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Plant Cell Wall Structure
Plant Cell Wall Structure
external image cell-wall.gif The cell wall is an organelle that is only in a plant cell. The cell wall has a rigid layer of a non-living, tough, yet flexible, material called cellulose. Although the cell wall is tough, it still lets in materials that are needed for the cell, such as water and oxygen. It supports and protects the inter cell from harmful substances and materials. The wall has to support the cell because if there is no support, the plant will not grow upward like a normal plant would.

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Cell Wall

The plant cell wall is a remarkable structure. It provides the most significant difference between plant cells and other eukaryotic cells. The cell wall is rigid (up to many micrometers in thickness) and gives plant cells a very defined shape. While most cells have a outer membrane, none is comparable in strength to the plant cell wall. The cell wall is the reason for the difference between plant and animal cell functions. Because the plant has evolved this rigid structure, they have lost the opportunity to develop nervous systems, immune systems, and most importantly, mobility.
The cell wall is composed of cellulose fiber, polysaccharides, and proteins. In new cells the cell wall is thin and not very rigid. This allows the young cell to grow. This first cell wall of these growing cells is called the primary cell wall. When the cell is fully grown, it may retain its primary wall, sometimes thickening it, or it may deposit new layers of a different material, called the secondary cell wall.
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Plant Cell Wall


One of the most important distinguishing features of plant cells is the presence of a cell wall. The relative rigidity of the cell wall renders plants sedentary, unlike animals, whose lack of this type of structure allows their cells more flexibility, which is necessary for locomotion. The plant cell wall serves a variety of functions. Along with protecting the intracellular contents, the structure bestows rigidity to the plant, provides a porous medium for the circulation and distribution of water, minerals, and other nutrients, and houses specialized molecules that regulate growth and protect the plant from diseashttp:micro.magnet.fsu.edu/cells/plants/cellwall.html

A cell wall is a tough, flexible and sometimes fairly rigid layer surrounding a cell, located external to the cell membrane, which provides the cell with structural support, protection, and acts as a filtering mechanism. A major function of the cell wall is to act as a pressure vessel, preventing over-expansion when water enters the cell. They are found in plants, bacteria, fungi, algae, and some archaea. Animals and protozoa do not have cell walls.

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Cell Wall - What's it for?

Cell wall diagram
Cell wall diagram
While cell membranes might be around every cell, cell walls made of cellulose are only found around plant cells. Cell walls are made of specialized sugars called cellulose. Cellulose provides a protected framework for a plant cell to survive. It's like taking a water balloon and putting it in a cardboard box. The balloon is protected from the outside world. Cellulose is called a structural carbohydrate (complex sugar) because it is used in protection and support.

Cell walls also help a plant keep its shape. While they do protect the cells, cell walls and cellulose also allow plants to grow to great heights. While you have a skeleton to hold you up, a 100-foot tall redwood tree does not. It uses the strong cell walls to maintain its shape. For smaller plants, cell walls are slightly elastic. Wind can push them over and then they bounce back. Big redwoods need strength in high winds and sway very little (except at the top).

The plant cell wall is a remarkable structure. It provides the most significant difference between plant cells and other eukaryotic cells. The cell wall is rigid (up to many micrometers in thickness) and gives plant cells a very defined shape. While most cells have a outer membrane, none is comparable in strength to the plant cell wall. The cell wall is the reason for the difference between plant and animal cell functions. Because the plant has evolved this rigid structure, they have lost the opportunity to develop nervous systems, immune systems, and most importantly, mobility.
The cell wall is composed of cellulose fiber, polysaccharides, and proteins. In new cells the cell wall is thin and not very rigid. This allows the young cell to grow. This first cell wall of these growing cells is called the primary cell wall. When the cell is fully grown, it may retain its primary wall, sometimes thickening it, or it may deposit new layers of a different material, called the secondary cell wall.
On the whole, each cell's cell wall interacts with its neighbors to form a tightly bound plant structure. Despite the rigidity of the cell wall, chemical signals and cellular excretions are allowed to pass between cells.
http://library.thinkquest.org/C004535/cell_wall.html

Cell Wall - What's it for?

