#organelles

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.

View Details

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.

View Details

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.

View Details

Basics of Cell Biology

Which of the following organelles is involved in detoxification?

Detoxification of endogenous metabolites and exogenous xenobiotics requires oxidative handling and compartmentalization to protect cellular components. Peroxisomes, single membrane organelles containing enzymes imported via peroxisomal targeting signals PTS1 C-terminal SKL tripeptide and PTS2 N-terminal recognized by shuttling receptors PEX5 and PEX7, house acyl-CoA oxidases that beta-oxidize very-long-chain fatty acids C22 and above and branched-chain fatty acids like phytanic acid generating acetyl-CoA for mitochondria plus hydrogen peroxide H2O2 as byproduct. Catalase, core component with heme prosthetic group and highly abundant crystal, rapidly dismutates H2O2 to water and oxygen preventing oxidative damage to lipids and DNA; absence or peroxisome biogenesis disorders such as Zellweger syndrome cause accumulation of VLCFA and neurological defects and liver dysfunction. Additionally D-amino acid oxidase, urate oxidase in non-primate organisms, and enzymes for plasmalogen ether lipid synthesis provide antimicrobial and membrane functions. Smooth ER complements via cytochrome P450 monooxygenase family CYP3A4, CYP2D6 hydroxylating xenobiotics increasing solubility, followed by conjugation via glucuronosyltransferases. Nucleus safeguards genome, Golgi processes glycans, lysosomes degrade polymers, but oxidative detoxification is hallmark of peroxisomes together with smooth ER collaboration.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 12: Peroxisomes and Detoxification.

View Details

Basics of Cell Biology

Which of the following is not a characteristic of eukaryotic cells?

Eukaryotes are defined by compartmentalization: nuclear envelope with nuclear pore complexes regulating RNA export and protein import via importins, linear chromosomes packaged with histone octamers H2A, H2B, H3, H4 into nucleosomes acetylated and methylated for epigenetic control, membrane-bound organelles including ER continuous with nucleus for secretory protein folding, Golgi for glycosylation, lysosomes for degradation, mitochondria for energy, and in plants chloroplasts for photosynthesis. Ribosomes are 80S in cytosol composed of 40S small subunit with 18S rRNA and 60S large with 28S, 5.8S, 5S rRNA. Division involves regulated G1, S where DNA replicates once per cycle under geminin-Cdt1 control, G2, and M phase mitosis with spindle assembly checkpoint Mad2, BubR1 ensuring chromosome alignment, followed by cytokinesis. Binary fission involving FtsZ contractile ring that constricts single circular chromosome segregation is prokaryotic strategy lacking mitotic spindle, CDK control, and nuclear envelope breakdown. Therefore binary fission as primary mode is not characteristic of eukaryotic cells which rely on mitosis-meiosis.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 1: Eukaryotic Characteristics vs Binary Fission.

View Details

Basics of Cell Biology

Which of the following statements about mitochondria is incorrect?

Mitochondria originated from alphaproteobacterial endosymbiont engulfed by archaeal host over 1.5 billion years ago, retaining circular mitochondrial DNA, bacterial-type transcription and translation machinery including 55S mitoribosomes, and binary fission mediated by Drp1 and dynamin-like proteins, with genomes encoding core subunits of oxidative phosphorylation. Inner membrane houses respiratory chain complexes I to IV transferring electrons from NADH and FADH2 to oxygen producing proton gradient used by F0F1 ATP synthase to generate ATP, central to aerobic metabolism. This organelle occurs in virtually all eukaryotes capable of aerobic or facultative anaerobic metabolism: animal cells with high demand like cardiomyocytes containing thousands mitochondria, plant leaf cells where mitochondria coexist with chloroplasts and participate in photorespiration converting glycolate, yeast, algae, protozoa, only missing in some anaerobic protists that contain hydrogenosomes derived from mitochondria. Claiming presence only in animal cells contradicts inclusion in plant root and shoot cells, fungi, and broad eukaryotic distribution, as microscopy and biochemistry clearly demonstrate mitochondrial markers across kingdoms.

Ref: Cooper and Hausman, The Cell, 8th ed., Chapter 4: Mitochondria Distribution in Eukaryotes.

View Details

Basics of Cell Biology

Which of the following best describes the function of lysosomes?

