Functional Anatomy of Prokaryotic and Eukaryotic Cells


5. Describe and give the function of:

· Capsule

· Slime layer

· Flagella

· Axial filament

· Fimbriae

· Pili (sex pili)

6. Describe the chemical composition of the glycocalyx.

Typical structures of a prokaryotic cell

Structures External To The Cell Wall



1. The glycocalyx (capsule, slime layer, or extra cellular polysaccharide) is a gelatinous polysaccharide and/or polypeptide covering.

2. Capsules may protect pathogens from phagocytosis.

3. Capsules enable adherence to surfaces, prevent desiccation, and may provide nutrients.




1. Flagella are relatively long filamentous appendages consisting of a filament, hook, and basal body.

2. Prokaryotic flagella rotate to push the cell.

3. Motile bacteria exhibit taxis; positive taxis is movement toward an attractant, and negative taxis is movement away from a repellent.

4. Flagellar (H) protein functions as an antigen.



Arrangement of bacterial flagella


The structure of a prokaryotic flagellum


The filament is composed of the globular protein flagellin, which is arranged in several intertwined chains that form a helix around a hollow core.


Flagellin can vary in structure and is used to identify some pathogenic bacteria serologically. The flagellar antigens are referred to as H antigens.


E. coli may express any of at least 50 different variants; serovars (serological variants) identified as O157:H7 are associated with food borne epidemics ( O antigens are somatic antigens and are lipopolysaccharide complexes associated with the cell wall).


Flagella are anchored by pairs of rings associated with the plasma membrane and cell wall. Gram positive bacteria have only the inner pair of rings.


Flagella and bacterial motility


Axial Filaments


1. Spiral cells that move by means of an axial filament (endoflagellum) are called spirochetes.

2. Axial filaments are similar to flagella, except that they wrap around the cell.


Axial filaments

Fimbriae And Pili


1. Fimbriae and pili are short, thin appendages.

2. Fimbriae help cells adhere to surfaces.

3. Pili join cells for the transfer of DNA from one cell to another.