Functional Anatomy of Prokaryotic and Eukaryotic Cells


2. List the three most common shapes of prokaryotic (bacterial) cells; give the different arrangements and specific examples of each.

The Prokaryotic Cell

Bacteria are unicellular and most multiply by binary fission.

Bacterial species are differentiated by morphology, chemical composition, nutritional requirements, biochemical activities, and source of energy.

The Size, Shape, And Arrangement Of Bacterial Cells

Most bacteria are 0.2 um in diameter and 2-8 um in length.

The three basic bacterial shapes are coccus (spherical), bacillus (rod-shaped), and spiral (twisted), however pleomorphic bacteria can assume several shapes.

Arrangement of cocci

Cocci may be oval, elongated, or flattened on one side.

Cocci may remain attached after cell division. These group characteristics are often used to help identify certain cocci.


Cocci that remain in pairs after dividing are called diplococci.




Cocci that remain in chains after dividing are called streptococci.




Cocci that divide in two planes and remain in groups of four are called tetrads.





Cocci that divide in three planes and remain in groups cube like groups of eight are called sarcinae.




Cocci that divide in multiple planes and form grape like clusters or sheets are called staphylococci.


Since bacilli only divide across their short axis there are fewer groupings.

Bacillus is a shape (rod shaped) but there is also a genus of bacteria with the name Bacillus. You wouldn't confuse the two, since you know the rules for writing the genus and species names of organisms, right????


Most bacilli appear as single rods. Diplobacilli appear in pairs after division.






Streptobacilli appear in chains after division.






Some bacilli are so short and fat that they look like cocci and are referred to as coccobacilli.


Spiral bacteria

Spiral bacteria have one or more twists.



Vibrios look like curved rods.





Spirilla have a helical shape and fairly rigid bodies.






Spirochetes have a helical shape and flexible bodies. Spirochetes move by means of axial filaments, which look like flagella contained beneath a flexible external sheath.

Other shapes

Stella are star-shaped.
Haloarcula, a genus of halophilic archaea, are rectangular.