Cellular wall
The cell wall is a tough, rigid layer that is located on the outside of the plasma membrane in cells of plants, fungi, algae, bacteria, and archaea. The cell wall gives rigidity to the cell, protects its contents, functions as a mediator in all its relationships with the environment, acts as a cell compartment, and supports osmotic forces and growth. Furthermore, in the case of fungi and plants, it defines the structure and gives support to the tissues and many other parts of the cell.
The cell wall is built from various materials, depending on the kind of organism. In plants, the cell wall is composed primarily of a carbohydrate polymer called cellulose, a polysaccharide, and can also act as a carbohydrate store for the cell. In bacteria, the cell wall is made up of peptidoglycan. Among the archaea, cell walls with different chemical compositions occur, including S-layers of glycoproteins, pseudopeptidoglycan, or polysaccharides. Fungi have chitin cell walls, and algae typically have walls built from glycoproteins and polysaccharides. However, some species of algae may have a cell wall made of silicon dioxide. Other accessory molecules integrated into the cell wall are often present.
Plant cell wall
The plant cell wall is a complex structure or organelle that, apart from supporting plant tissues, has the ability to condition cell development.
Structure
The plant cell wall has three fundamental parts:
- Primary Pared: It is present in all plant cells, usually measures between 100 and 200 nm of thickness and is the product of the accumulation of 3 or 4 successive layers of cellulose microfibillas, composed between 9 and 25 % of cellulose. The primary wall is created in the cells once its division is over, generating the fragmoplast, a cell wall that will divide the two daughter cells. The primary wall is adapted to cell growth because microfibillas slide between them, producing a longitudinal separation, while the protoplast puts pressure on them.
- Secondary: It is a layer adjacent to the plasma membrane, although it does not exist in all types of cell wall. It forms once cell growth has stopped and is related to the specialization of each cell type. Unlike the primary wall, it contains a high proportion of cellulose, as well as lignin or suberine. It is not deformable, and does not allow cell growth. In woody tissues it is much thicker than the primary wall.
- Medium laminilla: It is a layer that binds the primary walls of two contiguous cells; it is formed mainly by pectin and hemicelulose, but in the older cells it is often ignited.
Composition
The composition of the plant cell wall varies in different cell types and in different taxonomic groups. In general terms, the plant cell wall is composed of a network of carbohydrates, phospholipids, and structural proteins embedded in a gelatinous matrix composed of other carbohydrates and proteins.
Carbohydrates
The main component of the plant cell wall is cellulose. Cellulose is a fibrillar polysaccharide that is organized into microfibrils and represents between 15% and 30% of the dry weight of plant cell walls.
Cellulose microfibrils are bound by non-fibrillar carbohydrates that are generically called hemicellulose. The major components of hemicellulose are xyloglycans (XiGs) and glucuronarabinoxylans (GAXs).
Pectin is another important component of cell walls. It is a non-fibrillar polysaccharide, rich in D-galacturonic acid, heterogeneously branched and highly hydrated. The major components of pectin are: homogalacturonans (HGA) and rhamnogalacturonans I (RG I). The pectin matrix determines the porosity of the wall and provides fillers that modulate the pH of the wall.
Protein
The plant cell wall is also made up of structural proteins. These proteins are rich in one or two amino acids, have domains with repeated sequences, and are glycosylated to a greater or lesser degree. For most of the structural proteins of the plant wall, it has been proposed that they have a fibrillar structure and that they are immobilized by covalent bonding between them or with carbohydrates. These proteins are known to accumulate in the wall at different stages of development and in response to different stress conditions.
Structural proteins of the plant cell wall are considered: extensins or hydroxyproline-rich proteins (HRGPs), proline-rich proteins (PRPs), glycine-rich proteins (GRPs) and arabinogalactans (AGPs).
Included in the polysaccharide and protein network are various soluble proteins, some of which are enzymes related to the production of nutrients such as glucosidase, enzymes related to wall metabolism such as xyloglucan-transferases, peroxidases and laccases, defense-related proteins, transport proteins.
Other polymers
Lignin is a rigid amorphous polymer, which results from the union of various acids and phenylpropyl alcohols. They are, after cellulose, the most abundant component of plant walls. They accumulate in secondary walls, although occasionally they form part of the median lamella of necrotic tissues.
Suberin and cutin are complex polymers composed of long-chain fatty acids, which are linked to each other by ester linkages, creating a rigid three-dimensional network. They accumulate in some secondary walls and, in exceptional cases, in primary walls.
Waxes do not provide rigidity, but impermeability to water to the tissues on which they are deposited. Also lignin and cutin confer a partial degree of impermeability.
Biogenesis of the plant cell wall
The plant cell wall is formed during cell division, from vesicles that come from the Golgi apparatus. These vesicles, filled with the components of the cell wall, are located in the phragmoplast, which is a cytoskeletal arrangement typical of dividing cells. In the phragmoplast, the vesicles of the Golgi apparatus fuse and form the cellular plate, which grows from the interior of the dividing cell until it comes into contact with the lateral walls..
Once formed, the cell wall grows by deposition of successive layers of cellulose. In each layer, the orientation of the cellulose microfibrils is guided by the cytoskeleton, more precisely by the cortical microtubules, which align the complex responsible for cellulose synthesis, which is cellulose synthase.
