Cell Biology

Basics of Cell Biology

Which of the following best describes a plastid?

Plastids comprise diverse double-membrane organelles in plant and algal lineages originating from primary endosymbiosis of cyanobacterium engulfed by eukaryote over billion years ago, retaining circular plastid genome encoding photosystem components and ribosomal RNAs and ability to divide by binary fission via FtsZ ring. Differentiation depends on tissue and environmental cues: chloroplasts in mesophyll contain thylakoid stacks grana where photosystem II, cytochrome b6f, photosystem I perform light reactions generating proton gradient for ATP synthase and NADPH for Calvin cycle fixing CO2 via Rubisco, chlorophyll a and b absorbing 680 and 700 nm light; chromoplasts develop from chloroplasts during fruit ripening accumulating carotenoids lycopene in tomato and beta-carotene in pepper conferring orange-red coloration and antioxidant nutritional value attracting frugivores; amyloplasts in roots and seeds store amylose and amylopectin starch granules and function as statoliths sedimenting for gravity perception; etioplasts in dark-grown seedlings hold prolamellar body. Unlike mitochondria focused on oxidative phosphorylation, plastids specialize in pigment storage and photosynthesis.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 14: Plastids, Chloroplasts, and Photosynthesis.

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Basics of Cell Biology

Which scientist contributed to the cell theory by stating that all cells arise from pre-existing cells?

Cell doctrine progressed through microscopic observations of plant tissues by Matthias Schleiden in 1838 stating plants consist of cells and Theodor Schwann 1839 extending to animals proposing cells as basic units with nuclei, but mechanism of new cell formation remained speculative with crystallization theory suggesting spontaneous formation. Rudolf Virchow, German pathologist analyzing diseased tissues and thrombosis, in 1855 aphorism Omnis cellula e cellula challenged spontaneous generation, arguing cells originate only by division of pre-existing cells via mitosis involving chromatin condensation, metaphase plate alignment, sister chromatid separation driven by microtubule kinetochores, and cytokinesis via actin-myosin contractile ring. This principle placed cell division at heart of growth, wound healing, embryonic development, and tumorigenesis where uncontrolled proliferation causes malignancy. Pasteur's swan-neck flask experiments disproved microbial spontaneous generation supporting biogenesis, while Schleiden and Schwann established structural unity. Virchow's contribution completed three pillars of cell theory taught today: all organisms composed of cells, cell as basic unit, and cells arise from pre-existing cells.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 17: Virchow and Omnis Cellula e Cellula.

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Basics of Cell Biology

Which of the following statements about endoplasmic reticulum is false?

Endoplasmic reticulum is continuous reticulum of sheets and tubules emanating from outer nuclear membrane, enclosing single lumen bounded by phospholipid bilayer containing translocon Sec61 for cotranslational import. Rough ER domain densely studded with 80S ribosomes docking via SRP receptor synthesizes secretory and membrane proteins that fold aided by chaperone BiP, protein disulfide isomerase PDI forming disulfides, and undergo initial N-glycosylation by oligosaccharyltransferase. Smooth ER abundant in hepatocytes and steroidogenic cells lacks ribosomes, houses lipid synthesis enzymes including HMG-CoA reductase for cholesterol, phosphatidylcholine synthesis, plus cytochrome P450 family for drug metabolism and calcium pump SERCA storing calcium released via IP3 receptors for signaling. ER exists exclusively in eukaryotes where endomembrane system compartmentalizes; prokaryotes lack internal organelles exporting proteins directly across plasma membrane. Classification as simple single-membrane vesicle like lysosome understates its network complexity, paired cisternae forming double membrane appearance around lumen, contact sites with mitochondria and plasma membrane for lipid exchange. Moreover claim of prokaryotic exclusivity reverses reality.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 12: Endoplasmic Reticulum Organization.

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Basics of Cell Biology

Which organelle is responsible for the production of ribosomal RNA?

