CELL WALL: CHARACTERISTICS, FUNCTIONS AND STRUCTURE - BIOLOGY - 2025

The cell wall is a thick and resistant structure that delimits certain types of cells and is found surrounding the plasma membrane. It is not considered as a wall that prevents contact with the outside; It is a complex, dynamic structure and is responsible for a significant number of physiological functions in organisms.

The cell wall is found in plants, fungi, bacteria, and algae. Each wall has a structure and composition typical of the group. In contrast, one of the characteristics of animal cells is the lack of a cell wall. This structure is mainly responsible for giving and maintaining the shape of cells.

The cell wall acts as a protective barrier in response to osmotic imbalances that the cell environment may present. In addition, it has a role in communication between cells.

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 cell integrity against osmotic forces that can burst the cell.

-In the case of multicellular organisms, it helps the formation of tissues and participates in cell communication

Cell wall in plants

Structure and composition

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 repeating glucose units, linked together by β – 1,4 bonds. Each molecule contains about 500 glucose molecules.

The rest of the components include: homogalacturonan, rhamnogalacturonan I and II and hemicellulose polysaccharides such as xyloglucans, glucomannan, xylan, among others.

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

Hemicellulose binds via hydrogen bonds to cellulose. These interactions are very stable. For the rest of the components, the mode of interaction is not yet well defined.

You can differentiate between primary and secondary cell wall. The primary is thin and somewhat malleable. After cell growth stops, the deposition of the secondary wall occurs, which can change its composition with respect to the primary wall 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 wall biosynthesis process is complex. It involves approximately 2000 genes that participate in the construction of the structure.

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

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

Function

The cell wall in plants has functions analogous to those performed by the extracellular matrix in animal cells, such as maintaining cell shape and structure, connecting tissues, and cell signaling. Below we will discuss the most important functions:

Regulate turgor

In animal cells - which lack a cell wall - the extracellular environment poses a major challenge in terms of osmosis.

When the concentration of the medium is higher compared to the cell 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) the water enters and the cell can explode.

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

The turgor pressure created by the cell wall helps keep plant tissues rigid.

Cell connections

Plant cells are able to communicate with each other through a series of "channels" called plasmodesmata. These pathways connect the cytosol of both cells and exchange materials and particles.

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

Signaling routes

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

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 are produced, such as phytoalexins, which are antimicrobial substances.

Cell wall in prokaryotes

Structure and composition in eubacteria

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

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

For their part, Gram positive bacteria only have a plasma membrane and the cell wall is much thicker. These are usually rich in teichoic 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, work by preventing the formation of the bonds in the bacterial cell wall. When a bacterium loses its cell wall, the resulting structure is known as a spheroplast.

Structure and composition in archaea

Archaea differ in wall composition from bacteria, mainly 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 components of the polymer are lN-acetyltalosaminuronic acid linked to N-Acetylglucosamine.

They contain a number of rare lipids, such as glycerol-bound isoprene groups 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 bonds found are of the ester type, while in archaea they are of the ether type. The glycerol backbone is typical of this domain.

There are some species of archaea, such as Ferroplasma Acidophilum and Thermoplasma spp., That do not have a cell wall, despite living in extreme environmental conditions.

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

Synthesis

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

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

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

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

Features

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

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

Cell wall in fungi

Structure and composition

Most cell walls in fungi have a fairly similar composition and structure. They are formed from gel-like carbohydrate polymers, 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 exhibits 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 outside the cell by way of excretion through vesicles.

Features

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

References

1. Albers, SV, & Meyer, BH (2011). The archaeal cell envelope. Nature Reviews Microbiology, 9 (6), 414–426.
2. Cooper, G. (2000). The Cell: A Molecular Approach. 2nd edition. Sinauer Associates.
3. Forbes, BA (2009). Microbiological diagnosis. Panamerican Medical Ed.
4. Gow, NA, Latge, JP, & Munro, CA (2017). The fungal cell wall: structure, biosynthesis, and function. Microbiology spectrum 5 (3)
5. Keegstra, K. (2010). Plant cell walls. Plant physiology, 154 (2), 483–486.
6. Koebnik, R., Locher, KP, & Van Gelder, P. (2000). Structure and function of bacterial outer membrane proteins: barrels in a nutshell. Molecular microbiology, 37 (2), 239–253.
7. Lodish, H., Berk, A., Zipursky, SL, Matsudaira, P., Baltimore, D., & Darnell, J. (2000). Molecular cell biology 4th edition. National Center for Biotechnology Information, Bookshelf.
8. Scheffers, DJ, & Pinho, MG (2005). Bacterial cell wall synthesis: new insights from localization studies. Microbiology and Molecular Biology Reviews, 69 (4), 585–607.
9. Showalter, AM (1993). Structure and function of plant cell wall proteins. The Plant Cell, 5 (1), 9–23.
10. Valent, BS, & Albersheim, P. (1974). The structure of plant cell walls: On the binding of xyloglucan to cellulose fibers. Plant Physiology, 54 (1), 105-108.
11. Vallarino, JG, & Osorio, S. (2012). Signaling role of oligogalacturonides derived during cell wall degradation. Plant signaling & behavior, 7 (11), 1447–1449.