Plant Cell Wall Basics The diffuse layer model The stratified layer model Xyloglucan molecules are hydrogen bonded to and cross-link cellulose microfibrils. The cellulose-xyloglucan network is emeshed in a non-covalently cross-linked pectic network. Xyloglucan molecules are hydrogen bonded to the surface of cellulose microfibrils but do not directly cross link them. The tightly-bound xyloglucan is surrounded by a layer of less-tightly bound polysaccharides. The cellulose and xyloglucan are embedded in a pectic matrix. Xyloglucan molecules are hydrogen bonded to and cross-link cellulose microfibrils. The cellulose-xyloglucan lamellae are separated by strata of pectic polysaccharides. Much research is sitll required to provide a complete description of the primary wall at the molecular level. Moreover, there is increasing evidence that primary walls are dynamic structures whose composition and architecture changes during plant growth and development. Secondary cell walls Plants form two types of cell wall that differ in function and in composition. Primary walls surround growing and dividing plant cells.

These walls provide mechanical strength but must also expand to allow the cell to grow and divide. The much thicker and stronger secondary wall (see figure on right), which accounts for most of the carbohydrate in biomass, is deposited once the cell has ceased to grow. The secondary walls of xylem fibers, tracheids, and sclereids are further strengthened by the incorporation of lignin. The evolution of conducting tissues with rigid secondary cell walls was a critical adaptive event in the history of land plants, as it facilitated the transport of water and nutrients and allowed extensive upright growth. Secondary walls also have a major impact on human life, as they are a major component of wood and are a source of nutrition for livestock. In addition, secondary walls may help to reduce our dependence on petroleum, as they account for the bulk of renewable biomass that can be converted to fuel. Nevertheless, numerous technical challenges must be overcome to enable the efficient utilization of secondary walls for energy production and for agriculture. Primary and secondary walls contain cellulose, hemicellulose and pectin, albeit in different proportions.

Approximately equal amounts of pectin and hemicellulose are present in dicot primary walls whereas hemicellulose is more abundant in grasses (e.g., switchgrass). The secondary walls of woody tissue and grasses are composed predominantly of cellulose, lignin, and hemicellulose (xylan, glucuronoxylan, arabinoxylan, or glucomannan). The cellulose fibrils are embedded in a network of hemicellulose and lignin. Cross-linking of this network is believed to result in the elimination of water from the wall and the formation of a hydrophobic composite that limits accessibility of hydrolytic enzymes and is a major contributor to the structural characterisitics of secondary walls. Xylan, which accounts for up to 30% of the mass of the secondary walls in wood and grasses contributes to the recalcitrance of these walls to enzymic degradation. A high xylan content in wood pulp increases the economic and environmental costs of bleaching in paper manufacturing. Thus, reducing the xylan content of secondary walls and altering xylan structure, molecular weight, ease of extractability, and susceptibility to enzymic fragmentation are key targets for the genetic improvement of plants.

However, progress in these areas is limited by our incomplete understanding of the mechanisms of xylan biosynthesis. Xylans have a backbone of 1,4-linked β-D-xylosyl residues with short [α-D-glucosyluronic acid (GlcA), 4-O-methyl-α-D-glucosyluronic acid (MeGlcA), α-L-arabinosyl, O-acetyl, feruloyl, or coumaroyl] sidechains (see figure). Xylan synthesis requires the coordinated action of numerous enzymes, including glycosyl transferases (GTs) that elongate the backbone and add side chain residues (see left figure on right). None of these GTs have been purified and biochemically characterized, although several candidate genes have been identified. Little is known about the factors that regulate secondary wall polysaccharide biosynthesis and the mechanisms that control the assembly of these polysaccharides into a functional wall. Moreover, the mechanisms required to initiate and terminate xylan synthesis and to control the chain length of the xylan are as yet unidentified.

We are interested in several aspects of primary and secondary walls including: The primary structure of wall polysaccharides The conformation and interactions of wall polysaccharides The biosynthesis of wall polysaccharides The molecular and genetic mechanisms involved in wall formation Developmentally-associated changes in wall structure The role of the wall in plant-pathogen interactions We use the following plants and suspension-cultured plants cells as sources of cell walls: Plants Molecular Expressions Cell Biology: Plant Cell Structure - Cell Wall 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 disease.

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. Characteristics of Cell Walls | eHow Characteristics of Cell Walls  Email Save A cell wall is an additional layer found in specific organism's cells. Among its major functions is the ability to regulate the water and materials that enter and leave a cell.

Depending on the organism, a cell wall may be composed of different materials or possess additional features. Composition A cell wall surrounds every cell of a plant, fungi, algae and bacteria. Animal cells, including those of humans, do not contain cell walls. The cell wall is a rigid extracellular layer that rests outside of the cell membrane. The materials that comprise a cell wall vary between species. Cellulose, for example, makes up most of the cell wall in plants, while algae cell walls are mostly made of glycoproteins and polysaccharides. Strength A cell wall provides addition strength to its cell. It limits the entry of larger molecules into the cell, which would otherwise cause significant damage. The cell is also able to retain water that it would have lost with just a membrane. An organism can hold and maintain its physical shape due to the cell wall. In plants and multicellular organisms with multiple plant cells join to each other by their cell walls. Transporting Materials A cell wall safely transports materials between cells. In multicellular organisms, cells connect to one another by using the cell wall. Through small pores in cell walls called "Plasmodesmata," the cytoplasm of one cell is connected to another cell adjacent to it.

Once connected, materials in the cytoplasm such as ctyosol, organelles, vesicles and the cytoskeleton can transfer between cells. Additionally, key nutrients such as water and carbon dioxide are distributed throughout an organism through cell wall transfers. Secondary Cell Wall A secondary cell wall is in some plants, algae and single cell protists. The wall is essentially a thicker layer of cellulose or other material such as lignin or suberin. The cell wall forms inside the primary cell wall after the cell grows fully. The wall provides added rigidity to the cell, and is waterproof. In plant cells, an additional layer exists outside the primary and secondary cell wall. Called the middle lamella, the layer contains pectin, which is a gelling agent. The middle lamella connects multiple cells. Plant Cell – Everything about Plant Cell by admin Cells are the fundamental units of life. All living organisms are composed of cells, which are needed for normal function. According to scientists, cells provide structure and stability, but they also provide energy to an organism. Cells are so small that they cannot be seen without magnification. There are two primary types of cells known to exist. They are eukaryotic and prokaryotic cells.

