Cell wall: types, characteristics and functions
Let's see what the cell wall looks like in different forms of life, and its functions.
The cell is the basic unit of life. Every entity considered to be alive has at least one cell in its body structure, from the most basal prokaryote to the human being, which seems to be made up of 30 million million cells (84% of them red Blood cells).
Every cell must be able to nourish itself, grow, multiply, differentiate, signal, recognize the environment (chemotaxis) and evolve, i.e. its genome must vary over generations.
In addition to these functions, it should be noted that the cell has DNA in its structure 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 the dehydrated cell protoplasm. Through transcription and translation processes, the information present in the genes is transformed into a chain of amino acids, the basic units of all protein material.
For all these processes to take place, the cell must have an internal homeostatic equilibrium, i.e. remain relatively constant despite environmental changes. The plasma membrane delimits this unit from the rest of the environment 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 all about the cell wall.
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 things, 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..
Although in all cells it is the plasma membrane that delimits the inside of the cell from the outside, various taxa of living beings have chosen to cover these structural units with an insoluble matrix of secreted macromolecules. This matrix or extracellular wall not only provides structural support to cells and tissues, but also allows the maintenance of the cell in the environment, the formation of adhesions and special interactions, and dictates the functionality of different strains within the same living being.
The composition of the cell wall varies among the different taxa of living organisms that have it.. Therefore, we will tell you the particularities of this formation in bacteria, fungi and plants separately.
Cell wall in bacteria
In bacteria, the cell corresponds to the whole of its body.. Therefore, these microorganisms usually have special structures (such as cilia, flagella and fimbriae) that other multicellular organisms do not have in most of their tissues. While we have aggregate structures that allow us to locomote, bacteria must manage with a single cell body to perform all 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 less demanding structures (such as cell walls), which cover the membrane and allow the cellular unit to maintain its integrity. In addition to protection from the outside, the bacterial wall prevents the cell from bursting or deformation 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, as the fungal wall is made of chitin and the plant wall is made of cellulose. In any case, the premise and functionality are similar in all these taxa.
2. Cell wall in fungi
In biology, the term "fungus" or Fungi is used to designate a taxon of eukaryotic organisms that includes molds, yeasts and mushroom-producing organisms. They may look like plants, but they differ from plants in that they are heterotrophic, i.e., they obtain organic matter directly from the environment and do not organic matter directly from the environment and are unable to 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 phylogenetically closer to animals than to plants or prokaryotes.
Having clarified this point, it should be noted that, as we have already said, the cell wall of fungi is composed of chitin, the cell wall of fungi is composed of chitin.... This compound is a type of carbohydrate, which is itself formed by subunits of β-(1,4)-N-acetylglucosamine (in basidiomycetes and ascomycetes), although in zygomycetes it is present in the form of poly-β-(1,4)-N-acetylglucosamine chitosan).
In addition to chitin or chitosan, the cell wall of fungi also contains glucans, polymeric polymers of chitosan. also contains glucans, glucose polymers that serve to connect the various chitin chains.. Finally, this structure also contains enzymes necessary to synthesize and destroy the wall and structural proteins.
3. Cell wall in plants
The cell wall of plants is the most generally known, as it is often used as the main distinction between the cell of the kingdom Animalia and Plantae. The most important function of this tough and resistant extracellular matrix is to withstand the osmotic pressure of the cellular environment, the product of the difference in concentrations between the internal and external environment.The most important function of this hard and resistant extracellular matrix is to support the osmotic pressure of the cellular environment, which is the result of the difference in concentrations between the internal and external environment.
When the extracellular medium is hypotonic (has a lower concentration of solutes than the cell), water enters the cell, causing it to swell or turgor. From a chemical point of view, equilibrium is sought between the hypotonic external solution and the hypertonic cytoplasm, i.e., that both become isotonic with fluid exchange. Without cell walls (which can withstand pressures several times higher than atmospheric pressure), plant cells would swell due to water ingress and would eventually explode. and would eventually explode.
To withstand these pressures, the cell wall must be strong and rigid. In addition, it has three different layers:
- Primary cell wall: this is a thin, flexible layer, which develops as the plant cell grows.
- Secondary cell wall: when the cell stops growing and the primary cell wall has completely formed, the secondary cell wall begins to be synthesized. This layer is not found in all cell types within an organism.
- Middle lamella: a layer of calcium and magnesium pectins that binds two adjacent cell walls together.
In the growing primary cell wall, the most important synthesis materials are cellulose (a polymer composed of more than 10,000 glucose monomers), hemicellulose (mostly of the xyloglucan type) and pectin.. Interestingly, 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, some 450 gigatons.
In the plant cell environment, cellulose fibrils are embedded in a matrix consisting of proteins and the two other polysaccharides already mentioned, 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 their rigidity and strength.
As you may have noticed, the work of the cell wall goes far beyond the plant kingdom. Bacteria (except mycoplasmas) and fungi also have it and, although their composition is different, the rationale is the same: to prevent the cell from suffering mechanical stresses or exploding due to osmotic imbalances.
In addition to this vital task, the cell wall in plants (especially the secondary wall) also acts as the "partition walls" of tissue construction, since its hardness, low malleability and potential for union with adjacent structures give this extracellular matrix all the properties necessary to maintain organized tissues. Without the cell wall, the life of plants, prokaryotes and fungi would be impossible.
(Updated at Mar 28 / 2023)