Nucleolus is non-membrane-bound condensate within nucleus formed by liquid-liquid phase separation around nucleolar organizer regions on chromosomes 13,14,15,21,22 containing tandem arrays of 45S rRNA genes 400 copies in humans. RNA polymerase I, with transcription factors UBF and SL1, transcribes 47S pre-rRNA precursor containing 18S, 5.8S, 28S sequences separated by internal and external transcribed spacers. Co-transcriptional processing involves snoRNPs C/D box guiding 2'-O-methylation and H/ACA box guiding pseudouridylation, endonucleolytic cleavages generating mature rRNAs. 5S rRNA transcribed by Pol III in nucleoplasm imports. Ribosomal proteins, 33 for 60S and 21 for 40S plus assembly factors, imported from cytoplasm, combine hierarchically to pre-60S and pre-40S particles exported via CRM1 and RanGTP. Tripartite morphology fibrillar center containing Pol I, dense fibrillar component where processing occurs, granular component for assembly reflects this vectorial flow. Nucleolar stress with impaired rRNA synthesis stabilizes p53 via MDM2 sequestration. Rough ER, Golgi, mitochondria perform translation, glycosylation, and respiration, not ribosomal RNA synthesis.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 6: Nucleolus and rRNA Synthesis.

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Basics of Cell Biology

Which of the following types of transport requires ATP?

Membrane transport classification depends on thermodynamics and mechanism. Simple diffusion of lipophilic gases O2, CO2, steroids occurs directly through hydrocarbon core partitioning driven solely by concentration gradient without protein. Facilitated diffusion via channels like aquaporins and carriers like GLUT1 enhances polar solute permeation down gradient, increasing rate and specificity, still entropy-driven, saturable, no ATP hydrolysis. Osmosis is water movement across semipermeable membrane toward solute, also passive. Active transport moves substrates against electrochemical potential requiring energy coupling. Primary active transport directly hydrolyzes ATP by P-type ATPases Na+/K+ ATPase with phosphorylation intermediate pumping 3 Na+ outward and 2 K+ inward maintaining resting potential -70 mV, SERCA pumping Ca2+ into sarcoplasmic reticulum, V-type V-ATPase acidifying lysosomes. Secondary active transport uses sodium gradient energy to drive glucose uptake SGLT1 symport or Ca2+/Na+ exchange. ABC transporters also primary. Consequently active transport uniquely requires ATP or preexisting ion gradient powered originally by ATP, distinguishing it from passive modalities essential for nerve impulse and volume regulation.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 11: Active Transport Requires ATP.

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Basics of Cell Biology

Which of the following is a characteristic of heterochromatin?

Heterochromatin denotes highly condensed chromatin domains resistant to nuclease digestion, visible as electron-dense clumps along nuclear periphery associated with lamina and around nucleolus, comprising 10 to 20 percent mammalian genome depending on cell type. Enriched in repetitive elements including LINE-1, satellite repeats at centromeres and telomeres, and developmentally silenced genes, it carries repressive histone modifications such as H3K9 trimethylation deposited by SUV39H1 and SUV39H2, recognized and bound by hetero protein HP1 alpha that drives liquid-liquid phase separation and chromatin compaction through oligomerization, and H3K27 trimethylation for facultative heterochromatin marking developmental regulators and inactive X chromosome coated by Xist long noncoding RNA recruiting PRC2 complex with EZH2 catalytic subunit. DNA methylation by DNMT3A/B at CpG dinucleotides further locks silent state. Functionally transcription is minimal due to exclusion of RNA polymerase II and transcriptional activators, replication occurs late in S phase, high nucleosome density, suppressing transposon mobilization and illegitimate recombination preserving genome integrity during cell division. Euchromatin contrasts as gene-rich, loosely packed, DNase hypersensitive, early replicating. Transcriptionally inactive nature is defining characteristic distinguishing constitutive versus facultative heterochromatin types regulated developmentally and environmentally.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 4: Heterochromatin and Transcriptional Silencing.

