Cell Wall Function, Structure and Diagram

The cell wall is a structural layer of a cell that is outside the cell membrane and has the primary function of providing the cell with rigidity, support, and protection from mechanical stress. The cell wall function as a filtering mechanism and this semi-rigid thick layer known as the cell wall function in defining the cell’s shape.

Also, when water enters the cell, the function of the cell wall is to act as a pressure vessel in order to prevent over-expansion of the cell.

All organisms are made up of single or multiple cells which are responsible for crucial functions such as reproduction, excretion, and metabolism. A cell is the basic biological unit of all organisms. These cells are composed of various structures called organelles which are embedded in the cytoplasm of the cell.

The content of a cell has to be separated from the external environment, therefore, in some organisms, the cell is surrounded by a cell wall and cell membrane.

However, not all organisms have cells with cell walls. The cell wall is present in organisms like plants, protists (algae and molds), and some bacteria and is absent in the animal cell as well as cells of heterotrophic protists. T

he cell membrane surrounds the cell and separates it from the external environment while the cell wall is another layer that surrounds the cell and is next to the cell membrane. This cell wall can be flexible, tough, and sometimes rigid.

Generally, cell walls are found in most prokaryotic cells (such as algal cells and fungal cells) and eukaryotic cells such as plant cells.

The cell wall of bacteria is made up of peptidoglycan whereas, the algal cell wall is made up of glycoproteins and polysaccharides like agar and carrageenan that are not present in the plant cell walls of land plants.

The cell wall of fungi is made up of N-acetylglucosamine polymer chitin. In the case of archaea, the archaeal cell wall is made up of various compositions and may be made of polysaccharides, glycoprotein S-layers, and pseudopeptidoglycan.

It permits multicellular organisms to build and hold a definite form or shape and in plants, the mechanical properties, and chemical composition of the wall are associated with the cell growth and morphogenesis of plants.

It also acts as a filtering mechanism as it limits toxic large molecules from gaining entrance into the cell. Also, by retaining water and preventing osmotic lysis, the wall permits the creation of stable osmotic environments. However, the properties, composition, and form of the cell wall depend on growth conditions and may change during the cell cycle.

Cell wall Structure

• All cell walls have two structural layers which are the primary cell wall and the middle lamella. However, some cells have an additional layer known as the secondary wall.
• The structure of the cell wall is flexible and rigid.
• Permeability to small molecules is a basic characteristic of the cell wall structure.
• Organisms differ in the chemical composition and structural organization of their cell wall.
• All cell wall structure is basically made up of components such as cellulose fibers which are embedded in a water-saturated matrix of polysaccharides and structural glycoproteins.

Structural layers of the cell wall

All cell walls are made up of basically two layers which are the primary wall and the middle lamella. However many cells have an additional layer known as the secondary wall.

Primary walls are the layers that contain cellulose which is laid down by cells that are growing and dividing. The primary wall of growing cells is usually thinner and less rigid than those of cells that have stopped growing. They are thinner and less rigid during growth to enable cell wall expansion. Between the primary walls of adjacent cells, the middle lamella act as a cementing layer.

A plant cell that is fully grown may retain its primary wall or may produce a secondary wall as an additional rigidifying layer of various compositions. This secondary wall is usually responsible for the mechanical support of most plants and the mechanical properties of wood.

The thick secondary wall has a permanent stiffness as well as load-bearing capacity. Whereas, the primary wall is thin and is only capable of exerting a structural supportive role when the vacuoles in the cell are filled with water such that turgor pressure is exerted against the wall. Therefore, when there is a loss of turgor pressure as a result of water loss from the plant cells, the flowers, and leaves of the plant wilts.

Rigidity and flexibility

The wall of most cells are flexible and thus bend instead of holding a fixed shape. However, despite its flexibility, it has tensile strength. Cell walls enable the apparent rigidity of primary plant tissues.

The rigidity of the primary plant tissues is therefore not a result of the wall stiffness because it’s the cell wall structure and the hydraulic turgor pressure that creates the rigidity of the plant tissue. Therefore, the apparent rigidity of the wall is due to the inflation (a result of the passive uptake of water) of the cell that is contained within.