Lysosomes are terminal degradative compartments of endocytic and autophagic pathways, acidified to pH 4.5-5.5 by vacuolar V-ATPase multi-subunit proton pump consuming ATP to pump protons into lumen generating electrochemical gradient used for secondary active transport of metabolites. They contain soluble acid hydrolases including acid phosphatase marker enzyme, cathepsin B, D, L proteases, sulfatases, beta-hexosaminidase, lipases, and nucleases delivered from trans-Golgi network via mannose-6-phosphate receptor M6PR pathway binding GlcNAc-1-phosphotransferase modified N-glycans. Endocytosed cargo internalized via clathrin coated pits, caveolin vesicles, or macropinocytosis traverses early endosomes Rab5 positive with PI3P, late endosomes Rab7 positive with lysobisphosphatidic acid, then fuses with lysosomes forming endolysosomes where hydrolysis releases amino acids, monosaccharides, fatty acids exported via cystinosin and other transporters for reuse supporting growth and signaling. Macroautophagy induced by starvation forms double membrane autophagosomes via ULK1, Beclin1, ATG machinery engulfing damaged mitochondria and aggregates, fusing with lysosomes for clearance. Defects cause lysosomal storage diseases like Tay-Sachs, Pompe, and Gaucher illustrating central importance.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 13: Lysosomes and Acid Hydrolases.

View Details

Basics of Cell Biology

Which of the following is absent in prokaryotic cells?

Prokaryotes comprise bacteria and archaea lacking nucleus defined by nuclear envelope and possessing simpler architecture optimized for rapid division. They contain cell envelope with plasma membrane of phosphoglycerides and hopanoids, peptidoglycan wall providing shape and protection against turgor, nucleoid region where single circular chromosome typically 1-6 megabases supercoiled and organized by histone-like proteins HU, Fis, SMC condensins occupies cytoplasm without separation, and 70S ribosomes for coupled transcription translation. RNA species messenger, transfer, and ribosomal exist transiently. Metabolic processes such as aerobic respiration and photosynthesis occur across plasma membrane or intracytoplasmic membranes where electron transport components quinones, cytochrome bc1, cytochrome oxidase generate proton motive force for ATP synthase. Mitochondria, organelles bounded by outer membrane with VDAC porins and inner membrane with cristae housing respiratory complexes I-V, TCA cycle enzymes in matrix, and own circular mitochondrial DNA encoding 13 respiratory proteins, are absent in prokaryotic cells. Evolutionary theory traces mitochondria to endosymbiosis of alpha-proteobacteria engulfed by proto-eukaryote, providing compartmentalization in eukaryotes absent in prokaryotes which rely on membrane invaginations rather than dedicated powerhouses.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 1: Prokaryotic vs Eukaryotic Organization.

View Details

Glycolipid and Phospholipid

Which structure is involved in lipid trafficking between organelles?

Efficient movement lipids between organelles employs both vesicular carriers clathrin COPII COPI and non vesicular lipid transfer membrane contact sites where domain organization guides sorting specificity and lipid gradients. Vesicular pathway buds donor via coat proteins packaging lipids proteins, while lipid driven sorting trans Golgi network concentrates sphingolipids cholesterol GPI anchored proteins into rafts destined apical plasma membrane demonstrated sorting influenza hemagglutinin placental alkaline phosphatase independent cytosolic sorting signals tyrosine dileucine. Flotillin caveolin oligomers stabilize such domains recruiting cargo clustering via oligomerization forming carriers. Non vesicular transfer uses lipid transfer proteins CERT ceramide FAPP2 glucosylceramide OSBP ORP family sterol PI4P exchange at ER Golgi MCS tethering VAPA VAPB. Cytoskeleton provides tracks vesicle motors kinesin dynein, ribosomes synthesize proteins not traffic lipids. Thus lipid rafts function sorting platforms influencing lipid flow directionality polarity signaling, linking membrane biophysics secretory pathway organization relevant congenital disorders glycosylation lipid storage diseases affecting trafficking polarity maintenance cell migration immune synapse formation concepts examined cell biology biochemistry trafficking questions thoroughly for organelle communication mechanisms.

Ref: Simons & Sampaio, Cold Spring Harb Perspect 2011, Lipid raft sorting trafficking platforms and MCS.

View Details

Membrane Transportation

Which ATP pump is involved in generating acidic pH inside organelles like lysosomes?

