The cell varies from simple prokaryotes to humans, who have around 30 trillion cells, with 84% of these being red blood cells (Sánchez Amador, 2021). They all share functions such as nourishing, growing, multiplying, differentiating, signaling, recognizing the environment (chemotaxis), and evolving, which implies changes in their genome. The cellular structure highlights the presence of DNA in chromosomes, either free (prokaryotes) or in a nuclear membrane (eukaryotes), essential for protein synthesis, constituting 80% of dehydrated cellular protoplasm. Through processes of transcription and translation, genetic information is transformed into chains of amino acids, the fundamental units of proteins. For proper functioning, according to Sánchez Amador (2021), the cell maintains internal balance, regulated by the plasma membrane and accessory structures, ensuring its integrity and adaptation to the environment.
What is the Cell Wall?
The extracellular matrix surrounding all plant cells and also present in most prokaryotes, fungi, and other organisms considered "evolutionarily simple" is known as the cell wall (Sánchez Amador, 2021). This structure is located externally to the plasma membrane (Rivera Díaz, 2023). Despite its name suggesting an impenetrable barrier, Zita Fernandez (2018) mentions that the cell wall is characterized by its dynamism, playing a crucial role in cell relationships with its environment.
Cell Membrane vs. Cell Wall
The main difference between these two structures lies in the fact that the cell membrane is formed by two layers of phospholipids and proteins (Rivera Díaz, 2023). This structure delimits cells to safeguard their content from the external environment. In contrast, the cell wall is a characteristic structure of plant cells, offering robust structural support and additional protection. According to Rivera Díaz (2023), the cell wall is considerably more rigid than the cell membrane.
The Cell Wall: Support, Regulation, and Protection
The cell wall plays multiple functions and pursues various objectives (Rivera Díaz, 2023). It is essential for providing structural support, confining and delimiting cellular space, contributing to cellular stability (Rivera Díaz, 2023). Additionally, it plays a crucial role in maintaining internal cellular pressure, regulating the concentration of dissolved substances that influence water entry through the cell membrane (Zita Fernandes, 2018). The absence of the cell wall would lead the cell to swell uncontrollably, even leading to bursting. Moreover, it acts as a protective shield by controlling the entry of external agents that could damage the cell, being crucial for organism interaction with its environment (Zita Fernandes, 2018). Its role in signaling and defense is crucial for cellular communications and signaling mechanisms (Rivera Díaz, 2023). According to Rivera Díaz (2023), fewer defense mechanisms are activated in case of rupture.
Plant Cell Wall
The cell wall of plants is widely recognized, commonly used as the main distinction between Animalia and Plantae kingdom cells (Sánchez Amador, 2021). Outside the plasma membrane of the plant cell is a cell wall mainly composed of cellulose and pectin (Zita Fernandes, 2018). According to Sánchez Amador (2021), the most crucial function of this extracellular matrix is to sustain the osmotic pressure of the cellular environment, which arises due to concentration differences between the internal and external milieu.
When the extracellular environment is hypotonic, meaning it has a lower solute concentration than the cell, water enters the cell, resulting in swelling or turgor (Sánchez Amador, 2021). Chemically, the cell seeks to reach equilibrium between the hypotonic external solution and its hypertonic cytoplasm, so that both become isotonic through fluid exchange. Without the cell wall, capable of resisting pressures several times higher than atmospheric pressure, plant cells would swell due to water entry and eventually burst. According to Sánchez Amador (2021), to withstand these pressures, the cell wall must be strong and rigid.
The cell wall presents three layers (Sánchez Amador, 2021). The primary cell wall, a thin and flexible layer that develops during cell growth (Sánchez Amador, 2021). In the process of primary cell wall growth, key synthesis materials are cellulose (a polymer of over 10,000 glucose monomers), hemicellulose (mainly of the xylglucan type), and pectin (which acts as glue between cellulose fibers) (Zita Fernandes, 2018; Sánchez Amador, 2021). The secondary cell wall, synthesized after the cell has completed its growth and the primary wall is fully formed, not present in all cell types within an organism (Sánchez Amador, 2021). Finally, according to Sánchez Amador (2021), the middle lamella, a layer of calcium and magnesium pectins that joins two cell walls of adjacent cells.