Cell wall diagram
Cell wall diagram
While cell membranes might be around every cell, cell walls made of cellulose are only found around plant cells. Cell walls are made of specialized sugars called cellulose. Cellulose provides a protected framework for a plant cell to survive. It's like taking a water balloon and putting it in a cardboard box. The balloon is protected from the outside world. Cellulose is called a structural carbohydrate (complex sugar) because it is used in protection and support.

Cell walls also help a plant keep its shape. While they do protect the cells, cell walls and cellulose also allow plants to grow to great heights. While you have a skeleton to hold you up, a 100-foot tall redwood tree does not. It uses the strong cell walls to maintain its shape. For smaller plants, cell walls are slightly elastic. Wind can push them over and then they bounce back. Big redwoods need strength in high winds and sway very little (except at the top).

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Cell wall
Cell wall

Cell wall
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- Most commonly found in plant cells
- Controls turgity
- Extracellular structure surrounding plasma membrane
- Primary cell wall: extremely elastic
- Secondary cell wall: forms around primary cell wall after growth is complete
Plasma membrane
Plasma membrane

Plasma membrane
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- Outer membrane of cell that controls cellular traffic
- Contains proteins (left, gray) that span through the membrane and allow passage of materials
- Proteins are surrounded by a phospholipid bi-layer.

www.biology4kids.com/files/cell_wall.html

A cell wall is a tough, flexible and sometimes fairly rigid layer surrounding a cell, located external to the cell membrane, which provides the cell with structural support, protection, and acts as a filtering mechanism. A major function of the cell wall is to act as a pressure vessel, preventing over-expansion when water enters the cell. They are found in plants, bacteria, fungi, algae, and some archaea. Animals and protozoa do not have cell walls.
The materials in a cell wall vary between species, and in plants and fungi also differ between cell types and developmental stages. In plants, the strongest component of the complex cell wall is a carbohydrate, the glucose polymer called cellulose. In bacteria, peptidoglycan forms the cell wall. Archaean cell walls have various compositions, and may be formed of glycoprotein S-layers, pseudopeptidoglycan, or polysaccharides. Fungi possess cell walls of the glucosamine polymer chitin, and algae typically possess walls constructed of glycoproteins and polysaccharides. However the diatoms have a cell wall composed of silicic acid. Often, other accessory molecules are found anchored to the cell wall.



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http://www.astrographics.com/GalleryPrintsIndex/GP2164.html


Growing plant cells are surrounded by a polysaccharide-rich primary wall. This wall is part of the apoplast which itself is largely self-contiguous and contains everything that is located between the plasma membrane and the cuticle. The primary wall and middle lamella account for most of the apoplast in growing tissue. The symplast is another unique feature of plant tissues. This self-contiguous phase exists because tube-like structrues known as plasmodesmata connect the cytoplasm of different cells.

Plants differ in shape and size. These differences result from the different morphologies of the various cells that make up the vegetative and reproductive organs of the plant body. Changes in tissue and organ morphology that occur during plant growth and development result from controlled cell division and growth together with modification and structural reorganiztion of the wall, and the synthesis and insertion of new material into the existing wall.

Some of the functions of the primary wall:
    • Structural and mechanical support.
    • maintain and determine cell shape.
    • resist internal turgor pressure of cell.
    • control rate and direction of growth.
    • ultimately responsible for plant architecture and form.
    • regulate diffusion of material through the apoplast.
    • carbohydrate storage - walls of seeds may be metabolized.
    • protect against pathogens, dehydration, and other environmental factors.
    • source of biologically active signalling molecules.
    • cell-cell interactions.

