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Cell Envelope

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An Introduction

The protective outermost cover of prokaryotes is called a cell envelope. A cell envelope involves a cell wall, cell membrane and in some cases, outer membrane. In simple words, it is a union of an inner cell and a wall of bacteria cells that surrounds the cell. 

 

This thick layer provides the structural rigidity of a cell. Prokaryotes usually inhabit a dilute atmosphere. Therefore the concentration of inner macromolecules is higher than the outer part. So, it acts as a single protective unit of a bacterium cell. The cytoplasmic membrane, which divides the inside of the cell from its external environment, regulates the flow of nutrients, maintains the right intracellular milieu, and prevents the loss of the cell's contents, is the only trait shared by all cells. Many essential cellular processes are performed by the cytoplasmic membrane, including energy synthesis, protein secretion, chromosomal segregation, and efficient active transport of nutrients. It's a typical unit membrane made up of proteins and lipids, and it's quite similar to the membrane that encircles all eukaryotic cells. It appears as a triple-layered structure of lipids and proteins that fully surrounds the cytoplasm in electron micrographs.

 

Classification of Cell Envelope

Based on gram staining, bacteria are classified into two parts. 

  • Gram-Positive

This group of bacteria has a thick Peptidoglycan layer that prevents the crystal purple dyes from leaking out while the gram staining process takes place. 

 

They also contain Lipoteichoic and Teichoic acids. Teichoic acid is responsible for giving a negative charge to gram-positive bacteria because this acid shares a Phospodeister bond with monomers.  

  • Gram-Negative

Unlike gram-positive, these bacteria have a thin Peptidoglycan layer. Because of this structure, this layer cannot contain the crystal purple dye. It means they can be easily washed off when the gram staining process occurs. 

 

Gram-negative bacterial cell envelope consists of phospholipids and lipopolysaccharides. The latter provides a negative charge to gram-negative bacteria. It also safeguards the bacteria from certain drugs.

 

Mycobacteria

Apart from these two types of bacteria, there is another one that is entirely different from the two. The cell envelope of these bacteria does not possess the outer membrane. Instead, it has an exclusive cell wall made of arabinogalactan, peptidoglycan and mycolic acid that acts as an external barrier. 

 

Bacteria without a Cell Wall composed of Peptidoglycans

Bacteria without a cell wall are composed of peptidoglycans. The obligate intracellular bacteria in the Chlamydiaceae family have a peculiar morphology in that their infectious forms have no detectable levels of peptidoglycan in their cell walls. Instead, the extracellular forms of these gram-negative bacteria rely on a layer of disulfide bond cross-linked cysteine-rich proteins situated between the cytoplasmic and outer membranes, similar to the peptidoglycan layer in other gram-negative bacteria, to preserve structural integrity. The disulfide cross bond is absent in the bacterium's internal forms, making this form more mechanically brittle.

 

The bacterial class mollicutes have no cell walls in their cell envelopes. Mycoplasma and ureaplasma are the most common pathogenic bacteria in this group. L-form bacteria are strains of bacteria that lack cell walls yet are produced from bacteria that do.

 

Components of Cell Envelope

The bacterium cell wall consists of mainly three layers, the glycocalyx, cell wall and lastly plasma membrane. The composition of a glycocalyx varies for different types of bacteria. The loose sheath-like one is called a slime layer, and the tough and thick one is called a capsule. Also, the Plasma membrane is a thin and semi-permeable layer that contracts with the outer part of the cell. The composition of plasma membranes remains the same even for eukaryotes.  

 

It consists of another membrane-like structure, the mesosome. Mesosome is the extended part of this plasma membrane. They help in DNA replication, the formation of cell walls and daughter cell distribution.  

 

Another component of the bacterial cell wall is a pill, a long structure made of pilin protein. Also, there is another small bristle-like fibre, fimbriae. It helps the bacteria to attach to different surfaces. 

 

Bacterium cells also comprise a varied arrangement of flagella. More specifically, the bacterial flagellum is divided into three sections, filament, hook and the basal body.  

 

The most gram-positive bacteria cell wall consists of a cell wall, a cytoplasmic membrane and a capsule. 

 

The most gram-negative bacteria cell wall consists of a cell wall, a cytoplasmic-membrane, outer membrane, periplasmic space and capsule. 

 

Purpose of Cell Envelope

The bacterial cell wall conducts a series of functions. 

  • An essential function of it is to withstand the hydrostatic pressure of cells.