The cell wall flexibility can be observed when plants wilt in order for their leaves and stem to droop. Also, it can be seen in seaweeds that bend in water currents. A prokaryotic cell and eukaryotic cell with a cell wall obtain strength from a flexible cell membrane that presses against a rigid wall. In plants, there is a secondary layer which is a thicker additional layer of cellulose.

This additional layer increases the wall rigidity and may be formed by lignin or suberin. In the xylem wall, lignin forms the secondary wall whereas, in cork walls, the suberin forms the secondary wall. Lignin and suberin are rigid compounds that are waterproof which make the secondary wall stiff. Therefore, wood and the bark cells of trees possess secondary walls. Also, other plant parts such as the leaf stalk may have secondary walls in order to resist the strain of physical forces.

Permeability

The cell wall structure is said to be permeable. One important factor that governs the transportation of molecules through the wall is the pH of the cell. Small molecules including small proteins are transported through the primary wall. However, the primary wall is not freely permeable to molecule sizes estimated to be 30-60 kDa.

Components

• Matrix polysaccharides
• Cellulose
• Proteins
• Plastics

The primary and secondary wall has a basic architecture that is the same even though they are different in their structural and chemical composition. They both consist of cellulose fibers of tensile strength that are embedded in structural glycoproteins and a water-saturated matrix of polysaccharides.

Matrix polysaccharides

The stretches of pure galacturonic acid residues are tightly cross-linked by calcium ions in cell walls. As they do so, they leave the rhamnose-containing segments in a more open and porous configuration. The semi-rigid gel characteristics of the cell wall matrix are created by this cross-linking which is a process used in the preparation of jellied preserves.

There are two main types of cell wall matrix polysaccharides. There are the hemicellulose and the pectins or pectin polysaccharides. These two types of matrix polysaccharides are both synthesized in the Golgi apparatus. They are sent in small vesicles to the surface of the cell and are then secreted into the cell wall.

Types of matrix polysaccharides

• Hemicelluloses
• Pectic polysaccharides

Hemicelluloses

This type of matrix polysaccharides is made up of glucose molecules that are arranged end to end as in cellulose. The glucose molecules are arranged with other uncharged sugar and short side chains of xylose attached to one side of the ribbon. Whereas, the other side of the ribbon is tightly attached to the surface of cellulose fibrils. Hence, the microfibrils are covered with hemicellulose and thereby hindered from sticking together in an uncontrolled manner. Its been shown that during growth, the hemicellulose molecules regulate the expansion rate of primary walls.

Pectic polysaccharides

This type of matrix polysaccharide (pectic) is different from hemicellulose in important respects. These polysaccharides are heterogeneous, branched, and highly hydrated. As a result of galacturonic acid residues, the pectic are negatively charged. The galacturonic acid and rhamnose sugar molecules form the linear backbone of all pectic polysaccharides.

This linear backbone of the pectic comprises pure galacturonic acid residues that are interrupted by segments. The galacturonic acid and rhamnose residues alternate in these segments and complex branched sugar side chains are attached to these latter segments. Since pectic polysaccharides are negatively charged, they tightly bind to positively charged ions (cations).

Cellulose

There are several thousand molecules of glucose that are linked end to end to form cellulose. Each cellulose molecule has a flat ribbon-like structure as a result of the chemical link between the individual glucose subunits. This flat ribbonlike structure permits adjacent molecules to bind together into microfibrils. The microfibrils range in length is about 2-7 micrometers.

There are enzymes that float in the cell membrane that synthesize the cellulose fibrils. The cellulose fibrils are then arranged in a rosette configuration where each rosette seems capable of spinning a microfibril into the wall.

During this spinning process, there are new glucose subunits that are added to the growing end of the fibril. As that happens, the rosette is pushed around the cell on the cell membrane surface. Then, around the protoplast, its cellulose fibril becomes wrapped. Therefore, each plant cell can make its own cellulose fibril cocoon.

