Secondary cell walls contain a wide range of additional compounds that modify their mechanical properties and permeability. The main functions of the cell wall are to provide structure, support, and protection for the cell. A cell wall is defined as a rigid, external layer that is specifically designed to provide structural support and rigidity.  In multicellular organisms, they permit the organism to build and hold a definite shape. The LPS layer surrounds the peptidoglycan layer and acts as an endotoxin (poison) in pathogenic bacteria (disease causing bacteria).  Microfibrils are held together by hydrogen bonds to provide a high tensile strength. Their composition, properties, and form may change during the cell cycle and depend on growth conditions. Animal cells however, do not have a cell wall. Cellulose microfibrils are produced at the plasma membrane by the cellulose synthase complex, which is proposed to be made of a hexameric rosette that contains three cellulose synthase catalytic subunits for each of the six units.  Oomycete cell walls also contain the amino acid hydroxyproline, which is not found in fungal cell walls. A cell wall is a rigid, semi-permeable protective layer in some cell types. Plants later evolved various genes from CesA, including the Csl (cellulose synthase-like) family of proteins and additional Ces proteins.  However, "the dead excrusion product of the living protoplast" was forgotten, for almost three centuries, being the subject of scientific interest mainly as a resource for industrial processing or in relation to animal or human health.. Pictured: A cross-section of a plant cell with the cell wall highlighted. The cell wall separates the interior contents of the cell from the exterior environment. In plants, a secondary cell wall is a thicker additional layer of cellulose which increases wall rigidity. The middle lamella is also the outermost layer and it acts as an interface between the other neighbouring cells and glues them together. The gram-negative bacterial cell wall also contains an outer layer of lipopolysaccharides (LPS). Cell walls also limit the entry of large molecules that may be toxic to the cell. The inclusion of additional polysaccharides in algal cells walls is used as a feature for algal taxonomy. The names originate from the reaction of cells to the Gram stain, a test long-employed for the classification of bacterial species. Their glycoproteins are rich in mannose. The cell wall in gram-positive bacteria contains several layers of peptidoglycan. From the outermost layer of the cell wall, these layers are identified as the middle lamella, primary cell wall, and secondary cell wall. The cells are held together and share the gelatinous membrane called the middle lamella, which contains magnesium and calcium pectates (salts of pectic acid). Cell walls in some plant tissues also function as storage deposits for carbohydrates that can be broken down and resorbed to supply the metabolic and growth needs of the plant. The primary cell is situated closest to the inside of the cell and is the first-formed cell wall. Both cutin and suberin are polyesters that function as permeability barriers to the movement of water. A major function is to act as pressure vessels, preventing over-expansion of the cell when water enters. And just like the cell walls in plants, they provide structural support and prevents desiccation. The compositions of the cell wall usually vary along with organisms. An extracellular matrix (ECM) is also present in metazoans. Gram-positive bacteria have a thick peptidoglycan layer and gram-negative bacteria have a thin peptidoglycan layer. Cell walls of the epidermis may contain cutin. What is the function of the cell wall? , In brown algae, phlorotannins may be a constituent of the cell walls..  Cell walls are present in most prokaryotes (except mollicute bacteria), in algae, fungi and eukaryotes including plants but are absent in animals. These compounds are rigid and waterproof, making the secondary wall stiff. This layer primarily consists of pectins. This molecule gives the cell wall rigidity and helps to give bacteria shape. There are broadly speaking two different types of cell wall in bacteria, called gram-positive and gram-negative. The cell walls of archaea have various compositions, and may be formed of glycoprotein S-layers, pseudopeptidoglycan, or polysaccharides. 2. , Up to three strata or layers may be found in plant cell walls:. It is mainly made up of cellulose, allowing the wall to stretch for the purpose of growth. This outer covering is positioned next to the cell membrane (plasma membrane) in most plant cells, fungi, bacteria, algae, and some archaea. Most Archaea are Gram-negative, though at least one Gram-positive member is known.. 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