Animals, plants and fungi are the best examples of organisms that include eukaryotic cells. So, the cell is the simplest unit of life, while the plant cell represents the crucial element for the existence of life. There are numerous interesting facts you should know about plant cells. Below you can read some of them. Source A plant cell is the structural and functional unit for all plant organisms. There are millions of cells in an organism. A plant cell is extremely small. It is only visible on a microscope. As already mentioned, a plant cell can be included in the category of eukaryotic cells. These are the cells that have a well-defined nucleus. There are some important differences between plant and animal cells. However, there are also some similarities worth to be known. Plant cells have unique features that facilitate the process of photosynthesis. Through photosynthesis sunlight, water and carbon dioxide are transformed into energy, water and oxygen. This is an important process for the support of all forms of life. The main function of a plant cells is to carry all processes that are needed to facilitate plant life. The structure of the plant cell includes cellulose, chloroplasts, chlorophyll and vacuoles.

Plasma membrane, nucleus, nucleolus and mitochondria are also included in the structure of a plant cell. The endoplasmic reticulum, Golgi apparatus and microtubules are other features of the plant cell. Both plant and animal cells include nucleus, cell membrane and mitochondria, which are part of their structure. There are two types of reproduction in plants. They can be asexual, when the cell splits in two, and sexual, when two cells combine to create one living cell. Plant cells have cell walls in which they can store water. Cell walls of plants are made up of cellulose, hemicellulose and pectin. Source So, there are both similarities and differences between plant and animal cells. They are both basic units of life, which compose all living organisms. However, plant and animal cells may have a different function and a different structure. by admin All plant cells are characterized by several common features, such as chloroplasts, a cell wall, and a large vacuole. These features also differentiate plant cells from animal cells. In addition, there is a number of specialized types of plant cells that are found only in vascular plants.

These include parenchyma cells, collenchyma cells, and sclerenchyma cells. 1. Parenchyma cells The term parenchyma is derived from the Greek word para, which means “beside”, and en + chein, meaning “to pour in”. Parenchyma cells are the biochemistry machines of the plant. They are usually described as the typical plant cell, because they are not very specialized. These cells are commonly found in leaves, roots, and stems, usually having a spherical shape with only primary cell walls and highly functional cytoplasm. Source Parenchyma cells play an important role in food storage, photosynthesis, and aerobic respiration, meaning that they synthesize and store organic products in the plant and are the place where most of the plant’s metabolism takes place. These cells are alive at maturity and are responsible for a wide range of biochemical processes. Most parenchyma cells have the ability to differentiate into other cell types under special conditions 2. Collenchyma cells The term collenchyma is derived from the Greek word kola, meaning “glue”. Collenchyma cells have a support function in plants, particularly in young plants.

They help to support plants while not restraining growth due to their lack of secondary walls and the absence of a hardening agent in their primary walls. This means that their main function is to provide support for parts of the plant that are still growing, such as the stem. Source Collenchyma cells form strands or continuous cylinders just below the surfaces of stems or leaf stalks. They are similar to parenchyma cells in the sense that they are alive at maturity. To better understand what collenchyma cells do, imagine a stretchable support, without elastic snap-back. 3. Sclerenchyma cells The term sclerenchyma is derived from the Greek word skleros, which means “hard”. Sclerenchyma cells are the ones providing rigid support for the plant, because they have a hardening agent and are much more rigid. These cells develop an extensive secondary wall that is invested with lignin (the main chemical component of wood), making it extremely hard. Besides support, sclerenchyma cells have other functions, which include discouraging herbivory, and conduction. Source There are two types of sclerenchyma cells – fiber and sclereid.

Fiber cells are long, slender cells that usually form strands or bundles. Sclereid, also known as stone cells, occur singly or in groups and have various forms. Most sclerenchyma cells are dead cells at maturity. Sclerenchyma cells cannot survive for long because lignin also makes the wall waterproof, therefore making it impossible for the cells to exchange materials long enough for active metabolism. These are the main and most important types of specialized plant cells. Other specialized plant cell types include xylem, phloem, and epidermis cells. Plants have a wide variety of mechanical properties that may inspire engineers to design new materials. Lorna Gibson, the Matoula S. Salapatas Professor of Materials Science and Engineering at MIT says: ‘If you look at engineering materials, we have lots of different types, thousands of materials that have more or less the same range of properties as plants. But here the plants are, doing it arranging just four basic constituents. So maybe there’s something you can learn about the design of engineered materials.” Source Engineers claim that some plants such as bamboo, maples, palm trees or potatoes inspire them, being a great example of precise engineering on a microscopic scale.

The way their cell walls are arranged and what they’re made of, some plants may be sturdy as an oak, while others as flimsy as a reed. Source To Lorna Gobson, many cell walls’ components resemble certain manmade materials such as cellulose, hemicellulose, which can be strong and stiff as manufactured polymers. Additionally, cells in woods are aligned similarly to engineering honeycombs, while polyhedral cell configurations found in apples look like industrial foams. Source Gibson researched plants’ natural mechanics and focused on three main plant materials: oak, cedar and woods, parenchyma cells found in root vegetables and fruits and coconut trees. She and other scientists studies these plants and their cells and analyzed two main mechanical properties in each plant: strength and stiffness. by admin Plants are multicellular, eukaryotes organisms. They have membrane-bound organelles. A plant cell has a rigid cell wall, a central vacuole, plasmodesmata and plastids, which make it unique among all other type of eukaryotes cells. Plant cells sustain the photosynthesis process, during which oxygen is produced. Plant cells are formed by different parts.

All of them play an important role in the proper function of the cell. 1. Cell membrane The cell membrane allows waste material to exit the cell. The membrane surrounds the cytoplasm of the cell. It actually forms a barrier between the inside of the cell and the outside area. The cell membrane regulates the movement of materials into and out of the cell. Having under consideration its function, the cell membrane is believed to be in a way similar to the skin. 2. Cell wall The cell wall surrounds the cell. This is a rigid layer placed externally to the cell membrane. It offers structural support and protection to the entire cell. The cell wall also controls the amount of water that enters the cell. It allows the circulation of minerals and nutrients. The cell wall has the ability to sense the presence of pathogenic microbes, as well, being able to control the development of tissues within the cell. 3. Plasmodesmata Plasmodesmata are microscopic channels of plants traveling the cell walls. It enables communication within the cell. 4. Tonoplast The tonoplast surrounds a vacuole. Vacuoles are found in the cytoplasm, being membrane-bound compartments with storage functions. 5.

Plastids Plastids are some of the most important parts of the plant cell. They are responsible with photosynthesis, storage and synthesis. 6. Golgi Complex Golgi complex is an organelle found in plant cells. It processes macromolecules, including proteins and lipids. 7. Chloroplast, Leucoplast, Chromoplast Chloroplasts are organelles that conduct photosynthesis. They absorb light. Leucoplasts are plastids that are non-pigmented. Chromoplasts are plastids with role in pigment synthesis and storage. 8. Ribosome, Lysosome Ribosomes are complexes of RNA and protein. Lysosomes are organelles which contain digestive enzymes. 9. Mitochondrion, Microtubule, Microfilament, Microbody Mitochondrion is a membrane-enclosed organelle. Microtubules are components of the cytoskeleton. Microfilaments are the smallest filaments of the cytoskeleton. Microbodies are organelles of a globular shape. 10. Cytoplasm, Nucleus, Nuclear envelope The cytoplasm is a semi-transparent fluid which fills cells. The nucleus contains the cell’s genetic material.

