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Primary function of cell wall is

Primary function of cell wall is


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A. Controlling volume B. Providing shape C. It's selective permeability D. Protection against bursting To me, all the given options seem correct except C.

A and D are correct when the cell is placed in hypotonic solutions which makes the cell swell up and cell wall applies wall pressure antagonistic to turgor pressure to prevent cell bursting. B is also correct as cell wall acts as exoskeleton, so please tell me the most appropriate option.


  • The primary function of a cell wall is B. providing shape. Cell wall provides the cell with both structural support and protection. Protection against bursting by osmosis is a secondary function, which is otherwise provided by the plasma membrane.

  • Cell wall provides shape, rigidity, and turgidity to plant cells. Animal cells do not have cell walls because they have endoskeleton. The cell wall maintains cell shape, almost as if each cell has its own exoskeleton.


4: Bacteria - Cell Walls

  • Contributed by Linda Bruslind
  • Senior Instructor II and Lead Advisor (Microbiology) at Oregon State University
  • Sourced from Open Oregon State

It is important to note that not all bacteria have a cell wall. Having said that though, it is also important to note that most bacteria (about 90%) have a cell wall and they typically have one of two types: a gram positive cell wall or a gram negative cell wall. The two different cell wall types can be identified in the lab by a differential stain known as the Gram stain. Developed in 1884, it&rsquos been in use ever since. Originally, it was not known why the Gram stain allowed for such reliable separation of bacterial into two groups. Once the electron microscope was invented in the 1940s, it was found that the staining difference correlated with differences in the cell walls. Here is a website that shows the actual steps of the Gram stain. After this stain technique is applied the gram positive bacteria will stain purple, while the gram negative bacteria will stain pink.

Overview of Bacterial Cell Walls

A cell wall, not just of bacteria but for all organisms, is found outside of the cell membrane. It&rsquos an additional layer that typically provides some strength that the cell membrane lacks, by having a semi-rigid structure.

Both gram positive and gram negative cell walls contain an ingredient known as peptidoglycan (also known as murein). This particular substance hasn&rsquot been found anywhere else on Earth, other than the cell walls of bacteria. But both bacterial cell wall types contain additional ingredients as well, making the bacterial cell wall a complex structure overall, particularly when compared with the cell walls of eukaryotic microbes. The cell walls of eukaryotic microbes are typically composed of a single ingredient, like the cellulose found in algal cell walls or the chitin in fungal cell walls.

The bacterial cell wall performs several functions as well, in addition to providing overall strength to the cell. It also helps maintain the cell shape, which is important for how the cell will grow, reproduce, obtain nutrients, and move. It protects the cell from osmotic lysis, as the cell moves from one environment to another or transports in nutrients from its surroundings. Since water can freely move across both the cell membrane and the cell wall, the cell is at risk for an osmotic imbalance, which could put pressure on the relatively weak plasma membrane. Studies have actually shown that the internal pressure of a cell is similar to the pressure found inside a fully inflated car tire. That is a lot of pressure for the plasma membrane to withstand! The cell wall can keep out certain molecules, such as toxins, particularly for gram negative bacteria. And lastly, the bacterial cell wall can contribute to the pathogenicity or disease &ndashcausing ability of the cell for certain bacterial pathogens.


What Is the Function of the Cell Wall?

The cell wall gives cells shape, enables plant growth, prevents bursting from water pressure, keeps out water and pathogens, stores carbohydrates and sends signals to cells. The flexible cell wall surrounds plant cell membranes.

Plant cell walls perform many functions. Their main task is to support proper plant growth. This is accomplished by the cell wall creating a skeleton-like frame that enables plants to grow vertically and develop a rigid stem. Cell walls vary considerably in thickness and organization, which accounts for the wide range of plant shapes and sizes on the planet. They consist of two layers ‰ÛÓ a primary cell wall, which supports the cell as it matures, and a rigid secondary cell wall that appears after the primary wall stops growing. The primary cell wall is thinner and more flexible than the secondary wall. Internally, the primary and secondary walls have a similar physical composition. Over the course of a plant's life, they perform complementary functions to keep the plant healthy and vibrant.

