Cell wall

A cell wall is composed of a polymer shell surrounding the cells of plants, bacteria, fungi, algae and some Archaea. Animals and protozoa do not have cell walls. Cell wall outside the cell membrane, which in turn contains the cell interior. It is formed as a deposition product of living cells. The cell wall structure and provides protection and also acts as a filter. A main function of the cell wall is to act as a pressure vessel; it prevents a burst of the cell, if a result of the osmotic gradient water leaks.

• 2.1 Structure of the plant cell wall 2.1.1 middle lamella
• 2.1.2 primary wall
• 2.1.3 secondary wall
• 2.1.4 Tertiary wall

• 2.3.1 pectin
• 2.3.2 hemicellulose
• 2.3.3 cellulose
• 2.3.4 cell wall proteins

Properties

The cell walls of various organisms serve similar purposes. The wall gives cells rigidity and strength, which provides protection against mechanical stress. Multicellular organisms it makes it possible to build a mold and to obtain ( morphogenesis ). The cell wall also limits the entry of large, potentially toxic molecules. It allows further to produce an osmotically stable environment since excessive water absorption from the environment is prevented, which is a burst of the cell would result in the cell wall and is capable of storing water. The composition, characteristics and form of the cell wall may vary during the cell cycle and are dependent on growth conditions.

Strength of the cell walls

The strength of the cell walls is often overestimated. In most cells, the cell wall is flexible, it will bend rather than to maintain a certain shape, but it has a considerable tensile strength. The stability arises from an interplay between turgor pressure and tensile strength of the cell wall. Once the turgor decreases due to water deficiency ( wither ), leaves and stems begin in unlignified (not lignified ) to hang plants. According to John Howland a good comparison for the cell wall is a wicker basket in which a balloon is inflated ( the plasma membrane ), the pressure exerted from the inside ( turgor ). Such a basket is very strong and resistant to mechanical damage.

Plant cell walls

Plant cell walls have two main functions: they serve both the cell as a shaping element and provide stability. In addition, the cell wall maintains the turgor pressure was pushing the protoplasts with about 0.5-1 MPa against the cell wall. They enclose the cell completely and protect it. Plant cell walls consist of cellulose fibrils that are embedded in a matrix of pectins, hemicelluloses, proteins and partly lignin. Through the cell walls through the individual cells are connected by plasmodesmata.

The entirety of the cell walls and the cell spaces called apoplast and produced by release of substances from the living body of the cell.

Structure of the plant cell wall

From the outside inwards:

• Middle lamella
• Primary wall
• Secondary wall
• Tertiary wall

Middle lamella

The middle lamella consists mainly of pectins, which are incorporated into the still liquid cell plate in the region of the phragmoplast. It has gel character and is of small extent.

Primary wall

The primary wall consists of pectins, cellulose, hemicellulose and proteins (mainly glycoproteins like extensin ). After cell division cellulose is superposed in the form of microfibrils of the middle lamella and primary wall forms. The fibrils form while no structure. Therefore, the primary wall is elastic, whereby the plant cell is not limited in their growth. In Kollenchymgewebe it comes to partial thickening of the primary wall. However, the cell continues to be viable.

Cellulose is present in the primary ( and still growing ) cell wall with a share of 8-14 %. It is in the form of microfibrils arranged confusedly (as scattering texture). These fibrils are embedded in a matrix which mainly consists of hemicellulose and pectin. The most common hemicellulose in the primary cell wall xyloglucan. The Hemicellulosestränge associated with several cellulose fibrils, as well as the cellulose fibrils with each other, both in each case via hydrogen bonds. This results in a network. In this, the remaining components are integrated as sub-structure: on Ca2 - and Mg2 ions cross-linked pectins and Isodityrosin bridges networked Extensins. The gel-like Matrixpolimere and incorporated therein, skeleton-forming cellulose fibrils lead to a very high tensile strength and still plastically deformable composite material. Also, by the reversibility of the cross-linking hydrogen bonds a reconstruction of the network is possible.

The structure of the cell walls of grasses ( Poales ) differs greatly from the one described here, xyloglucan and pectin are less frequent and are partially replaced by Glucuronarabinoxylan, a hemicellulose. This is available to as type II cell wall, in contrast to type I cell wall that occur in typical dicots and monocots and most gymnosperms.

During the growth of the primary cell wall expands after an acidification by auxin by a turgor - driven movement of the solid cellulose microfibrils within the weaker hemicellulose / pectin matrix, catalyzed by expansins. The stable final state of the primary cell wall will be referred to as Sakkoderm.

Secondary wall

The secondary wall is formed only when the cell has completed its growth. It consists largely of densely packed cellulose microfibrils and hemicelluloses, which are wrapped in mineral substances and especially in lignins. The microfibrils are superposed parallel. Multiple layers cross each other there. In the secondary cell wall, there are recesses ( pits ), which allow the connection between the individual cells.