The nuclear envelope is formed by two cellular membranes. Source Besides the aforementioned plant cell parts, DNA, chromatin and RNA should also be presented. DNA is the nucleic acid that contains genetic information, chromatin is a complex of DNA and protein, while RNA has an important role in translating genetic information from DNA to proteins. by admin Most cells are not visible with the naked eye. However, with microscopes of various types, plant cells can be readily viewed and studied. In young parts of plants and fruits, cell shapes are generally round, while in older sections, the cells are somewhat boxlike with up to 14 sides as they become packed together. A cell, regardless if it belongs to a plant, animal or human, is generally defined as the structural and functional unit of life. Think of your school building. Each building is made up a number of classrooms. Each classroom has four walls. Each wall is made of bricks. Each brick is the smallest unit of your school building. There are structural and functional differences between plants and animals. These are manifested in the form of plant cell and animal cell differences.

Plant Cell Diagram Plant Cell Walls Next Section STRUCTURAL ISSUES The polysaccharide and glycoprotein components found in plant cell walls have been well characterized structurally. We need now to understand how these components are organized into the three-dimensional matrix needed for plant cell walls to perform their functions. The most characteristic component found in all plant cell walls is cellulose. It consists of a collection of β-1,4-linked glucan chains that interact with each other via hydrogen bonds to form a crystalline microfibril ( Somerville, 2006 ). In addition to cellulose, plant cell walls contain several matrix polysaccharides that are grouped into two general categories: (1) the pectic polysaccharides include homogalacturonan, and rhamnogalacturonan I and II ( Harholt et al., 2010 ) and (2) the hemicellulosic polysaccharides include xyloglucans, glucomannans, xylans, and mixed-linkage glucans ( Scheller and Ulvskov, 2010 ).

Plant cell walls also contain many proteins and glycoproteins, including various enzymes and structural proteins ( Rose and Lee, 2010 ). For example, arabinogalactan proteins are structurally complex molecules found on the plasma membrane and in the cell wall; they are thought to play important roles in recognition and signaling events at the cell surface ( Ellis et al., 2010 ). Despite tantalizing evidence for their involvement in many important recognition and signaling events, few details are available regarding how they are recognized or how recognition leads to signal transmission. How are wall components organized into a functional matrix? Over the years, several models have been proposed to explain the organization of wall components ( Keegstra et al., 1973 ; Carpita and Gibeaut, 1993 ; Somerville et al., 2004 ). Most of the models have focused on understanding the organization of components in primary cell walls that would allow regulated reorganization of wall components during cell growth and differentiation. Hemicellulosic polysaccharides are known to bind tightly to cellulose microfibrils via hydrogen bonds and most wall models have incorporated this interaction as one important feature of cell wall architecture.

Less is known about how the pectic polysaccharides interact with other components in plant cell walls, but there is increasing awareness of their importance in primary cell walls, where they are most abundant. The dynamic nature of plant cell walls is an important feature that is lacking from most models. As cells grow and differentiate, new wall material is laid down near the plasma membrane and older wall material is pushed outward. This process has the potential to create a wall where the composition and architecture are not uniform across the wall. For example, pectic polysaccharides are thought to be deposited early after cell division, leading to a middle lamella that is rich in pectins; other components are deposited later. This differentiation of the wall may be especially important for protein and glycoprotein components, such as AGPs that may change as cells mature and differentiate. Information about such heterogeneity is lost when tissues are ground and subjected to biochemical analysis. Thus, to fully understand the dynamic nature of plant cell walls at the molecular level, new visualization techniques are needed that reveal the three-dimensional complexity of the walls on individual cells as well as the ability to monitor any changes as a function of developmental time and space.

One important tool that will help in such studies is an array of antibodies and carbohydrate-binding proteins that can be used to visualize specific epitopes within plant cell walls ( Pattathil et al., 2010 ). Preliminary analysis supports the hypothesis that every cell type has a distinct array of wall components, but much more work and even greater resolution will be needed to gain the desired information about the three-dimensional organization of cell wall components. Previous Section Next Section BIOSYNTHETIC ISSUES Probably the biggest gap in our knowledge about cell walls relates to biosynthesis of the various wall components. It has been estimated that more than 2000 genes are required for the synthesis and metabolism of cell wall components ( McCann and Rose, 2010 ). Identification of the genes responsible for wall biosynthesis and characterization of the biochemical and biological functions of the gene products that mediate wall biosynthesis are important areas of current research activity. Finally, as the process of wall biosynthesis is revealed, it will be important to understand how these processes are regulated, at both the biochemical and the transcriptional level. One important feature of plant cell wall biosynthesis is that it involves multiple cellular compartments ( Fig. 1 ).

Specifically, cellulose is synthesized at the plasma membrane with the insoluble cellulose microfibrils being deposited directly into the extracellular matrix. On the other hand, matrix polysaccharides and various glycoproteins are synthesized in the endomembrane system, with the polymers being delivered to the wall via secretory vesicles ( Fig. 1 ). Components synthesized in different locations must be assembled into a functional wall matrix. Although very little is known about this assembly process, it seems likely that it is a mediated event, most probably requiring proteins of various kinds. View larger version: Download as PowerPoint Slide Figure 1. Schematic representation of the key events in cell wall biosynthesis. Cellulose biosynthesis occurs at the plasma membrane in large complexes visualized as rosettes. The synthesis of matrix polysaccharides and glycoproteins occurs in the Golgi where the products accumulate in the lumen before transport to the cell wall via vesicles. The regulation of these biosynthetic events is an important issue that needs more study.

Abbreviations used in the figure: CesA, cellulose synthase proteins that form the rosette; NDP-sugar, nucleotide sugars that act as donors for the sugars that go into polysaccharides; Csl, cellulose synthase-like proteins that are known to be involved in hemicellulose biosynthesis. Cellulose biosynthesis involves a large multisubunit complex containing at least three different cellulose synthase enzymes and probably other proteins ( Guerriero et al., 2010 ). These proteins form a complex that appears in the plasma membrane as a rosette structure that is thought to transfer Glc from cytosolic UDP-Glc to produce multiple extracellular glucan chains that eventually coalesce into a cellulose microfibril ( Fig. 1 ). While much has been learned about cellulose biosynthesis in the past two decades ( Somerville, 2006 ; Guerriero et al., 2010 ), many questions remain. For example, at the biochemical level, how is glucan chain polymerization initiated? How are the individual sugar molecules, or the growing chains, transported across the plasma membrane while still maintaining the membrane potential characteristic of living cells?