In many plants, the cell walls of neighboring cells can fuse, reaching a thickness of approximately 0.1 micrometers (Rivera Díaz, 2023). In the plant cellular environment, cellulose fibrils are immersed in a matrix consisting of proteins and the other two polysaccharides, hemicellulose, and pectin (Sánchez Amador, 2021). Sánchez Amador mentions that while the distribution of these three polysaccharides is homogeneous in the primary wall, in the secondary wall, 80% corresponds to cellulose, explaining its rigidity and strength.
Fungal Cell Wall
In biology, the term "fungus" or Fungi is used to refer to a taxon of eukaryotic organisms comprising molds, yeasts, and mushroom-producing organisms (Sánchez Amador, 2021). Although their appearance may resemble that of plants, they are distinguished by being heterotrophs, meaning they obtain organic matter directly from the environment without carrying out photosynthesis. Furthermore, in correspondence with Sánchez Amador (2021), they differ from animals thanks to the presence of the cell wall in their cells, in contrast to the plasma membrane that delimits the latter.
The integrity of the fungal cell wall is presented as a dynamic structure (Zita Fernandes, 2018). The components of this wall are synthesized in the plasma membrane through enzymes (Zita Fernandes, 2018). Phylogenetically, fungi are closer to animals than plants (Sánchez Amador, 2021). It is important to highlight that the fungal cell wall is mainly composed of chitin, a carbohydrate formed from β-(1,4)-N-Acetyl-Glucosamine subunits in basidiomycetes and ascomycetes, while in zygomycetes it is presented as poly-β-(1,4)-N-Acetyl-Glucosamine chitosan. In addition to chitin or chitosan, the fungal cell wall also contains glucans, glucose polymers that connect chitin chains. Finally, according to Sánchez Amador (2021), this structure has enzymes for wall synthesis and destruction, as well as structural proteins.
Internally, the cell wall presents a framework of glucans and chitosans, forming a kind of basket around the fungal cell (Zita Fernandes, 2018). The surface of the cell wall varies among species due to glycoproteins. Chitin, a linear polymer of the sugar N-Acetyl-Glucosamine, composes this structure. According to Zita Fernandes (2018), it is worth mentioning that the cell wall of fungi that cause diseases presents strategies to evade the defense system of the affected organism, and its components are an ideal target for designing drugs to treat mycosis.
Bacterial Cell Wall
The cell represents the entire body of bacteria, so these microbes possess special structures (cilia, flagella, and fimbriae) not found in most tissues of multicellular organisms (Sánchez Amador, 2021). Unlike organisms that have aggregated structures for locomotion, Sánchez Amador (2021) mentions that bacteria must perform all their functions with a single cell body.
The same applies to protection against external stressors (Sánchez Amador, 2021). While humans have specific tissue for lining and protection (skin), bacteria require other less demanding structures (cell walls) that cover the membrane and allow the cellular unit to maintain its integrity. In addition to protection from external factors, the bacterial cell wall prevents the cell from bursting or deforming due to turgor (swelling due to concentration changes between the cytoplasm and the medium). According to Sánchez Amador (2021), this cell wall is composed of peptidoglycan (murein), formed by polysaccharide chains linked by unusual peptides with D-amino acids.
Cell Wall in Archaea
Archaea, like bacteria, are prokaryotic cells (Zita Fernandes, 2018). However, their cell wall presents a unique composition since it is formed by pseudomurein. This structure is composed equally of sugars, including N-Acetyl-Glucosamine and N-Acetyl-Talosaminuronic Acid in a 50%, and L-amino acids such as Alanine, Lysine, and Glutamic Acid in the other 50%. Unlike bacterial murein, Zita Fernandes (2018) mentions that archaea do not incorporate D-amino acids and opt to use N-Acetyl-Talosaminuronic Acid instead of N-Acetyl-Muramic Acid.
References
Rivera Díaz, A. G. (2023, enero 31). Función de la Pared Celular. Plataforma Educativa Luca: Curso en línea y Aprendizaje Esperado; Plataforma Educativa Luca. https://www.lucaedu.com/funcion-de-la-pared-celular/
Sánchez Amador, S. A. (2021, junio 9). Pared Celular: Tipos, Características y Funciones. Psicología y Mente. https://psicologiaymente.com/cultura/pared-celular
Zita Fernandes, A. (2018, agosto 24). Pared Celular. Significados. https://www.significados.com/pared-celular/
