35 public questions tagged with this topic.
The plasma membrane is selectively permeable and regulates the movement of substances in and out of the ll. This follows from NCERT principle where the relation explains the outcome clearly for students in simple steps.
Ref: NCERT Biology Textbook for Class XI and XII (Botany section), Chapter: Morphology and Anatomy of Flowering Plants, Topic: Plant structure and tissue systems.
The bacterial capsule provides protection against phagocytosis by host immune lls. 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.
Monerans, including bacteria and archaebacteria, are prokaryotic and lack a well-defined nucleus. This follows from NCERT principle where the relation explains the outcome clearly for students in simple steps.
Ref: NCERT Biology Textbook for Class XI and XII (Botany section), Chapter: Morphology and Anatomy of Flowering Plants, Topic: Plant structure and tissue systems.
Sperm midpiece is short segment posterior to neck connecting head to principal piece of flagellum, characterized by helical arrangement of elongated mitochondria wrapped around outer dense fibers and axoneme. Mitochondria generate ATP via oxidative phosphorylation fueling dynein-dependent flagellar beating, essential for progressive motility through female tract. Though glycolysis in principal piece also contributes, midpiece mitochondrial respiration supports hyperactivation and sustained motility. Structural defects in mitochondrial sheath cause asthenozoospermia. Midpiece therefore function
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 7: Sperm midpiece mitochondria, ATP production and flagellar motility.
HeLa displays epithelial-like morphology reflecting origin from cervical keratinocytes. In vitro, HeLa forms adherent monolayer of polygonal cells with cobblestone pattern, distinct cell borders, prominent nucleoli, and high nuclear to cytoplasmic ratio typical of carcinoma. Cells lack extensive spindle architecture seen in fibroblast-like populations such as NIH-3T3 which grow elongated and align in parallel arrays. HeLa retains some epithelial characteristics including expression of cytokeratins, presence of microvilli on apical surface, and formation of weak tight junctions but fails to dev
Ref: ATCC CCL-2 HeLa morphology epithelial-like; Freshney Ch.16 Epithelial vs fibroblast morphology characteristics.
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.
Cytoskeleton provides scaffold and track system built from protein polymers regulated by nucleation factors, GTPases, and post-translational modifications. Actin filaments, 7 nanometer diameter polarized structures polymerizing from ATP-G-actin mediated by Arp2/3 complex branching at 70 degrees and formins nucleating straight cables for filopodia, associate with myosin II for contractility at cortex and myosin V for short-range vesicle transport near cell periphery, driving lamellipodia extension and endocytic invagination. Microtubules, 25 nanometer hollow tubes of alpha-beta tubulin dimers w
Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 16: Cytoskeleton and Intracellular Transport.
Three cytoskeletal filament systems differ chemically mechanically and kinetically defining cytoplasmic organization. Actin filaments polar 7 nm helical polymers of ATP G actin assembling barbed plus fast pointed minus slow exhibiting treadmilling ATP hydrolysis dependent turnover myosin motors. Microtubules polar 25 nm tubes of GTP tubulin dimers showing dynamic instability and kinesin dynein transport highways plus end tracking. Intermediate filaments 10 nm rope like nonpolar exceptionally stable structures without polarity unique to metazoans. Lack of polarity stems from assembly pathway: p
Ref: Alberts et al., Molecular Biology of the Cell, Chapter 16: Intermediate filaments lack polarity distinction from actin and microtubules.
Cytoskeletal polymers provide complementary mechanical functions. F actin, 7 nanometer double helical polymer of ATP loaded G actin, drives protrusion and contraction via myosin II and generates branched networks via Arp2/3 at 70 degrees. Microtubules, 25 nanometer hollow cylinders of alpha beta tubulin heterodimers with GTP cap, resist compression and act as long range highways for kinesin plus end and dynein minus end motors transporting organelles and chromosomes. Intermediate filaments, 10 nanometer non polar fibers, provide outstanding tensile strength and resilience. Assembly hierarchy i
Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 16: Cytoskeleton and Intermediate Filaments.
Cytoskeletal homology across domains revealed by structural biology although sequence similarity low. FtsZ protein essential division highly conserved tubulin homolog. Crystal structures show globular N terminal domain with Rossmann fold GTPase active site containing T1 to T6 loops coordinating GTP plus magnesium, H7 helix central, C terminal domain involved filament contacts. Organization identical to alpha beta tubulin dimer architecture. Both polymerize head to tail forming protofilaments, binding GTP promotes assembly, hydrolysis to GDP promotes disassembly causing dynamic instability. Tub
Ref: Erickson et al., Microbiol Rev 2010, FtsZ tubulin homolog structural and functional homology.
Envelope architecture differentiates Gram staining groups morphologically and chemically. Gram-negative bacteria possess dual membrane system separated by periplasm containing thin peptidoglycan two to six nanometers and proteins, hydrolytic enzymes. Inner membrane phospholipid bilayer standard contains respiratory chain. Outer membrane asymmetric: inner leaflet phosphatidylethanolamine phosphatidylglycerol, outer leaflet predominantly lipopolysaccharides LPS molecules comprising lipid A glucosamine disaccharide with six acyl chains anchoring membrane and conferring endotoxin activity stimulat
Ref: Silhavy et al., Cold Spring Harbor Perspect Biol 2010, Gram-negative outer membrane contains LPS.
The axoneme of a eukaryotic cilium or flagellum usually has a 9+2 arrangement of peripheral doublets and central microtubules. A centriole has nine peripheral triplets connected to a proteinaceous hub by radial spokes.
Ref: NCERT Class 11 Biology Chapter 8: Cell: The Unit of Life Cytoskeleton Cilia Flagella Centrosome Nucleus and Chromosomes
