fails because prokaryotes lack a nucleus, misconstruing that all cells require nuclear protection for genetic material during division. To verify understanding, recall that cell theory applies universally but prokaryotes achieve functions without organelles. A transferable check is to identify how genetic continuity is maintained in cells without nuclei through direct cytoplasmic division.
7. A team isolated an archaeal species from a hypersaline lake and compared it to a non-halophilic bacterium under increasing NaCl concentrations. The archaeon maintained cell shape and did not lyse even at 4 M NaCl. Chemical analysis of the archaeal cell envelope revealed an S-layer (paracrystalline protein lattice) but no detectable peptidoglycan. (Terms: S-layer = surface protein layer; 4 M = 4 mol/L.) Which evolutionary adaptation would be expected given the described conditions?
   Answer: Selection for an S-layer that stabilizes the cell envelope in high ionic strength environments This question tests evolutionary adaptations in prokaryotic cell envelopes, particularly in archaea. Prokaryotic structures like the S-layer provide a paracrystalline protein lattice that stabilizes the cell envelope, especially in extreme environments without peptidoglycan. In the vignette, the archaeon from a hypersaline lake maintains shape at 4 M NaCl due to its S-layer, unlike the non-halophilic bacterium. Selection for an S-layer is expected as it stabilizes the envelope in high ionic strength, matching choice A. Choice B is incorrect because chloroplasts are eukaryotic organelles acquired via endosymbiosis, and prokaryotes do not acquire them; this misconstrues prokaryotic energy adaptations. To verify understanding, note that archaeal envelopes often lack peptidoglycan but use S-layers for rigidity. A transferable check is to evaluate envelope components: presence of S-layers indicates adaptation to extremes, aligning with cell theory's functional unit principle.
8. Investigators compared a bacterium (Species P) and a yeast cell after treatment with an antibiotic that covalently binds the D-Ala–D-Ala terminus of peptidoglycan precursors, blocking cross-linking. Both cultures were placed in an isotonic medium (no net water movement) and then shifted to hypotonic medium. Species P rapidly lysed after the shift, whereas the yeast remained intact and continued to bud. The team concluded that the antibiotic's lethality depended on disrupting a structure unique to many prokaryotes. In the context of cell theory (cells are the fundamental unit of life and are bounded by a plasma membrane), which feature best supports the conclusion that Species P is a cellular organism despite lacking membrane-bound organelles?
   Answer: A phospholipid bilayer that maintains an internal environment distinct from the extracellular space This question tests understanding of cell theory's fundamental principle that all cells possess a plasma membrane. Cell theory states that cells are the basic unit of life and are bounded by a plasma membrane that maintains an internal environment distinct from the external environment. In the vignette, Species P (a bacterium) rapidly lysed when shifted to hypotonic medium after antibiotic treatment disrupted its peptidoglycan cell wall, demonstrating that it relies on both a plasma membrane and cell wall to maintain integrity. The correct answer (B) identifies the phospholipid bilayer as the essential feature that defines Species P as a cellular organism according to cell theory, regardless of whether it has membrane-bound organelles. Answer choice A is incorrect because prokaryotes lack membrane-enclosed nuclei, which contradicts the premise that Species P lacks membrane-bound organelles. To verify understanding, remember that cell theory's definition of a cell requires a plasma membrane boundary, not internal compartmentalization.
9. Researchers monitored chromosome segregation in a rod-shaped bacterium using fluorescent tags. When a drug disrupted the actin-like protein MreB, cells became spherical and showed frequent mis-segregation of the circular chromosome during division, even though DNA replication initiation remained normal. No nucleus was observed. Core cell theory idea: hereditary information is transmitted to daughter cells. Based on the vignette, which structure is most critical for the prokaryote's function in reliably passing genetic material to progeny?
   Answer: MreB-associated cytoskeletal organization that helps position cellular components during division The skill being tested is applying cell theory's hereditary transmission to prokaryotic division. Cell theory states that hereditary information is reliably passed to daughter cells during division. In the vignette, MreB disruption causes chromosome mis-segregation in the rod-shaped bacterium. Choice A is correct because MreB organizes the cytoskeleton to position components for accurate segregation in prokaryotes. Choice B fails as it describes eukaryotic spindles, misconstruing prokaryotic segregation which uses actin-like proteins without kinetochores. To verify understanding, contrast prokaryotic and eukaryotic chromosome segregation. A transferable check is to identify cytoskeletal roles in prokaryotes for genetic continuity without nuclei.
