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CELL BIOLOGY

What is cell wall? Definition, composition and types

What is the definition of cell wall?

The cell wall is a rather rigid wall that surrounds the plant cells, located outside the cell membrane, provides the cell with structural support, protection against biotic factors (pathogens) and abiotic factors (stress mechanical, osmotic), and acts as a filtering mechanism.

The cell wall is found in plants, fungi, bacteria and algae. Each wall has a structure and a typical composition of the group. In contrast, one of the characteristics of animal cells is the lack of cell wall. Cell wall is mainly responsible for giving and maintaining the shape of the cells. It also hinders expansion as water enters the cell.

General characteristics

  • The cell wall is a thick, stable and dynamic barrier that is found in different groups of organisms.
  • The presence of this structure is vital for the viability of the cell, its shape and, in the case of harmful organisms, it participates in its pathogenicity.
  • Although the composition of the wall varies depending on each group, the main function is to maintain cellular integrity against osmotic forces that can burst the cell.
  • In the case of multicellular organisms, it helps in tissue formation and participates in cell communication.

Structure and composition

Plant cell wall

The cell walls of plant cells are composed of polysaccharides and glycoproteins, organized in a three-dimensional matrix.

The most important component is cellulose. It consists of repeated glucose units, linked together by β – 1,4 bonds. Each molecule contains about 500 glucose molecules.

The rest of the components include: homogalacturonan, ramnogalacturonan I and II and hemicellulose polysaccharides such as xyloglycans, glucomannans, xylanes, among others.

The wall also has components of a protein nature. Arabinogalactan is a protein that is found in the wall and is related to cell signaling.

Hemicellulose binds by means of hydrogen bonds to cellulose. These interactions are very stable.

The primary is thin and somewhat flexible. After cell growth stops, secondary wall deposition occurs, which can change its composition with respect to the primary or remain unchanged and only add extra layers.

In some cases, lignin is a component of the secondary wall. For example, trees have significant amounts of cellulose and lignin.

Synthesis

The biosynthesis process of the wall is complex. It involves approximately 2000 genes that participate in the construction of the structure.

Cellulose is synthesized in the plasma membrane to be deposited directly outside. Its formation requires several enzymatic complexes.

The rest of the components are synthesized in membranous systems located inside the cell (such as the Golgi apparatus) and excreted by means of vesicles.

Function

The cell wall in plants has similar functions to those performed by the extracellular matrix in animal cells, such as maintaining cell shape and structure, connecting tissues and cell signaling. 

Let’s  discuss the most important functions of plant cell wall………:

Regulate the turgidity

In animal cells – which lack a cell wall – the extracellular environment poses an important challenge as far as osmosis is concerned.

When the concentration of the medium is higher compared to the cellular interior, the water in the cell tends to flow out. Conversely, when the cell is exposed to a hypotonic environment (higher concentration within the cell) water enters and the cell can explode.

In the case of plant cells, solutes found in the extracellular environment are lower than within the cellular interior. However, the cell does not explode because the cell wall is pressed. This phenomenon causes the appearance of certain mechanical pressure or cellular turgor.

The turgor pressure created by the cell wall helps keep the tissues of plants rigid

Connections between cells

Plant cells are able to communicate with each other through a series of “channels” called plasmodesmata. These pathways allow connecting to the cytosol of both cells and exchanging materials and particles.

This system allows the exchange of metabolic products, proteins, nucleic acids and even viral particles.

Signaling pathways

In this intricate matrix there are molecules derived from pectin, such as oligogalacturonids, which have the ability to trigger signaling pathways as defense responses. In other words, they function as the immune system in plants.

Although the cell wall forms a barrier against pathogens, it is not totally impenetrable. Therefore, when the wall is weakened, these compounds are released and “warn” the plant of the attack.

In response, the release of reactive oxygen species occurs and metabolites, such as phytoalexins, which are antimicrobial substances are produced.

Cell wall in prokaryotes

a) In eubacteria

Structure and composition

The cell wall of the eubacteria has two fundamental structures, which are distinguished by the famous Gram stain.

