Definition
Domain Bacteria: prokaryotic (no nucleus), typical cell walls, found almost everywhere.
Domain Archaea: prokaryotic but structurally/genetically distinct from bacteria — many live in extreme environments (hot springs, salt lakes).
Domain Eukarya: organisms with a true nucleus — contains all four eukaryotic kingdoms.
Remember
Archaea and Bacteria look similar under a microscope (both prokaryotic, no nucleus), but differ enough in cell chemistry and genetics to be placed in separate domains.
Definition
Protista: mostly unicellular eukaryotes.
Fungi: eukaryotic, mostly multicellular, heterotrophic (absorb food rather than making or ingesting it) — this is why fungi are classed as heterotrophs even though they don't "eat" like animals.
Plantae: multicellular, photosynthetic (make their own food).
Animalia: multicellular, heterotrophic, ingest their food.
Remember
Viruses show non-living characteristics too: they cannot reproduce, grow, or carry out metabolism on their own — only inside a host cell. This is why they sit outside the traditional three-domain classification altogether, rather than fitting into any one kingdom.
Rule
The seven taxonomic ranks, broadest to narrowest: Kingdom, Phylum, Class, Order, Family, Genus, Species.
Binomial nomenclature: every organism gets a two-part scientific name — Genus (capitalized) then species (lowercase), both italicized — e.g. .
Remember
A scientific name works the same in every language, avoiding the confusion of local common names — this is exactly why binomial nomenclature makes communication between scientists worldwide reliable.
Biodiversity — the variety of organisms in an ecosystem — keeps ecosystems stable and resilient, and is the source of food, medicine and materials humans depend on; losing species reduces an ecosystem's ability to recover from disturbance.
Quick Recap
- ●Three domains: Bacteria, Archaea (both prokaryotic), Eukarya (has a nucleus)
- ●Four eukaryotic kingdoms: Protista, Fungi (heterotroph, absorbs food), Plantae (photosynthetic), Animalia (heterotroph, ingests food)
- ●Viruses show non-living traits outside a host — they sit outside the three-domain system
- ●Seven ranks: Kingdom, Phylum, Class, Order, Family, Genus, Species
- ●Binomial nomenclature: italicized Genus + species, universal across languages
- ●Biodiversity keeps ecosystems stable and provides food, medicine and materials