Primary walls are the major textural component of plant-derived foods. The ripening of fruits and vegetables is associated with changes in wall structrue and composition. Plant-derived beverages often contain significant amounts of wall polysaccharides. Some wall polysaccharides bind heavy metals, stimulate the immune system or regulate serum cholesterol. Wall polysaccharides are used commercially as gums. gels, and stabilizers. Thus, cell wall structure and organization is of interest to the plant scientist, the food processing industry and the nutritionist.

cell wall - a thick, rigid membrane that surrounds a plant cell. This layer of cellulose fiber gives the cell most of its support and structure. The cell wall also bonds with other cell walls to form the structure of the plant.
One of the most important distinguishing features of plant cells is the presence of a cell wall. The relative rigidity of the cell wall renders plants sedentary, unlike animals, whose lack of this type of structure allows their cells more flexibility, which is necessary for locomotion. The plant cell wall serves a variety of functions. Along with protecting the intracellular contents, the structure bestows rigidity to the plant, provides a porous medium for the circulation and distribution of water, minerals, and other nutrients, and houses specialized molecules that regulate growth and protect the plant from disease.
Plant Cell Wall Structure
Plant Cell Wall Structure
Cell walls are significantly thicker than plasma membranes and were visible even to early microscopists, including Robert Hooke, who originally identified the structures in a sample of cork, and then coined the term cells in the 1660s. The thickness, as well as the composition and organization, of cell walls can vary significantly. Many plant cells have both a primary cell wall, which accommodates the cell as it grows, and a secondary cell wall they develop inside the primary wall after the cell has stopped growing. The primary cell wall is thinner and more pliant than the secondary cell wall, and is sometimes retained in an unchanged or slightly modified state without the addition of the secondary wall, even after the growth process has ended.
The main chemical components of the primary plant cell wall include cellulose (in the form of organized microfibrils; see Figure 1), a complex carbohydrate made up of several thousand glucose molecules linked end to end. In addition, the cell wall contains two groups of branched polysaccharides, the pectins and cross-linking glycans. Organized into a network with the cellulose microfibrils, the cross-linking glycans increase the tensile strength of the cellulose, whereas the coextensive network of pectins provides the cell wall with the ability to resist compression. In addition to these networks, a small amount of protein can be found in all plant primary cell walls. Some of this protein is thought to increase mechanical strength and part of it consists of enzymes, which initiate reactions that form, remodel, or breakdown the structural networks of the wall. Such changes in the cell wall directed by enzymes are particularly important for fruit to ripen and leaves to fall in autumn.
The secondary plant cell wall, which is often deposited inside the primary cell wall as a cell matures, sometimes has a composition nearly identical to that of the earlier-developed wall. More commonly, however, additional substances, especially lignin, are found in the secondary wall. Lignin is the general name for a group of polymers of aromatic alcohols that are hard and impart considerable strength to the structure of the secondary wall. Lignin is what provides the favorable characteristics of wood to the fiber cells of woody tissues and is also common in the secondary walls of xylem vessels, which are central in providing structural support to plants. Lignin also makes plant cell walls less vulnerable to attack by fungi or bacteria, as do cutin, suberin, and other waxy materials that are sometimes found in plant cell walls.
A specialized region associated with the cell walls of plants, and sometimes considered an additional component of them, is the middle lamella (see Figure 1). Rich in pectins, the middle lamella is shared by neighboring cells and cements them firmly together. Positioned in such a manner, cells are able to communicate with one another and share their contents through special conduits. Termed plasmodesmata, these small passages penetrate the middle lamella as well as the primary and secondary cell walls, providing pathways for transporting cytoplasmic molecules from one cell to another.

cell wall-(plant cells only): Plant cells have a rigid, protective cell wall made up of polysaccharides. In higher plant cells, that polysaccharide is usually cellulose. The cell wall provides and maintains the shape of these cells and serves as a protective barrier. Fluid collects in the plant cell vacuole and pushes out against the cell wall. This turgor pressure is responsible for the crispness of fresh vegetables.

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Cell Wall - What's it for?

Cell wall diagram
Cell wall diagram
While cell membranes might be around every cell, cell walls made of cellulose are only found around plant cells. Cell walls are made of specialized sugars called cellulose. Cellulose provides a protected framework for a plant cell to survive. It's like taking a water balloon and putting it in a cardboard box. The balloon is protected from the outside world. Cellulose is called a structural carbohydrate (complex sugar) because it is used in protection and support.

