**Cell Wall Structure and Composition**:
– The cell wall is a structural layer surrounding some cell types.
– It provides support, shape, protection, and acts as a selective barrier.
– Helps cells withstand osmotic pressure and mechanical stress.
– Composition varies across organisms and can include materials like cellulose, chitin, glycoproteins, and peptidoglycan.
– Major carbohydrates in primary plant cell walls include cellulose, hemicellulose, and pectin.
– Secondary cell walls in plants contain cellulose, xylan, and lignin.
– Fungal cell walls consist mainly of chitin, glucans, and proteins.
– Algal cell walls contain polysaccharides, mannans, xylans, and alginic acid.
**Properties and Functions of Cell Walls**:
– Cell walls provide rigidity, strength, and protection.
– Linked to plant cell growth and morphogenesis.
– Allow organisms to maintain shape and stable osmotic environments.
– Limit entry of toxic molecules and help retain water.
– Cell walls are flexible but have tensile strength due to hydraulic turgor pressure and wall structure.
– Primary plant cell walls are permeable to small molecules, with size exclusion limits.
– Cell walls change in composition and properties during the cell cycle.
**Evolution and Diversity of Cell Walls**:
– Cell walls evolved independently in many groups, with different compositions.
– Plant and algae have cellulose cell walls, while fungi have chitin-glucan-protein cell walls.
– Evolutionary developments like CesA cellulose synthase and Csl proteins in plants.
– Fungi possibly acquired 1,3-β-glucan synthesis pathway via horizontal gene transfer.
– Algal cell walls may contain sporopollenin, calcium ions, and silica frustules.
**Formation and Structure of Cell Walls**:
– Middle lamella is laid down first during cytokinesis, followed by primary cell wall deposition.
– Cellulose microfibrils are aligned differently in primary and secondary walls.
– Cells communicate through plasmodesmata in the cell wall.
– Fungal cell walls are synthesized at the plasma membrane and contain glycosylated structural proteins.
– Archaeal cell walls lack peptidoglycan, with some having pseudopeptidoglycan and an S-layer.
**Other Cell Coverings and Comparative Analysis**:
– Prokaryotic cell walls are made of peptidoglycan and are essential for bacterial survival.
– Algae have mucilage sheaths, diatoms have silica frustules, and some algae encase cells in calcium carbonate skeletons.
– Slime molds like Dictyostelids have cellulose walls and three-layered spore walls.
– Various eukaryotic cell walls differ in composition and structure, with unique adaptations for environmental challenges.
A cell wall is a structural layer that surrounds some cell types, found immediately outside the cell membrane. It can be tough, flexible, and sometimes rigid. Primarily, it provides the cell with structural support, shape, protection, and functions as a selective barrier. Another vital role of the cell wall is to help the cell withstand osmotic pressure and mechanical stress. While absent in many eukaryotes, including animals, cell walls are prevalent in other organisms such as fungi, algae and plants, and are commonly found in most prokaryotes, with the exception of mollicute bacteria.
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| Plant cell diagram | |
 Components of a typical plant cell:
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The composition of cell walls varies across taxonomic groups, species, cell type, and the cell cycle. In land plants, the primary cell wall comprises polysaccharides like cellulose, hemicelluloses, and pectin. Often, other polymers such as lignin, suberin or cutin are anchored to or embedded in plant cell walls. Algae exhibit cell walls composed of glycoproteins and polysaccharides, such as carrageenan and agar, distinct from those in land plants. Bacterial cell walls contain peptidoglycan, while archaeal cell walls vary in composition, potentially consisting of glycoprotein S-layers, pseudopeptidoglycan, or polysaccharides. Fungi possess cell walls constructed from the polymer chitin, specifically N-acetylglucosamine. Interestingly, diatoms have a unique cell wall composed of biogenic silica.