Cells are the functional unit of all living organisms on this planet.
Each cell is made up of many small structures called ‘ organelles ‘, which are specialized to carry out vital functions.
The cell wall is a rigid layer that surrounds some types of cells and is located outside the cell membrane.
A cell wall is an important and distinct organelle, present in plants, bacteria, algae, and fungi.
It also turns out to be a special feature that helps us distinguish a plant cell from an animal cell.
Cell wall function
Among the functions of the cell wall are:
- It gives the cell a defined shape and structure.
- Provides structural support.
- Protection against infections and mechanical stress.
- It separates the inside of the cell from the outside environment.
- It allows the transport of substances and information from the inside of the cell to the outside and vice versa.
- It also helps in osmotic regulation.
- Avoid water loss.
- The physiological and biochemical activity of the cell wall aids in cell-cell communication.
- It prevents the cell from breaking due to pressure.
- It helps in the diffusion of gases inside and outside the cell.
- It also provides mechanical protection against pathogens.
The composition of the cell wall differs from species to species.
In plants, the cell wall is made up of polysaccharides which are complex carbohydrates built from monosaccharides.
In simple words, it is made up of cellulose, hemicellulose, pectic polysaccharides, lignin, protein, certain lipids, and water.
In the case of algae, cell walls contain polysaccharides or a variety of glycoproteins, or both.
Fungi have a cell wall consisting mainly of chitin and other polysaccharides, and the cell walls of bacteria are made of peptidoglycan, which is made up of polysaccharide chains cross-linked by unusual peptides containing D-amino acids.
Fungi cell wall
The fungal cell wall consists of chitin and other polysaccharides.
They do not have cellulose in their cell walls.
Species of fungi that possess a cell wall have a plasma membrane and three layers of cell wall material that surround it.
These layers are composed exclusively of α- and β-linked mannoproteins, chitins, and glycans and serve many functions, including providing cell stiffness and shape, metabolism, ion exchange, and interactions with host defense mechanisms.
Cell wall composition varies between fungal species, but a major component of many fungal cell walls is β1,3-glucan.
Mechanism of action of treatments
Current drugs are aimed at inhibiting the enzymes that synthesize the cell membrane wall of pathogens, without affecting the patient’s body.
The action is exerted by inhibiting the synthesis of b (1-3) D-glucan, which is an essential component of the fungal cell wall.
The echinocandins, caspofugin, micafungin, and anidulafungin are lipopeptides that inhibit fungal growth in a non-competitive manner.
Β1,3-d-glucan synthase blocks the synthesis of β1,3-glucan.
This decreases cell structural integrity and morphology and ultimately results in osmotic lysis of the cell.
Inhibition of glucan synthesis is believed to result in a loss of 1,3-β-glucan synthase enzymatic activity and this in turn compromises the structure and osmolarity of the cell wall.
Through this mechanism of action, the integrity of the liposome is maintained in human cells, and therefore, results in minimal toxicity to the host.
Bacterial cell wall
The cell walls of bacteria contain an ingredient known as peptidoglycan (also known as murein).
But the bacterial cell wall also contains additional ingredients, making this a very complex structure overall.
The cell walls of eukaryotic microbes are typically made up of a single ingredient, such as cellulose found in the cell walls of algae or chitin in the cell walls of fungi.
The bacterial cell wall also fulfills several functions, in addition to providing general resistance to the cell.
It protects the cell from osmotic lysis, as the cell moves from one environment to another or carries nutrients from its environment.
Since water can move freely through the cell membrane and cell wall, the cell is at risk of an osmotic imbalance, which could put pressure on the relatively weak plasma membrane.
The cell wall can avoid certain molecules, such as toxins, particularly for gram negative bacteria.
And finally, the bacterial cell wall can contribute to the pathogenicity or disease-causing ability of the cell for certain bacterial pathogens.
The cell wall of bacteria is broadly classified into two types: gram-positive and gram-negative.
The two different types of cell walls can be identified in the laboratory by a differential stain known as a Gram stain.
Developed in 1884, and which has been in use ever since. Originally, it was not known why the Gram stain allowed such a reliable separation of bacteria into two groups.
Once the electron microscope was invented in the 1940s, the difference in staining was found to correlate with differences in cell walls.
Gram negative bacteria have thinner cell walls, and are composed of few layers of peptidoglycans and are surrounded by a lipid membrane that contains lipopolysaccharides and lipoproteins.
This molecule is unique for the composition of the bacterial cell wall.
Peptidoglycan is a polymer made up of double sugars and amino acids (the subunits of proteins).
This molecule provides rigidity to the cell wall and helps shape bacteria.
Peptidoglycan molecules form sheets that enclose and protect the bacterial plasma membrane.
Gram positive bacteria have a thick cell wall.
The cell wall in Gram positive bacteria contains several layers of peptidoglycan, which is a molecule made of sugars and polypeptides that forms a mesh-like structure.
These stacked layers increase the thickness of the cell wall.
The composition of the cell wall of gram positive bacteria is as follows:
- Multiple layers of peptidoglycan.
- Theic acid.
The Gram-negative cell wall also has peptidoglycan, but a much thinner layer that is sandwiched between not one, but two plasma membranes.
Finally, the mycobacterial cell wall has a single plasma membrane, a thin layer of peptidoglycan, and a layer of mycolic acids, which are special lipids unique to mycobacteria.
In gram-negative bacteria, the cell wall contains a much lower percentage of peptidoglycan.
The gram-negative bacterial cell wall also contains an outer layer of lipopolysaccharides.
The lipopolysaccharide layer surrounds the peptidoglycan layer and acts as an endotoxin in pathogenic bacteria.
The lipopolysaccharide coating also protects gram-negative bacteria against certain antibiotics, such as penicillins.
The composition of the cell wall of gram negative bacteria
- External membrane.
Cell walls are good targets for antibiotics because bacteria need them to survive and human cells don’t.
The discovery of penicillin and how it could selectively kill bacteria without harming patients was a breakthrough in the 1940s, leading to a drastic decrease in the number of people who died from bacterial infections.
Eventually, scientists discovered that penicillin kills bacteria by inhibiting the synthesis of cell walls.
Many different antibiotics work by inhibiting the synthesis of N-acetylglucosamine acid and N-acetylneuraminic acid, two sugars that make up peptidoglycan, preventing bacteria from producing it.
This is especially important when bacteria are dividing, as they need a new cell wall material for the new cell that is forming.
As the bacterium begins to replicate, it first elongates to about twice its normal size.
As this happens, more peptidoglycan is being made to compensate for the extra surface area.
But when these antibiotics are present, the peptidoglycan cannot cross-link properly, so the cell wall is very weak in places.
Eventually enough osmotic pressure builds up for the bacteria to burst and die.
That means that all of these antibiotics that inhibit peptidoglycan synthesis are bactericidal because they kill bacteria by attacking their cell wall.