Cell Wall in a modern Plant Cell

 Cell Wall in a modern Plant Cell

Basically, a cell wall provides an additional layer of protection on top of the cell membrane, and it is most common in plants, bacteria, fungi, and algae. Animals and protozoans do not have cell wall structures. The outstanding role of a plant cell wall is the maintenance of a plant cell structure and shape. The rigid nature of the cell wall helps it in performing the role of cell protection perfectly (Keegstra, pp 483-486, 2010). For instance, the cell wall protects pathogens like plant viruses from entering the plant cell. Another key role that the cell wall plays is providing a medium of transmission of molecules between individual cells. The main components of a plant cell are carbohydrates like cellulose, hemicellulose, and pectin. The other components are proteins, although in small amounts and some minerals like silicon. Cellulose forms complex cellulose that contains thousands of glucose monomers that form long chains. The long chains, in turn, lead to the formation of cellulose microfibrils, which are useful in monitoring the growth of a cell by limiting or allowing cell expansion.

The structure of a plant cell consists mainly of three layers, that is, the middle lamella, the primary cell wall, and the secondary cell wall. Even though it is the outermost layer, the middle lamella lies in between and holds together cell walls of two adjacent walls and, therefore, the name, middle lamella. Primary cell walls have equal amounts of cellulose, pectin, and hemicelluloses, while the secondary cell wall does not have any pectin but instead have more cellulose (Sorensen et al., pp 366-372, 2010).

So far, scientists have been able to discover many ways in which the cell wall functions in plants. However, research is still needed to establish more and more functions of the cell wall in plants. Recently, researchers are working on establishing the actual number of genes that are required in the biosynthesis process of a cell wall. So far, it is estimated that approximately 2000 genes are enough to carry out the biosynthesis process (Popper et al., pp 567-590, 2011). Micro- and nanomechanical methodologies will be useful in analyzing the plant cell wall fully. Another interesting area of study in the plant cell wall that scientists can consider focusing on is how regulating genes can affect the cell wall as well as the cell wall functions..

Introduction

A cell wall is an extracellular matrix that is found around every cell of a plant. Even though it appears as an inactive product in the plant cell that performs only structural and mechanical purposes, it is the main determinant of all the properties that differentiate between a plant cell and an animal cell. Some plant cell walls trace their origin from the ancient prokaryotic, while others trace their origin from algal ancestors of plants. In rare cases, the origin of some polymers is related to specific land type tissues, but the recent understanding of the cell wall origin is limited to the structure and functions of many cell wall functions (Pauly et al., pp 304-311, 2012).

The cell wall is surrounded by the cell membrane, and it is only found on a plant cell and few other organisms. The plant cell is protected from the outside pressures by the cell wall through a structural carbohydrate known as the cellulose. Unlike the other sugars, cellulose cannot dissolve in water, and therefore it forms long chains that give support to plants (Ding et al., pp 1055-1060, 2012). Human beings cannot easily digest and convert cellulose into energy, but only cows and other herbivores that possess a particular type of bacteria have the ability to digest and convert it into energy. This paper will focus on giving a thorough analysis of a cell wall structure, components, functions, and interactions with other organelles.

The Structure and Functions of the Cell Wall

A cell wall is mostly found in plant cells such as bacteria, fungi, archaea, and some algae. However, animal cells do not have a cell wall. There are key functions of a cell wall in a plant cell, among them being offering support, protection, and maintaining the cell structure. The composition of a cell wall varies depending on the organism. In-plant cells, the cell wall contains strong chains of carbohydrates known as the polymer cellulose. Cellulose is rich in wood and cotton fiber and, therefore, useful in paper production (Underwood, pp 85, 2012).  Cell walls in bacteria mainly encompass sugar and amino acid polymer known as the peptidoglycan. In fungal cell walls, the main composition is proteins, glucans, and chitin.

The cell wall structure

The plant cell wall has three key layers; the middle lamella, primary cell wall, and the secondary cell wall. The middle lamella and the primary cell wall are present in all plant cells; however, not all plant cells have a secondary cell wall. The middle lamella layer of the cell wall is the outermost part and contains polysaccharides known as pectins. The pectin acid is useful in binding cell walls of adjacent cells together (Perkins, pp 65, 2012). The primary cell wall is the layer that is found between the middle lamella and the plasm membrane in growing plants. It contains cellulose microfibrils, which are found in a gel-like matrix of hemicellulose pectin polysaccharides and hemicellulose fibers. The primary cells are responsible for providing the strength and flexibility necessary during plant cell growth.

The secondary cell wall is the layered sandwich between the primary cell wall and the plasma membrane in some type of plants. When the primary cell wall reaches the maturity stage where it can no longer grow, it may become rigid and form the secondary cell wall. The resulting rigid layer is responsible for strengthening and supporting the plant cell (Pettoline et al., pp87-93, 2012). However, some secondary cell walls contain lignin, which is responsible for strengthening the cell wall and enhancing water conductivity in plant vascular tissue cells.

