Biological Classification

From two kingdoms to five kingdoms

Linnaeus used just two kingdoms, Plantae and Animalia. This put bacteria, fungi and single-celled creatures in the wrong places. In 1969 R.H. Whittaker proposed five kingdoms: Monera, Protista, Fungi, Plantae and Animalia.

Criteria Whittaker used

• Cell type: prokaryotic or eukaryotic
• Cell wall: present or absent, and what it is made of
• Body: one cell or many cells (tissue level)
• Nutrition: makes own food (autotroph) or takes food (heterotroph)
• Reproduction and evolutionary relationships

Comparing the five kingdoms

Kingdom Monera

All Monera are bacteria: tiny, one-celled, with no nucleus. They live almost everywhere, even in hot springs and deep ice.

Shapes of bacteria

• Coccus: round
• Bacillus: rod
• Vibrio: comma
• Spirillum: spiral

Archaebacteria

Special bacteria of harsh places: halophiles (very salty water), thermoacidophiles (hot, acidic springs) and methanogens (marshes and the gut of cows, where they make methane in biogas). Their cell wall is different, which helps them survive.

Eubacteria (true bacteria)

• Cyanobacteria (blue-green algae): make food by photosynthesis; some like Nostoc and Anabaena fix nitrogen in special cells called heterocysts; they can cause water blooms.
• Chemosynthetic autotrophs: get energy by oxidising chemicals like nitrates or ammonia; they recycle nitrogen, iron and sulphur.
• Heterotrophic bacteria: the most common; decomposers, curd making, antibiotics; some cause disease (cholera, typhoid, tetanus, citrus canker).

Bacteria reproduce mostly by binary fission; they form spores in bad times and can swap DNA by a simple sexual process.

Mycoplasma

The smallest living cells known. They have no cell wall, can live without oxygen and cause diseases in plants and animals.

Kingdom Protista and its five groups

Protists are one-celled eukaryotes, mostly living in water. They link simple cells to plants, animals and fungi.

• Chrysophytes (diatoms, golden algae): glassy silica walls that fit like a soap box; their dead walls pile up as diatomaceous earth, used in polishing and filtering. Chief producers of the ocean.
• Dinoflagellates: mostly marine, two flagella (one along the body, one across a groove), stiff cellulose plates. Gonyaulax can multiply so fast that sea turns red (red tide) and fish die.
• Euglenoids: fresh water, no wall but a flexible protein layer called pellicle. Euglena makes food in light but eats small organisms in the dark.
• Slime moulds: feed on rotting leaves; in good times cells join into a big moving mass called a plasmodium; in bad times form tough spores.
• Protozoans: animal-like, heterotrophs. Four types: amoeboid (Amoeba, Entamoeba), flagellated (Trypanosoma, sleeping sickness), ciliated (Paramoecium) and sporozoans (Plasmodium, malaria).

Kingdom Fungi and its four classes

Fungi are eukaryotic heterotrophs with cell walls of chitin. Except for one-celled yeast, the body is made of long threads called hyphae; a network of hyphae is a mycelium. Some hyphae have no cross walls and many nuclei (coenocytic).

They feed as saprophytes (on dead matter), parasites (on living hosts) or in partnerships (lichens, mycorrhiza with plant roots).

Reproduction: vegetative (fragmentation, fission, budding), asexual (spores such as conidia, sporangiospores, zoospores) and sexual (oospores, ascospores, basidiospores). Sexual reproduction has three steps: plasmogamy (cytoplasm joins), karyogamy (nuclei join) and meiosis. Sometimes there is a stage with two nuclei per cell, the dikaryon.

Four classes

• Phycomycetes: water or damp places; hyphae without cross walls; zoospores or aplanospores; e.g. Mucor, Rhizopus (bread mould), Albugo.
• Ascomycetes (sac fungi): spores (ascospores) inside sacs called asci; e.g. yeast, Penicillium, Aspergillus, Claviceps, Neurospora; morels and truffles are eaten.
• Basidiomycetes (club fungi): spores (basidiospores) on club-shaped basidia; e.g. mushroom Agaricus, bracket fungi, rust (Puccinia) and smut (Ustilago).
• Deuteromycetes (imperfect fungi): only the asexual stage is known; e.g. Alternaria, Colletotrichum, Trichoderma. Many are decomposers.

Plantae and Animalia in brief

Plantae: eukaryotic, chlorophyll, cellulose walls; a few are partly heterotrophic (insect-eating plants like bladderwort, parasites like Cuscuta). Their life cycle alternates between a gametophyte and a sporophyte generation. Animalia: many-celled heterotrophs without walls; they eat food, store it as glycogen or fat, and most can move. Both kingdoms get their own lessons next.

Lichens: a partnership

A lichen is a symbiotic association (a close partnership that helps both) between an alga (the phycobiont) and a fungus (the mycobiont). The alga makes food by photosynthesis; the fungus gives shelter and absorbs water and minerals. They grow on bare rocks and bark and start soil formation. Lichens do not grow in polluted air, so they are good indicators of air pollution.

Viruses, viroids and prions

These are not placed in any kingdom because they are not truly cells.

Viruses

A virus is a piece of genetic material (DNA or RNA, never both) inside a protein coat called a capsid, made of units called capsomeres. Outside a living cell it is like a crystal and does nothing. Inside a host cell it takes over the cell to make copies of itself, often killing the cell. Plant viruses usually have RNA; animal viruses have RNA or DNA; viruses that attack bacteria (bacteriophages) usually have double-stranded DNA. Ivanowsky (1892) found the tobacco mosaic agent; Beijerinck called it a contagium vivum fluidum; Stanley showed viruses can be crystallised. Diseases: common cold, flu, polio, AIDS, mumps, measles, and plant diseases like leaf curl and mosaic.

Viroids

Found by T.O. Diener (1971). A viroid is free RNA with no protein coat, much smaller than a virus. It causes potato spindle tuber disease.

Prions

Prions are misfolded proteins with no nucleic acid at all. They cause brain diseases such as mad cow disease (BSE) in cattle and Creutzfeldt–Jakob disease in humans.

Try it: see Monera and Fungi at home

Keep a slice of moist bread in a closed box for 4–5 days. The black fuzz is Rhizopus, a fungus: look for thin threads (hyphae) and black dots (sporangia). Put a drop of curd on a clean glass: those tiny rods under a school microscope are Lactobacillus, bacteria. In the 3D free-play step, tap each organism to check its shelf.

Key formulas and definitions

• Five kingdoms: Monera, Protista, Fungi, Plantae, Animalia (Whittaker, 1969)
• Monera = prokaryotes; Protista, Fungi, Plantae, Animalia = eukaryotes
• Fungi wall = chitin; plant wall = cellulose; Monera wall = peptidoglycan (not cellulose)
• Lichen = alga (phycobiont) + fungus (mycobiont)
• Virus = DNA or RNA + protein coat; viroid = RNA only; prion = protein only

Worked examples

Common mistakes

• Calling blue-green algae 'algae' of kingdom Plantae or Protista. They are cyanobacteria in Monera.
• Saying fungi have cellulose walls. Fungal walls are made of chitin.
• Writing that a virus contains both DNA and RNA. It has only one of them.
• Placing viruses in Monera. Viruses are not in any of the five kingdoms.