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

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 conde

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 incl

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 transc

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 potentia

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 c

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

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 prot

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 procen

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, procee

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 int

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 outw

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

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