Proteins

There are a small number of proteins contained in the wall. However, they still serve a number of crucial functions. The most well-known group is the hydroxyproline-rich glycoproteins. They are shaped like rods with connector sites and a typical example is an extensin.

Extensin has about 45% hydroxyproline and 14% serine residues that are distributed along its length. Most of the serine residues carry galactose sugar whereas every hydroxyproline carries a short side chain of arabinose sugars.

This produces long molecules that resemble bottle brushes. These long molecules towards the end of the primary wall formation, are secreted into the wall, and at the time that cell growth stops, they become cross-linked into a mesh. By regulating the time at which the cross-linking of extensin molecules occur, plants are able to control their ultimate size.

The cell wall contains a variety of enzymes in addition to the structural proteins. Enzymes that cross-link extensin, cutin, and lignin are the most notable ones while the other enzymes help to protect the plants from fungal pathogens.

They do this by breaking fragments off of the fungal wall. These fragments, sequentially induce a defense response in underlying cells. Moreso, cell wall-degrading enzymes are responsible for the softening of ripe fruit and the dropping of leaves in the autumn.

Plastics

The cell wall plastics are lignin, suberin, and cutin. They are made of a variety of organic compounds that are cross-linked into tight three-dimensional networks. These three-dimensional networks strengthen the wall and make it more resistant to attacks from bacteria and fungi.

Types of plastics

• Lignin
• Cutin
• Suberin

Lignin

The general name lignin is given to the diverse group of polymers of aromatic alcohols. Lignin accounts for up to 30% of the dry weight of plants and is deposited mostly in secondary walls. Hence, providing the rigidity of land vascular plants. Therefore, the diversity of the cross-links between the polymers and the tightness it causes makes lignin a powerful barrier for most microbes to be able to penetrate.

Cutin

Cutin is a complex biopolyester that is made up of fatty acids and aromatic compounds. It is the major component of the cuticle. The cuticle is the waxy water-repelling surface layer of the wall. It is normally exposed to the environment aboveground. Cutin plays a crucial role in the defense mechanism of plants. It reduces the wettability of stems and leaves and by doing so affects the ability of fungal spores to germinate.

Suberin

Suberin is also a complex biopolyester that is made up of aromatic compounds and fatty acids. Together with waxes, suberin act as a surface barrier of underground parts. Cells close to wounds stimulate the synthesis of suberin which seals off the wound surfaces and protects underlying cells from dehydration.

Cell Wall Diagram

The Function of Cell wall

1. The cell wall maintains and gives the cell a definite shape.
2. It gives the cell mechanical strength and rigidity.
3. The cell wall helps to control the expansion of cells during water intake.
4. It helps to prevent cellular water loss.
5. It controls and regulates the intercellular transportation of substances.
6. The cell wall function as a barrier between the interior component of the cell and the external environment.
7. It protects the cell against infective organisms.
8. Cell walls have a signaling device against attack from microbes.
9. The cell wall function as a food reservoir.

It maintains and gives the cell a definite shape

Cell walls are responsible for the various shapes of some cells. Maintaining the cell shape is therefore one function of the cell wall. Isolated protoplasts whose boundary is the cell membrane are more or less spherical.

The cell membrane is the most common site for the synthesis of cellulose. Cellulose is one of the cell walls components and an additional component of the wall may include lignin or pectin. These wall components surround the cell membrane and thereby the shape of the cells is determined by the cell wall.

Cell walls give the cell mechanical strength and rigidity

The functions of the cell wall are to give mechanical strength and rigidity to the cell. For instance, the non-lignified wall gets strength from the cellulose microfibrils. The mechanical strength of the walls of plants is a result of the skeletal framework created by cellulosic microfibrils.

The most strength is, however, gotten from these tissues- sclerenchyma and collenchyma. Collenchyma cells are living and their walls are thickened with components such as cellulose, protein, pectin, and hemicellulose.

These cells retain their protoplast even when mature and according to the need of developing organs can regulate the orientation and deposition of cell wall materials.