Primary Cell Walls

Primary cell walls are comprised mostly of a complex carbohydrate called cellulose. Cellulose is a complex sugar that provides cells with shape and protection against outside harm including bacteria and dehydration. This cell wall also contains a group of polysaccharides, which breaks down into pectins and cross-linking glycans. Pectins, or pectic saccharides, congeal into a gel-like substance as they bind with neighboring polymers. Pectic saccharides provide cell walls with immunity and protection against environmental harms. They also facilitate cell recognition and enable plant cells to combine with each other. Cross-linking glycans bond with cellulose molecules by forming hydrogen bonds. As they form a network of bonds, cross-linking glycans give strength to the cellulose, which builds cell rigidity and structure. Primary cell walls also contain small amounts of protein, which produces enzymes that help cells grow, break down and change. These enzymes are responsible for common plant behavior, such as changing leaf color in autumn or when under stress.

Secondary Cell Walls

Secondary cell walls, which form inside primary cell walls as plants grow, have a similar composition to primary cell walls. However, they contain additional substances that aid in various plant functions. One such substance is lignin, which is a group of hard polymers. Lignin gives cell walls their rigid shape. It also provides cell walls with an extra layer of defense against bacteria and fungi. Lipids in the secondary cell wall, such as wax and cutin, keep cells from absorbing too much water.

Surrounding the cell wall is a structure called the middle lamella, which is comprised mostly of pectins. The middle lamella acts as a binding agent that connects plant cells to their neighbors. This special glue-like substance has tiny passageways called plasmodesmata, which are essentially channels for inter-cellular communication. Plasmodesmata cross between the inner and outer cell walls, and they also branch out into a network that connects to other cells. Through these passageways, the middle lamella lets cells share vital nutrients and minerals. Collectively, the components of the cell wall work together to give the plant strength and immunity and regulate growth and development. Cell walls are constantly growing and changing to meet the plant's needs as it progresses through its life cycle.


Cell Membrane

The cell membrane (also known as the plasma membrane) is a barrier that surrounds a cell, separating its interior from the outside environment. A membrane is a selective barrier which allows certain things to pass through but not others—for this reason, the cell membrane is said to be selectively permeable. This selective permeability is essential to the functioning of the cell, allowing for the maintenance of homeostasis. Large molecules like carbohydrates or amino acids are not able to move passively across the cell membrane, and require energy to be actively transported. This prevents foreign substances from penetrating the membrane and causing damage to a cell or an organelle. Cell membranes also play a role in cellular communication, as well as the detection of external signals sent to the cell.

While the cell membrane is essential to controlling the movement of substances in and out of the cell, it also plays an important role in compartmentalizing the cell. Both prokaryotic and eukaryotic cells are surrounded by cell membranes, but eukaryotes also possess membrane-bound organelles like mitochondria or lysosomes. In these cases, the cell membrane defines a space separate from the rest of the cell where specific processes such as cellular respiration take place.

Membrane Structure

The cell membrane is made up of proteins, lipids, and carbohydrates, with a majority of the cell membrane being made of a phospholipid bilayer (two layers of phospholipids). Proteins are interspersed throughout the cell membrane, being classified as either integral (integrated within the membrane) or peripheral (temporarily adhering to the surface). Monotopic proteins are anchored to the membrane from one side, while polytopic proteins pass through the entirety of the membrane.

Lipids

The main lipid found in the cell membrane is known as a phospholipid—a type of lipid consisting of a phosphate group and glycerol "head" and two fatty acid "tails". The head is hydrophilic while the tails are hydrophobic, making phospholipids amphipathic compounds (having both hydrophilic and hydrophobic regions). The outside of the membrane is surrounded by polar fluids, so the hydrophilic head orients itself to face outward. As a result, the hydrophobic "tails" orient themselves towards the inside of the membrane. The phospholipids are held together by weak interactions between the tails, letting individual phospholipids to move within the membrane and allowing for the membrane to be fluid and flexible.