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Basics of Cell Biology

Which of the following organelles has a double membrane?

Double membrane architecture provides compartments for energy transduction and genetic separation. Mitochondria possess outer membrane containing porins VDAC1-3 allowing metabolite exchange up to 5 kDa and TOM complex for importing nuclear-encoded proteins, and inner membrane highly impermeable and folded into cristae via MICOS complex and ATP synthase dimer rows generating curvature, housing electron transport chain Complexes I to IV and ATP synthase utilizing chemiosmotic proton gradient established by proton pumping for ATP synthesis. Intermembrane space accumulates protons. Matrix contains TCA cycle enzymes, mtDNA nucleoids, mitoribosomes. Outer membrane derived evolutionarily from host vesicle, inner from endosymbiont. Golgi apparatus, lysosomes, peroxisomes are single membrane organelles performing processing without chemiosmosis. Chloroplasts also double membrane with thylakoids inside. Nuclear envelope is double membrane with pores. Identification of mitochondria as double membrane organelle underlies understanding of apoptosis regulation via cytochrome c release from intermembrane space through Bax/Bak pores and calcium handling at ER-mitochondria contact sites essential for metabolism.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 14: Double-Membrane Organelles, Mitochondria.

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Basics of Cell Biology

Which of the following is true about the Fluid Mosaic Model?

Fluid Mosaic Model postulated by Singer and Nicolson in 1972 integrated thermodynamic considerations and experimental data that membranes behave as two-dimensional fluid solutions. Phospholipids and sphingolipids undergo rapid lateral diffusion measured 1-3 µm2/s, rotation around axis, transbilayer flip-flop rare without flippase catalysis, demonstrated by Frye and Edidin fusion of human and mouse fibroblasts where surface antigens intermix within 40 minutes, and later single-particle tracking revealing Brownian, confined, and directed motions due to cytoskeletal fences. Integral membrane proteins float in lipid sea, some tethered via ankyrin to spectrin cortex, others partitioning into cholesterol-rich ordered rafts enriched in sphingomyelin for signaling. Peripheral proteins attach via ionic interactions or lipid anchors myristoylation, palmitoylation. Lipid asymmetry is maintained by ATP-dependent P4-ATPases. Membrane is not static nor rigid, lipids and proteins can move laterally, asymmetry persists except during apoptosis, enabling processes like immune synapse, vesicular budding, and hormone reception that require reorganization of components in plane of membrane.

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 10: Fluid Mosaic Model, Lateral Diffusion.

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Basics of Cell Biology

What is the role of centrioles in animal cells?

Centrioles are cylindrical non-membranous organelles formed of nine triplet microtubule blades arranged radially around central symmetry, each triplet composed of A, B, C complete and partial tubules, approximate dimensions 200 nanometer diameter and 500 nanometer length, embedded in pericentriolar material comprising pericentrin, CEP192, and gamma-tubulin ring complexes that nucleate microtubule asters. Duplication begins at G1 to S transition via master kinase PLK4 recruitment to mother centriole, SAS-6 cartwheel assembly providing nine-fold symmetry template, STIL and CPAP elongating procentriole in orthogonal orientation. During late G2 and early mitosis, centrosomes separate driven by kinesin-5 Eg5 and dynein, forming bipolar spindle poles that nucleate dynamic microtubules capturing kinetochores via Ndc80 complex attachment, ensuring accurate segregation of sister chromatids to daughter cells preventing aneuploidy and chromosomal instability. In interphase, mother centriole templates basal body anchoring primary cilium axoneme containing 9+0 microtubules for Hedgehog signaling and motile cilia 9+2 for mucus clearance. They do not produce proteins, lipids, or ATP; synthesis belongs to ribosomes, ER, and mitochondria respectively.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 17: Centrosomes, Centrioles, and Spindle Formation.

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Basics of Cell Biology

Which of the following statements about bacterial cells is false?

