Cell Envelope: Types, Composition, Functions

The cell membrane, cell walls and outer membrane make up the cell envelope. This envelope is usually a characteristic of bacteria and other prokaryotes. It consists of the cell wall of the bacterium and its inner cell wall. The cell’s cell envelope is responsible for its structural integrity. The cell is protected by the internal turgor pressure created by a high level of macromolecules within the cell.

Different types of Bacterial Cell Envelopes

The bacterial cell envelopes are of the following three types:

Gram-positive Cell Envelope

A cell envelope of gram-positive bacteria contains a thick layer of peptidoglycan. This allows for crystal violet dyes to be retained during gram-staining. The cell wall is embedded with lipoteichoic and teichoic acid.

Gram-Positive bacteria characteristics

1. They are covered with thick peptidoglycan and cytoplasmic membranes.
2. Gram-positive bacteria is the genus of bacteria and a member of the phylum Firmicutes.
3. These bacteria retain the color from the crystal violet stain that is used in gram staining. These bacteria are purple-colored and give positive results in the Gram stain test.
4. These bacteria do not have an outer membrane.
5. They have a lower lipid level and a higher teichoic acid content.
6. They can move with the aid of locomotion organs like flagella and cilia.
7. Teichoic acid is found in the walls of Streptococcus aureus as well as Streptococcus flavius.
8. Actinomyces and Clostridium are just a few examples.

Gram negative Cell Envelope

Peptidoglycan forms a thin layer on the cell walls of Gram-negative bacteria. Because of this, their cell walls are unable to retain crystal violet stain during gram staining. Phospholipids and lipopolysaccharides make up the outer membrane.

General Characteristics of Gram-Negative Bacteria

1. The cell wall is thin without an outer layer.
2. Belonging to the phylum Firmicutes, Gram-negative bacteria are the genus of bacteria family.
3. Gram-Negative bacteria are the bunch of aerobic bacteria that do not hold the crystal violet dye during the procedure of Gram staining. Instead, they are pink when examined under the microscope.
4. Lipid is present in a high percentage in them.
5. Almost all types of amino acids are present.
6. They contain less amount of muramic acid.
7. Streptomycin is the sensitive compound for them.
8. They are ribonucleate, magnesium, and teichoic acid deficient.
9. Lipopolysaccharides, flagella and sialic acid are found in them.

Mycobacteria

Mycobacteria’s cell envelope is different from that of gram-positive and gram-negative bacteria. Although it lacks an outer membrane, mycobacteria have a cell wall made of peptidoglycan and arabinogalactan.

Composition of the Cell Envelope

1. The cell envelope includes the cell membrane, cell wall, and outer membrane.
2. Gram-positive bacteria’s cell envelope includes the cell wall, cytoplasmic, and capsule.
3. Gram-negative bacteria’s cell envelope includes the cell wall, outer membrane, cytoplasmic and capsule.

Functions of the Cell Envelope

Here are the essential functions of the cell’s envelope:

1. The bacterial cell wall is responsible for maintaining the cell’s shape and preventing it from collapsing. It also forms filamentous extensions, which can be seen in motile bacteria.
2. The movement of substances within and outside the cell is controlled by the cell membrane.
3. An extension of the plasma membrane in the cell wall creates a special structure called a mesosome. These extensions take the form of lamellae, tubules, and vesicles. These extensions aid the cell in many functions, including the synthesis of cell walls and DNA replication.

Note: A nuclear membrane is a double membrane found enclosing the cell’s nucleus. It is responsible for separating the chromosomes and the rest of the cells. There are small holes, or pores, in the nuclear membrane that allow certain materials to pass between the nucleus and cytoplasm. It is found in the eukaryotic cells. Therefore, the nuclear membrane is found in animal cells and plant cells.