Cell elongation occurs in the axis perpendicular to that of the microfibrils of the wall layer that is being deposited, hence the synthesis of the wall and the orientation of the cellulose microfibrils is directly related to cell size.
Plant Cell Wall Interactions
The cell wall is the outermost organelle of the cell and the interactions between cells and between tissues depend on it. As with the extracellular matrix of animals, adhesion to the substrate depends on the cell wall of plants, which is decisive in the case of some plant organs that are mobile, such as pollen.
On the other hand, the wall is in constant communication with the cell interior, this interaction between the wall and protoplast is dynamic and transmits signals to the interior of the cell, which account for the conditions of the extra-cytoplasmic environment. In the other direction, from the inside out, the protoplast regulates the state of the wall at all times, depending on tissue development and environmental conditions.
During the phenomenon known as plasmolysis, which is the separation of the living protoplast from the cell wall by a hyperosmotic effect, the physical interaction between the cell wall and the protoplast becomes evident; when this physical interaction is lost, the cell becomes incapable of responding to the attack of pathogens and loses its cellular differentiation.
Cell wall of algae
Like the cell walls of higher plants, the cell wall of algae is composed of carbohydrates such as cellulose and glycoproteins. The presence of some polysaccharides in algae walls, it is used as a diagnostic character in the taxonomy of algae.
- Polysaccharides sulfonated like agarosa, are presented on the walls of red algae.
- Manosyl: Microfiblical shape on the cell wall of some green algae of the genres Codium,Acetabulary, Porphyra and Bangia among others.
- Alginyl acid: it is a common polysaccharide on the cell wall of the brown algae.
The group of diatom algae synthesize their cell walls (also known as frustules or valves) using silicic acid (specifically orthosilicic acid, H4SiO4). The acid polymerizes intracellularly, and then the wall comes out to protect the cell. Compared to organic cell membranes produced by other groups, they require less energy (approximately 8%) for synthesis, which is cost saving to the cell, and possibly explains the higher growth rates in diatoms.
Cell wall of fungi
Fungi contain cell walls composed of glucosamine and chitin, the same carbohydrate that makes arthropod exoskeletons tough. Chitin is distributed in microfibrillar bundles to give cells rigidity and, in turn, maintain their shape and prevent osmotic lysis, this function is similar in plants. In some fungi, the chitin of the cell wall can be replaced by other polysaccharides such as mannans, galactosans or chitosans. Another function of the cell wall is to limit the entry of molecules that can be toxic to the fungus, such as synthetic fungicides or those produced by floors. The composition, characteristics and shape of the fungal cell wall varies during their life cycle and also depends on the growth conditions.
The composition of the cell wall of fungi has been used for classification and for industrial application. Oomycetes are a group of saprotrophic plant pathogens that have abnormal cellulose cell walls. Until recently they were believed to be fungi, but structural and molecular data have led to their reclassification as protists of the Heterokonta clade. This is a group of protists that includes autotrophs such as brown algae and diatoms.
Bacterial cell wall
The bacterial cell wall is made of peptidoglycan (also called murein), which is made up of polysaccharide chains intersected by unusual peptides containing D amino acids. Bacterial cell walls are different from the walls of plants and fungi which are made of cellulose and chitin, respectively. They are also different from the walls of Archaea, which do not contain peptidoglycan. The cell wall is essential for the survival of many bacteria, and the antibiotic penicillin can kill bacteria by inhibiting a step in peptidoglycan synthesis.
There are two different types of cell wall in bacteria, called Gram-positive and Gram-negative, respectively. The names come from their reaction to the Gram stain, a test widely used in the classification of bacterial species.
- In the Gram-positive bacteria the cell wall contains a thick layer of peptidoglycan as well as theic acids, which are polymers of glycerol or ribitol phosphate. Theic acids join peptidoglycan or cytoplasmic membrane.
- In the Gram-negative bacteria, the peptidoglycan layer is relatively thin and is surrounded by a second outer lipid membrane containing lipopolisaccharides and lipoproteins. The peptidoglycan layer joins the outer membrane through lipoproteins.
Most bacteria have a Gram-negative cell wall, and only Firmicutes and Actinobacteria (previously known as low GC Gram-positive bacteria and high GC Gram-positive bacteria, respectively) have Gram-positive walls. These differences in structure can produce differences in antibiotic susceptibility, for example, vancomycin can only kill Gram-positive bacteria and is ineffective against Gram-negative pathogens, such as Haemophilus influenzae or Pseudomonas aeruginosa.
Some bacteria do not have a cell wall such as Mycoplasmas or Mollicutes.
Cell wall of archaea
Unlike bacteria, the archean cell wall does not contain peptidoglycan or ester bonds, instead there are peptide, carbohydrate and/or glycoprotein compounds and ether bonds. They are of four types:
- Pared of glucoproteins, common in hyperthermophiles, hyperhalophiles and methamogens
- Pared of pseudopeptidoglycan, typical of methanebacteria
- Capa S proteica, as in Methanomicrobia and Desulfurococcus
- Pair of polysaccharides, as in Methanosarcina and Haloccus
Archae lipids have ether linkages between glycerol and the hydrophobic side chains of isoprene. The general disposition is a lipid monolayer very resistant to disintegration that allows them resistance to extreme conditions.
Some archaea do not have a wall, such as the class Thermoplasmata.