Cell Morphology Quiz

Study Notes

Prokaryotic cells, bacteria and archaea, have three major shapes: coccus (spherical), rod or bacillus (cylindrical), and spirillum (curved or spiral).
Some cells form characteristic arrangements after cell division, like chains of Streptococcus, cubes of Sarcina, and grapelike clusters of Staphylococcus.
Unusual shapes include spirochetes (tightly coiled), appendaged bacteria, and filamentous bacteria.
There are many variations on the basic morphological types.
There is no direct relationship between morphology and the physiology, ecology, phylogeny, or other properties of a prokaryotic cell.
Morphology may be influenced by selective forces to optimize certain factors:
- Nutrient uptake: Smaller cells or cells with high surface-to-volume ratios (e.g., appendaged cells) maximize nutrient uptake.
- Swimming motility: Helical or spiral-shaped cells are efficient at swimming in viscous environments or near surfaces.
- Gliding motility: Filamentous bacteria exhibit gliding motility.

Prokaryotes range in size from 0.2 µm to >700 µm in diameter.
Most cultured rod-shaped bacteria are between 0.5 and 4.0 µm wide and 600 µm in diameter.
Small size benefits bacteria by providing a higher surface-to-volume ratio.
Small size allows for faster nutrient exchange and growth rates.
Evolutionary changes are often more rapid in smaller organisms.
Small cells are advantageous for rapid evolution and adaptation.