Functions of the cell envelope

The bacterial cell envelope, which includes different membranes and other structures surrounding and protecting the cytoplasm, is far from simple. Except for cells of higher species, the bacteria live in a frequently hostile and uncertain environment. Bacteria have created a sophisticated and intricate cell envelope that shields them from the outer environment. It also allows a selective flow of nutrients and waste products between the interior and exterior of the cell. The following article concerns the composition, types, and essential functions of the cell envelope in bacteria. 

Composition of a cell envelope

What is the bacterial cell envelope composed of? The cell envelope comprises the bacteria’s cell wall and cell membrane. It also consists of an outer membrane, mainly present in a particular type of bacteria. 

Since the late 1830s, scientists have known that all living organisms are made up of basic units called cells. The plasma membrane defines the cell as a discrete entity with a defined boundary. That is to say; the biological membrane contains the basis of the living state; it is a vital characteristic of all living organisms.

This membrane comprises phospholipids, carbohydrates, proteins, and cholesterol. The cell walls of bacteria are made of a large polymer called peptidoglycan. This polymer is composed of disaccharide N-acetylglucosamine-N-acetyl-muramic acid connected by the pentapeptide side chains. 

The outer membrane in certain bacteria( gram-negative) comprises phospholipids and glycolipids ( especially lipopolysaccharides). 

Types of cell envelope

The structure, composition, and functions of the cell envelope vary in different types of bacteria. The types of cell envelopes found in bacteria can be segregated into

The cell envelope of Gram-negative bacteria

The Gram-negative bacterial cell envelope consists of an outer membrane, a peptidoglycan cell wall, and an inner membrane. There is also an aqueous periplasmic layer between the membranes. 

The cell wall of Gram-negative bacteria contains a thin layer of peptidoglycan. Additionally, the cell wall has an outer membrane facing the outside environment.   

The inner leaflet of the outer membrane has a phospholipid layer. In contrast, the outer leaflet is made of glycolipid, particularly Lipopolysaccharide. The gram-negative cell wall has a negative charge due to the highly charged structure of lipopolysaccharides. Many of the antigenic features of bacteria are due to the chemical structure of lipopolysaccharides in the outer membrane. This feature is typically unique to individual bacterial strains, i.e. the subspecies.

The gel-like periplasm layer consists of peptidoglycan and a high concentration of proteins. 

The cell envelope of Gram-positive bacteria

The cell envelope of Gram-positive bacteria varies from the Gram-negative cell envelope in various respects. Firstly, they lack the outer membrane covering the cell wall. 

However, there is a layer of cell-surface proteins in certain Gram-positive bacteria, forming an S-layer. A capsule of polysaccharides often protects the S-layer. 

Polyalcohols are present in the Gram-positive cell wall, known as teichoic acid. Some of these polyalcohols are linked to lipids, thus forming lipoteichoic acids. These lipoteichoic acids connect the cytoplasmic membrane and the peptidoglycan. 

The cell envelope of Acid-fast bacteria

The cell envelope of Mycobacteria is unlike that of gram-positive or gram-negative bacteria. Their cell envelope lacks the outer membrane found in gram-negative bacteria, but it does include a substantial peptidoglycan-arabinogalactan-mycolic acid wall that acts as an outer permeability barrier. As a result, a unique ‘pseudo periplasm’ compartment is considered to exist between the cytoplasmic membrane and this external barrier.

An example of acid-fast bacteria is Mycobacteria. Due to the high amount of mycolic acid present in Mycobacteria has a poor absorption capacity for most stains, except the acid-fast stain.

Functions of cell envelope

The functions of the cell envelope in bacteria are as follows:

• The cell envelope provides a shape or structural integrity to the bacterial cell. The rigid skeleton of a cell is due to the presence of the large polymer, peptidoglycan, in its cell envelope. This polymer gives rigidity to the cell and maintains its shape. 
• Another function of the cell envelope is to help transport nutrients between the cell’s interior and the outer environment. 
• It acts as a shield and protects the bacterial cell from the outer uncertain and hostile environment. 
• In many bacteria, the cell envelope provides the surface upon which many chemical reactions occur. 
• The flagella are formed from the cell envelope in the case of motile bacteria. 
• In the case of Gram-positive bacteria, the S-layer helps in adhesion and biofilm production. On the other hand, the capsule helps the bacteria avoid phagocytosis by the host.

Conclusion

The cell envelope of a bacterial cell consists of several layers. These layers act as a shield to protect the bacteria from the uncertain outer environment. The cell envelope of bacteria can be segregated into different types based on their structure and composition. They are the cell envelopes of Gram-positive bacteria, Gram-negative bacteria, and Acid-fast bacteria. There are several functions of the cell envelope. Its primary role is to protect the bacterial cell and provide structural integrity.