At a cell biological level, how are the cellulose microfibrils properly oriented? It is known that cortical microtubules are important in determining cellulose microfibril orientation ( Wightman and Turner, 2010 ), but the molecular details of how is this accomplished remain unclear. The biosynthesis of matrix polysaccharides and glycosylation of various cell wall glycoproteins occur in the Golgi membranes ( Fig. 1 ). Although recent advances have enhanced our understanding of the synthesis of these molecules ( Ellis et al., 2010 ; Harholt et al., 2010 ; Scheller and Ulvskov, 2010 ), many important questions remain. At a biochemical level, we must identify and characterize the enzymes needed to synthesize the diverse array of matrix components. For example, it has been estimated that more than 65 different enzymes are required to synthesize the pectic polysaccharides known to exist in plant cells ( Harholt et al., 2010 ). Yet only a few of them have been identified and characterized, partly because of the inherent difficulty of the problem.

Two basic strategies are available for identifying the biochemical and biological functions associated with the many gene sequences that have been identified as candidates for involvement in wall biosynthesis. The first is expression of a cloned gene followed by measurement of the biochemical activity of the resulting protein. Expression of the gene is relatively easy, but measuring the resulting biochemical activity is difficult, largely because of the extreme specificity of the glycosyl transferase enzymes. Many of the substrates that donate sugar molecules are commercially available, but few of the acceptor molecules are. The latter are often complex carbohydrates that are difficult to produce in the laboratory. Finally, there is growing evidence that many wall biosynthetic enzymes act in multienzyme complexes so that in vitro assays may require the action of multiple enzymes. A second strategy for exploring gene function is reverse genetics using the power of model systems such as Arabidopsis (Arabidopsis thaliana; Liepman et al., 2010 ). However, this approach is often complicated by the presence of multiple genes encoding a particular enzyme, so that double, triple, or even higher-order mutants are needed.

Once mutants are obtained, some mutant plants have no visible phenotype, but even when mutants have morphological changes, detailed analysis is needed to define the biochemical changes in wall components and to connect them to the morphological changes. Although progress is being made in identifying and characterizing the genes required for the synthesis of wall matrix components ( Ellis et al., 2010 ; Guerriero et al., 2010 ; Harholt et al., 2010 ; Scheller and Ulvskov, 2010 ), little is known about how the production and accumulation of wall components are regulated. It is clear that synthesis of wall components is regulated in very specific ways to produce the diversity of cell shapes and functions that characterize a living plant. But understanding how this regulated deposition of wall components is accomplished is a major challenge. One important aspect of controlling this overall process is regulation of carbon flow to the nucleotide sugars that are the sugar donors for cell wall polymers ( Reiter, 2008 ). How this flow of carbon is regulated, i.e. biochemical controls, transcriptional controls, or both, and how much this regulation contributes to overall regulation of wall deposition are yet unknown.

Another likely point of regulation is the activities of the glycan synthases and glycosyltransferases that assemble wall polysaccharides from the nucleotide sugars. One attractive hypothesis posits that the quantity of these enzymes is regulated by controlling gene expression, probably in a coordinated manner, so that all of the enzymes needed for the production of a particular wall component are coordinately regulated. In addition, it seems likely that the activities of many enzymes may be controlled by phosphorylation or other mechanisms. The quantities and rates of cellulose deposition may be controlled in part by the location and cycling of the rosettes that mediate cellulose synthesis, whereas the orientation of cellulose microfibrils is determined by interactions with the cytoskeleton ( Wightman and Turner, 2010 ; see Fig. 1 ). Other potential regulatory points are the delivery and assembly steps. For example, the delivery of matrix components from the Golgi to the cell surface may be regulated by controlling the activity of the secretory system. It has been suggested that cells have feedback mechanisms that sense the status of the cell wall and control wall deposition events in response to need (see Seifert and Blaukopf, 2010 , for a recent update).

However, many important questions remain regarding how these feedback mechanisms operate. Previous Section Next Section CONCLUDING REMARKS One final issue relevant to both the structure of plant cell walls and the biosynthesis of wall components is the evolutionary relationships of cell walls from the many plant species and their algal progenitors. While most work on structure and biosynthesis has focused on angiosperms, especially model systems such as Arabidopsis ( Liepman et al., 2010 ) and crop plants, recent work on cell walls from algae and primitive plants have begun to yield interesting insight into the evolution of cell walls and their components ( Popper and Tuohy, 2010 ; Sørenson et al., 2010 ). Such studies may lead to important insights into the functional relationships among the various wall components. A major conclusion from this brief summary is that the plant community faces many challenges in understanding cell wall structure, function, and biosynthesis. New biophysical and visualization methods will be needed to understand the organization of components in the wall of a single cell.

With respect to the challenges of understanding cell wall biosynthesis and its regulation, molecular biology, molecular genetics, and genomics has already provided many powerful new tools so that rapid progress can be expected. Previous Section Next Section Footnotes ↵ 1 This work was supported by the Division of Biological and Environmental Sciences in the Office of Science at the U.S. Department of Energy via the Department of Energy Great Lakes Bioenergy Research Center, and from the Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences in the Office of Science at the Department of Energy via the Michigan State University-Department of Energy Plant Research Laboratory. Cell Wall - Biology Encyclopedia - cells, plant, body, human, system, different, organisms, used, types Cell Wall - Biology Encyclopedia Cell Wall Photo by: Sebastian Kaulitzki With very few exceptions, all cells are enveloped by an extracellular matrix composed of proteins , carbohydrates , and other substances. Owing to its exceptional strength and its ability to control cell shape, the extracellular matrix of eubacteria, algae, fungi, and plants is called the cell wall. The composition of cell walls varies widely among these kingdoms and the species within them, but the central functions are similar for most organisms.

Cell walls provide rigidity and protection. For multicellular organisms, the cell wall also binds different cells together. Plants use their cell wall as part of their system for maintaining their shape and stiffness. The plant concentrates ions and other substances within the cell, which pulls in water by osmosis . The cell swells, pressing tightly against the cell wall. The swelling increases rigidity, or turgor , while the wall keeps the cell from bursting. Eukaryotic Cell Walls Eukaryotic organisms, such as algae, fungi, and higher plants, have multilayered cell walls composed in large part of either cellulose or chitin . Cellulose and chitin are polysaccharides , meaning they are composed of many linked sugar molecules. Cellulose is a polymer of glucose , which contains only carbon, hydrogen, and oxygen, while chitin is a polymer of N-acetylglucosamine, a sugar that contains nitrogen as well. Both cellulose and chitin are linear, unbranched polymers of their respective sugars, and several dozen of these polymers are assembled into large crystal-like cables, called microfibrils, that spool around the cells.

