Cell wall: types, characteristics and functions

The cell is the basic unit of life. Every entity that is considered alive has at least one cell in its body structure, from the most basal prokaryote even the human being, who seems to be made up of 30 million million cells (84% of them globules red).

Every cell must be able to nourish itself, grow, multiply, differentiate, signal, recognize the environment (chemotaxis) and evolve, that is, its genome must vary over generations.

In addition to these functions, it should be noted that the cell has DNA in the form of chromosomes, which may be free in the cytoplasm (prokaryotes) or enclosed by a nuclear membrane (eukaryotes). This DNA contains all the information necessary for the synthesis of proteins, which make up 80% of dehydrated cell protoplasm. Through transcription and translation processes, the information present in genes is transformed into a chain of amino acids, the basal units of all protein material.

For all these processes to take place, the cell must present an internal homeostatic balance, that is, it must remain relatively constant despite environmental changes. The plasma membrane delimits this unit from the rest of the medium and modulates the entry and exit of substances, but there are other accessory structures that promote the protection and integrity of the cell. Here we tell you everything about

cell wall.

• Related article: "The most important parts of the cell and organelles: an overview"

What is the cell wall?

The cell wall can be defined as an extracellular matrix that surrounds all plant cells (Kingdom Plantae). However, it is also present in most prokaryotes, fungi and other living beings, which are usually considered "evolutionarily simple".

On the other hand, animal cells do not have a cell wall and their only delimitation with respect to the environment is the plasma membrane.

Despite the fact that in all cells it is the plasma membrane that delimits the interior of the cell from the exterior, various taxa of living beings have chosen to cover these structural units with an insoluble matrix of macromolecules secreted. This matrix or extracellular wall not only provides structural support to cells and different tissues, but also allows the maintenance of the cell in the environment, the formation of adhesions and special interactions and dictates the functionality of various strains within the same being alive.

The composition of the cell wall varies between the different taxa of living beings that present it. Therefore, we tell you the peculiarities of this formation in bacteria, fungi and plants separately.

1. Cell wall in bacteria

In bacteria, the cell corresponds to the whole of your body. For this reason, these microorganisms usually have special structures (such as cilia, flagella and fimbriae) that the rest of multicellular beings do not have in most of their tissues. While we have aggregate structures that allow us to locomotion, bacteria must contrive with a single cell body to perform all of their vital functions.

Something similar happens with protection against external stressors. Although we have an entire tissue dedicated to lining and protection (skin), bacteria need other structures less demanding (such as cell walls), which cover the membrane and allow the cell unit to maintain its integrity. In addition to protecting the exterior, the bacterial wall prevents the cell from exploding or deforming due to turgor (swelling due to changes in concentration between the medium and the cytoplasm).

The bacterial cell wall is composed of peptidoglycan (murein), which in turn is made up of polysaccharide chains, interconnected by unusual peptides containing D-amino acids. The chemical composition is the essential differentiator between the walls in the different kingdoms, since that of fungi is formed by chitin and that of plants by cellulose. However, the premise and functionality are similar in all these taxa.

• You may be interested in: The 3 types of bacteria (characteristics and morphology)

2. Cell wall in fungi

In biology, the term "fungus" or Fungi it is used to designate a taxon of eukaryotic organisms that includes molds, yeasts and living beings that produce mushrooms. They could look like plants, but they differ from these in that they are heterotrophs, that is, that obtain organic matter directly from the environment and cannot photosynthesize.

On the other hand, they differ from animals by the presence of the cell wall in their cells, since we remember that the delimitation in the latter ends with the plasma membrane. Between two waters, fungi are considered more phylogenetically close to animals than to plants or prokaryotes.

Once this point has been clarified, it should be noted that, as we have already said, the cell wall of fungi is composed of chitin. This compound is a type of carbohydrate, which is formed by subunits of β- (1,4) -N-acetylglucosamine (in basidiomycetes and ascomycetes), although in zygomycetes it is present in the form of chitosan poly-β- (1,4) -N-Acetylglucosamine).

In addition to chitin or chitosan, the cell wall of fungi It also contains glucans, glucose polymers that serve to connect the different chitin chains. Finally, this structure also has enzymes necessary to synthesize and destroy wall and presents structural proteins.

3. Cell wall in plants

The cell wall of plants is the best known at a general level, as it is usually used as the main distinction between the cell of the Animalia kingdom and the Plantae. The most important function of this tough and resistant extracellular matrix is ​​to withstand the osmotic pressure of the cellular environment, product of the difference in concentrations between the internal and external environment.

When the extracellular medium is hypotonic (it has a lower concentration of solutes than the cell), water enters the cell, causing its swelling or turgor. From a chemical point of view, a balance is sought between the hypotonic external solution and the hypertonic cytoplasm, that is, that both become isotonic with the exchange of fluids. Without cell walls (which withstand pressures several times higher than atmospheric), plant cells would swell due to the entry of water and they would end up exploding.

To withstand these pressures, the cell wall must be strong and rigid. In addition, it has three different layers:

• Primary cell wall: it is a thin and flexible layer, which develops as the plant cell grows.
• Secondary cell wall: when the cell stops growing and the primary cell wall is fully formed, the secondary wall begins to be synthesized. This layer is not found in all cell types within the same organism.
• Middle lamella: it is a layer of calcium and magnesium pectins that joins two cell walls of cells adjacent to each other.

In the growing primary cell wall, the most important synthesis materials are cellulose (a polymer made up of more than 10,000 glucose monomers), hemicellulose (mostly of the xyloglucan type) and pectin. It should be noted that, curiously, cellulose is the most abundant biopolymer on Earth, since plants contain in their tissues (in the form of carbon molecules) 80% of the biomass of the entire planet, about 450 gigatons.

In the plant cell environment, cellulose fibrils are embedded in a matrix, which consists of proteins and the other two polysaccharides already named, hemicellulose and pectin. While the distribution of these three polysaccharides is homogeneous in the primary wall, in the secondary wall 80% of them correspond to cellulose, hence its rigidity and strength.

Resume

As you may have seen, the work of the cell wall goes far beyond the kingdom of plants. Bacteria (except mycoplasmas) and fungi also possess it and, although their composition is different, the The rationale is the same: prevent the cell from suffering mechanical stress or from exploding due to imbalances osmotic.

In addition to this vital work, the cell wall in plants (especially the secondary one) also acts as "partitions" for the construction of tissues, since its hardness, little malleability and potential for binding with adjacent structures give this extracellular matrix all the properties necessary to maintain tissues organized. Without the cell wall, the life of plants, prokaryotes and fungi would be impossible.