Friday, February 24, 2006

Prokaryotic cell envelopes and appendages.


(EM thin section image from here.)


Chapter 2 of Microbe deals with the outer layers of bacterial cells. It explains how these are complex in structure.
Important points are
The composition, content and function of the cell membrane.
Protection of the cell membrane in Gram positive and Gram negative bacteria.
The operation and assembly of the flagellum

Other links with images
Cells alive


Study question:

How are bacterial flagella and pili (=fimbrae) different from one-another in terms of structure, mode of assembly, and function?

4 Comments:

Blogger Archeak9 said...

Structure:
Flagella:
15nm diam x 15um long . Helical polymer with Left hand twist. Made of Flagellin monomers 53Kd. Assembly has 5.5 monomers per turn around hollow core, creating impression of 11 protofilaments. Also connective hook and basal body (Proton gradient powered motor ). Sometimes covered with membranous sheath.

Pili:
3-10nm diam (sex pili at large end) x several micrometers. Made from pilins (15-16Kd monomers), helically arranged.

Function:
Flagella:
propulsion(counter clockwise) tumbling/change direction (clockwise)
Pili:
1/ attachment to solids,
2/ mating SSDNA & protein transfer conduit.
3/ motility by adhesion and disassembly from membrane end.

Mode of assembly:
Flagella:
subunits pass through hollow core and add to end
Pili:
subunits attached/detached at membrane end.

Found a comment about flagella protofilaments disociating under clockwise rotation resulting in tumbling but this seems to contradict the helical structure 5.5 monomers per turn description???

March 01, 2006 8:49 pm  
Blogger Microbe Pundit said...

I find it interesting that the axial filaments on spiral bacteria are (apparently) homologous to flagella.

Microbe Pundit.

March 06, 2006 5:26 pm  
Anonymous corona said...

Structure of flagella

Flagella are composed of three parts: the basal body, hook and filament. The basal body is made up of 15 proteins with 4 rings (G-ves) or 2 (G+ves) which embed in the membrane. The hook acts a join and is made of a single protein. The protein is the hollow ‘tail’ of the flagella which is made of flagellin. This is very rigid and is able to spontaneously aggregate in the right ionic environment.

Structure of pili

In contrast, the structure of pili is less complex. Pili are straight rods made of pilin protein that also extend from the cell.

Mode of assembly of flagella

The first step in the assembly of flagella is that the basal body is assembled and embedded into the cell envelope. The hook is subsequently attached. Flagellin molecules are subsequently passed through the structure and are added at the tip to create the filament. Interestingly, there is a limit to how long the filament can be and some bacteria (such as E.coli) are able to limit its length.

Mode of assembly of pili

Pili are assembled by the addition of pillin proteins at the membrane edge (c.f. flagella where subunits are added at the tip)

Function of flagella

Flagella contribute to the motility of bacterial cells by rotating and acting as a bacterial motor. This process is driver by the re-entry of protons into the cell (which is like proton motive force in reverse).

Function of Pili

Pili have numerous functions including attachment, transfer of proteins and nucleic acids to other cells and motility. Pili motility is not provided by a rotating mechanism (like flagella) but instead by extension and subsequent depolymerisation and retraction. Pili are also an important antigen.

March 03, 2007 3:02 pm  
Anonymous Anonymous said...

How wide are cilia?
Hw wide are "flagella" of Eukaryotic cells?

Pundit Question

March 04, 2007 7:47 pm  

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