Cell walls also help a plant keep its shape. While they do protect the cells, cell walls and cellulose also allow plants to grow to great heights. While you have a skeleton to hold you up, a 100-foot tall redwood tree does not. It uses the strong cell walls to maintain its shape. For smaller plants, cell walls are slightly elastic. Wind can push them over and then they bounce back. Big redwoods need strength in high winds and sway very little (except at the top).

Another Hole in the Wall

Holes in the cell wall
Holes in the cell wall
A cell wall is not a fortress around the delicate plant cell. There are small holes in the wall that let nutrients, waste, and ions pass through. Those holes are called plasmodesmata. These holes have a problem: water can also be lost. But even when the plant cell loses water, the basic shape is maintained by the cell walls. So if a plant is drooping because it needs water, it can recover when water is added. It will look just the same as when it started.

More Than Walls in Plants

You may hear about cell walls in other areas of biology. Bacteria also have a structure called a cell wall. Fungi and some ptotozoa also have cell walls. They are not the same. Only plant cell walls are made out of cellulose. The other walls might be made from proteins or a substance called chitin. They all serve the same purpose of protecting and maintaining structure, but they are very different molecules. websitee :)
Plant cells are usually enclosed by a more or less rigid cell wall containing cellulose. Only a few algae as well as some protists and endosperm cells have no cell wall. Some algal groups contain other structural substances than cellulose. The structure of cell walls could well be compared to that of reinforced concrete: the scaffolding substance, cellulose in plants, iron in concrete is embedded in an amorphous ground substance, the matrix.
The cell wall has a number of functions: it lends the cell stability, it determines its shape, influences its development, protects the cell against pathogens (viruses, bacteria, fungi, etc.) and counterbalances the osmotic pressure. The cell wall of elongating cells is still elastic, a property that is lost in fully differentiated cells. It is therefore distinguished between a primary and a secondary wall.
The **primary cell wall** is laid out during the first division of the cell. It develops normally between the two daughter cells during early telophase.
The early stage of the new cell wall is the cell plate, a lamella-like structure in the former equatorial plane of the mitotic apparatus. Electron microscopic studies show that it develops by fusion of numerous vesicles. The plate grows centrifugally until it reaches the longitudinal lateral walls of the mother cell. Electron dense material is deposited at both its sides. The thus developing structure is called the phragmoplast. It is the immediate precursor of the primary wall.
The at first still elastic and flexible secondary wall develops by successive encrustation and deposition of cellulose fibrils and other components as soon as the cell has stopped growth. While the primary wall structure is the same in nearly all cell types and species, are cell type and species-specific differences typical for the secondary cell wall.
In order to understand the cell wall properties have its chemical and physical structures to be understood. Among them are:
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the structure of cellulose,
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the significance and structure of matrix polysaccharides,
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the significance and meaning of polymers of other classes of substances (like lignin and glycoproteins),
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a knowledge of accompanying substances: low-molecular oligosaccharides and polysaccharides, enzymes and lipids.



In addition arise questions according to their biosynthesis and its regulation (by the phytohormone auxine, for example) as well as to the properties of substances secreted by or through the cell wall. Among the latter is the formation of appositions (cuticle, etc.), the excretion of gelatinous substances and the expression of receptor molecules that serve the species-specific cell-cell recognition and that, too, play a role in host-parasite interactions.


