Biological Classification

In this chapter: history of classification, the five-kingdom system, Kingdoms Monera, Protista and Fungi in detail, a brief look at Plantae and Animalia, and viruses, viroids and lichens.

From two kingdoms to five

Aristotle made the first attempt, dividing plants into trees, shrubs and herbs, and animals into those with red blood and those without. Linnaeus used two kingdoms, Plantae and Animalia. This system did not separate prokaryotes from eukaryotes, unicellular from multicellular, and photosynthetic from non-photosynthetic organisms. Fungi, for example, were placed with plants despite having very different walls and nutrition.

R.H. Whittaker (1969) proposed five kingdoms: Monera, Protista, Fungi, Plantae and Animalia. His criteria were cell structure, body organisation (thallus), mode of nutrition, reproduction and phylogenetic relationships.

Kingdom Monera (bacteria)

Bacteria are the most abundant micro-organisms and are found almost everywhere, including hot springs, deserts, snow and deep oceans. By shape they are coccus (spherical), bacillus (rod), vibrio (comma) and spirillum (spiral).

Archaebacteria

They live in the harshest habitats: halophiles (extreme salt), thermoacidophiles (hot springs) and methanogens (marshy areas and the gut of ruminants such as cows and buffaloes). Methanogens produce methane (biogas) from cattle dung. Their different cell-wall structure helps them survive extreme conditions.

Eubacteria ("true bacteria")

• Have a rigid cell wall; motile forms have a flagellum.
• Cyanobacteria (blue-green algae): photosynthetic autotrophs with chlorophyll a like plants; unicellular, colonial or filamentous; colonies often surrounded by a gelatinous sheath; can form blooms in polluted water. Some, like Nostoc and Anabaena, fix atmospheric nitrogen in specialised cells called heterocysts.
• Chemosynthetic autotrophs oxidise inorganic substances (nitrates, nitrites, ammonia) and use the energy to make ATP. They recycle nitrogen, phosphorus, iron and sulphur.
• Heterotrophic bacteria are the most abundant: decomposers, curd-making (Lactobacillus), antibiotic producers, nitrogen fixers in legume roots, and pathogens causing cholera, typhoid, tetanus and citrus canker.
• Reproduce mainly by fission; under unfavourable conditions produce spores. A primitive form of DNA transfer (sexual reproduction) also occurs.

Mycoplasma lack a cell wall entirely. They are the smallest living cells known, can survive without oxygen, and many are pathogenic in plants and animals.

Kingdom Protista

All single-celled eukaryotes are placed under Protista, though the boundaries of the kingdom are not well defined. Protists form a link with plants, animals and fungi. They have a well-defined nucleus and membrane-bound organelles; some have flagella or cilia. Reproduction is asexual and sexual (by cell fusion and zygote formation).

The four protozoan groups

• Amoeboid: freshwater, sea or moist soil; move and capture prey using pseudopodia. Marine forms have silica shells. Parasitic Entamoeba.
• Flagellated: free-living or parasitic, with flagella. Trypanosoma causes sleeping sickness.
• Ciliated: aquatic, with thousands of cilia. A cavity (gullet) opens outside; cilia beat to steer food into it. Example: Paramoecium.
• Sporozoans: have an infectious spore-like stage. Plasmodium causes malaria.

Kingdom Fungi

Fungi are heterotrophic, mostly saprophytic; some are parasitic (like Puccinia, which causes wheat rust); some symbiotic (lichens with algae, mycorrhiza with plant roots). Except yeast (unicellular), fungi are filamentous: the body is made of long threads called hyphae, forming a network called mycelium. Some hyphae are continuous tubes with many nuclei (coenocytic); others have septa. The cell wall is made of chitin and polysaccharides.

Reproduction

• Vegetative: fragmentation, fission, budding.
• Asexual: spores called conidia, sporangiospores or zoospores.
• Sexual: oospores, ascospores and basidiospores, produced in fruiting bodies. The cycle involves plasmogamy (fusion of protoplasm of two motile or non-motile gametes), karyogamy (fusion of two nuclei) and meiosis in the zygote, giving haploid spores. In ascomycetes and basidiomycetes, an intermediate dikaryotic stage (n + n, two nuclei per cell) occurs; this is the dikaryophase.

Common trap: in Ascomycetes the sexual spores are endogenous (inside asci); in Basidiomycetes they are exogenous (on the basidium). Conidia in Ascomycetes are exogenous asexual spores.

Kingdoms Plantae and Animalia (brief)

Plantae: eukaryotic, chlorophyll-containing, cellulose cell wall. A few are partially heterotrophic: insectivorous plants such as bladderwort and Venus fly trap, and parasites such as Cuscuta. Includes algae, bryophytes, pteridophytes, gymnosperms and angiosperms. The life cycle shows alternation of generations between the diploid sporophyte and haploid gametophyte.

Animalia: heterotrophic eukaryotes, multicellular, no cell wall. Holozoic nutrition (ingestion). Food stored as glycogen or fat. Definite growth pattern, most capable of locomotion. Sexual reproduction by copulation of male and female, followed by embryological development.

Viruses, viroids and lichens

Viruses

• Not fully living: they have no cell structure and are inert outside a host cell. Inside a living cell they take over the machinery to replicate, killing the host.
• D.J. Ivanowsky (1892) found that the organism causing mosaic disease of tobacco was smaller than bacteria; it passed through bacteria-proof filters.
• M.W. Beijerinck (1898) called the infectious fluid contagium vivum fluidum ("infectious living fluid").
• W.M. Stanley (1935) showed viruses could be crystallised; crystals consist largely of proteins.
• A virus has a protein coat (capsid, made of capsomeres arranged in helical or polyhedral form) and genetic material that is either RNA or DNA, never both.
• Plant viruses usually have single-stranded RNA; animal viruses may have single- or double-stranded RNA or double-stranded DNA; bacteriophages (viruses of bacteria) usually have double-stranded DNA.
• Diseases: mumps, smallpox, herpes, influenza, AIDS in humans; mosaic formation, leaf rolling and curling, yellowing and vein clearing, dwarfing and stunted growth in plants.

Viroids

Discovered by T.O. Diener (1971) as the cause of potato spindle tuber disease. Smaller than viruses: free RNA without a protein coat, of low molecular weight.

Lichens

Symbiotic associations between algae (phycobiont, which prepares food) and fungi (mycobiont, which provides shelter and absorbs mineral nutrients and water). The association is so close that a lichen looks like a single organism. Lichens do not grow in polluted areas, so they are good pollution indicators.

NEET focus

• Whittaker's criteria and the comparison table of five kingdoms.
• Archaebacteria types and methanogens; heterocysts in Nostoc/Anabaena; mycoplasma.
• Protist groups with their examples: diatomaceous earth, red tides (Gonyaulax), pellicle (Euglena), protozoan diseases.
• Fungal classes: mycelium type, spore type (endogenous vs exogenous) and examples.
• Virus discoveries (Ivanowsky, Beijerinck, Stanley), viroids (Diener), lichens as pollution indicators.

Practice questions