In woods, lignification further strengthens the cellulosic walls. Therefore, the wall is responsible for the large size and structural strength of a woody plant. In these plants, the cell wall components contribute 95% of the dry weight of wood.

Hence, the rigidity and strength of the whole plant are a result of the cell wall. Cellulose is absent in some species and in such cases, the strengthening role is assumed by other polysaccharides and may form microfibrils.

It helps to control the expansion of cells during water intake

Cell walls function in the control of cell expansion. The elastic (reversible) and plastic (irreversible) nature of the wall is a major factor in the growth of plant cells. When water moves into the cell, it causes the protoplast to press against the wall. Thereby extending the cell wall.

The turgor pressure in the turgid cells presses the wall to expand. However, the cell wall at flaccid condition regains its original position due to its elastic nature.

The wall expands in a plastic manner when the critical turgor pressure exceeds in a growing cell. Hence, leading to a permanent increase in the growth and volume of the cell.

The turgor pressure is the driving force behind the elastic stretching of the cell wall. This pressure exerted by neighboring cells in a multicellular plant has much control in the extension of the cell.

Furthermore, when lignin is laid down over the cell wall’s whole area, the cell may stop elongating. However, there are areas where lignin is deposited to a part of the wall only, like in some phloem fibers and primary tracheids. In such cases, growth and elongation progress in the non-lignified areas.

It controls and regulates the intercellular transportation of substances

Functions of the cell wall include the control and regulation of intercellular transportation of substances.  The transport of substances by cell walls between plant cells occurs in two ways:

1. Apoplastic transport
2. Symplastic transport

Substances are moved in apoplastic transport between neighboring cells through the matrix of the cell wall and across the cell membrane. Whereas, substances are moved in a symplastic transport through the plasmodesmata present between cells. These plasmodesmata involved in symplastic movement exist in pit pairs as well as in the sieve pores of sieve tubes.

Moreso, the substance size in apoplastic transport is very significant. There is a sieve-like structure formed by the microfibrils and matrix polymer of the wall. This structure hinders the entry of large molecules and microbes.

It is through the aqueous channel present in the matrix that movement of smaller molecules, small proteins, ions, and polysaccharides occur. However, apoplastic movement is resisted by lignified walls, the cuticle present on the epidermis, and the Casparian strip of endodermis.

The movement of substances is affected by the charges of molecules. The molecules that are negatively charged, neutral or uncharged move freely.

Whereas, the movement of molecules that are positively charged is retarded because they tend to bind with the negatively charged polymers in the cell wall. The primary wall, therefore, provides a means of easy transport.

It functions as a barrier between the interior component of the cell and the external environment

All organisms are made up of cells which are the basic biological unit of all organisms. An organism may be a single cell or be made up of multiple cells. These cells are composed of organelles that are embedded in the cytoplasm of the cell. The content of a cell has to be separated from the external environment, therefore, in some organisms, the cell is surrounded by a cell wall and cell membrane.

However, not all organisms have cells with cell walls. The cell wall is present in organisms like plants, protists (algae and molds), and some bacteria and is absent in the animal cell as well as cells of heterotrophic protists.

The cell membrane surrounds the cell and separates it from the external environment while the wall is another layer that surrounds the cell and is next to the cell membrane. Therefore, one major function of the cell wall is to create a barrier between the external environment and the organelles or components of the cell

It protects the cell against infective organisms

One crucial function of the cell wall is to protect the cell against infective organisms. Pathogenic microbes such as bacteria, fungi, and viruses tend to attack the epidermal walls of higher plants.

These microbes penetrate the host plant either through the stomata opening, an opportunistic entry via wounds or breaks in the wall, or by the enzymatic dissolution of a part of the wall. Due to such attacks, the cell wall has passive and active defense mechanisms to protect the cell against attack.

The wall act as an effective and passive physical barrier such that the pores on the cell wall are too small for tiny microorganisms to enter. In the epidermal wall, there are cuticle, cutin, lignin, and silica (in grasses) that serve as an effective barrier against fungi. Fungi have to secrete cutinase and pectinase in order to penetrate the middle lamella and cutin respectively. Silica forms a mechanical barrier in grasses and is toxic to fungi.