  • Periplasmic space between the plasma membrane and cell wall helps the gram-positive bacteria to digest things. 

  • This cell wall is porous, thus helping the large molecules to move in and out of cells. 

  • It helps in protein secretion, transportation of nutrients, etc. 

 

Multiple Choice Questions

1. Which one lacks a cell wall?

  1. Bacterial cells

  2. Plant cells

  3. Protozoa

  4. Algae

 

2. What is S-layer?

  1. Surface layer

  2. Secondary layer

  3. Soluble layer

  4. Solid layer

Answers:  1-c), 2-a)

 

Fill in the Blanks

1. Cord factor is a _______________ derivation. 

2. Glycerol residues present in ______________ acids. 

Answers: 1-mycolic acid, 2-Teichoic. 

 

Conclusion

A cell envelope is a union of an inner cell and a wall of bacteria cells. It contains the cytoplasmic membrane, which divides the inside of the cell from its external environment. It regulates the flow of nutrients, maintains the right intracellular milieu, and prevents the loss of contents.


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FAQs on Cell Envelope

1. What is the Function Cell Envelope?

The cell envelope is the bacterial cell's outermost coat, and it serves a variety of purposes including cell protection, communication with the outside world, cellular shape preservation, cell stability and stiffness, and cell metabolism, growth, and division. Its functions include cell defence, maintaining cellular structure, and facilitating cell development and metabolism. Carbohydrate-based and carbohydrate-containing macromolecules are important components of the cell envelope, and they play an important role in all of these processes, regardless of whether the cell envelope is Gram-positive, -negative, mycobacterial, or archaeal.

2. Which Eukaryotic and Prokaryotic Cell has a Plasma membrane?

The plasma membrane, a double layer of lipids that divides the cell interior from the outside environment, is found in both prokaryotic and eukaryotic cells. Phospholipids, which are specialised lipids, make up the majority of this double layer.


A phospholipid is composed of two hydrophobic, water-fearing fatty acid tails and a hydrophilic, water-loving phosphate head. Phospholipids form a double-layered structure naturally, with their hydrophobic tails pointing inside and their hydrophilic heads facing outward. Many biological membranes have this energetically advantageous two-layer structure, known as a phospholipid bilayer.

3. What is the difference between a positive and a negative E Coli gram?

Escherichia coli is a gram-negative, rod-shaped bacteria. Warm-blooded creatures have it in their lower gut. E Coli is mostly harmless. E. coli is a facultative anaerobic bacterium that is Gram-negative and rod-shaped. Theodor Escherich was the first to characterise this bacterium in 1885. The majority of E. coli strains inhabit people and animals' gastrointestinal tracts as natural flora. However, certain strains of E. coli have developed into pathogenic E. coli as a result of virulence factors acquired via plasmids, transposons, bacteriophages, and/or pathogenicity islands. Serogroups, pathogenicity mechanisms, clinical signs, and virulence factors can all be used to classify pathogenic E. coli.

4. What is the use of Gram Stain?

Gram staining is a bacteriological laboratory method that divides bacteria into two categories based on the physical features of their cell walls (gram-positive and gram-negative). Because these microbes produce widely varied responses that do not follow their evolutionary groupings, Gram staining is not utilised to categorise archaea, previously archaebacteria.


Some organisms are gram-variable (meaning they can stain either negative or positive), while others are not visible because neither of the Gram dyes can be used to stain them. In a contemporary environmental or molecular microbiology lab, genetic sequences and other molecular methods, which are significantly more specific and informative than differential staining, are used for the majority of identification.

5. What makes a membrane different from an envelope?

The primary distinction between the nuclear membrane and the nuclear envelope is that the nuclear membrane serves as a selective barrier between the nucleoplasm and the cytoplasm, whereas the nuclear envelope serves as a structure that separates the nucleus' content from the cytoplasm. In addition, the nuclear membrane is a lipid bilayer, whereas the nuclear envelope is made up of two nuclear membranes and nuclear pores.


In all eukaryotic cells containing a nucleus, the nuclear membrane and nuclear envelope are two structural components of the nucleus.

6. Which Eukaryotes have a Cell Membrane?

Not all eukaryotes have a cell membrane. Only a few protists, plants and fungi cells have it.

7. What is E Coli gram, Positive or Negative?

Escherichia coli is a rod-shaped, gram-negative bacterium. It is seen in the lower intestine of warm-blooded organisms. Mostly E Coli is harmless.