|Microorganism Category|Cellular Characteristics|Nuclear Features|Structural Features|Morphological Features|Special Behaviors or Functions|
| ---- | ---- | ---- | ---- | ---- | ---- |
|Bacteria|Unicellular prokaryotes|No true nucleus, circular DNA in nucleoid region|Cell wall (in most cases, composition varies like peptidoglycan in some), may have flagella, pili etc.|Coccus (spherical), bacillus (rod - shaped), spirillum (spiral - shaped) etc.|Can perform various metabolic processes like fermentation, nitrogen - fixation; some are pathogenic, others are beneficial in digestion, decomposition |
|Archaea|Unicellular prokaryotes|No true nucleus, circular DNA in nucleoid region|Cell wall (but lacks peptidoglycan), unique membrane lipids|Diverse shapes similar to bacteria|Adapted to extreme environments such as hot springs, salt lakes; unique metabolic pathways |
|Fungi|Unicellular (e.g., yeasts) or multicellular (e.g., molds)|Eukaryotic, have a true nucleus with nuclear membrane|Cell wall (made of chitin), complex cellular organelles|Yeasts are usually round or oval; molds form filamentous hyphae|Decompose organic matter; some are used in food production (e.g., yeast in baking), some are pathogenic to plants and animals |
|Protozoa|Unicellular eukaryotes|Eukaryotic, have a true nucleus with nuclear membrane|May have structures for movement like pseudopods, flagella, cilia|Diverse shapes depending on the locomotion structure|Heterotrophic, some are free - living in water, some are parasitic in animals and cause diseases |
|Algae|Unicellular or multicellular eukaryotes|Eukaryotic, have a true nucleus with nuclear membrane|Cell wall (composition varies), chloroplasts for photosynthesis|Varied shapes including spherical, filamentous|Photosynthetic, produce oxygen; form the base of aquatic food chains |
|Viruses|Acellular|Neither prokaryotic nor eukaryotic, just nucleic acid (DNA or RNA)|Protein coat (capsid) surrounding nucleic acid, some have an envelope|Simple geometric shapes like icosahedral, helical|Obligate intracellular parasites, can only reproduce by using the cellular machinery of host cells; can cause diseases in various organisms |
|Multicellular Animal Parasites (Helminths)|Multicellular|Eukaryotic, have true nuclei with nuclear membrane|Complex body structures with tissues and organs|Flatworms are flat - bodied, roundworms are cylindrical|Parasitic lifestyle, live in or on host organisms and obtain nutrients at the host's expense, causing various diseases |