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Classification of the Five Kingdoms

• Before the three domains of life (Bacteria, Archaea and Eukarya) were introduced and then widely accepted as the highest rank in the classification of life on Earth, most people thought there to be five kingdoms at the top of the classification hierarchy

• These five kingdoms include:

  • Prokaryota

  • Protoctista

  • Fungi

  • Plantae

  • Animalia

Prokaryota

• This kingdom includes bacteria and blue-green bacteria

• The main features of all organisms within Prokaryota include:

  • Most are unicellular (some can be found as filaments of cells or groupings of similar cells known as colonies)

  • Their cells have cell walls (not made of cellulose) and cytoplasm but no nucleus or mitochondria

  • They vary in size over a wide range: the smallest are bigger than the largest known viruses and the largest are smaller than the smallest known single-celled eukaryotes

  • Their cells divide by binary fission

• Blue-green bacteria and some bacteria are autotrophic (they are photosynthetic)

• Many bacteria are heterotrophic (feeding by decomposing living or dead organic materials)

A typical bacterial cell

• The other kingdoms can all be classified within the domain Eukarya, which can be divided into the following four kingdoms:

  • Protoctista

  • Fungi

  • Plantae

  • Animalia

• Organisms from each of these four kingdoms have distinct characteristics and features, but share similarities in that they have cells with membrane-bound nuclei separating genetic material from the cytoplasm, and compartmentalisation within their cells as a result of the presence of other organelles

The four kingdoms within the Eukarya domain: protoctista, fungi, plantae and animalia

Kingdom Protoctista

• All Protoctista are eukaryotic, and this broad group of cellular life encompasses all eukaryotic cells that do not belong to the other three eukaryotic kingdoms

• Members of this kingdom show great diversity in all aspects of life including structure, life cycle, feeding and trophic levels and well as modes of locomotion

• Protoctists can exist as single-celled organisms or as a group of similar cells

• A group of Protoctista known as protozoa possess cells similar to animal cells

  • Their cells have no cell wall

• Another group of Protoctista known as algae possess cells similar to plant cells

  • Their cells have cellulose cell walls and chloroplasts

Two examples of protoctist cells

Kingdom Fungi

• All fungi are eukaryotic cells

• The cells of fungi:

  • Possess non-cellulose cell walls (often made of the polysaccharide chitin)

  • Don’t have cilia

• Fungi are heterotrophs:

  • They use organic compounds made by other organisms as their source of energy and molecules for metabolism

  • They obtain this energy and carbon by digesting dead/decaying matter extracellularly or from being parasites on living organisms

• Fungi reproduce using spores that disperse onto the ground nearby

• Fungi have a simple body form:

  • They can be unicellular (like the common baker’s yeast Saccharomyces cerevisiae

  • Some consist of long threads called hyphae that grow from the main fungus body and form a network of filaments called the mycelium

  • Larger fungi possess fruiting bodies that release large numbers of spores (this is how many fungi reproduce)

• The mould found on bread is actually a fungus: Rhizopus nigricans

A typical fungal cell

The structure of a fungus with its hyphae, mycelium and fruiting bodies

Kingdom Plantae

• Plants are multicellular eukaryotic organisms

• Plant cells:

  • All have cell walls composed of cellulose

  • Possess large (and usually permanent) vacuoles that provide structural support

  • Are able to differentiate into specialized cells to form tissues and organs

  • Possess chloroplasts that enable photosynthesis (not all plant cells have chloroplasts)

  • Can sometimes have flagella

• They are autotrophs

  • This means they can synthesize their organic compounds and molecules for energy use and building biomass from inorganic compounds

• Plants have complex body forms

  • They have branching systems above and below the ground

A typical plant cell

Kingdom Animalia

• Animals are also multicellular eukaryotic organisms

• Animal cells:

  • Are able to differentiate into many different specialised cell types that can form tissues and organs

  • Have small temporary vacuoles (for example, lysosomes)

  • Have no cell walls

  • Sometimes have cilia

• They are heterotrophs and have a wide range of feeding mechanisms

• Communication within their complex body forms takes place through a nervous system and chemical signalling

A typical animal cell

Five Kingdoms Comparison Table