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#cell wall

22 public questions tagged with this topic.

Transgenic Plant -2

Polygalacturonase enzyme is involved in:

Polygalacturonase is a major pectinase induced during tomato ripening, leaf abscission and pod dehiscence, directly responsible for fruit softening. Primary cell wall and middle lamella contain homogalacturonan composed of α-1,4-linked D-galacturonic acid residues partially methylesterified. During breaker to red stage, ethylene upregulates endo-polygalacturonase, pectin methylesterase and cellulase synergistically. Endo-PG hydrolyzes internal glycosidic bonds, reducing polymer length, increasing water solubility, decreasing cell-to-cell adhesion, facilitating separation of cells and access for other hydrolases. This leads to loss of firmness, increase in locular juice, over-softening during transport and susceptibility to Botrytis and Rhizopus. Antisense suppression of PG mRNA, as used in Flavr Savr, blocks translation via formation of double-stranded RNA degraded by Dicer, leaving other ripening processes like lycopene synthesis and sugar accumulation intact. Thus PG exemplifies how targeted manipulation of a single wall-modifying enzyme can decouple textural change from color, flavor and aroma development, providing rational control of post-harvest physiology without compromising sensory quality.

Ref: Alberts Molecular Biology of the Cell 6th ed Chapter 20 Plant Cell Walls; Taiz & Zeiger Plant Physiology, PG pectin degradation.

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Microbial Physiology

What is the function of Teichoic acids in Gram-positive bacteria?

Wall teichoic acids covalently attached to peptidoglycan via phosphodiester to N-acetylmuramic acid and lipoteichoic acids anchored to membrane diacylglycerol through glycolipid are hallmark anionic glycopolymers of Gram-positive envelope, comprising up to 60 percent of wall dry weight. Their polyanionic phosphate backbone creates high density of negative charge at surface, enabling binding of divalent cations like magnesium and calcium, concentrating them near membrane for activity of enzymes and protection against cationic antimicrobial peptides. By controlling local ionic environment, teichoic acids modulate activity of autolysins, preventing uncontrolled wall degradation, and regulate insertion of penicillin-binding proteins and cell division machinery through spatial cues. They also function as phage receptors, contribute to biofilm formation via electrostatic interactions, and influence host immune detection via TLR2. In contrast, porin function belongs to Gram-negatives, endotoxin activity derives from lipopolysaccharide lipid A, and peptidoglycan degradation is carried out by lysozymes and lytic transglycosylases. Hence providing structural support via cation homeostasis and ion transport regulation captures multifaceted roles.

Ref: Brown et al., Ann Rev Microbiol 2013, Teichoic Acids; Percy & Grundling, Ann Rev Microbiol 2014, LTA Functions.

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Microbial Physiology

The Gram-negative bacterial cell wall consists of:

Gram-negative cell wall is thin but complex, optimized for protection while maintaining permeability for nutrients. It comprises inner plasma membrane, narrow periplasmic space containing single or few layers of peptidoglycan only about 2 to 6 nanometers thick, and outer membrane distinguished by asymmetry. Outer membrane outer leaflet consists of lipopolysaccharide anchored by lipid A, while inner leaflet contains phospholipids; integral outer membrane proteins such as porins, TonB-dependent receptors, and OmpA form beta-barrels. Lipopolysaccharide confers negative charge, stabilizes outer membrane via bridging divalent cations, and provides endotoxin activity and serotype specificity. Thick peptidoglycan, characteristic of Gram-positives, is absent; instead, thin layer provides shape but contributes less mechanical strength than in Gram-positives. Monolayer lipid and teichoic acid-only models do not reflect Gram-negative dual membrane architecture. Assembly of outer membrane requires Lpt pathway transporting LPS from inner membrane across periplasm to outer leaflet and Bam complex folding beta-barrel proteins. This organization explains staining behavior, antibiotic susceptibility, and resistance to detergents and bile in enteric organisms.

Ref: Silhavy et al., The Bacterial Cell Envelope, 2nd ed.; Vollmer & Seligman, Trends Microbiol 2010, Gram-Negative Wall.

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Microbial Physiology

Which archaeal species contains pseudomurein in its cell wall?

Distribution of pseudomurein within Archaea is limited to certain methanogenic euryarchaeota that build rigid wall sacculus, whereas many other archaea rely solely on proteinaceous S-layers. Methanobrevibacter species, including human commensal Methanobrevibacter smithii and rumen isolate Methanobrevibacter ruminantium within order Methanobacteriales, synthesize pseudomurein composed of N-acetylglucosamine and N-acetyltalosaminuronic acid linked beta-1,3 and crosslinked with tetrapeptides containing L-amino acids. Related genera like Methanobacterium and Methanothermobacter share this trait, distinguishing them from methanococci and methanosarcinales that lack walls. Sulfolobus, a crenarchaeon, possesses S-layer and tetraether lipids but not pseudomurein. Thermococcus, also euryarchaeon but within Thermococcales, possesses only S-layer or glycoprotein sheath. Thus among choices presented, Methanobrevibacter clearly contains pseudomurein, exemplifying lineage-specific wall innovation, while statement that all listed genera possess it would be incorrect because envelope diversity across archaea is extensive and pseudomurein evolved only within methanobacteriales and related orders. Interestingly, Methanobacteriales also produce unique coenzymes such as coenzyme M and methanofuran essential for methanogenesis, linking wall chemistry with specialized metabolism, and environmental surveys detect pseudomurein-encoding genes only within this lineage, confirming narrow distribution that supports using pseudomurein as chemotaxonomic marker for this group.