Proteins

Function of the proteins is to form channels in membranes that allow the passage of specific molecules or ions act as enzymes to increase the rate of cellular reactions (and modify proteins in blood or extracellular space) act as receptors that detect the presence of specific molecules or ions in the external environment and interact with proteins in other membranes, generating sites of attachment between membranes and cells

Integral membrane proteins - exposed to interior AND exterior

  • Form channels (or pores or pumps), receptors (that recognize & respond to hormones), or adhesion points
  • Also can be cell-surface markers, such as glycoproteins which have carbohydrates that act as labels attached to the external side (these labels allow cells to recognize each other and viruses use the labels as “docks” to enter and infect cells)
  • Can span membrane at least once and cross it several times
  • They are permanently embedded and can only be removed through expenditure of large amounts of energy or digestions

Peripheral membrane proteins - exposed to one side (interior OR exterior) provide structural support to membranes

  • Participate in transmitting cell signaling events
  • Alter the topology of membranes in the secretory pathway
  • Can be enzymes
  • Associate with the head groups of specific phospholipids or portions of integral membrane proteins (hence the name “peripheral”)

Unlike integral proteins, the association is impermanent - they can be easily removed by changing the composition of the membrane or the morphology or charge of the protein

Some proteins in the outer leaflet form covalent links (through the amino acids in their C-terminuses) with the head groups of phospholipids these are the proteins that act as enzymes.

Carbohydrates

Membrane carbohydrates account for approximately 2-10 % of the mass of the cell membrane. They are confined mainly to the non-cytosolic surface on the extracellular surface of the cells. They are covalently bonded to proteins and lipids, forming glycoproteins/proteoglycans and glycolipids, respectively.

Cytoskeleton

Structural support (maintaining shape & preventing damage) for the cell membrane is provided by cytoskeleton.

  • Sits directly under the cell membrane and is composed of a “mesh” of actin filaments
  • Interacts with integral membrane proteins by limiting the diffusion of membrane proteins and providing a stable framework to which membrane proteins attach
  • Prevents damage to membranes when external forces pull or push on integral membrane proteins
  • Microtubules that form unique structures (ex: 9 + 2 arrangement for cilia)

Membrane Fluidity

The cell membrane follows the fluid mosaic model or the Singer-Nicholson model, with the phospholipid bilayer behaving more like a fluid than a solid. Lipids and proteins can move laterally within the bilayer, and the pattern ("mosaic") of lipids and proteins constantly changes. However, other models for the cell membrane were initially developed before being disproven. One former model known as the sandwich model or Davson-Danielli model proposed that the phospholipid bilayer was surrounded by proteins, as opposed to having proteins which are integrated within the phospholipid bilayer. However, there were several limitations to this model—it did not account for the selective permeability of the membrane, and it assumed that all membranes were of a uniform thickness. Further experimentation disproved the model, and led to the development of the fluid mosaic model.

The fluidity of the cell membrane is determined by a variety of factors, including the presence of unsaturated fatty acid tails and the presence of cholesterol. Saturated fatty acids can pack much more densely because they are saturated with hydrogen and have straight structures as a result. Unsaturated fatty acids have kinks because of their double bonds, resulting in a bent chain which can pack much less densely. Cholesterol is a sterol (steroid alcohol) which packs between phospholipids, reducing the permeability of the membrane and increasing its rigidity. Steroids have four rigid carbon rings which interact with and stabilize the fatty acid tails of the phospholipids. It is referred to as a bidirectional regulator--at low temperatures it increases fluidity by preventing the fatty acids from coming together and crystalizing (liquid to solid) while at high temperatures it decreases fluidity by immobilizing some groups in the fatty acid tail and increasing the melting point of the tails.