The first group is made up of Gram negative bacteria. In this type the double membrane. The cell wall is thin and is surrounded on both sides by an inner and outer plasma membrane. The classic example of a Gram negative bacterium is E. coli.

On the other hand, Gram positive bacteria only have a plasma membrane and the cell wall is much thicker. These are usually rich inTeichoic acids and mycolic acids. An example is the pathogen Staphylococcus aureus.

The main component of both types of walls is peptidoglycan, also known as murein. The units or monomers that compose it are N-acetylglucosamine and N-acetylmuramic acid. It is composed of linear chains of polysaccharides and small peptides. Peptidoglycan forms strong and stable structures.

Some antibiotics, such as penicillin and vancomycin, act by preventing the formation of bacterial cell wall bonds. When a bacterium loses its cell wall, the resulting structure is known as spheroplast.

 b) In arches

Structure and composition

The archaea differ in the composition of the wall with respect to bacteria, primarily because they do not contain peptidoglycan. Some archaea have a layer of pseudopeptidoglycan or pseudomurein.

This polymer is 15-20 nm thick and is similar to peptidoglycan. The polymer components are N-Acetylglucosamine-linked N-acetyltalosaminuronic acid.

They contain a series of rare lipids, such as groups of isoprenes attached to glycerol and an additional layer of glycoproteins, called the S layer. This layer is often associated with the plasma membrane.

Lipids are different than in bacteria. In eukaryotes and bacteria, the links found are of the ester type, while in the archaea they are of the ether type. The glycerol skeleton is typical of this domain.

There are some species of archaea, such as Ferroplasma Acidophilum and Thermoplasma spp., Which do not have a cell wall, although they live in extreme environmental conditions.

Both eubacteria and archaea have a large layer of proteins, such as adhesins, which help these microorganisms to colonize different environments.

Synthesis

In Gram-negative bacteria the components of the wall are synthesized in the cytoplasm or in the inner membrane. The construction of the wall occurs outside the cell.

The formation of peptidoglycan begins in the cytoplasm, where synthesis occurs the precursor nucleotides of the wall components.

Subsequently, the synthesis continues in the cytoplasmic membrane, where the compounds of lipid nature are synthesized.

The synthesis process ends inside the cytoplasmic membrane, where polymerization of the peptidoglycan units occurs. Different enzymes participate in this process.

Functions

Like the cell wall in plants, this structure in bacteria performs similar functions to protect these unicellular organisms from lysis against osmotic stress.

The outer membrane of Gram negative bacteria helps translocation of proteins and solutes, and signal transduction. It also protects the organism from pathogens and provides cellular stability.

Fungal cell wall

Structure and composition

Most cell walls in fungi have a fairly similar composition and structure. They are formed from carbohydrate polymers similar to a gel, intertwined with proteins and other components.

The distinctive component of the fungal wall is chitin. It interacts with glucans to create a fibrous matrix. Although it is a strong structure, it has a certain degree of flexibility.

Synthesis

The synthesis of the main components – chitin and glucans – occurs in the plasma membrane.

Other components are synthesized in the Golgi apparatus and in the endoplasmic reticulum. These molecules are carried to the outside cell by excretion pathways through vesicles.

Functions

The cell wall of fungi determines their morphogenesis, their cell viability and their pathogenicity. From an ecological point of view, it determines the type of environment in which a certain fungus can live or not.

Types of cell wall

The primary wall

It is plastic of 1 to 3 microns thick, including cellulose, hemicelluloses and pectic compounds.
In this primary wall, the microfibrils are arranged without order (scattered texture) thus forming a network in the meshes of which is the amorphous matrix.

The primary wall is the first formed and the only one for undifferentiated cells. It is able to grow in length and thickness.

The secondary wall

It is rigid and can reach a considerable thickness in certain supporting tissues. It is applied against the primary wall and inside it. It is rigid and therefore no longer allows cell growth.

It is formed of cellulose microfibrils and a matrix like the primary wall, but the microfibrils are arranged in a regular manner, describing highly corrected propellers with respect to the long axis of the cell. These microfibrils have been arranged in successive layers for which the winding direction of the helices changes abruptly from one stratum to another.

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