The intact cell walls also carry out active defense mechanisms where it creates an effective barrier by depositing lignin and sometimes suberin around the site of parasitic infection. The suberin and lignin deposited thereby seals the path of the pathogen’s penetration. Also, the death of cells around the site of infection prevents further penetration.

Furthermore, an effective barrier is provided when there is a dynamic deposition of callose, a cell wall component at the site of penetration of fungal haustorium adjacent to the cell membrane. Studies show that as a result of the pathogenic attack, the wall polysaccharides degrade to oligosaccharides. From injured tissues, these oligosaccharides diffuse out to the neighboring cells and stimulate the active defensive mechanisms.

Cell walls have a signaling device against attacks from microbes

The cell wall has a signaling device against the attack from microbes. It can react in response to attack from pathogens. In order to prevent the entry and establishment of a pathogen, the wall triggers active defense mechanisms.

The production of phytoalexin by higher plants is the most common example. Higher plants when stimulated by the attack of a pathogen produce phytoalexins which are nonspecific toxins. The chemical structure of this toxin varies from plant genera and takes an active part in the plant’s defense mechanism.

In response to the signal send forth from injured tissues, a cell apart from producing phytoalexin can produce inhibitors of microbial proteins. Proteinases that originate from microbes degrade the protein of the host while the inhibitors prevent the protein from degradation.

It is said that oligosaccharides and pectic polysaccharides are the signals for the production of phytoalexin and proteinase inhibitors.

Studies show that as a result of the pathogenic attack, the cell wall polysaccharides degrade to oligosaccharides. From injured tissues, these oligosaccharides diffuse out to the neighboring cells and stimulate the active defensive mechanisms.

Also, oligosaccharides have a role in the extension and differentiation of the cell. Furthermore, the wall has active and passive mechanisms against larger predators.

The active defense mechanism in plants as earlier discussed is triggered when insects attack and the wall is stimulated. In response, the plant thus generates the same inhibitors of microbial proteinase. The passive defense mechanism involves the presence of lignin on the wall which makes the tissues of the plant tough and unpalatable to higher animals. Also, the epidermal cells have silicon which decreases the palatability as well.

It functions as a food reservoir

Cell wall functions as a food reservoir. There is usually a substantial amount of food in a seed in order to provide nutrition to the young seedling. The seedling depends on the food in the seed until it is able to photosynthesize independently.

This food may be stored within the cell or may remain stored as a cell wall component. The following polysaccharides of the wall which include xyloglucans, mannans, and galactans are examples of food stored as cell wall components.

Mannans, for instance, are cell wall storage polysaccharides. The endosperm of Apiaceae and Ericaceae has approximately 90% mannose. Glucose and mannose which form glucomannans are present in the endosperm of Iridaceae and Liliaceae. Galactose and mannose forming galactomannans are present in the endosperm of Fabaceae.

In species where the food is stored in cotyledons, the xyloglucans are found there. Whereas, galactans are found in the seed of Lupinus as cell wall food reserve.

In the seeds of cereal grasses and other species, the endosperm cell walls are rich in glucans and other polysaccharides.

During seed germination, enzymes easily digest these glucans and polysaccharides to produce simple sugars that nourish the growing embryo.  However, aside from serving as a food reservoir the cell wall function in transpiration, translocation, secretion, absorption, etc.

Types of cell wall

• Plant cell wall
• Algal cell wall
• Fungal cell wall
• Bacterial cell wall
• Archaeal cell wall

Cell walls are found in all kingdoms except the Animalia (animal) kingdom. Hence, the various types of cell walls include the cell wall that is present in plants, fungi, protists, bacteria, and archaea.

Plant cell wall

Cell walls are found in plants and the plant cell walls are made of cellulose (gotten from glucose), lignin, hemicellulose, and pectin. The cell wall of plants consists of a primary membrane and an optional secondary membrane.

This primary wall of plants is made of pectin whereas the secondary wall is made of lignin which is a very large and complex organic molecule. Lignin is the basic component that makes the wood of trees and aids the cells involved in the xylem (water transport system) waterproof and strong.