Cellulose microfibrils form the scaffold of all plant cell walls. At least two types of primary walls are found among the species of flowering plants (angiosperms). In the Type I walls of eudicots and some monocots, the microfibrils are tethered together by sugars called xyloglucans, and this framework is embedded in a gel of pectins , another type of polysaccharide. The pectins establish several of the wall's physical characters, such as electrical charge, density, porosity, enzyme and protein distribution, and cell-to-cell adhesions . Pectins are used commercially to thicken jellies and jams. The Type II walls of cereal grains and other monocot relatives tether the microfibrils with different sugars, and is relatively pectin-poor. The hardness of wood comes from lignin , which is impregnated between the cellulose microfibrils. Lignin is a phenolic compound, chemically related to benzene.

The cell walls of fungi are diverse among the taxonomic groups, but most contain chitin microfibrils embedded in a polysaccharide matrix and covered with a loose coating of additional molecules combining sugars and peptides (amino acid chains). However, the cell walls of the Oomycetes contain cellulose instead of chitin. Different groups of fungi can be distinguished partly by the composition of their cell wall components. Cellulose forms a substantial part of the microfibrillar framework of most algae, although some contain other polysaccharides instead. These microfibrillar networks are embedded in a thick gel of polysaccharides of immense diversity. Three important classes of algae, the Chlorophyceae (green), Rhodophyceae (red), and Phaeophyceae (brown), can be distinguished to a certain extent based on their polysaccharide constituents. Alginic acid and fucans are found in brown algae, whereas agarose and carrageenan are found predominately in red algae. Several of these polysaccharides are used as thickening and stabilizing agents in a variety of foods.

Bacterial Cell Walls In eubacteria, the cell wall is composed of one or more layers of a peptidoglycan, called murein. A peptidoglycan is a combination of peptides and sugars. Murein is composed of the sugars N-acetylglucosamine and N-acetylmuramic acid. To murein are linked peptide extensions that are crosslinked to form the netlike wall. The antibiotic penicillin shuts down the enzyme that creates these cross-links, thus preventing bacterial growth. Many bacteria produce a capsule to the exterior of the murein wall, composed of a diverse selection of molecules, including polypeptides and several complex carbohydrates, which may include cellulose. Bacteria with this outer capsule do not absorb a particular dye, called Gram stain, and therefore known as Gram-negative bacteria. Bacteria lacking the outer capsule do absorb the dye and are called Gram-positive bacteria. The Gram stain is a basic tool for identifying bacteria. Escherichia coli bacteria in the human large intestine are Gram-negative bacteria.

In contrast to eubacteria, archaea possess a pseudomurein wall, with a different set of sugars, no D-amino acids, and exterior layers of proteins, glycoproteins, and polysaccharides similar to those found in higher organisms. SEE ALSO Nicholas C. Carpita Bibliography Cell wall & Plasmodesmata - APBiology Cell wall & Plasmodesmata 6 Citations Overview The cell wall, an tough, rigid and semi-permeable extracellular structure responsible for cell protection and support, is found in plants, bacteria, archaea, fungi and some protists. Cell wall main functions may include: - Structural and mechanical support - Providing a chemically buffered environment - Maintenance of cell shape - Resistance to pressure of cell - Rate and direction of growth control - Regulate diffusion, a porous medium for circulation of water, minerals, and other small nutrients - Rigid building blocks from which stable structures can be produced - Storage of carbohydrates - Protection against pathogens and other environmental factors - Source of active signaling molecules - Cell to cell interactions Plant Cell Walls In plants, the cell wall provides support to individual cells by containing the pressure created in the central vacuole and protecting the plant cells against invading bacteria and fungi.

The wall consists of cellulose fibers (cellulose microfibril), which provides tensile strength, embedded in a network of highly branched carbohydrates. These cellulose fibers are bound by polysaccharides called hemicelluloses. The initial cell wall, the primary cell well, is relatively soft and flexible. As the cell grows and matures, this layer expands and additional layers of cellulose fibers and branched carbohydrates are laid down between the primary cell wall and the plasma membrane of the cell creating a new wall layer, more rigid and thicker than the first. This is called the secondary cell wall. The secondary cell wall can be reinforced by lignin, a hard substance assembled for alcohols, surrounded by cellulose fibers. It is commonly found in plants and trees, hence why, wood is useful to make strong structures like houses and furniture. The walls of adjacent primary cell walls are held together by a layer of gel-like polysaccharides, called pectin. This polysaccharide material forms the middle lamella. Other Types of Cell Walls Algae This simple organization of plants represents the process of evolution. Algal cell walls lack long, fibrillary carbohydrates. It’s mostly composed of glycoproteins and cellulose.

Polymers like mannanes and xlyanes, as well as alginic acid and other alginates are found in different types of algal walls. Fungi Fungal cell walls are complex structures made mostly of chitin, glucans, and many other polysaccharides especially mannoproteins that engage in extensive cross-linking to give extra rigidity. Fungi do use cellulose. The wall changes constantly during cell division and growth. Bacteria The bacterial cell wall is predominantly made of peptidoglycan, made from polysaccharides cross linked by certain peptides. It does not contain cellulose and chitin, like some of the other cell walls. Plasmodesmata Plasmodesmata are microscopic junctions between cells in plants that help facilitate communication and transport of materials between plant cells, establishing living bridges. Some plants with plasmodesmata are charophyceans, Charales and Coleochaetales, as well as all embryophytes. Though cells are permeable and can transport to some small soluble proteins, the plasmodesmata offer direct connection between cells transporting all sorts of materials that are important for cell function.

Plasmodesmata are known for transporting proteins, messenger RNA, and viral genomes. Structure: Most plants will have anywhere from 1000 to 100,000 plasmodesmata in between cells. These bridges are 50-60nm wide at the midpoint. There are three layers, which include the plasma membrane, the cytoplasmic sleeve, and the desmotubule. Plasmalemma extends continually from around the cell to form the plasma membrane, which is similar to the phospholipid bilayer. The cytoplasmic sleeve is a fluid-filled space, enclosed by the plasmalemma and cytosol. The desmotubule is a cylindrical membrane lined channel that links the cisternae of the endoplasmic reticulum. Links Overview of Cell Walls - [ [1] ] Plant Cell Walls - [ [2] ] Russel, Wolfe, Hertz, and Starr. Biology: The Dynamic Science Encyclopedia Britannica Online Plant Physiology Plant Physiology (Biology 327) - Dr. Stephen G. Saupe; College of St. Benedict/ St.

John's University; Biology Department; Collegeville, MN 56321; (320) 363 - 2782; (320) 363 - 3202, fax; ssaupe@csbsju.edu Plant Cells I. A Water Balloon in a Box This is a good model for a plant cell. Thus, there are two major components to a plant cell: A. The Box This part of the cell is analogous to the cell wall. Like a cardboard box, the cell wall is relatively rigid, it is non-living and highly structured. Its more obvious functions are to support and protect the cell. It is produced by the protoplast (see below). If you stack up a bunch of boxes you generally cant make a very large tower before it comes crashing down. Since a plant is essentially constructed of numerous small boxes, why dont they fall apart, too? The answer is glue! Plants glue their cells together with pectic polysaccharides. These carbohydrates, which make up the outermost layer of the plant cell wall (called the middle lamella), bind adjacent cells together.