If it comes to lignification of the secondary wall, the cell dies (formation of sclerenchyma ). Secondary cell walls are typical of the xylem. There are also secondary sealing wall layers, for example in epidermal cells the cuticle, prevent the drying out of the plant. They contain cutin, waxes, or in the case of cork suberin also.

Secondary cell walls contain a wide range of additional compounds that change their mechanical properties and permeability. As the walls of the cork cells in the bark of trees are impregnated with suberin, as the outer part of the primary cell wall of Pflanzenepidermis usually impregnated with wax and cutin and forms a permeability barrier. Suberin also forms the permeability barrier in primary roots, known as Casparischer strips. Secondary walls - especially in grasses - may also contain microscopic silica crystals that strengthen the wall and protect against herbivores.

The cell walls of some plant tissue also function as bearings of carbohydrates, which can be recycled and added monomerized to support metabolism and growth. For example, the cell walls of the endosperm of the seed of cereals and grasses and glucans rich in polysaccharides that can be easily degraded by the enzymes during germination of the seeds to simple sugars to feed the growing embryo. Cellulose microfibrils, however, can not be digested easily by the plants.

The most important polymers from which timber is ( predominantly secondary cell wall ) are:

• Cellulose, 35-50 %
• Xylan, 20-35 %, a hemicellulose
• Lignin, 10-25%, a phenolic polymer complex, the intermediate spaces in the cell wall between cellulose, hemicellulose and pectin filling and makes them hydrophobic, and solid

Tertiary wall

The innermost layer is also called " tertiary cell wall ". She is thin but very resistant and covers the cell wall inward from; she has a warty surface and is rich in pectins and hemicelluloses. It differs from the secondary cell wall composition and texture.

Emergence of the plant cell wall

Only during cell division, new cell walls are formed. First the cell wall panel is formed in the equatorial plane and from that of the Phragmoplast. This is caused by the confluence of many Golgi vesicles and is retained as the middle lamella of the cell wall after its completion. Now microfibrils are superposed on both sides in a random scattering texture and so form the primary walls. The individual fibrils are connected to one another via hydrogen bonds. Since the cell is still growing it comes to the surface growth of the cell wall. The stretchability of the cell wall is due to the scattering structure of the fibrils. By stretching it comes to wall thinning, which is compensated by the application of further fibrils. The growth of the primary wall ends at the extent of the cell. After the surface growth of the cell wall is now using the thick growth. It can be applied in parallel and layered microfibrils, with the fibrils abutting layers usually intersect ( parallel texture ). The resulting secondary wall makes up the bulk of the cell wall and gives it stability. However, it is not, as the primary wall, stretchable. Towards the end of the growth of the cell wall is a last layer, the final slat or tertiary wall, plotted. This consists of hemicellulose and protopectin.

Chemical composition

Most involved in the construction of the plant cell wall carbohydrates are cellulose, hemicellulose and pectin. Its shares sharply differ to some extent. These three substances are fractions, so they include a number of different polysaccharides. Essentially, they consist of only seven different glycosidically linked monosaccharides: D -glucose, D -galactose, D -galacturonic acid, L- rhamnose, L- fucose, D -xylose, L- arabinose. Other substances such as lignin and suberin can be added for a small part also make polypeptides ( 5-10%).

The cell wall is thus mainly from the following groups of substances:

Pectin

Pectins can be solved with relatively mild media from the cell wall, eg with hot water and complexing agents or with potassium chlorate and nitric acid ( Schulz Esches mixture ) or after an exposure of specific enzymes ( pectinases). The cellulose-containing cell walls resist this treatment, but not the pectin middle lamella, so that the cells can also be separated from each other ( maceration ). Chemically, it is located at Pectin is a heterogeneous fraction, after extraction, one can distinguish between: Homogalacturonanen (1,4- α -D - galacturonans, polygalacturonic acid ), Rhamnogalacturonanen ( branched copolymers of galacturonic acid and rhamnose with various additional sugar residues ), arabinans (1,5- α -L- Arabinosylketten ) and galactans (1,4- β -D - Galactosylketten ). The carboxyl groups of pectin are linked by Ca2 and Mg2 on salt bridges. Some carboxyl groups, however, are as esters with methanol before and can thus no longer form salt bridges. These salt bridges are relatively reversibly binding to prevent pectin is elastic and easily changeable. A distinction is made between protopectin, which makes up most of the middle lamella and consists mainly of galacturonic acid and rhamnose as well as pectin, which consists of hochmethyliertem galacturonan and is found in the cell walls of many fruits in large quantities.

Hemicellulose

Hemicelluloses can be solved from the cell wall by alkaline treatment. Their name comes from the fact that you have kept them earlier mistaken for an intermediate of cellulose synthesis. Hemicellulose is the predominant part of the matrix in the primary wall. In hemicelluloses are just as in Pectin is a heterogeneous mixture of different polysaccharides, whose composition can vary greatly, main ingredient are usually xyloglucans, but also occur arabinogalactans and glucomannans. They are composed of pentoses like D -xylose and L- arabinose, and hexoses such as D- glucose, D- mannose and D-galactose.