Moreso, structural proteins which are in about 1-5% are found in a majority of plant walls. These structural proteins are classified as glycine-rich proteins (GRPs), hydroxyproline-rich glycoproteins (HRGP), proline-rich proteins (PRPs), and arabinogalactan proteins (AGP). These structural proteins are usually concentrated in cell corners and in specialized cells.

Also, the walls of the epidermis may possess cutin and there is also suberin in the Casparian strip of the endodermis cork and roots cells of the plant bark. This suberin and cutin are polyesters that serve as permeable barriers to the movement of water.

However, the composition of proteins, carbohydrates, and secondary compounds varies among plants as well as the age and type of cell. Additionally, there are numerous enzymes in the plant cell wall that cut, trim, and cross-link wall polymers. These enzymes include esterases, hydrolases, transglycosylases, and peroxidases

Furthermore, the inside components of the plant cell take up water and press against the rigid wall. This is called the turgor pressure and it is the secret to the plant’s stability. However, the wall of plants is flexible enough to allow the plant to retain plasticity and grow. The thickness of the plant wall is about 0.1µm to several µm. It has to possess sufficient tensile strength to be able to withstand internal osmotic pressure.

Layers

In multicellular plants, the different layers and placement of the wall are due to the protoplasm. There are up to three strata or layers found in the cell walls of plants. They include:

1. The primary wall
2. Secondary wall
3. Middle lamella

The primary wall

This layer of the cell wall is usually thin flexible and extensive. The primary wall is formed as the cell grows.

Composition

Pectin, hemicellulose, and cellulose are the major carbohydrate in the primary wall of the plant. Through the hemicellulosic tethers, the cellulose microfibrils are linked in order to form the cellulose-hemicellulose network.

This cellulose-hemicellulose network is embedded in the pectin matrix. Xyloglucan tends to be the most common hemicellulose found in the primary cell wall. Whereas, in the walls of grass, pectin and xyloglucan are reduced in abundance and are partially replaced by another type of hemicellulose called the glucuronarabinoxylan.

The primary wall through a mechanism called acid growth is able to extend and grow. Acid growth is mediated by expansins which are extracellular proteins that are activated by conditions that are acidic. These expansins modify the hydrogen bonds between cellulose and pectin.

This whole mechanism helps to increase the extensibility of the cell wall. Cutin and wax are found in the outer part of the plant’s primary wall which forms a permeable barrier called the plant cuticle.

Secondary wall

This layer is thick and is formed inside the primary wall after the cell is grown fully. However, the secondary wall is not found in all types of cells. Some cells like the conducting cells in the xylem have a secondary wall that is made of lignin. This lignin waterproofs and strengthens the wall.

Composition

The secondary wall is made of lignin which is a very large and complex organic molecule. Also, secondary walls have a broad range of additional compounds. These compounds modify their permeability and mechanical properties. The main polymers that make up the secondary wall (wood) include:

• A type of hemicellulose called xylan which is about 20%-35%
• Cellulose which is about 35%-50%
• Lignin is a complex phenolic polymer and is about 10%-25%. Lignin enters the spaces in the cell wall between hemicellulose, cellulose, and pectin components in order to drive out water and strengthen the wall.
• In grasses, the secondary wall may have microscopic silica crystals which may protect the grass from herbivorous animals by strengthening the wall.

Middle lamella

Plants walls are made of the middle lamella layer which is rich in pectins and is the outermost layer that binds and forms the interface between adjacent plant cells.

The function of the cell wall in plants

• The function of the cell wall in plant tissues is to serve as the storage for carbohydrates. The carbohydrates stored are broken down and reabsorbed to supply the growth and metabolic needs of the plant.
• In grasses, the secondary wall may have microscopic silica crystals which may protect the grass from herbivorous animals by strengthening the wall.

How is the plant cell wall formed?

The first layer that is laid down is the middle lamella which is formed during cytokinesis from the cell plate. Then, the primary wall is deposited inside the middle lamella. The primary wall appears as a composition of cellulose microfibrils that are aligned at all angles.