Significance of Acid-Fast Cell Wall Components to the Initiation of Body Defenses

The body has two immune systems: the innate immune system and the adaptive immune system.

  1. Innate immunity is an antigen-nonspecific defense mechanisms that a host uses immediately or within several hours after exposure to almost any microbe. This is the immunity one is born with and is the initial response by the body to eliminate microbes and prevent infection.
  2. Adaptive (acquired) immunity refers to antigen-specific defense mechanisms that take several days to become protective and are designed to react with and remove a specific antigen. This is the immunity one develops throughout life.

Initiation of Innate Immunity

To protect against infection, one of the things the body must initially do is detect the presence of microorganisms. The body does this by recognizing molecules unique to microorganisms that are not associated with human cells. These unique molecules are called pathogen-associated molecular patterns or PAMPs. Pathogenic Mycobacterium species such as Mycobacterium tuberculosis and Mycobacterium leprae release mycolic acid, arabinogalactan, and peptidoglycan fragments from their acid-fast cell wall. (Because all microbes, not just pathogenic microbes, possess PAMPs, pathogen-associated molecular patterns are sometime referred to as microbe-associated molecular patterns or MAMPs.)

These PAMPS bind to pattern-recognition receptors or PRRs on a variety of defense cells of the body causing them to synthesize and secrete a variety of proteins called cytokines. These cytokines can, in turn promote innate immune defenses such as inflammation , phagocytosis, activation of the complement pathways , and activation of the coagulation pathway .

Inflammation is the first response to infection and injury and is critical to body defense. Basically, the inflammatory response is an attempt by the body to restore and maintain homeostasis after injury. Most of the body defense elements are located in the blood, and inflammation is the means by which body defense cells and body defense chemicals leave the blood and enter the tissue around an injured or infected site.

Body defense cells called macrophages , and dendritic cells have pattern recognition receptors such as toll-like receptors on their surface that are specific for the peptidoglycan fragments and mycolic acids in the acid-fast cell wall and/or to NODs in their cytoplasm that are specific for peptidoglycan fragments. The binding of these cell wall components to their corresponding pattern recognition receptors triggers the macrophages to release various defense regulatory chemicals called cytokines, including IL-1 and TNF-alpha. The cytokines then bind to cytokine receptors on target cells and initiate inflammation and activate both the complement pathways and the coagulation pathway.

Innate immunity will be discussed in greater detail in Unit 5.

Initiation of Adaptive Immunity

Proteins and polysaccharides associated with the acid-fast cell wall function as antigens and initiate adaptive immunity. An antigen is defined as a molecular shape that reacts with antibody molecules and with antigen receptors on lymphocytes. We recognize those molecular shapes as foreign or different from our body's molecular shapes because they fit specific antigen receptors on our B-lymphocytes and T-lymphocytes, the cells that carry out adaptive immunity.

The actual portions or fragments of an antigen that react with antibodies and with receptors on B-lymphocytes and T-lymphocytes are called epitopes . An epitope is typically a group of 5-15 amino acids with a unique shape that makes up a portion of a protein antigen, or 3-4 sugar residues branching off of a polysaccharide antigen. A single microorganism has many hundreds of different shaped epitopes that our lymphocytes can recognize as foreign and mount an adaptive immune response against.

The body recognizes an antigen as foreign when epitopes of that antigen bind to B-lymphocytes and T-lymphocytes by means of epitope-specific receptor molecules having a shape complementary to that of the epitope. The epitope receptor on the surface of a B-lymphocyte is called a B-cell receptor and is actually an antibody molecule. The receptor on a T-lymphocyte is called a T-cell receptor (TCR).

There are two major branches of the adaptive immune responses: humoral immunity and cell-mediated immunity.

  1. Humoral immunity: Humoral immunity involves the production of antibody molecules in response to an antigen and is mediated by B-lymphocytes. Through a variety of mechanisms, these antibodies are able to remove or neutralize microorganisms and their toxins after binding to their epitopes.
  2. Cell-mediated immunity: Cell-mediated immunity involves the production of cytotoxic T-lymphocytes, activated macrophages, activated NK cells, and cytokines in response to an antigen and is mediated by T-lymphocytes. These defense cells help to remove infected cells and cancer cells displaying foreign epitopes.

