Cell Wall

Cell Wall Definition

A cell wall can be defined as a rigid layer that encircles the contents of the cell. This is a semi-permeable layer that acts as a protective layer in certain types of cells. As such, the cell wall will be seen outside of the cell membrane in the cells of the following organisms:

  • Plant cells
  • Fungi
  • Bacteria
  • Algae
  • Some archaea

The animal and protozoan cells do not contain a cell wall. It’s to be noted that the structure of the cell wall has evolved several different times among various groups of organisms. Also, the components that make up this cell wall would differ between the types of organism.

Cell Wall Structure:

As mentioned above, the composition of cell wall will differ from one species to another. Following are the makeup of cell wall in various organisms:

Cell Wall in Plant Cells:

In the plant cells, the cell wall is mainly composed of cellulose, which is the most abundant macromolecule available on the planet. As such, the cell wall in the plant cells has three layers. The first layer is the middle lamella. This is the first unit of the cell wall that will be formed during cell division. This is the outermost layer and is rich in pectin. This will join together the nearby cells, thereby, holding them together.

The second layer is the primary cell wall that will be formed after the middle lamella. The components of the primary cell wall include pectin, hemicellulose, and glycoproteins. This layer will contain a network of cellulose microfibrils, which will appear as a gel-like matrix. This is a thin and extensible layer that is flexible in nature. The third layer of the cell wall is the secondary cell wall that is found inside the primary cell wall. This is a thick and rigid layer and contains cellulose, hemicellulose, and lignin.

Apart from these, the plant cell walls would also contain the following elements:

  • Hydrolases
  • Esterases
  • Peroxidase
  • Transglycosylases
  • Structural proteins
  • Silica crystals

Cell Wall in Fungal Cells:

The species of fungi that have cell walls would contain a plasma membrane surrounded by three layers of cell wall material encircling it. As such, these layers will be composed of chitin, glucans, and mannoproteins.

Cell Wall in Bacterial Cells:

In general, the bacterial cells can be classified as gram-positive and gram-negative based on the reactions of the cells to the process called “gram staining”. As such, the gram-positive bacterial cells would contain a thick cell wall that is composed of several layers of peptidoglycan and teichoic acids. On the other hand, the gram-negative bacterial cells would contain a thin cell wall, which is composed of peptidoglycans. In addition, these cell walls will be encircled by a lipid membrane that is made up of lipopolysaccharides and lipoproteins.

Cell Wall in Algal Cells:

Algae, as such are a diverse group and we can notice this diversity in the cell walls too. For instance, the green algae would contain cell walls that are similar to plant cell wall, whereas, the brown and red algae would contain cellulose and polysaccharides or fibrils in their cell walls. The other components that can be found in algal cell walls are:

  • Mannans
  • Xylans
  • Alginic acid
  • Silicic acid

Cell Wall in Archaeal Cells:

Although archaeal cells resemble bacterial cells in several cells, the cell walls in archaeal cells would hardly contain peptidoglycan. The cell walls in archaeal cells are diverse in nature and are composed of the following molecules:

  • Pseudopeptidoglycan
  • Polysaccharides
  • Glycoproteins
  • Surface-layer proteins

Cell Wall Functions:

The functions of the cell wall are as follows:

  • Providing strength to the cells
  • Protecting the cell from damage
  • Providing shape to the cells
  • Aiding the cells to maintain their shapes
  • Offering protection against pathogens
  • Allowing the passage of small molecules into the cells
  • Preventing the passage of large molecules into the cells
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