The middle lamella which is a gelatinous membrane that contains calcium pectate and magnesium holds cells together. Cells, therefore, share the middle lamella and interact through plasmodesmata. The plasmodesmata are interconnecting channels of cytoplasm which connects to the adjacent cells’ protoplasts across the cell wall.

A secondary wall in some plants and cell types is formed after a maximum stage in development has been reached. It is formed between the primary wall and the plasma membrane. The cellulose microfibrils of the secondary wall are aligned parallel in layers.

However, with each additional layer, the orientation changes slightly making the structure of the wall helicoidal. Cells that have the secondary wall can be rigid and cells interact with one another through pits in the secondary wall that permits the plasmodesmata to connect cells via the secondary walls. A typical example of cells that possess a secondary wall is the gritty sclereid cells in quince fruit and peer.

Algal cell wall

The cell wall is present in the algae. Its wall is similar to the plant primary wall and is made of cellulose and other polysaccharides. In addition to cellulose, the algal cell wall can have xylan or mannan. The additional polysaccharides included in the algal wall are used as a characteristic for the algal taxonomy.

The cell wall of some algae has certain components. A typical example is the brown algae that have alginic acid that absorb water and forms a tasteless gum. This gum is used in the cosmetic and food industries. Diatoms also use silica gotten from hydrated silicic acid to strengthen their wall.

Mannan forms microfibrils in the algal wall in a lot of marine green algae and some red algae. Examples of such marine green algae include the genera Codium, Dasycladus, and Acetabularia, while some of the red algae include Bangia and Porphyra.

Alginic acid, on the other hand, is a common polysaccharide that occurs in the brown algal wall. Most algae have sulfonated polysaccharides in their wall e.g carrageenan, funoran, furcelleran, agarose, and porphyran. Furthermore, calcium ions and sporopollenin are the other compounds that may occur in algal walls.

Fungal cell wall

Cell walls are found in fungi. The cell wall of fungi is made up of chitin which is a polysaccharide that is similar to cellulose. However, chitin contains nitrogen (acetyl-amin) groups rather than hydroxyl groups. One can find chitin in the exoskeletons of arthropods where it is mixed with sclerotin.

The majority of true fungi have cell walls that consist of a large amount of chitin and other polysaccharides. However, there is no cellulose in the walls of true fungi. The cell wall in fungi is the outer-most layer that is external to the cell membrane. It is a matrix of basically 3 components which include:

1. Chitin
2. Glucans
3. Proteins

Bacterial cell wall

Cell walls are found in bacteria and a majority of them except mycoplasma and L-form bacteria have cell walls. The cell wall of bacteria is made up of peptidoglycan (also called murein) which is derived from glycan polysaccharide chains that are connected by amino acids. This molecule forms a strong barrier around the cell and is found only in bacteria. Thus, making it an excellent target for the immune system and therapeutics.

The bacterial cell wall is the main feature that distinguishes gram-positive bacteria and gram-negative bacteria. During gram staining, the gram stain will stain the thick peptidoglycan-rich wall of the gram-positive bacteria purple whereas it will stain the gram-negative bacteria pink.

The cell wall of bacteria is different from the walls of fungal and plant walls. The cell wall is key for the survival of many bacteria. In order to kill bacteria, the antibiotic penicillin prevents the cross-linking of peptidoglycan which results in the weakening and lysis of the wall. Also, lysozyme enzymes can damage the cell wall of bacteria.

Gram staining has been a long-used test for the classification of bacterial species and based on the reaction of bacterial cells to gram staining, there are two types of bacterial cell walls. They include:

1. Gram-positive wall
2. Gram-negative wall

The gram-positive bacteria are classified so because of their thick cell wall that contains many layers of peptidoglycan and teichoic acid. Gram-negative bacteria, on the other hand, have a wall that is thin with a few layers of peptidoglycan that is surrounded by a second lipid membrane. This additional lipid membrane consists of lipoproteins and lipopolysaccharides.

