What is Cell envelope? Components and Classification

Cell envelope is the combined structure of cell membrane and cell wall. In some cases, a third layer is also present in the cell envelope. The third layer is usually present in the cell envelope of the bacteria in the form of the glycocalyx, cell wall, and plasma membrane (cell membrane). The main function of the cell envelope is to provide structural integrity to the cell, the cell envelope also provides selective permeability and protection from harmful agents. In prokaryotic cells, the cell envelope functions as a protective unit against the internal turgor pressure built because of the macromolecule’s high concentration. The cell envelopes are classified into two types that are gram-positive and gram-negative. Apart from these two types, there is an additional type of cell envelope that is found only in the mycobacteria. In this article, we will look at the components of the cell envelope and its classification.

Key Terms: Cell, Cell Membrane, Function, Compotenent, Cell Wall, Mycobacteria

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What is a cell envelope?

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The cell envelope is generally composed of two layers but in several bacteria, the envelope has an additional layer called the glycocalyx. So, in bacteria, the cell envelope consists of three layers that are glycocalyx, cell wall, and cell membrane. The cell envelope of different cells can totally vary from one another in their structure and functions, but some common functions are performed by every cell envelope like providing structure to the cell. It is also the site of exchange of materials from the environment outside the cell. The structural integrity of the cell is maintained by the cell wall of the cell, whereas the cell membrane is responsible for the selective permeability of the cell that helps the cell by letting the nutrients pass in and stopping the harmful agents that are not needed by the cell. The glycocalyx of the bacterial cell is present as the outermost layer of the cell envelope and provides rigidity to the bacterial cell. In addition, it also prevents the bursting of the cell because of turgor pressure built in the cell because of the macromolecules in it.

Cell Envelope

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Components of cell envelope

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The cell envelope is composed of:

• Glycocalyx

It is kind of a mucilage sheath present as the outermost layer of the cell envelope, this layer is only present in bacteria. The glycocalyx consists of non-cellulosic polysaccharides and can or cannot contain proteins. There are two possible forms of glycocalyx based on their structures one is the slime layer and the other is the capsule. In bacteria with a slime layer, the glycocalyx is present as a loose sheath of mucilaginous covering, in the capsule the glycocalyx is thick and tough. The main function of the glycocalyx is to prevent the desiccation of the cell, phagocytosis of the cell itself, protection from other harmful microorganisms.

• Cell wall

The cell wall is the middle layer of the cell envelope and is placed between the glycocalyx and cell membrane of the bacterial cell. The main function of the cell wall is to provide shape and structural support to the cell. The bursting of the bacterial cells is also prevented by the cell wall. The cell wall is the outermost covering of the cell except in the case of cells with glycocalyx and mycoplasma.

• Cell membrane

The cell membrane is also known as the plasma membrane of the cell. The structure of the cell membrane is made up of phospholipids and proteins. The lipids provide permeability to the membrane and protein provides the structure. The plasma membrane is a semi-permeable or selectively permeable membrane which allows the passage of only selective material through it. It is also the innermost layer of the cell envelope that performs the function of the boundary for the cytoplasm of the cell. The plasma membrane is in direct contact with the cytoplasm of the cell. It is also mounted with the receptor molecules that respond to the different chemicals trying to enter the cell.

Components of the cell envelope

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Functions of cell envelope

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Some main functions of the cell envelope are as listed below:

• The cell envelope maintains the shape of the cell.
• The in and out transport of useful or useless substances is also regulated by the cell envelope.
• In the case of motile bacteria, the flagella are formed from the cell envelope itself.
• The cell envelope is responsible for the prevention of cell bursting.
• Mesosomes are also formed by the extension of the plasma membrane. Mesosomes help the cell in DNA replication and cell division.

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Classification of cell envelope

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The cell envelope is classified into two parts based on gram staining which are:

Gram-positive

The gram-positive bacteria have the cell envelope composed of the cell wall with a thick layer of peptidoglycan.