10. A lab compared two bacterial strains during exposure to a lytic bacteriophage. Strain X adsorbed phage normally, but phage DNA injection failed and infection did not proceed. Sequencing identified a loss-of-function mutation in a gene required for synthesis of lipopolysaccharide (LPS). Strain X is Gram-negative and lacks internal membrane-bound organelles. Core cell theory idea: the cell membrane defines the boundary of the living unit. Based on the vignette, which structure is most critical for the prokaryote's function in this interaction?
    Answer: Outer membrane LPS that serves as a surface component required for phage entry The skill being tested is applying cell theory's cell boundary concept to prokaryotic interactions. Cell theory highlights the cell membrane as the defining boundary of the living unit, regulating interactions like phage entry. In the vignette, the LPS mutation in Gram-negative strain X blocks phage DNA injection, preventing infection. Choice D is correct because outer membrane LPS is crucial for phage adsorption and entry in prokaryotes. Choice B fails as prokaryotes lack a nuclear pore complex, misconstruing viral genome import which does not occur in bacteria. To verify understanding, examine how prokaryotic envelopes differ from eukaryotic ones in pathogen interactions. A transferable check is to identify membrane components essential for prokaryotic boundaries without internal organelles.
11. A team examined a soil bacterium that survives desiccation by producing an extracellular capsule. In drying conditions, wild-type cells formed hydrated microcolonies and retained viability, while a capsule-deficient mutant showed extensive cell clumping and loss of viability. When both strains were rehydrated, only the wild-type rapidly resumed growth. (Terms: capsule = extracellular polysaccharide layer; desiccation = severe drying.) Which structure is most critical for the prokaryote's function that improved survival in the described environment?
    Answer: Capsule, because it can retain water and provide protection during desiccation stress This question tests prokaryotic structures for environmental survival, focusing on extracellular adaptations. Prokaryotes often produce capsules, polysaccharide layers that retain water and protect against desiccation, aiding viability in dry conditions. In the vignette, the soil bacterium's capsule enables hydrated microcolonies and survival during drying, unlike the deficient mutant that clumps and loses viability. The capsule is critical for water retention and protection, improving survival as in choice A. Choice B fails because the smooth endoplasmic reticulum is eukaryotic for lipid synthesis, not present in prokaryotes; this misconstrues organelle roles in bacteria. To verify understanding, note capsules are external and non-membrane-bound, unique to prokaryotes. A transferable check is to evaluate extracellular structures: if they provide physical protection without internal compartments, they align with prokaryotic adaptations per cell theory.
12. A marine bacterium was grown in progressively higher NaCl concentrations. Cells with an intact S-layer (a paracrystalline surface protein array) maintained shape and showed less lysis at 1.2 M NaCl than an isogenic S-layer knockout. Both strains lacked internal membrane-bound organelles. Defined term: M = mol/L. Core cell theory idea: cells are bounded units whose structures support survival. Which structure is most critical for the prokaryote's function under these high-salt conditions?
    Answer: S-layer that provides an external protective lattice contributing to envelope stability The skill being tested is understanding prokaryotic structures supporting survival per cell theory. Cell theory posits that cellular structures enable bounded units to withstand environmental stresses. In the vignette, the S-layer enhances stability in high-salt conditions for the marine bacterium. Choice D is correct because the S-layer provides a protective lattice aiding envelope integrity under osmotic stress. Choice B fails as prokaryotes lack a nucleolus, misconstruing rRNA transcription as a salt adaptation mechanism. To verify understanding, examine prokaryotic surface layers versus eukaryotic junctions. A transferable check is to assess how external structures maintain prokaryotic integrity without organelles.
13. A clinician cultured a bacterium from a urinary tract infection. In Gram stain, cells appeared pink and were sensitive to an antibiotic that disrupts peptidoglycan crosslinking. Electron microscopy showed two membranes surrounding a thin cell wall layer. Core cell theory idea: all cells are bounded by membranes and contain internal components necessary for life. Based on the vignette, which prokaryotic structure most directly explains the staining and envelope observations?