Plant cell walls


Composition

external image 250px-Plant_cell_wall_diagram.svg.pngexternal image magnify-clip.pngMolecular structure of the primary cell wall in plants.
The major carbohydrates making up the primary (growing) plant cell wall are cellulose, hemicellulose and pectin. The cellulose microfibrils are linked via hemicellulosic tethers to form the cellulose-hemicellulose network, which is embedded in the pectin matrix. The most common hemicellulose in the primary cell wall is xyloglucan. In grass cell walls, xyloglucan and pectin are reduced in abundance and partially replaced by glucuronarabinoxylan, a hemicellulose. Primary cell walls characteristically extend (grow) by a mechanism called acid growth, which involves turgor-driven movement of the strong cellulose microfibrils within the weaker hemicellulose/pectin matrix, catalyzed by expansin proteins. The outer part of the primary cell wall of the plant epidermis is usually impregnated with cutin and wax, forming a permeability barrier known as the plant cuticle.
Secondary cell walls contain a wide range of additional compounds that modify their mechanical properties and permeability. The major polymers that make up wood (largely secondary cell walls) include cellulose (35 to 50%), xylan, a type of hemicellulose, (20 to 35%) and a complex phenolic polymer called lignin (10 to 25%). Lignin penetrates the spaces in the cell wall between cellulose, hemicellulose and pectin components, driving out water and strengthening the wall. The walls of cork cells in the bark of trees are impregnated with suberin, and suberin also forms the permeability barrier in primary roots known as the Casparian strip. Secondary walls - especially in grasses - may also contain microscopic silica crystals, which may strengthen the wall and protect it from herbivores.
Plant cells walls also contain numerous enzymes, such as hydrolases, esterases, peroxidases, and transglycosylases, that cut, trim and cross link wall polymers. Small amounts (1-5%) of structural proteins are found in most plant cell walls; they are classified as hydroxyproline-rich glycoproteins (HRGP), arabinogalactan proteins (AGP), glycine-rich proteins (GRPs), and proline-rich proteins (PRPs). Each class of glycoprotein is defined by a characteristic, highly repetitive protein sequence. Most are glycosylated, contain hydroxyproline (Hyp) and become cross-linked in the cell wall. These proteins are often concentrated in specialized cells and in cell corners. Cell walls of the epidermis and endodermis may also contain suberin or cutin, two polyester-like polymers that protect the cell from herbivores.[2] The relative composition of carbohydrates, secondary compounds and protein varies between plants and between the cell type and age.
Up to three strata or layers may be found in plant cell walls:[3]
  • The **middle lamella**, a layer rich in pectins. This outermost layer forming the interface between adjacent plant cells and glues them together.
  • The primary cell wall, generally a thin, flexible and extensible layer formed while the cell is growing.
  • The secondary cell wall, a thick layer formed inside the primary cell wall after the cell is fully grown. It is not found in all cell types. In some cells, such as found xylem, the secondary wall contains lignin, which strengthens and waterpoofs the wall.
Cell walls in some plant tissues also function as storage depots for carbohydrates that can be broken down and resorbed to supply the metabolic and growth needs of the plant. For example, endosperm cell walls in the seeds of cereal grasses, nasturtium, and other species, are rich in glucans and other polysaccharides that are readily digested by enzymes during seed germination to form simple sugars that nourish the growing embryo. Cellulose microfibrils are not readily digested by plants, however.

Formation

The middle lamella is laid down first, formed from the cell plate during cytokinesis, and the primary cell wall is then deposited inside the middle lamella. The actual structure of the cell wall is not clearly defined and several models exist - the covalently linked cross model, the tether model, the diffuse layer model and the stratified layer model. However, the primary cell wall, can be defined as composed of cellulose microfibrils aligned at all angles. Microfibrils are held together by hydrogen bonds to provide a high tensile strength. The cells are held together and share the gelatinous membrane called the middle lamella, which contains magnesium and calcium pectates (salts of pectic acid). Cells interact though plasmodesma(ta), which are inter-connecting channels of cytoplasm that connect to the protoplasts of adjacent cells across the cell wall.
In some plants and cell types, after a maximum size or point in development has been reached, a secondary wall is constructed between the plant cell and primary wall. Unlike the primary wall, the microfibrils are aligned mostly in the same direction, and with each additional layer the orientation changes slightly. Cells with secondary cell walls are rigid. Cell to cell communication is possible through pits in the secondary cell wall that allow plasmodesma to connect cells through the secondary cell walls.