Adaptive immunity will be discussed in greater detail in Unit 6.


Primary function of cell wall is - Biology

Table 1 provides the components of prokaryotic and eukaryotic cells and their respective functions.

Table 1. Components of Prokaryotic and Eukaryotic Cells and Their Functions
Cell Component Function Present in Prokaryotes? Present in Animal Cells? Present in Plant Cells?
Plasma membrane Separates cell from external environment controls passage of organic molecules, ions, water, oxygen, and wastes into and out of the cell Yes Yes Yes
Cytoplasm Provides structure to cell site of many metabolic reactions medium in which organelles are found Yes Yes Yes
Nucleoid Location of DNA Yes No No
Nucleus Cell organelle that houses DNA and directs synthesis of ribosomes and proteins No Yes Yes
Ribosomes Protein synthesis Yes Yes Yes
Mitochondria ATP production/cellular respiration No Yes Yes
Peroxisomes Oxidizes and breaks down fatty acids and amino acids, and detoxifies poisons No Yes Yes
Vesicles and vacuoles Storage and transport digestive function in plant cells No Yes Yes
Centrosome Unspecified role in cell division in animal cells source of microtubules in animal cells No Yes No
Lysosomes Digestion of macromolecules recycling of worn-out organelles No Yes No
Cell wall Protection, structural support and maintenance of cell shape Yes, primarily peptidoglycan in bacteria but not Archaea No Yes, primarily cellulose
Chloroplasts Photosynthesis No No Yes
Endoplasmic reticulum Modifies proteins and synthesizes lipids No Yes Yes
Golgi apparatus Modifies, sorts, tags, packages, and distributes lipids and proteins No Yes Yes
Cytoskeleton Maintains cell’s shape, secures organelles in specific positions, allows cytoplasm and vesicles to move within the cell, and enables unicellular organisms to move independently Yes Yes Yes
Flagella Cellular locomotion Some Some No, except for some plant sperm.
Cilia Cellular locomotion, movement of particles along extracellular surface of plasma membrane, and filtration No Some No

Like a prokaryotic cell, a eukaryotic cell has a plasma membrane, cytoplasm, and ribosomes, but a eukaryotic cell is typically larger than a prokaryotic cell, has a true nucleus (meaning its DNA is surrounded by a membrane), and has other membrane-bound organelles that allow for compartmentalization of functions. The plasma membrane is a phospholipid bilayer embedded with proteins. The nucleolus within the nucleus is the site for ribosome assembly. Ribosomes are found in the cytoplasm or are attached to the cytoplasmic side of the plasma membrane or endoplasmic reticulum. They perform protein synthesis. Mitochondria perform cellular respiration and produce ATP. Peroxisomes break down fatty acids, amino acids, and some toxins. Vesicles and vacuoles are storage and transport compartments. In plant cells, vacuoles also help break down macromolecules.

Animal cells also have a centrosome and lysosomes. The centrosome has two bodies, the centrioles, with an unknown role in cell division. Lysosomes are the digestive organelles of animal cells.

Plant cells have a cell wall, chloroplasts, and a central vacuole. The plant cell wall, whose primary component is cellulose, protects the cell, provides structural support, and gives shape to the cell. Photosynthesis takes place in chloroplasts. The central vacuole expands, enlarging the cell without the need to produce more cytoplasm.

The endomembrane system includes the nuclear envelope, the endoplasmic reticulum, Golgi apparatus, lysosomes, vesicles, as well as the plasma membrane. These cellular components work together to modify, package, tag, and transport membrane lipids and proteins.

