Biological Classification

Early classification by Linnaeus used only two kingdoms — Plantae and Animalia. This was unsatisfactory because it grouped together very different organisms (e.g. bacteria, fungi and green plants were all 'plants'). In 1969, R.H. Whittaker proposed the widely used Five Kingdom Classification: Monera, Protista, Fungi, Plantae and Animalia.

Whittaker used several criteria: cell structure (prokaryotic vs eukaryotic), body organisation (cellular vs multicellular), mode of nutrition (autotrophic vs heterotrophic), reproduction and phylogenetic (evolutionary) relationships.

Kingdom Monera includes all prokaryotes — the bacteria. They are the most abundant micro-organisms, with cells that have no true (membrane-bound) nucleus or organelles. By shape, bacteria are coccus (spherical), bacillus (rod), vibrio (comma) or spirillum (spiral). Important groups:

• Archaebacteria — live in extreme habitats: halophiles (salty), thermoacidophiles (hot acidic springs) and methanogens (in marshy areas and the gut of ruminants, where they produce methane).
• Eubacteria ('true bacteria') — have a rigid cell wall and, if motile, a flagellum. Includes the photosynthetic cyanobacteria (blue-green algae, which have chlorophyll and can fix nitrogen, e.g. Nostoc, Anabaena) and chemosynthetic bacteria.
• Mycoplasma — the smallest living cells, which completely lack a cell wall, can survive without oxygen, and many cause disease.

Kingdom Protista includes all single-celled eukaryotes. Being eukaryotic, their cells have a well-defined nucleus and membrane-bound organelles. Protists form a link between the simple monerans and the complex plants, fungi and animals. Most are aquatic. They reproduce both asexually and sexually (by cell fusion and zygote formation). The main groups are:

• Chrysophytes — include diatoms and golden algae (desmids). Diatoms are photosynthetic and have hard, indestructible cell walls of silica that form two overlapping halves (like a soap box). Their accumulated cell-wall deposits over billions of years form diatomaceous earth, used in polishing and filtration.
• Dinoflagellates — mostly marine and photosynthetic, with cellulose plates on the body and two flagella. Red dinoflagellates (e.g. Gonyaulax) may multiply rapidly to cause a red tide, releasing toxins that can kill marine animals.
• Euglenoids — freshwater organisms (e.g. Euglena) with a flexible protein-rich body (pellicle) instead of a cell wall. They are photosynthetic in sunlight but feed as heterotrophs in the dark — showing a mix of plant and animal features (mixotrophic).
• Slime moulds — saprophytic protists that form an aggregation (plasmodium) under suitable conditions and produce spores that are very resistant.
• Protozoans — heterotrophic, animal-like protists: amoeboid (Amoeba; some, like Entamoeba, are parasites), flagellated (Trypanosoma, which causes sleeping sickness), ciliated (Paramecium, with cilia) and sporozoans (Plasmodium, which causes malaria).

Kingdom Fungi is a group of eukaryotic, heterotrophic organisms (e.g. moulds, mushrooms, yeasts). Except for unicellular yeast, the fungal body is made of long thread-like hyphae that form a network called a mycelium; the walls are made of chitin. Their nutrition is heterotrophic: saprophytic (on dead matter), parasitic (on living hosts) or symbiotic (with other organisms). They reproduce by vegetative means, and by asexual and sexual spores. Major classes are Phycomycetes (e.g. Rhizopus), Ascomycetes (sac fungi, e.g. Penicillium, yeast), Basidiomycetes (club fungi, e.g. mushrooms, rusts) and Deuteromycetes ('imperfect fungi').

Economic importance: fungi are valuable (yeast in bread and alcohol; Penicillium gives the antibiotic penicillin; mushrooms as food) but also harmful (causing diseases of plants, animals and humans, and food spoilage).

Lichens are symbiotic associations between a fungus (mycobiont) and an alga/cyanobacterium (phycobiont): the alga makes food by photosynthesis and the fungus provides shelter, water and minerals. Lichens are excellent pollution indicators — they do not grow in polluted areas.

Some entities are acellular (not made of cells) and lie at the border of living and non-living:

• Viruses — non-cellular, made of a protein coat (capsid) enclosing nucleic acid (DNA or RNA). They are inert outside a host but reproduce only inside a living host cell, so they are obligate parasites. The name (Latin for poison) was coined by Pasteur; Stanley showed they could be crystallised.
• Viroids — even smaller infectious agents made of free RNA only, with no protein coat (discovered by T.O. Diener; cause potato spindle tuber disease).
• Prions — infectious agents made of abnormally folded protein only, causing diseases such as mad-cow disease and CJD.