Cellulose

Cellulose is a linear, unbranched polymer of β -D-glucose monomers. The cellulose content is in the primary wall is about 10 %, and can be the secondary walls of over 90%. Cellulose molecules are in parallel bundles before, the elementary fibrils (or Micellarsträngen ), which are ordered to microfibrils, which can be bundled in macrofibrils. By hydrogen bonds, the fibrils are held tightly together and can have a high proportion of paracrystalline regions. By Parakristallinität fibrils obtain a tear strength which is comparable to that of steel. Also by Parakristallinität they are different as the matrix material, hardly hydrated. In the primary wall, the cellulose fibrils are randomly arranged in the plane, this is called scattering texture. In the secondary wall, however, the fibrils are applied in layers, parallel, then turned to the next layer in the alignment. This is called parallel texture.

Cellulose is water insoluble and thus can not be synthesized in the Golgi apparatus and will be transported by vesicles in the cell wall, such as pectins, hemicelluloses and cell wall proteins. Instead, cellulose is synthesized by the enzyme complex cellulose synthase directly as Elementarfibrille in the cell wall. Cellulose synthase is a transmembrane protein that acts as a hexameric rosette complex, mostly in rosette fields. The time required for the synthesis of glucose is provided in the form of Uridindiphosphoglucose ( UDPG ) in the cytoplasm. The orientation of the cellulose fibrils are important for cell growth and differentiation is effected by the cytoskeleton, in which the cellulose synthase moves on rails. Plants themselves can not degrade cellulose, herbivores and fungi but some have the needed for this enzyme cellulase. Technically cellulose is then cleaved by sulfuric acid to glucose ( saccharification of wood ).

Cell wall proteins

In addition to numerous enzymes, such as hydrolases, esterases, peroxidases and transglycosylases that are involved in the assembly and modification of the cell wall (especially in primary walls ), make structural proteins (1-5% ) from the main part of the cell wall proteins. A distinction is made between the glycine-rich proteins ( GRP), proline-rich proteins ( PRP), arabinogalactan protein (AGP) and hydroxyproline glycoproteins ( HRGP ). The hydroxyproline glycoproteins ( HRGP ) are probably the most common and best studied. Each class of glycoproteins defined by a characteristic, highly repetitive protein sequence. Most are glycosylated, contain hydroxyproline ( Hyp ) and are cross-linked in the cell wall. The relative proportion of carbohydrates, secondary compounds and proteins varies between different plants, cell type and age.

Permeability

The primary cell wall of most plant cells is semi-permeable and allows the passage of small molecules and proteins; by gel permeation chromatography, the maximum size was estimated to be 30-60 kDa. Especially water and carbon dioxide will be distributed throughout the plant cell wall by the cell wall apoplastic transport. The pH is an important factor in the transport of molecules through the cell walls.

Bacterial cell walls

The bacterial cell wall separates the actual cell with its surrounding cell membrane from the environment. The cell wall is both robust enough to maintain the cell geometry as a protection against adverse environmental conditions to serve. On the other hand it is also flexible enough so as not to interfere with cell growth, cell division and transport processes in the cell into and out of the cell.

Due to the high concentration of soluble substances in the cytoplasm of the cell is formed in an osmotic pressure of up to 1.5 MPa, which must be compensated by the cell wall.

In addition, the cell wall provides protection against phages and pathogenic bacteria to the immune system of their hosts and must withstand aggressive metabolites of competing microorganisms.

Bacteria can be classified with the so-called Gram stain roughly according to their cell wall structure. Gentian violet dye is in Gram-positive bacteria due to their multi-layer cell wall is not washed out, therefore, these bacteria appear blue, whereas the Gram-negative bacteria with a very thin cell wall are not colored because of the leaching of the dye. These can then be dyed red with the dye fuchsin. Gram-positive and Gram-negative bacteria differ in the structure of their cell walls. For gram-positive, it consists of many layers of the so-called murein ( peptidoglycan ), in which (lipo ) teichoic acids and proteins are embedded.

In the cell membrane of Gram-negative bacteria is ( inner membrane ), only a thin peptidoglycan layer, on which a second, outer cell membrane is superposed, which is different in chemistry and structure of the cell membrane. This outer membrane proteins by pull as porins, and it has on the outside lipopolysaccharide ( LPS), which is why it is also called lipopolysaccharide. The lipid A of LPS can act as an endotoxin and is a virulence factor of pathogenic bacteria.

Depending on the species, additional protein layers are formed (see S-layer in Archaea), capsules or slime layers.

Cell walls in fungi

The cell walls that surround the cells in some fungi, consisting of chitin, from which also the exoskeleton of insects has been established. Similar to plant the cell walls also serve here the reinforcement, so that the cells can maintain their shape.