Generating acidic interior in endocytic compartments essential for cargo sorting, iron release from transferrin, ligand receptor dissociation, viral entry and lysosomal hydrolysis demands active proton translocation against roughly two pH units. Main machinery responsible is vacuolar proton ATPase V-type, large composite enzyme about 900 kilodaltons composed of peripheral V1 complex A3B3CDE3FG3H where A3B3 hexamer contains catalytic ATP hydrolysis sites at alternating interfaces, and integral membrane Vo complex including a subunit with two half channels, c-ring of nine to ten copies c, c double prime providing proton binding glutamate essential carboxylates, subunits d, e and accessory Ac45. Hydrolysis drives rotation of central rotor DF coupled to c-ring, moving protonated glutamates from cytosolic half channel to luminal half channel releasing protons inside. Mechanism rotary without phosphoenzyme intermediate, inhibited by bafilomycin macrolide binding Vo. ABC transporters export xenobiotics, P-type Na+/K+ ATPase maintains plasma gradients not acidic organelle pH. Thus V-class proton pump, not ABC, creates acidic pH in lysosomes and related compartments supporting degradative pathways.

Ref: Forgac, V-ATPase Review, Acidification of Lysosomes and Endosomes by V-Class Proton Pump.

View Details

Cell Organelles

Which pair of organelles have the same pH and ionic composition?

Ionic milieu of subcellular compartments reflects membrane permeability architecture and active pumping. The mitochondrial outer membrane contains abundant 19-stranded beta-barrel voltage-dependent anion channels, VDAC porins, present at high density allowing nonselective diffusion of solutes up to about 4 to 5 kilodaltons, including chloride, potassium, sodium, small metabolites, ATP, ADP, NADH and citrate. Because VDAC pores remain predominantly open under physiological low potentials, the intermembrane space equilibrates rapidly with cytosol via free diffusion, sharing near-identical pH around 7.0 to 7.1, comparable potassium around 140 millimolar, sodium, chloride and metabolite concentrations, and redox potential. In contrast matrix maintains more alkaline pH around 7.8 due to proton pumping by Complexes I, III and IV, low calcium due to MCU regulation, and distinct adenine nucleotide composition due to selective ADP/ATP carrier. Cytosol versus lysosome comparison reveals stark differences: lysosomal lumen acidic pH 4.5 maintained by V-ATPase, accumulates calcium via TRPML1 and concentrated hydrolases. Endosomal lumen also progressively acidic, while mitochondrial matrix alkaline. Therefore cytosol and intermembrane space equivalence is mechanistically explained by VDAC porin porosity and absence of active ion pumping across outer membrane, creating Donnan equilibrium similar to cytosol.

Ref: Nicholls & Ferguson, Bioenergetics, 4th ed., Chapter 3: Outer Membrane Permeability and Cytosol-Equilibrated Intermembrane Space.

View Details

Cell Organelles

Endosomes are formed from:

Endosomal system is central node of vesicular traffic that integrates endocytic and biosynthetic routes to sort cargo after internalization or secretion. Classical early endosomes originate when clathrin-coated pits invaginate with assistance of AP2 adaptor, epsin and dynamin GTPase scission, or when caveolae containing caveolin internalize, delivering extracellular ligands, nutrient receptors such as transferrin receptor, signaling receptors like EGFR, and fluid-phase markers into cell. These primary vesicles rapidly lose coat through auxilin and Hsc70 and fuse homotypically via Rab5-GTP, EEA1 coiled-coil tether and phosphatidylinositol 3-phosphate to generate larger early sorting endosomes. However endosomes also receive substantial and essential input from trans-Golgi network, TGN, where newly synthesized lysosomal hydrolases tagged with mannose-6-phosphate and lysosomal membrane proteins LAMPs are sorted into clathrin and AP1, GGA vesicles that traffic and fuse with late endosomes marked by Rab7. This dual supply merges surface-derived endocytic material with Golgi-derived enzymes, ensuring lysosomes acquire complement of acidic hydrolases while plasma membrane cargo proteins are efficiently recycled via Rab4 and Rab11 routes. Thus endosomes represent convergence of plasma membrane-derived and TGN-derived pathways, with Golgi contributions indispensable for maturation, acquisition of degradative capacity and eventual fusion with lysosomes.

Ref: Alberts et al., Molecular Biology of the Cell, 6th ed., Chapter 13: Endosome Formation from Plasma Membrane and TGN.

View Details