Functions of the Plant Cell

The main responsibility of the cell wall in a plant cell is to protect the cell during expansion activities. Structural proteins, cellulose fibers, and other types of polysaccharides assist in maintaining the shape of a cell. The cell wall also provides support and strength to the cell as well as monitoring and controlling the direction towards which the cell grows. Another function of the cell wall is holding the turgor pressure. This is the pressure that develops when the composition of the cell pushes the plasma membrane against the cell wall (Albersheim et al., pp63-64, 2010). The pressure is important as it maintains the rigid and erective nature of the plant, although, on some occasions, it leads to a cell rupture.

A cell wall also plays a key role in sending reminder signals to the plant cells for it to start dividing and growing. A plant cell wall also regulates diffusion by monitoring the movement of some molecules such as proteins to pass in and out of the cell while hindering the movement of other substances. The cell wall also provides storage for carbohydrates, which are highly used during the plant cell growth process (Cosgrove, pp463-476, 2016). Through the plasmodesmata, cells are able to communicate with each other. Finally, a cell wall provides protection to the plant cell against viruses and other pathogens.

Interaction of the cell wall and other organelles

A plant cell wall provides support and protection to other tiny organelles in order to enhance their growth and survival. Some of the organelles that receive support and protection from the plant cell wall are as follows; the cell or plasma membrane which encloses the cytoplasm of a cell and protects its contents. Another internal organelle in the cytoplasm, which is a gel-like substance inside the cell membrane and performs the role of supporting and suspending organelles. A cytoskeleton is another internal structure, which is a network of fibers that runs throughout the cytoplasm (Keegstra, pp483-486, 2010). A plasmodesmaencompasses the tiny pores that provide a communication channel between individual plant cells as well as allowing the passage of molecules. Centrioles are internal structures that contain cell structures that are responsible for assembling microtubules during the process of cell division.

Chloroplasts provide a favorable platform for the process of photosynthesis to take place. Golgi complex is an internal organelle that is responsible for manufacturing, storing, and distributing certain types of cellular products. Endoplasmic Reticulum (ER) is an internal structure that provides a diverse network of membranes which comprises regions with ribosomes (rough ER) and regions without ribosomes (smooth ER). Mitochondria organelles are responsible for producing energy that is used during the respiration process. Nucleoli are circular structures that help in the processing of ribosomes (Perkins, pp56-58, 2012).

Intercellular Communications

Plasmodesma forms a channel of intermolecular communication through the cell wall. It is made of cell membrane linen that unites all connected cells with one continuous cell membrane. In the middle of each communication channel, there is a thin membranous tube that connects the endoplasmic reticula (ER) of two cells. Plasmodesma is known to perform the role of monitoring the passage of small molecules such as those of water, sugar, and amino acids by regulating the movement of the openings at each end of the channel (Popper et al., pp567-590, 2011).

Recent findings by scientists show that there are special fragments of cell wall polysaccharides known as oligosaccharides, which have the ability to produce certain responses in plant cells and tissues. A good example of oligosaccharides is a linear polymer of 10 to 12 galacturonic acid residue, which, when exposed to a plant cell, produces antibiotics known as phytoalexins (Ding et al., pp1055-1060, 2012). Other scientific experiments have shown that exposure of strips of tobacco stem to different types of cell wall fragments leads to the formation of stems in some and flowers in others. However, the amount of oligosaccharides required to induce a response in a plant cell is equal to the number of hormones required to induce a change in an animal cell.

Components of a Cell Wall

Cellulose – component of a cell wall that contains thousands of molecules linked together from one to another. The chemical structures between the individual molecule subunits give the structure of the cellulose a flat like a ribbon structure and thus allowing the adjacent molecules to join together into microfibrils, which are two to seven micrometers long. Cellulose fibrils are characterized by enzymes floating on the cell membrane denoting a rosette configuration. The rosette has the ability to spin a micro fibril into a cell wall (Albersheim et al., pp60, 2010). During the “spinning” process, when fresh glucose subunits are added to the ends of the fibrils, the rosette is caused to move around the surface of the cell membrane and thus wrapping its cellulose fibril around the protoplast. Therefore, it can be concluded that each plant cell makes its own cellulose fibril cocoon. The figure is a structure of plant cell cellulose.

Proteins – plant cells contain very small amounts of proteins but which performs very critical roles. Hydroxyproline-rich glycoprotein is an example of a plant cell protein that contains 45 percent hydroxyproline and 14 percent serine residues, which are distributed throughout its entire surface area (Pauly et al., pp304-311, 2012). For each hydroxyproline residue, there is a short side chain of arabinose sugars, while for each serine residue, there is galactose sugar. As a result, long molecules are attached at the cell wall towards the end of the cell wall formation and are later cross-linked into a mesh when the wall stops growing.

