Cell Envelope and its modifications

Most prokaryotic cells, especially bacteria have chemically complex cell envelope which consists of a tightly bound 3 layered structure.

• Glycocalyx (outermost)
  
• Cell wall
  
• Plasma membrane
  

Though each has different functions, all of them act together as a protective unit.

Bacteria can be

• Gram positive (those that take up gram stain) (contains teichoic acid) (eg: Staphylococci, Streptococci, Bacillus, Clostridium) [see wikipedia article about cell wall of Gram +ve]
  
• Gram negative (those that do not take up gram stain) (contains Lipopolysaccharides (LPS), also known as lipoglycans) (eg: Escherichia coli, Salmonella, Shigella) [See wikipedia article about cell wall of Gram -ve]
  

on the basis of differences in the cell envelopes and the manner in which they respond to the staining procedure developed by Gram.

Glycocalyx differs in composition and thickness. It actually is a network of polysaccharides that project from cellular surfaces. It can be:

• Slime layer (a loose sheath) (The function of the slime layer is to protect the bacteria cells from environmental dangers such as antibiotics and desiccation). (autoclaving or flushing with boiling water are the only certain methods of decontamination)
• Capsule (thick and tough) (prevents phagocytosis, ie engulfing solid particles by the cell membrane to form an internal phagosome)

Cell wall determines the shape of the cell and provides structural support (prevents bursting or collapsing) It is made up of peptidoglycan (also called murein)

Plasma Membrane is semi-permeable and interacts with outside world. It is similar to eukaryotic plasma membrane

The mesosome is a special membraneous structure formed by the extension of plasma membrane into the cell. They may be in the form of

• Vesicles
  
• Tubules
  
• Lamella
  

They help in

• Cell wall formation
  
• DNA replication and distribution to daughter cells
  
• Respiration
  
• Secretion
  
• Increasing surface area of plasma membrane and enzymatic content
  

(But actually whether they perform all these functions is in doubt [:D])

In some prokaryotes like cyanobacteria, there are other membraneous extensions called chromatophores that contain pigments. (Chromatophores are responsible for skin colouring in cold blooded animals too)

Motility:
Bacteria may be motile or non-motile. If motile, they will have thin filamentous extensions from the cell wall called flagella.

Flagellum : is different from eukaryotic flagellum. Is composed of three parts:

• Basal body (embedded in cell envelope)
  
• Hook (embedded in cell envelope)
  
• Filament (longest portion and extends from cell surface to the outside)
  

Based on the arrangement of flagella there can be bacteria of type:

• Monotrichous : single flagellum at one end
  
• Lophotrichous: many flagella at one end
  
• Amphitrichous : flagella at both ends
  
• Peritrichous: flagella all over the body.
  

Besides flagella there are structures, that do not play a role in motility:

• Pili: elongated tubular structures made of a special protein (oligomeric pilin proteins)
  (Pili connect a bacterium to another of its species, or to another bacterium of a different species, and build a bridge between the cytoplasms of the cells. This enables the transfer of plasmids between the bacteria. An exchanged plasmid can code for new functions, e.g., antibiotic resistance.)
  (During bacterial conjugation, a sex pilus emerging from one bacterium ensnares the recipient bacterium, draws it in, and eventually triggers the formation of a mating bridge, which establishes direct contact, merging the cytoplasms of two bacteria via a controlled pore. This pore allows for the transfer of bacterial DNA from the bacteria with the pilus (donor) to the recipient bacteria. Through this mechanism of genetic transformation, advantageous genetic traits can be disseminated amongst a population of bacteria. Not all bacteria have the ability to create sex pili, however sex pili can form between bacteria of different species.)
• Fimbriae: small bristle like fibres sprouting out of the cell. (helps bacteria attach to rocks in streams or to host tissues) (these are actually small pili that help to attach bacteria together when the pili is used for connecting)