    Answer: Outer membrane surrounding a thin peptidoglycan layer in the periplasm The skill being tested is identifying prokaryotic envelope structures aligning with cell theory. Cell theory states all cells are membrane-bound with internal components for life. In the vignette, the pink Gram stain and thin wall indicate Gram-negative structure. Choice D is correct because Gram-negative bacteria have an outer membrane and thin peptidoglycan, explaining staining and antibiotic sensitivity. Choice C fails as it describes plant cell walls with chloroplasts, misconstruing prokaryotic envelopes which lack such features. To verify understanding, recall Gram staining basis in envelope architecture. A transferable check is to distinguish prokaryotic wall compositions from eukaryotic ones.
14. A group investigated why a bacterium rapidly colonizes urinary catheters. Under flow conditions, wild-type cells adhered strongly to plastic, while a mutant lacking a pilin subunit gene showed markedly reduced adhesion despite normal growth rate and intact cell walls. Complementation restored adhesion. The authors proposed that a specific surface structure enabled initial attachment. Based on the vignette, which structure is most critical for the prokaryote's function in this setting?
    Answer: Pili (fimbriae) that mediate attachment to surfaces and host tissues This question tests knowledge of prokaryotic surface structures involved in adhesion. Pili (also called fimbriae) are hair-like protein appendages extending from the bacterial surface that mediate attachment to various surfaces including host tissues and medical devices like catheters. The vignette describes reduced adhesion in a mutant lacking a pilin subunit gene, with restoration upon complementation, clearly implicating pili as the critical adhesion structure. The correct answer (A) identifies pili/fimbriae as the structures mediating attachment to surfaces and host tissues. Answer choice D is incorrect because tight junctions are specialized cell-cell adhesion structures found between eukaryotic epithelial cells, not between bacterial cells—bacteria use different mechanisms for cell-cell adhesion and biofilm formation. To verify understanding, remember that pili are extracellular protein structures distinct from the cell wall and are commonly involved in bacterial pathogenesis through adhesion.
15. In a functional analysis, investigators tracked cell division in a bacterium exposed to a drug that inhibits peptidoglycan transpeptidases. Cells continued to elongate and initiate septa but frequently ruptured at the division site. The bacterium lacked mitochondria and a nucleus. Core cell theory idea: cell structures maintain the integrity of the living unit. Based on the vignette, which structure is most critical for successful completion of binary fission?
    Answer: Peptidoglycan cell wall that withstands turgor pressure during septation The skill being tested is recognizing structures for prokaryotic division integrity in cell theory. Cell theory highlights structures maintaining cell unit integrity during reproduction. In the vignette, inhibiting transpeptidases causes rupture at division sites. Choice A is correct because peptidoglycan withstands turgor during septation in prokaryotes. Choice B fails by referencing a nuclear envelope, misconstruing prokaryotic fission which lacks nuclear protection. To verify understanding, examine cell wall roles in bacterial division. A transferable check is to identify wall contributions to prokaryotic stability without eukaryotic cytoskeletons.
16. Researchers examined a bacterium that produces pili required for conjugation. When pilus assembly was disrupted, plasmid transfer to recipient cells decreased by >99%, but donor cell growth rate in rich medium was unchanged. The bacterium lacked a nucleus. Core cell theory idea: hereditary information can be transmitted between cells, affecting traits. Based on the vignette, which structure is most critical for the described function?
    Answer: Sex pilus that establishes cell-to-cell contact enabling DNA transfer during conjugation The skill being tested is recognizing structures for prokaryotic gene transfer per cell theory. Cell theory includes horizontal transmission of hereditary information between cells. In the vignette, pilus disruption blocks conjugation. Choice D is correct because sex pili enable DNA transfer in prokaryotes. Choice B fails as it describes eukaryotic mitosis, misconstruing bacterial conjugation without spindles. To verify understanding, differentiate vertical and horizontal inheritance. A transferable check is to identify appendages facilitating prokaryotic gene exchange.
17. Researchers isolated two bacterial strains from a hypersaline lake. Both strains grew well at 3.5 M NaCl, but only Strain H remained viable after rapid transfer to freshwater. Chemical analysis showed Strain H produced a thick extracellular polysaccharide layer and formed compact cell aggregates. When a gene required for polysaccharide export was knocked out, the mutant lost freshwater survival despite unchanged intracellular enzyme profiles. Which evolutionary adaptation would be expected given the described conditions and results?