Plastics are other components of a plant cell that contains a variety of organic compounds cross-linked together as three tight dimensional networks that makes cell walls stronger and more resistant to bacterial and fungal attacks. Lignin represents a diverse group of polymers of aromatic alcohols. It is mostly found on the secondary wall and accounts for 30 percent of the plant’s dry weight (Cosgrove, pp463-476, 2016). The diverse group of polymers, which results in the rigidity of the lignin, forms a reliable barrier against the penetration of microbes.

Cutin and suberin proteins are mainly made of aromatic compounds and fatty acids. Cutin is a major component of the cuticle and performs the role of reducing wetness in leaves and stems and thereby affecting the germination of fungal spores. Suberin, on the other side, functions with waxes as a surface barrier of underground parts (Sorensen et al., pp366-372, 2010).

Matrix polysaccharides

This component of the cell wall has two main classifications, such as the hemicellulose and the pectic polysaccharides. Hemicelluloses are glucose molecules with an end to end arrangement and some short side chains of xyloses and unprocessed sugars attached on one side of the ribbon. Hemicellulose molecules perform an important role in monitoring the rate of expansion of primary cells during plant cell growth. On the other hand, pectic polysaccharides are negatively charged because of the galacturonic acid residue, which combines with the rhamnose sugar molecules to make the backbone of all pectic polysaccharides. The cross-linking of the negatively charged pectic polysaccharides and the positively charged cations or ions leads to the formation of semi-rigid gel characteristics of a cell wall matrix (Pettoline et al., pp70, 2012).

Critical Review

Recently, many scientists have been involved in the study of cell wall structure and formation. The main objectives of the study has been to establish a deeper understanding of the complicated biological functions of a cell wall in a plant cell and also to sink deeper into the current issue of dependency of the society on the utilization of the cell wall raw materials in the biofuel, timber, pulp, and paper industries. Therefore, much focus will be on the expansion of the plant cell walls. A cell wall polymer, the cellulose, will influence the study of how the properties of the biosynthesis and the organization of the plant cell play a key role in the process of cell expansion (Underwood, pp87, 2012).

Also, recent studies are focusing on the understanding of various issues related to the process of cell expansion such as the establishment of the key players between cellulose synthase complexes and microtubules, establishing the interaction between cellulose microfibrils and other wall polymers, establishing the relationship between the cell wall and the microtubules,  the relationship between the microtubules and the cell wall architecture, the function of the cytoskeleton and the trafficking involved in cell wall maintenance, and finally analyzing the influence of hormones and sensing of the cell wall integrity in the cell wall regulation (Keegstra, pp483-486, 2010).

Questions and Methodologies

The questions will be based on the main objectives of the recent studies. The questions would be as follows;

1. Analyze the process of cell wall structure formation
2. Establish the key functions of the cell wall in a plant,
3. Establish the various industrial uses of the cell wall material.
4. Give various ways which can contribute to the cell wall expansion and continuous production of the cell wall material

Methodologies

For quite some time now, scientists have been using micro- and Nanomechanical techniques in analyzing the mechanical design and nanostructure of the plant cell wall. Much focus has been placed on the micro- and Nanomechanical procedures that are greatly used in analyzing primary and secondary cell walls from a biomechanics approach (Popper et al., pp567-590, 2011). The methodologies will also look into the new technological developments and advancements, especially in the field of Nanomechanics. More emphasis will be on the mechanical function of cellulose fibrils, matrix components, as well as the polymer interactions in secondary and primary cell wall interactions (Ding et al., pp1055-1060, 2012). The micro- and Nanomechanical techniques are useful in analyzing the plant cell wall fully and establishing answers to the study questions above.

Bibliography

Keegstra, Kenneth. “Plant cell walls.” Plant Physiology 154.2 (2010): 483-486.

Albersheim, Peter, et al. Plant cell walls. Garland Science, 2010.

Popper, Zoë A., et al. “Evolution and diversity of plant cell walls: from algae to flowering plants.” Annual review of plant biology 62 (2011): 567-590.

Pettolino, Filomena A., et al. “Determining the polysaccharide composition of plant cell walls.” Nature protocols 7.9 (2012): 1590.

Perkins, H. R. Microbial cell walls and membranes.Springer Science & Business Media, 2012.

Ding, Shi-You, et al. “How does plant cell wall nanoscale architecture correlate with enzymatic digestibility?.” Science 338.6110 (2012): 1055-1060.

Pauly, Markus, and Kenneth Keegstra.”Plant cell wall polymers as precursors for biofuels.” Current opinion in plant biology 13.3 (2010): 304-311.

Underwood, William. “The plant cell wall: a dynamic barrier against pathogen invasion.” Frontiers in plant science 3 (2012): 85.

Cosgrove, Daniel J. “Plant cell wall extensibility: connecting plant cell growth with the cell wall structure, mechanics, and the action of wall-modifying enzymes.” Journal of Experimental Botany 67.2 (2016): 463-476.

Sørensen, Iben, David Domozych, and William GT Willats. “How have plant cell walls evolved?.” Plant Physiology 153.2 (2010): 366-372.