Cooks use pectins extracted from the middle lamella to solidify jams and jellies. Pores or air spaces (called intercellular spaces) exist between adjacent cells because of the difficulty of packing of cells with rigid walls. Try and squeeze a bunch of irregularly boxes into a room without leaving any space between them! The intercellular spaces are important for gas exchange and water transport, some movements (i.e., sensitive plants - water moves into/out of theses spaces; nyctinastic movements - sleep movements) and freezing protection (i.e., water moves out of cells into the spaces to minimize cellular damage on freezing. Trivia note: prized ginseng roots have a translucent appearance - apparently obtained by freezing). Putting a cell in a box presents one major problem - how do cells talk to one another since they are now effectively isolated in their own small compartment? Plants solved this problem by putting windows in the box! Or in other words, there are lots of specialized pores through the wall called plasmodesmata that provide a cytoplasmic connection ("cytoplasmic bridges") between adjacent plant cells. Thus, the cytoplasm of a plant is essentially contiguous throughout the entire plant.

Imagine hopping on board a tiny submarine like in the films Fantastic Voyage or Innerspace. You can essentially travel from cell-to-cell throughout the plant without ever leaving the cytoplasm nor crossing any cell membranes. Cool! The plasmodesmata are 40-50 nm in diameter. The maximum sized object that can pass through as a molecular mass of 700-1000 daltons, which is equivalent to a molecule 1.5 - 2.0 nm. The plasmodesmata have a unique structure. The plasma membranes from adjacent cells are continuous through the pore. In addition, the ER (see below) is also continuous between adjacent cells. The narrow tubule of ER is called the desmotubule and in the center there may be dark staining rod (called then "central rod"). The cytoplasmic channel between desmotubule and membrane is called the "cytoplasmic sleeve." Protein fibers radiate from the desmotubule to the membrane like spokes on a bicycle. These fibers may act like a sieve or screen. Plasmodesmata are aggregated in pits and there are 1-15 per mm2.

They can make up as much as 1% of the cell surface. Plasmodesmata are formed during cell division (see cell notes) or secondarily after division. They may or may not be branched, too. B. The water balloon - this is analogous to the protoplast. The protoplast is everything inside the cell wall. It is the "living" part of the cell and includes: cytosol - matrix in which the other cell components are embedded; nucleus - the cell brain; ergastic (non-living) substances which includes crystals ("far out, man"), tannins, and starch grains. II. Separating the balloon from the box. Just as you can remove a balloon from a box, so too can a plant physiologist liberate the protoplast from the cell wall. This is accomplished by slicing up a leaf (or other tissue) and then floating it in a solution containing wall-digesting enzymes like pectinase, cellulase, and hemicellulase suspended in an inert osmotic agent (0.7 M mannitol or sorbitol) in a buffered solution. The enzymes degrade the components of the cell wall releasing the protoplasts.

This will be the focus of at least one or our lab sessions. III. Organelles in Both Plant and Animal Cells First, we will study the components of the protoplast, but will especially concentrate on those structures unique to plants. In the next lecture well tackle the cell wall. A. Plasma or Cell membrane Cell boundary; selectively permeable; bilayer of phospholipids with inserted protein. Phospholipids are unique molecules - they are amphipathic, meaning that they have both hydrophilic and hydrophobic regions. They have a glycerol backbone; one of the hydroxyls is bonded to a phosphate and another charged group, the other two hydroxyls are esterified to fatty acids. The fatty acids range in length from C14 - C24. One fatty acid is usually unsaturated and the other is saturated. The unsaturated fatty acid is kinked which helps to keep plant cell membranes fluid at cool temperatures. As a result plant phospholipids usually have a higher degree of unsaturation than animals. Hydrophobic interactions between the tail regions of the phospholipids hold the membrane together.

Some proteins are found: (1) just on the outside or inside surfaces of the membrane (peripheral proteins - non-covalent interactions and anchored proteins - covalently bound to lipids, etc); or (2) embedded in the membrane (integral protein), many of which span the membrane (trans-membrane proteins). Hydrophilic regions of the integral proteins are oriented to the outside of the membrane whereas hydrophobic regions are embedded within the phospholipid bilayer. Lipid soluble materials can readily pass through but charged or ionized substances (hydrophilic) pass through very slowly, if at all. The function of the membrane is to: (1) regulate traffic; (2) separate the internal from external environment; (3) serve as a platform on which some reactions can occur; (4) participate in some reactions (i.e., the membrane components are important intermediates or enzymes); and (5) provide some structural integrity for the cell. B. Nucleus The cell "brain". Surrounded by a double membrane (two phospholipid bilayers) - the nuclear membrane. Has pores. The structure of the pores is complex comprised of more than 100 proteins.

The pore opening is surrounding by a series of proteins and these are attached to a series radial spokes. Nucleoplasm - matrix within nucleus. DNA, which is found in the nucleus, may be condensed into chromosomes or not (chromatin). There may be one or more nucleolus (site of ribosome production). The nucleus is 5-20 mm in diameter. There is a layer of intermediate filaments (see below) just inside the nuclear envelope; called the nuclear lamina. C. Cytoplasm/cytosol The cytosol is the gel-like matrix within the cell in which the other structures are embedded. The cytoplasm refers to the cell contents inside the membrane. D. Mitochondria These organelles, like the nucleus and plastids, are double-membrane bound. They vary in shape from tubular (like sausages) to spherical. They reproduce by fission, have their own ribosomes and DNA (a circular loop like prokaryotic cells). The inner membrane has a larger surface area so it must be folded into finger-like projections (called cristae) to fit inside the outer membrane. Mitochondria are found in all eukaryotic cells.

They are the sites of cellular respiration - process by which energy is released from fuels such as sugar. The mitochondria are the power plant of the cell. They are small (1-5 mm) and generally numerous (500-2000 per cell). A popular misconception is that "plants have chloroplasts, animals have mitochondria." Plant cells, at least green plant cells (i.e., leaf cells), have both. Root cells only have mitochondria. Mitochondrial DNA which comprises about 200 kbases, codes for some of the genes required for cellular respiration including the 70S ribosomes and components of the electron transport system. The inner membrane differs from the plasma membrane in that it has a higher protein content (70%) and unique phospholipids (i.e., cardiolipin). E. Ribosome Sites of protein synthesis (translation). Two subunits; one large and the other small. Made in the nucleus from rRNA and protein. Ribosomes are tiny (0.25 mm) and numerous (5 - 50 X 1010 per cell).

