Most prokaryotes, fungi, and plants have cell walls. The cell wall is a thick, tough and slightly elastic structure located in the outer layer of the cell membrane, surrounding the protoplast inside, which is composed of the mucous complex. Some organisms also have a protective capsule composed of polysaccharide substances outside the cell wall, and the capsule itself can also serve as the nutrient of the cell. It can be used by cells when nutrients are scarce. There are some differences in the composition and function of cell walls in various organisms, that depending on different tissue types of the same organism or in different developmental stages of the same cell. Today we will briefly introduce the composition and structure of cell walls of four kinds of organisms.
Cyanobacteria Cell Wall
Cyanobacteria cells have cell wall mainly composed of peptidoglycan, which can be dissolved by lysozyme, similar to eubacteria. The cell wall of cyanobacteria also contains pectin, mucopolysaccharide, cyttoic acid, diaminoheptanoic acid (DAP), etc. The vast majority of cyanobacteria have thick or thin gelatinous sheaths outside the cell wall, so cyanobacteria were also called mucopolysaccharides. The cell walls of cyanobacteria contain cellulose and intracystic acid, which, like eukaryotic cells, also have bacterial characteristics.
Bacterial Cell Wall
Bacterial cell walls are mainly composed of peptidoglycan (also known as membranins, mucins or mucopoleptides), intracellular acids and special lipid complexes. The mechanical strength of bacterial cell wall depends on the presence of peptidoglycan, which can account for 10%~30% of the dry weight of bacteria. Except for bacteria without cell walls, almost all bacteria have polypeptidoglycans, but there are differences in the amount, which is the main reason that cell walls have a certain hardness and keep bacterial cells in a certain shape. Bacterial cell walls are usually distinguished by Gram staining into two types, Gram-positive (G+) and Gram-negative (G-). Gram+ bacterium cell wall is thicker, of which chemical composition is simple, generally containing 90% peptidoglycan and 10% teichoic acid. The cell wall is basically composed of a thick layer of peptidoglycan interspersed with teichoic acid. The cell wall of Gram- bacteria is thinner than that of Gram+ bacteria, and its chemical composition is more complex. It can be divided into the outer membrane and the thin peptidoglycan layer.
Fungus Cell Wall
The main components of fungal cell walls are polysaccharide chains (chitin, cellulose, glucan, mannan and other polysaccharide components) composed of hexose as well as proteins, lipids, melanin and inorganic salts. Electron microscopy showed that the yeast cell wall has two layers, the inner wall is a complex of chitin and β -glucan, they constitute the mechanical strength of the cell wall; The outer wall contains proteins that radiate along the cell’s surface.
The cell wall of filamentous fungi has about 4 layers: the innermost layer is radially arranged chitin microfilaments, which may also contain protein components; The outer layer is the protein layer, which may contain other components; The next layer is a rough web of glycoproteins embedded in the protein layer; The outermost layer is made of amorphous dextran. Yeast cell is the most common fungal cell, the thickness of yeast cell wall is 0.1~0.3μm, the weight of the cell dry weight of 18%~30%, mainly composed of d-glucan and D-mannan polysaccharides, containing a small amount of protein, fat, minerals.
Plant Cell Wall
The main components of plant cell walls are cellulose and pectin. Cell wall is one of the main characteristics that distinguish plant cells from animal cells. The main components of the plant cell wall are cellulose, pectin polysaccharide, hemicellulose, lignin, protein (including peroxidase, phosphatase, autolyase and other enzymes) and so on. Some specific types of plant cells have other chemical components as well. For example, the outer wall of the pollen cell wall is composed mainly of sporopollen and cellulose. Under the light microscope, plant cell walls can be divided into primary and secondary walls, but the two types of cell walls are actually difficult to strictly distinguish. Generally speaking, primary cell wall refers to the wall formed during the cell growth period after the mitotic generation of daughter cells. At this time, the wall contains more pectin and no lignin, allowing cell growth and expansion. The basic structure of the primary wall of different plants is similar, that is, cellulose microfibrils as the skeleton, hemicellulose and pectin and glycoprotein as the matrix, through the combination of covalent and non-covalent bonds, cross to form a highly complex network structure with strong tensile strength. There is usually an intercellular layer connecting the primary walls of two adjacent plant cells.