Kingdoms of Life: Classification & Characteristics
Kingdoms of Life
Biological Classification
In biology, a kingdom is a major subdivision of living organisms, categorized based on shared characteristics. The earliest classification into kingdoms dates back to Aristotle, who distinguished between the animal, vegetable, and mineral kingdoms.
Key historical classifications include:
- Carl Linnaeus (1735): Vegetalia and Animalia
- Ernst Haeckel (1866): Protista, Plantae, Animalia
- Whittaker (1969): Monera, Protista, Fungi, Plantae, Animalia
Kingdoms are further divided into phyla (singular: phylum for animals) or divisions (for plants and other organisms). These are then subdivided into classes, orders, families, genera, and species.
Example: Linnaean Classification of Humans
- Kingdom: Animalia (heterotrophic, eukaryotic, multicellular organisms without cell walls)
- Phylum: Chordata (organisms with a notochord)
- Class: Mammalia (organisms with mammary glands, homeothermic, with hair)
- Order: Primates (forward-facing eyes, opposable thumb)
- Family: Hominidae (developed brain and neocortex, stereoscopic vision)
- Genus: Homo (curved spine, bipedal)
- Species: Homo sapiens (thin skull bones, vocalization capacity)
Monera
The term Monera designates a clade of prokaryotic, single-celled organisms lacking defined nuclei. They are considered the oldest life forms.
General Characteristics:
- Cellular Level: Mostly unicellular, prokaryotic cells
- Nutrition: Osmotrophic; heterotrophic (saprophytic, parasitic, or symbiotic) or autotrophic (photosynthesis or chemosynthesis)
- Oxygen Use: Anaerobic, aerobic, or microaerophilic
- Reproduction: Asexual (binary fission); sexual (conjugation)
- Locomotion: Bacterial flagellum or absent
Domain Archaea:
- Archaeabacteria (Mendosicutes)
- Methanocreatrices
- Halophilic and thermoacidophilic bacteria
Domain Bacteria:
- Eubacteria (Tenericutes/Gracilicutes)
- Spirochaetae
- Thiopneutes
- Anaerobic phototrophic bacteria
- Cyanobacteria
- Chloroxybacteria
- Aerobic nitrogen-fixing bacteria
- Pseudomonads
- Omnibacteria
- Chemoautotrophic bacteria
- Myxobacteria
- Firmicutes
- Fermentative bacteria
- Aeroendospora
- Micrococci
- Actinobacteria
Protista
Protists are primarily aquatic or inhabit moist environments.
Characteristics:
- Cellular Organization: Eukaryotic, unicellular or multicellular
- Structure: Some (e.g., brown algae) exhibit tissue-like complexity. Unicellular forms often have cilia or flagella for movement.
- Nutrition: Autotrophic (photosynthesis) or heterotrophic; some are both.
- Oxygen Metabolism: Aerobic, with some secondarily anaerobic.
- Ecology: Important components of plankton, benthic communities, and soil.
Fungi
Fungi are eukaryotes that perform external digestion.
Characteristics:
- Cell Wall: Rigid, containing mannans, glucans, and chitin.
- Structure: Composed of hyphae (filaments) that form the mycelium.
- Identification: Colony appearance, hyphae type, sporophore placement, spores, rhizoids.
Classical Classification:
- Amoeboid slime molds (Myxomycota, Ascomycota, Plasmodiophoromycota)
- Absorptive fungi (Oomycota, Chytridiomycota)
- True fungi (Eumycota): Zygomycetes, Ascomycetes, Deuteromycetes, Basidiomycetes
General Characteristics:
- Cellular Level: Eukaryotic
- Nutrition: Osmotrophic
- Oxygen Metabolism: Aerobic or facultative anaerobic
- Reproduction: Sexual and asexual
- Organization: Multicellular (hyphae forming mycelium) or unicellular
Plantae
Plants, also known as embryophytes, are multicellular autotrophs.
Characteristics:
- Cellular Level: Eukaryotic
- Nutrition: Photosynthesis
- Oxygen Metabolism: Aerobic
- Reproduction: Sexual and asexual
- Structure: Multicellular, with tissues; cellulose cell wall; plasmodesmata
Divisions:
- Multicellular algae
- Terrestrial plants
- Bryophytes (mosses, liverworts)
- Vascular plants
- Pteridophytes
- Seed plants (Spermatophyta)
Plantae (Archaeplastida/Primoplantae) are characterized by the primary acquisition of chloroplasts.