Since ribosomes are not surrounded by a membrane, they are not considered to be "true" organelles. Some ribosomes are 'free' (produce proteins that remain in the cell) while others are attached to the ER (produce proteins for export). To export a protein, the mRNA and subunits of the ribosome bind together. A signal recognition particle (SRP) binds to specific amino acids in the newly forming protein. The SRP, which is bound to the protein/mRNA/ribosome, then binds to a receptor in the ER membrane. As the protein is made it is released into the lumen of the ER and the SRP sequence of the protein is snipped off. F. Endoplasmic reticulum A series of membranous tubes and sacs (cisternae) that run throughout the cell. Rough ER has ribosomes associated with it and is laminar while smooth ER lacks ribosomes and is tubular. Totally man.

The ER has several functions including: (1) synthesis of lipids and membranes (smooth ER); (2) serving as a site for the synthesis of proteins by the ribosomes (rough ER); (3) transport (a type of cell 'highway' system); and (4) support. G. Peroxisomes Membrane sac containing enzymes for metabolizing waste products from photosynthesis, fats and amino acids. Hydrogen peroxide is a product of metabolism in peroxisomes. Catalase, which breaks down the peroxide is also present and serves as a marker enzyme for these organelles. H. Glyoxisomes Membrane sac containing enzymes for fat metabolism. Especially common in seeds. Also contain catalase. I. Golgi apparatus Pancake- or pita bread-like stack of membranes. Particularly important in cells that produce materials for export (secretion). They have a polarity (cis - imports vesicles from ER; trans - exports vesicles). The Golgi is the site of processing and packaging cellular components.

Vesicles containing proteins, lipids and other materials, fuse with the Golgi (cis side), release contents, which then get processed, sorted, packaged and re-released from the other side (trans face). The Golgi also is active in synthesizing many cell components, especially carbohydrates and is involved in tagging proteins with carbohydrates and other side chains for sorting them to their final destination. There are two models for the movement of materials thru the Golgi: (1) Vesicle Migration Model - in this case a vesicle fuses with the cis side, then ultimately a new vesicle pinch off this stack and fuses with teh next one, and so on, until the vesicle reaches the trans side; and (2) Escalator Model - a vesicle fuses with the cis side and never leaves this stack. Rather, the stack on the trans side releases vesicles and then disintegrates while a new stack forms on the cis side. The original vesicle is now in the "second" stack, and so on until it reaches the trans side. Vesicles are tagged with various proteins to direct them to the appropriate locations. J.

Microtubules Hollow tubes made of a mix of alpha and beta tubulin, which are globular proteins. There are essentially 13 columns of proteins. The tubes are about 25 mm in diameter. Microtubules are involved in the cell cytoskeleton (for support), cell movements (cilia, flagella) and cell division (spindle). Assembly of microtubules is prevented by colchicine, an inhibitor derived from Crocus bulbs. Low calcium concentration favors the formation of microtubules K. Microfilaments Protein strands. Solid. Made from G-actin. Involved with the cell cytoskeleton. Main function is support. They are about 7 nm in diameter. L. Intermediate filaments These are similar to microfilaments. They are also made of protein in the keratin family; about 10 nm diameter. M. Cilia/flagella For cellular movements. Cilia = many, short; flagella = few, long. Have a 9+2 arrangement of microtubules. Prongs on the tubules are ATPases (dynein) to hydrolyze ATP to provide energy for movement. These are not particularly common in plants. N.

The Cytomembrane system The membranous organelles (ER, vesicles, golgi, cell membrane) comprise a group of organelles that cooperate and function together. For example, imagine the synthesis of cellulose in the cell wall of a plant. Cellulose synthesis requires the enzyme cellulose synthase. Ribosomes (rough ER) → makes enzyme → passes through RER to smooth ER → packaged into a vesicle → pinches off → to golgi (cis face) → processed → repackaged into vesicle → pinches off (trans face) → cell membrane → fuses → releases contents → cellulose synthase makes cellulose. O. Others Microbodies - a general term for any single membrane bound organelle typically derived from the ER that contain catalase and/or hydrogen peroxide producing enzymes. This includes the peroxisomes and glyoxisomes; Microsomes - a "biochemical" term for the fraction that is obtained from high speed centrifugation of cell homogenates. It includes membrane fragments and ribosomes. Oleosome (spherosomes) - these are lipid bodies.

The coolest thing about them is that they are encased by one-half of a cell membrane; in other words, just a single phospholipid layer. IV. Organelles Unique to Plants - Plastids Plastids are double membrane-bound organelles in plants. They contain their own DNA (in nucleoid region) and ribosomes. They are semi-autonomous and reproduce by fission similar to the division process in prokaryotes. If plastids only arise from other plastids and cant be built "from scratch", then where do they come from? The egg. Plastids are inherited cytoplasmically, primarily through the female - however, there are examples of paternal inheritance of plastids. The plastid DNA carries several genes including the large subunit of rubisco and those for resistance to some herbicides. The chemistry of the membranes differs from the plasma membrane - plastid membranes are comprised of glycosylglycerides rather than phospholipids (the phosphate in the polar head group in glycosylglycerides is replaced with galactose or a related sugar).

There are several types of plastids including: Proplastids - small, precursors to the other plastid types, found in young cells, actively growing tissues; Chloroplasts - sites of photosynthesis (energy capture). They contain photosynthetic pigments including chlorophyll, carotenes and xanthophylls. The chloroplast is packed with membranes, called thylakoids. The thylakoids may be stacked into pancake- like piles called grana (granum, singular). The "liquidy" material in the chloroplast is the stroma. A chloroplast is from 5-20 μm in diameter and there are usually 50-200 per cell. The chloroplast genome has about 145 Kbase pairs, it is smaller than that of the mitochondria (200 kbases).

About 1/3 of the total cell DNA is extranuclear (in the chloroplasts and mitochondria); Chromoplasts - non-photosynthetic, colored plastids; give some fruits (tomatoes, carrots) and flowers their color; Amyloplasts - colorless, starch-storing plastids; Leucoplast - another term for amyloplast; Etioplast - plastid whose development into a chloroplast has been arrested (stopped). These contain a dark crystalline body, prolamellar body, which is essentially a cluster of thylakoids in a somewhat tubular form. Plastids can dedifferentiate and convert from one form into another. For example, think about the ripening processing in tomato. Initially, green tomatoes have oodles of chloroplasts which then begin to accumulate lycopene (red) and become chromoplasts. Usually you find only chromoplasts or chloroplasts in a cell, but not both. V. Organelles Unique to Plants - Vacuoles This is the large, central cavity containing fluid, called cell sap, found in plant cells. The vacuole is surrounded by a membrane (tonoplast).