The cytoskeleton has three different types of protein elements. Microfilaments provide rigidity and shape to the cell, and facilitate cellular movements. Intermediate filaments bear tension and anchor the nucleus and other organelles in place. Microtubules help the cell resist compression, serve as tracks for motor proteins that move vesicles through the cell, and pull replicated chromosomes to opposite ends of a dividing cell. They are also the structural elements of centrioles, flagella, and cilia.

Animal cells communicate through their extracellular matrices and are connected to each other by tight junctions, desmosomes, and gap junctions. Plant cells are connected and communicate with each other by plasmodesmata.

Practice Question

In the context of cell biology, what do we mean by form follows function? What are at least two examples of this concept?


Primary function of cell wall is - Biology


Click on a link to obtain information on:

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 wall composition and architecture

Primary walls isolated form higher plant tissues and cells are composed predominantly of polysaccharides together with lesser amounts of structural glycoproteins (hydroxyproline-rich extensins) , phenolic esters (ferulic and coumaric acids), ionically and covalently bound minerals (e.g. calcium and boron), and enzymes. In addition walls contain proteins (expansins) that are believed to have a role in regulating wall expansion. Lignin, a macromolecule composed of highly cross-linked phenolic molecules, is a major component of secondary walls .

The major polysaccharides in the primary wall are :

Cellulose - a polysaccharide composed of 1,4-linked β-D-glucose residues

Hemicellulose - branched polysaccharides that are structurally homolgous to cellulose because they have a backbone composed of 1,4-linked β-D-hexosyl residues. The predominant hemicellulose in many primary walls is xyloglucan. Other hemicelluloses found in primary and secondary walls include glucuronoxylan, arabinoxylan, glucomannan, and galactomannan.

Pectin - a family of complex polysaccharides that all contain 1,4-linked α-D-galacturonic acid. To date three classes of pectic polysaccharides have been characterized: Homogalacturonans, rhamnogalacturonans, and substituted galacturonans .

The organization and interactions of wall components is not known with certainty and there is still considerable debate about how wall organization is modified to allow cells to expand and grow. Several models have been proposed to account for the mechanical properties of the wall:

The covalently cross-linked model

Peter Albersheim and colleagues in 1973 proposed that the wall matrix polymers (xyloglucan, pectin, and glycoprotein) are covalently linked to one another. The binding of xyloglucan to cellulose microfibrils results in a non-covalently cross-linked cellulose-hemicellulose network that gives the wall tensile strength. This model has been questioned because of the lack of evidence for the existence of covalent linkages between xyloglucan, pectin and glycoprotein.

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.

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

The primary structures of wall polysaccharides

Pectic polysaccharides (RG-I, RG-II, and oligogalacturonides) are released by treating walls with endopolygalacturonase. High molecular weight pectic polysaccharides are solubilized with aqueous buffers and chelators such as CDTA.

Xyloglucan oligosaccharides are solubilized by treating walls with endoglucanse. High molecular weight hemicelluloses (xylans, glucuronoxylans, arabinoxylans, glucomannans, and xyloglucan) are solubilized by treating walls with 1 and 4M KOH

The solubilized polysaccharides are then purified by size-exclusion and anion-exhchange chromatographies.

The primary sequence of a polysaccharide is known when the following have been determined :

The conformation and interaction of wall polysaccharides

These studies involve extensive use of 2D NMR spectroscopy and computer-aided molecular modeling techniques. We are also growing plants in a 13 C-enriched environment to generate isotopically labeled polysaccharides that can be characterized using 13 C NMR spectroscopy. Further information is available by clicking on Xyloglucan .

The biosynthesis of pectic polysaccharides

We have developed methods for the purification and assay of enzymes involved in pectin biosynthesis, and for enzymes involved in the formation of various nucleotide sugars. In addition we are using molecular techniques to clone and express numerous enzymes involved in cell wall biosynthesis. Further information can be obtained by clicking on Biosynthesis

Keegstra et al (1973) The structure of plant cell walls. III. A model of the walls of suspension-cultured sycamore cells based on the interconnections of the macromolecular components. Plant Physiol ., 51 , 188-196.

