25 public questions tagged with this topic.
The Golgi apparatus is responsible for modifying, sorting, and packaging proteins and lipids for transport. This follows from NCERT principle where relation explains outcome clearly for students.
Ref: NCERT Biology Textbook for Class XI and XII (Botany section), Chapter: Biology - Botany portion covering relevant concept, Topic: Plant structure, physiology and applications.
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.
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.
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.
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 h
Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 12: Peroxisomes and Detoxification.
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.
Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 1: Eukaryotic Characteristics vs Binary Fission.
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
Ref: Cooper and Hausman, The Cell, 8th ed., Chapter 4: Mitochondria Distribution in Eukaryotes.
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 c
Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 13: Lysosomes and Acid Hydrolases.
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 t
Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 1: Prokaryotic vs Eukaryotic Organization.
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 cave
Ref: Simons & Sampaio, Cold Spring Harb Perspect 2011, Lipid raft sorting trafficking platforms and MCS.
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 doub
Ref: Forgac, V-ATPase Review, Acidification of Lysosomes and Endosomes by V-Class Proton Pump.
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 aro
Ref: Nicholls & Ferguson, Bioenergetics, 4th ed., Chapter 3: Outer Membrane Permeability and Cytosol-Equilibrated Intermembrane Space.