Bacterial genome organization contrasts sharply with eukaryotic chromatin packed into chromosomes with histones. Escherichia coli, Bacillus subtilis, and most eubacteria possess single circular chromosome ranging 1 to 6 megabases, contour length about 1 millimeter compacted over 1000-fold into irregular nucleoid region via negative supercoiling induced by ATP-dependent DNA gyrase and topoisomerase I relaxation, bridging by nucleoid-associated proteins HU, H-NS, Fis, and condensin MukBEF complex. DNA replication initiates at single origin oriC regulated by DnaA-ATP binding to DnaA boxes, proceeds bidirectionally with leading and lagging synthesis coupled to membrane anchoring. Plasmids are extrachromosomal small circles conferring antibiotic resistance, virulence factors, conjugation ability. Linear chromosomes exist as rare exceptions in Borrelia burgdorferi spirochete causing Lyme disease and Streptomyces coelicolor, but require specialized telomere-like proteins and linear replication mechanisms. Claiming bacterial DNA is generally linear misrepresents dominant circular topology, which impacts replication termination at dif site resolved by XerCD recombinases and segregation by ParABS partitioning. Lack of nucleus enables coupled transcription-translation, and reproduction by binary fission using FtsZ tubulin homolog constricting septum without mitotic spindle typical for bacteria.

Ref: Madigan et al., Brock Biology of Microorganisms, 16th ed., Chapter 4: Bacterial Circular Chromosome.

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Basics of Cell Biology

Which component of the plasma membrane is responsible for fluidity?

Membrane fluidity describes lateral mobility, rotation, and flexing of components, parameter influencing permeability, fusion, and signaling complex formation. Phospholipids primarily determine fluidity through acyl chain composition: presence of cis double bonds as in oleoyl 18:1 and arachidonoyl 20:4 introduces kinks preventing close packing, lowering melting temperature Tm and increasing lateral diffusion coefficient about 10^-8 cm2/s measured by fluorescence recovery after photobleaching and fluorescence anisotropy. Shorter chains also increase fluidity. Cholesterol buffers fluidity by intercalating with rigid ring near chains, reducing motion at high temperature while preventing crystallization at low temperature, broadening phase transition. Proteins and carbohydrates can locally restrict diffusion via cytoskeletal corrals and lectin crosslinking, but bulk fluidity originates from lipids. Ribosomes associated with rough ER translation do not influence plasma membrane lipid order. Regulated desaturase enzymes SCD1 introducing double bonds adjust fluidity in response to cold, diet, and insulin signaling, essential for maintaining receptor tyrosine kinase activity and cold tolerance in poikilotherms.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 10: Phospholipids and Fluidity Regulation.

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Basics of Cell Biology

Which of the following is not a function of the cytoskeleton?

Cytoskeletal networks coordinate shape, division, and intracellular logistics. Actin filaments, 7 nm flexible helical polymers of ATP-G-actin nucleated by Arp2/3 at 70 degree branch and formins for linear cables, interact with 50 myosin classes for cortical contraction, formation of contractile ring with myosin II during cytokinesis, lamellipodial protrusion, and vesicle movement near periphery. Microtubules, 25 nm rigid hollow tubes of alpha-beta tubulin heterodimers assembling head to tail with GTP cap regulating dynamic instability, serve as polarized tracks for kinesin families moving outward to plus ends and dynein moving inward, positioning Golgi, endosomes, mitochondria, and assembling mitotic spindle with kinetochore capture ensuring chromosome segregation. Intermediate filaments, 10 nm non-polar rope-like polymers keratin, vimentin, desmin, lamins anchored via plectin and nesprin, provide mechanical resilience against shear stress. Together they govern intracellular transport, mechanical support, and motility. Protein synthesis uses ribosomal peptidyl transferase in cytoplasm and rough ER, not filaments, so translation is not cytoskeletal function.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 16: Cytoskeleton Functions Overview.

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