Functional Anatomy of Prokaryotic and Eukaryotic Cells


3. Describe and give the function of:

  • Capsule
  • Slime layer
  • Flagella
  • Axial filament
  • Fimbriae
  • Pili

4. What is flagellin? What is the H antigen?

Typical structures of a prokaryotic cell

Structures External To The Cell Wall


The glycocalyx (capsule, slime layer, or extra cellular polysaccharide) is a gelatinous polysaccharide and/or polypeptide covering. The exact chemical composition varies depending on the species.

Capsules are organized and firmly attached to the cell wall.

Capsules may protect pathogens from phagocytosis.

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

Slime layers are unorganized and loosely attached to the cell wall.

Extracellular polymeric substance (EPS) is the description of a glycocalyx that is a component of a biofilm.


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

Prokaryotic flagella rotate to push the cell.

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


Arrangement of Bacterial Flagella


The Structure of a Prokaryotic Flagellum

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.

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 and Bacterial Motility

Axial Filaments

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

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

Axial filaments

Fimbriae And Pili

Fimbriae and pili are short, thin appendages.

Cells may have many fimbriae, which help the cells adhere to surfaces.

Cells that have pili have only one or two.

Pili join cells either for the transfer of DNA from one cell to another (sex pili) or are used for special types of movement; twitching, seen in Pseudomonas aeurginosa, Neisseria gonorrhoeae and some strains of E. coli, or the gliding motility of myxobacteria.