Back to the water balloon in the box model - imagine the vacuole to be analogous to another water balloon inside our protoplast balloon. This water balloon is a separate entity that can be physically removed from the cell. The vacuole is penetrated by strands of cytoplasm - transvacuolar strands. The tonoplast and plasma membrane have different properties such as thickness (tonoplast thicker). Virtually every plant cell has a large, well-developed vacuole that makes up to 90% or more of the cell volume. Wow! Meristematic and embryonic cells are exceptions. Young tissues have many small vacuoles. As the cell grows the vacuoles expand and eventually coalesce. These small vacuoles appear to be derived from the Golgi. The central vacuole contains water, ions, organic acids, sugars, enzymes, and a variety of secondary metabolites. Among the hydrolytic enzymes are proteases (digest protein), ribonucleases (digest RNA) and glycosidases (break links between monosaccharides). These enzymes are typically not used for recycling cellular components but rather leak out on cell senescence.

There are smaller lytic vacuoles, which contain digestive enzymes, that are used for this purpose. Another type of vacuole, protein bodies, are vacuoles that store proteins. Why do plants cells have a large central vacuole? Important roles of the vacuoles are: A. Energetically efficient means to increase surface to volume ratio in the dendritic growth form Since 90% of the cell volume is vacuole, therefore 90% of the cell is water, which is relatively cheap in metabolic terms. Thus, it allows plants to get big with a minimal energy investment. Plants are particularly 'smart,' since, the cell wall, which comprises much of the remaining 10% or so of the cell is a polymer of glucose. Cellulose is a great bargain! It is stronger and cheaper than comparable polymers. Lets compare: polymer Probably a minor role; mostly in succulents C. Waste disposal The vacuole can be considered the cell cesspool. It contains many secondary metabolites including a variety of hydrolytic enzymes like the "marker enzyme" alpha mannosidase. In this regard, the vacuole is analogous to the lysosome.

Lets consider differences between plants and animals in terms of wastes. Plants have little waste. Their nutrients are in dilute form, they use them efficiently and hence, there is little left over. Plants remind me of my Dad who was a Marine during WWII. He said that the Marine philosophy at dinner was to "eat all you want, but all that you take." Plants do just that - most everything they "ingest" they use. The minimal wastes plants produce can be stored in the cells in the vacuoles (or disposed of in other ways - released as a gas into the air, leached out of roots or leaves). In contrast, animals rely on nutrients in a concentrated form. They are rarely selective about what they eat; much un-usable material is ingested along with the "good stuff". Thus, they have a large quantity of waste and needed to evolve specialized digestive/excretory systems to get rid of it. Further, animals dont have the luxury of having bulky vacuoles and internally storing wastes because these adaptations would limit motility. Some related ideas: Heterotrophic plants (like carnivorous plants) have excessive wastes.

The plant strategy is usually to discard the structure with the wastes (i.e., insect remains) and produce a new one. For example, pitcher plant leaves fill up with insect parts and the plant produces a new batch of leaves the next season. Plants are able to do this because of their architectural design; Humans start out life very much like a plant. Newborns are non-motile and breast-fed. Breast milk is dilute and nutritious. And, it is highly digestible leaving relatively few leftovers. Hence, newborn diapers are relatively innocuous. However, as babies become more animal-like and begin solid food, diapers are best left to the strong of stomach; Cell walls - have been suggested that the cell wall first evolved as a site for waste disposal for excess carbon. Whatdayathink? D. pH regulation The vacuole is a pool to dump excess protons. There is an active proton pump in the tonoplast. The cell sap has a pH of 2-5.7, whereas the cytosol is ca. 7.0. E. Storage of essential ions Ions are pumped into the vacuole for water balance. Potassium and calcium. in particular, are stored in the vacuole.

F. Cell enlargement Cell growth requires some force to allow for the cell to increase in size. Water pressure provides the force and it moves into the vacuole. For example, root hair enlargement is due entirely to vacuolar enlargement. G. Facilitates diffusion The cytosol essentially forms a thin coating around the large vacuole which in effect, increases the surface-to-volume ratio of the cytoplasm. It provides easier access and shorter diffusion distances between any part of the cytoplasm and the external environment of the cell. This can be particularly important for chloroplasts. H. Structural support The vacuole helps to maintain turgor pressure in plant cells due to the opposing forces of tensile strength of the wall vs. compression strength of water. VI. Study Guide: click here VII. References: Wiebe, H.H. 1978. The Significance of Plant Vacuoles. BioScience 28: 327-331. The Annual Review of Plant Physiology and Plant Molecular Biology is an excellent source of references. The Nuclear Family - article about the origin of mitochondria and chloroplasts. Scientific American. November 19, 2001. VIII.

Web Sites: Cell Wall |Function, Cell Wall Structure, Bacterial Cell Wall | Biology@TutorVista.com Protection against infection and mechanical stress. Separates interior of the cell from the outer environment. It enables transport of substances and information from the cell insides to the exterior and vice versa. Also helps in osmotic-regulation. The physiological and biochemical activity of the cell wall helps in cell-cell communication. It prevents the cell from rupturing due to tugor pressure. Aids in diffusion of gases in and out of the cell. Also provides mechanical protection from insects and pathogens. Cell Wall Structure Back to Top The composition of the cell wall differs from one species to the other. In bacteria the cell wall is made up of peptidoglycans. The Archean cell wall is made of glycoproteins and polysaccharides. In fungi cell walls are made of glucosamine and chitin. In algae it is composed of glycoproteins and polysaccharides. The plant cell wall is mainly composed of cellulose, hemicellulose, glycoproteins, pectins and lignin. Plant Cell Wall Back to Top Presence of Cell wall is the major difference between plant cell and animal cell.

Plant cell wall performs essential functions like providing shape, tensile strength and protection and also helps the cell develop tugor pressure to maintain the pressure of the cell contents. Plant cell walls are primarily made up of cellulose. cellulose is the most abundant macro-molecule on Earth. Plant cell wall consists of three layers: the primary cell wall, secondary cell wall and the middle lamella. The middle lamella - It is first layer formed during cell division. This layer is rich in pectin. It is the outermost layer, joins together adjacent plant cells and holds them together. The primary cell wall â It is formed after the middle lamella. It is composed of pectin compounds, hemicellulose and glycoproteins. The layer consists of a framework of cellulose micro-fibrils, in a gel-like matrix. It is thin, flexible and extensible layer. The secondary cell wall - It is a thick layer formed inside the primary cell wall. It is extremely rigid and provides strength. It is composed of cellulose,hemicellulose and lignin.

Composition of plant cell wall In the growing cell wall the carbohydrates are cellulose, hemicellulose and pectin as well as lignin, proteins and enzymes. The outer part of the primary cell wall of the plant epidermis usually forms a permeability barrier and is known as the plant cuticle. The secondary cell wall contains cellulose, xylan and lignin. Plant cell walls also contain enzymes such as hydrolases, esterases, peroxidase and transglycolases. also contain structural proteins and silica crystals. Bacterial Cell Wall

Question: What is the strong material found in the cell walls of plants?
Answer: Cellulose