Ref: Kandler, Microbiol Sci 1982, Pseudomurein Distribution; Doddema et al., Int J Syst Bacteriol 1982, Methanobrevibacter.

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Microbial Physiology

In Archaea, the S-layer is primarily composed of:

In many archaea, particularly those lacking pseudomurein or outer proteinaceous sheath, envelope consists of cytoplasmic membrane plus outermost crystalline surface layer that serves as cell wall equivalent. This S-layer is not composed of peptidoglycan, lipids, or teichoic acids characteristic of bacteria; it is assembled from one or two species of proteins or glycoproteins that self-organize into hexagonal or tetragonal lattice with nanometer-scale pores. Subunits are often heavily glycosylated with sulfated or branched oligosaccharides that confer additional stability against proteases and extremes. Lattice formation is driven by non-covalent interactions, capable of self-assembly in vitro, and anchored via stalk-like domains interacting with membrane lipids or pseudomurein. In Sulfolobus, S-layer consists of two glycoproteins SlaA forming outer canopy and SlaB forming membrane-anchoring stalks. In halophiles, single glycoprotein forms matrix. Thus proteinaceous or glycoprotein nature distinguishes archaeal S-layers from peptidoglycan-based walls and explains resistance to lysozyme, beta-lactams, and cell wall antibiotics targeting murein biosynthesis pathways. This mechanistic insight is relevant for competitive examinations such as CSIR-NET and GATE, where understanding molecular detail rather than memorization enables accurate interpretation of experimental data and pathway interconnections.

Ref: Sleytr et al., FEMS Microbiol Rev 2014, S-layers; Fagan & Fairweather, Nature Rev Microbiol 2014, S-layer Proteins.

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Microbial Physiology

Which bacterial structure prevents plasmolysis in hypertonic conditions?

Hypertonic surroundings draw water out of bacterial cytoplasm through semipermeable inner membrane, generating inward osmotic pressure that tends to detach inner membrane from wall and shrink cytoplasm, a process termed plasmolysis. Survival depends on mechanical resistance supplied by peptidoglycan, a covalently crosslinked heteropolymer forming a single macromolecule sacculus surrounding entire cell, often described as exoskeleton. Its glycan strands crosslinked via peptide bonds possess high tensile strength, allowing it to withstand internal turgor of 2 to5 atmospheres in Gram-negatives and up to 20 atmospheres in Gram-positives, plus external osmotic shifts. Capsular polysaccharide, while important for immune evasion and desiccation resistance, is gel-like, highly hydrated, and lacks load-bearing stiffness. Outer membrane of Gram-negatives contributes some rigidity but remains fluid and present only in one group. Cytoplasmic proteins alone cannot counteract membrane deformation. Experimental evidence shows lysozyme digestion of wall in isotonic sucrose yields osmotically fragile protoplasts that burst upon dilution. Bacteria also accumulate compatible solutes like glycine betaine, trehalose, and ectoine to balance osmolarity, but primary immediate defense against plasmolysis remains peptidoglycan.

Ref: Rojas et al., PNAS 2014, Stiffness of Bacterial Cell Wall; Alberts et al., Molecular Biology Cell, Cell Wall Mechanics.

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Microbial Physiology

The main function of Teichoic acids in Gram-positive bacteria is:

Gram positive wall comprises thick peptidoglycan crosslinked peptides providing rigidity against internal turgor up to 20 atmospheres. Embedded anionic polymers wall teichoic acids covalently linked phosphodiester to C6 hydroxyl N acetylmuramic acid and lipoteichoic acids anchored diacylglycerol into membrane extending outward. Structure polyglycerol phosphate or polyribitol phosphate modified D alanylation introducing positive charge modulating autolytic enzymes. Physiological roles encompass providing cell wall integrity by controlling autolysin Atl Atl mediated remodeling preventing premature lysis ensuring balanced expansion during growth, maintaining cation homeostasis binding magnesium essential for enzyme activity, influencing cationic antimicrobial peptide resistance by charge modification, serving phage receptor and adhesin for host colonization. Deletion tagO gene first step WTA synthesis lethal in Staphylococcus aureus without compensatory mutations highlighting essentiality. Unlike ion channels formed by proteins, teichoic acids not transporters nor replication regulators nor initiating peptidoglycan degradation but structural scaffolds regulating wall metabolism and physicochemical properties critical for survival in high osmolarity and immune evasion.

Ref: Brown et al., Nat Rev Microbiol 2013, Teichoic acids provide cell wall integrity in Gram-positive.

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Eukaryotic Cell - Cell Wall and Cell Membrane - Fluid Mosaic Model

Two cell-wall samples are chemically analysed: Sample P contains cellulose, galactans, mannans and calcium carbonate. Sa

Algal walls may contain cellulose, galactans, mannans and minerals such as calcium carbonate. In other plants, the wall is composed of cellulose, hemicellulose, pectins and proteins.

Ref: NCERT Class 11 Biology Chapter 8: Cell: The Unit of Life Eukaryotic Cell - Cell Wall and Cell Membrane - Fluid Mosaic Model

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