Therefore, the majority of bacteria have a gram-negative wall. The only bacteria with the gram-positive wall are the Actinobacteria and Firmicutes. These structural difference among bacteria results in a difference in antibiotic susceptibility. For example, gram-positive bacteria can be killed by vancomycin which happens to be ineffective against gram-negative bacteria.

Archaeal cell wall

The cell wall is also present in archaea which are made up of pseudo-peptidoglycan. The archaeal wall is unusual and there are basically four types of wall among archaea.

Type-one archeal wall

One type of wall among archaeae is made up of pseudopeptidoglycan which is also known as pseudomurein. This type is seen in some methanogen e.g methanothermus and methanobacterium. The structure of the archaeal pseudopeptidoglycan superficially resembles the bacterial peptidoglycan. However, there are some chemical differences.

Pseudopeptidoglycan similar to the peptidoglycan in the wall of bacteria is made up of polymer chains of glycan that are cross-linked by short peptide connections.

However, it is quite different from the peptidoglycan in that N-acetyltalosaminuronic acid replaces the sugar N-acetylmuramic acid. The two sugar in this wall type is bonded with a β,1-3 glycosidic linkage. Moreso, L-amino acids are the cross-linking peptides instead of the D-amino acids in bacteria.

Type-two archaeal wall

This second type of wall is common in Halococcus and Methanosarcina. It is made up of a thick layer of polysaccharides. In the case of Halococcus, the layer of polysaccharides may be sulfated. However, this wall structure is quite complex and not fully investigated.

Type-three archaeal wall

This type of wall is common in some methanogens, hyperthermophiles, and halobacterium. It consists of glycoprotein and there are proteins in the walls of halobacterium that have a high content of acidic amino acids. Due to this, the cell wall has an overall negative charge. Therefore, halobacterium tends to thrive only under high salinity conditions.

Type-four archaeal wall

This wall type is common in other archaea like Desulfurococcus and Methanomicrobium. It is made up of surface-layer proteins called S-layers which are common in bacteria. In bacteria, these layers serve as either an outer layer together with polysaccharides or as the sole component of the cell wall.

Cell wall vs cell membrane

The cell wall and cell membrane are two crucial organelles in living organisms. However, the cell wall is found only in fungi, plants, some bacteria, algae, and archaea whereas the cell membrane is found in all living organisms.

Originally, the cell well is referred to as the layer of polysaccharides that occurs outside the cell membrane. The cell membrane, on the other hand, is the cell’s outermost layer that envelopes the other organelles in the cell. Cell walls are rigid and serve structural functions and support whereas the cell membrane is flexible and can be changed when needed.

An organism will have both a cell wall and a cell membrane or just a cell membrane. Therefore, there is a difference between a cell wall and a cell membrane. The table below summarizes a comparison of cell wall vs cell membrane.

Differences between the cell wall and cell membrane

Cell wall	Cell Membrane
It is the outermost layer that is next to the cell membrane.	It is a bilipid layer that surrounds the content and organelles of the cell.
The composition of cell walls varies according to the species. It is made up of cellulose in the plant cells, chitin in fungal cells, and peptidoglycan in bacteria.	It is basically made up of a lipid bilayer of lipoproteins and carbohydrates.
Cell walls are rigid and serve structural functions and support	The cell membrane is flexible and can be changed when needed.
It is a thick structure that can be seen with a light microscope	It is a thin delicate structure that can only be seen with an electron microscope
The thickness of the cell wall varies between 0.1 μm to several μm depending on the composition	The thickness of the cell membrane varies between 7.5–10 nm.
It determines the shape of the cell and gives it protection.	It protects the protoplasm and maintains the  constant internal environment
It doesn’t exist in all cells. It is therefore present in only plants, bacteria, fungi, protists, and archaea	It is present in all cells
Cell walls are nonliving and are inactive metabolically	Cell membranes are living and metabolically active
cell walls don’t have cell surface receptors	The cell membrane possesses cell surface receptors
Macromolecules can pass through cell walls because they are completely permeable	Only certain molecules can pass through the cell membrane because it is semi-permeable or selectively-permeable.

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