Albersheim (1976) The primary cell wall. In Plant Biochemistry (Bonner and Varner eds) 3rd Edition, Academic Press, New York, pp 225-274.

Selvendran and ONeill (1985) Isolation and analysis of cell walls from plant material. In Methods of Biochemica l Analysis (Glick ed) Vol 32, John Wiley pp 25-133.

Carpita and Gibeaut (1993) Structural models of primary-cell walls in flowering plants - consistency of molecular structures with the physical properties of the wall during growth. Plant J ., 3 , 1-30.

Cosgrove (2001) Wall structure and wall loosening. A look backwards and forwards. Plant Physio l., 125 , 131-134.

Ebringerova (2006) Structural diversity and application potential of hemicelluloses. Macromol Symp 232, 1-12.

Ragauskus et al (2006) The path forward for biofuels and biomaterials. Science 311, 484-489.


Conclusions

To summarize, seedlings with T-DNA insertions in 17 of the 23 candidate genes that were selected in this study seemed to exhibit FTIR phenotypes. Gene expression analysis showed that WSR gene expression is modulated in response to ISX-induced CWD, with the modulation apparently sensitive to changes in THE1 activity. This connected the genes identified to the THE1-dependent CWI maintenance mechanism, suggesting that our approach has identified new components mediating CWI maintenance in Arabidopsis. Follow up studies with KO or KD lines for four candidate genes found cell wall phenotypes in adult plants for all four and effects on CWD responses for WSR1 and 4. These results also suggest strongly that a more detailed analysis of the remaining candidate genes identified, will probably yield interesting novel insights into the mode of action of the CWI maintenance mechanism and cell wall metabolism in general.


In complex organisms, tissues grow by simple multiplication of cells. This takes place through the process of mitosis in which the parent cell breaks down to form two daughter cells identical to it. Mitosis is also the process through which simpler organisms reproduce and give rise to new organisms.

Cells import nutrients to use in the various chemical processes that go on inside them. These processes produce waste which a cell needs to get rid of. Small molecules such as oxygen, carbon dioxide and ethanol get across the cell membrane through the process of simple diffusion. This is regulated with a concentration gradient across the cell membrane. This is known as passive transport. However, larger molecules, such as proteins and polysaccharides, go in and out of a cell through the process of active transport in which the cell uses vesicles to excrete or absorb larger molecules.


What are the main functions of the cell membrane?

The cell membrane is the outer covering of a cell and helps it maintain shape, as well as allows certain molecules to enter and leave the cell.

Explanation:

The cell membrane is made out of two layers of phospholipids, a type of lipid with a head and two tails. These molecules' structure allows the membrane to be semi-permeable, meaning only certain molecules can cross the membrane. This is important as cells need to quickly gets things like oxygen and water, and get rid of wastes like carbon dioxide.

This image shows more of the membrane's structure:

You can see that there are also protein channels that allow materials to go in and out of the cell. These can transport molecules that cannot normally cross the phospholipid bilayer.

The membrane separates the cell from its surrounding environment.

Explanation:

Molecules of the cell membrane are arranged in a sheet.

This membrane is called the fluid mosaic model as it is a mixture of phospholipids, cholesterol, proteins and carbohydrates.

Most of the membrane is composed of phospholipid molecules. These allow the membrane to be rather fluid.

Embedded in this membrane are proteins which give some structure to the membrane.

The 3rd components are proteins or glycolipids.

The membrane can seal itself if pierced by something very thin like a pin. But it will burst if it takes in too much water.

The proteins sort of float on the surface of the membrane like islands in the sea.

Cholesterol is also found in the membrane. It prevents lower temperatures from inhibiting the fluidity of the membrane and prevents higher temperatures from increasing fluidity.

The carbohydrates that are in plasma membranes are bound either to proteins or to the lipids. They form sites on the surface that allow the cells to recognize each other.
This is important because it all tells the immune system to determine whether a cell is foreign (non-self) or are body cells (self).