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?
posted by GMO Pundit at 8:56 pm
3 Comments:
I find it interesting that the axial filaments on spiral bacteria are (apparently) homologous to flagella.
Microbe Pundit.
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.
How wide are cilia?
Hw wide are "flagella" of Eukaryotic cells?
Pundit Question
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