Kingdom
A kingdom is one of the major divisions of living things in classification, such as Animalia, Plantae, Fungi, Protista, and the bacterial kingdoms.
Definition
A kingdom is one of the biggest groups scientists use to sort living things, such as the animal kingdom, the plant kingdom, or the fungi kingdom that mushrooms belong to because, unlike plants, they cannot make their own food from sunlight. The six-kingdom system commonly taught divides life into Archaebacteria, Eubacteria, Protista, Fungi, Plantae, and Animalia based on cell type (prokaryotic or eukaryotic), cell number (unicellular or multicellular), and how the organism gets food (autotroph or heterotroph); Plantae are multicellular autotrophs with cellulose cell walls, Fungi are heterotrophs with chitin walls that absorb nutrients, Animalia are multicellular heterotrophs without cell walls, and Protista is the catch-all for eukaryotes fitting none of the other three. The kingdom rank has shifted as phylogenetics advanced: Linnaeus recognized two kingdoms, Whittaker proposed five in 1969, and Woese's three-domain system split Monera into Bacteria and Archaea, while molecular phylogenies show Protista is not a natural group but a paraphyletic assemblage of independent eukaryotic lineages, replaced in modern systems by supergroups such as SAR, Archaeplastida, Amoebozoa, and Opisthokonta, which unites fungi and animals, making kingdom a convenient teaching category rather than a rank with a fixed evolutionary meaning.
Example
A dog belongs to the animal kingdom, a rose to the plant kingdom, a mushroom to the fungi kingdom, and yogurt bacteria to a bacterial kingdom. Ribosomal RNA comparisons place fungi and animals together in Opisthokonta, sharing a single posterior flagellum in motile cells, while plants belong to Archaeplastida with red and green algae, sharing a chloroplast acquired from a cyanobacterium; brown algae such as kelp sit in an entirely different supergroup, having acquired their chloroplasts by engulfing a red alga.
Key Insight
Fungi were once placed with plants because both are stationary, but DNA shows fungi are our closer relatives, one reason fungal infections are hard to treat without also harming human cells.