Cell Envelope

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 inner cell and wall of bacteria cells that surrounds the cell. 


This thick layer provides 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.   


Now, let us proceed with the other vital aspects of a cell envelope. 


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-positives, 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. 


Components of Cell Envelope

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The bacterium cell wall consists of mainly three layers, the glycocalyx, cell wall and lastly plasma membrane. The composition of 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, 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, mesosome. Mesosome is the extended part of this plasma membrane. They help in DNA replication, formation of cell wall 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, bacterial flagellum divided into three sections, filament, hook and basal body.  


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

Bacterial cell wall conducts a series of functions. 

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

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

  • This cell wall is porous, thus helps 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. 


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FAQ (Frequently Asked Questions)

1. What is the Function Cell Envelope?

Ans. Cell envelope is the outermost layer of the cells of bacteria. The function of it includes the protection of cells, maintaining cellular shape, allowing growth and metabolism of cells.

2. Which Eukaryotes have a Cell Membrane?

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

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

Ans. 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.