• Gram-positive bacteria show the retention (the peptidoglycan layer prevents the leaking) of the violet-colored dye at the time of the gram staining procedure.
• The actinobacteria belong to this group and are exclusively gram-positive bacteria, the actinobacteria are the bacteria with high gram-positive character.
• The firmicutes also show gram-positive staining but are with low gram-positive staining character.
• Some gram-positive bacteria may have some minute particles or structures of the cell wall of the bacteria that show negative staining with the same procedure of gram staining, the structures are embedded in the cell wall of the gram-positive bacteria these structures are teichoic acids.
• Some of these structures are also linked to the lipid resulting in the formation of the lipoteichoic acids. This lipid-linked teichoic acid is also responsible for the formation of links between the peptidoglycan layer and the cytoplasmic membrane of the cell.
• Many gram-positive bacteria have an additional layer of proteins present at the outside of the cell wall. This layer is called S-layer tiled proteins here tiled refers to the covering of the cell wall.
• The attachment and biofilm production is assisted by the S-layer of proteins. When the cell is cultured in the laboratory then most of the time the S-layer and the capsule of the cells are lost by the reductive evolution. That is the trait of having a capsule and the S-layer is lost because of the absence of positive selection.

Gram Positive and Gram Negative

Gram-negative

Gram-negative bacteria have the cell envelope composed of the cell membrane, cell wall, and a very thin layer of peptidoglycan.

• The layer is so thin that the layer can be considered as absent in gram-negative bacteria. So due to the lack of the peptidoglycan layer, the cell wall of the gram-negative bacteria is not able to retain the crystal violet dye at the time of the gram staining.
• The gram-negative bacteria have an additional layer of phospholipids and lipopolysaccharides which acts as the outermost layer in gram-negative bacteria, the lipopolysaccharides are responsible for the negatively charged membrane in gram-negative bacteria.
• The lipopolysaccharide in the outermost covering is a unique character. And responsible for the antigenic properties of the bacteria that show negative gram staining.
• Due to the presence of an extra layer of phospholipids, the lipid portion of the membrane becomes impermeable for the charged particles. But the ions, sugar, and amino acids are passively transported by the channels present in the membrane called porins.
• In the gram-negative bacteria, the periplasm contains the peptidoglycan, and proteins in higher concentration resulting in turning the periplasm into more gel-like instead of liquid.
• In gram-negative bacteria, the proteins help the transport of the substrate by allowing it to bind with itself and then pass through the plasma membrane and then go into the cytoplasm.
• These proteins are known as the transport and signal proteins. Many gram-negative bacteria also have the S-layer of protein along with a capsule which is lost at the time of laboratory cultivation.

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Mycobacteria

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The mycobacteria are the group of bacteria that is neither gram-positive nor gram-negative. The periplasm is absent in the mycobacteria but there is another significant wall of peptidoglycan-arabinogalactan-mycolic acid that acts as a “pseudo periplasm”. Pseudo Periplasm is present between the plasma membrane and cell wall of the bacteria. The most resilient way to identify the mycobacteria is Ziehl–Neelsen stain or acid-fast stain in this stain the bacilli are stained bright red and confirm that the stained bacteria is the mycobacteria. Mycobacteria are also called the “acid-fast bacteria”.

Mycobacteria

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Things to remember

• The cell envelope is generally composed of the cell wall, cell membrane, and glycocalyx.
• The main function of the cell envelope is providing structure to the cell and it is also the site of exchange of materials.
• The cell envelopes are classified into two types that are gram-positive and gram-negative based on gram staining.
• The cell wall is the outermost covering of the cell except in the case of cells with glycocalyx and mycoplasma.
• It is kind of a mucilage sheath present as the outermost layer of the cell envelope, this layer is only present in bacteria.
• The lipids provide permeability to the membrane and protein provides the structure.
• The gram-positive bacteria have a cell envelope composed of the cell wall with a thick layer of peptidoglycan.
• The gram-negative bacteria have an additional layer of phospholipids and lipopolysaccharides which acts as the outermost layer in gram-negative bacteria.
• The mycobacteria are the group of bacteria that is neither gram-positive nor gram-negative.

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