    Answer: Selection for an extracellular capsule that slows osmotic water influx during hypoosmotic shock This question tests understanding of prokaryotic adaptations to osmotic stress. Prokaryotes can produce extracellular structures like capsules or slime layers that provide protection against environmental stresses, including osmotic shock. In the vignette, Strain H survives transfer from hypersaline to freshwater conditions due to its thick extracellular polysaccharide layer, which forms a protective barrier that slows water influx during hypoosmotic shock. The correct answer (B) identifies selection for an extracellular capsule as the evolutionary adaptation that enables survival in fluctuating osmotic conditions. Answer choice C is incorrect because prokaryotes lack nuclei, and nuclear compartmentalization is not a mechanism for preventing osmotic lysis—the cell wall and capsule serve this function in bacteria. To verify understanding, remember that prokaryotic capsules are external to the cell wall and provide additional protection beyond what the peptidoglycan layer offers.
18. To test whether a newly discovered microbe follows cell theory, researchers measured whether it can generate offspring without pre-existing cells. Under sterile conditions with only inorganic salts and carbon source, no new cells appeared unless a small inoculum was added. Once inoculated, the population increased by binary fission. The organism lacked a nucleus. Core cell theory idea: cells arise from pre-existing cells. Which feature best exemplifies this principle as applied to the microbe?
    Answer: Population growth occurring only after inoculation with existing cells The skill being tested is applying cell theory's origin principle to microbes. Cell theory asserts that cells arise only from pre-existing cells, refuting spontaneous generation. In the vignette, no growth occurs without inoculum, but binary fission follows addition. Choice A is correct because it demonstrates dependence on existing cells for population growth. Choice B fails by suggesting organelles enable spontaneous formation, misconstruing that prokaryotes lack organelles yet follow cell theory. To verify understanding, consider historical experiments disproving abiogenesis. A transferable check is to test if an organism requires pre-existing cells for propagation.
19. A microbiology group monitored binary fission in a rod-shaped bacterium using a fluorescent fusion to FtsZ, a tubulin-like protein. When an FtsZ inhibitor was added, cells continued to elongate but failed to form a midcell constriction and produced no daughter cells. DNA replication markers remained detectable. (Terms: binary fission = prokaryotic cell division producing two cells; FtsZ = protein that forms a contractile ring at the division site.) Based on the vignette, which structure is most critical for the prokaryote's function that was blocked by the inhibitor?
    Answer: The FtsZ ring, because it is required to recruit division machinery and complete cytokinesis in bacteria This question tests comprehension of prokaryotic cell division mechanisms in the context of cell theory. Cell theory emphasizes that cells arise from pre-existing cells through division, and in prokaryotes, binary fission involves the FtsZ protein forming a contractile ring at the division site to facilitate cytokinesis. In the vignette, the FtsZ inhibitor prevents midcell constriction and daughter cell production in the rod-shaped bacterium, despite continued elongation and DNA replication. The FtsZ ring is most critical as it recruits division machinery for cytokinesis, explaining the block in choice D. Choice B fails because the mitotic spindle is a eukaryotic structure for chromosome segregation, and prokaryotes do not use it; this highlights the misconception of applying eukaryotic division to bacteria. To verify, recall that FtsZ is homologous to tubulin but functions without spindles in prokaryotes. A transferable check is to compare division proteins: if tubulin-like and ring-forming, it's prokaryotic; if spindle-involved, it's eukaryotic.
20. In a structural comparison experiment, a bacterium and a protozoan were exposed to a detergent that solubilizes cholesterol-rich membranes. The protozoan plasma membrane was disrupted at low detergent concentrations, while the bacterial plasma membrane remained largely intact. The bacterium lacked internal membrane-bound organelles. Core cell theory idea: all cells have membranes, but composition varies. Which conclusion best follows from the vignette?
    Answer: Bacterial membranes generally lack cholesterol, reducing susceptibility to cholesterol-targeting detergents The skill being tested is comparing membrane compositions across cell types in cell theory. Cell theory notes universal membranes with domain-specific variations. In the vignette, bacterial membranes resist detergent due to lacking cholesterol. Choice C is correct because prokaryotic membranes typically lack cholesterol, conferring resistance. Choice B fails by stating a nuclear envelope protects, misconstruing prokaryotic structure without nuclei. To verify understanding, recall lipid differences between domains. A transferable check is to evaluate how membrane composition affects prokaryotic resilience.

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