🦠 Biological Classification Study Notes

2.1 From Two Kingdoms to Five

Classification began for practical use, then grew scientific. The big idea of this chapter: as our criteria improved, the number of kingdoms grew from two to five.

The road to Whittaker

• Aristotle — earliest scientific attempt: plants into trees, shrubs, herbs; animals into those with red blood and those without.
• Linnaeus — Two Kingdom system: Plantae & Animalia. It did not distinguish prokaryotes/eukaryotes, unicellular/multicellular, or photosynthetic (green algae) / non-photosynthetic (fungi). Inadequate.

So newer criteria were added — cell structure, nature of cell wall, mode of nutrition, habitat, reproduction, and evolutionary (phylogenetic) relationships.

R.H. Whittaker (1969) — Five Kingdom Classification

Kingdoms: Monera, Protista, Fungi, Plantae, Animalia. Criteria used: cell structure, thallus/body organisation, mode of nutrition, reproduction, phylogenetic relationships. A later three-domain system split Monera into two domains (→ a six-kingdom view).

Why old 'Plants' broke down

Earlier, 'Plants' lumped together everything with a cell wall — bacteria, BGA, fungi, mosses, ferns, gymnosperms, angiosperms. This wrongly grouped prokaryotes with eukaryotes, unicellular with multicellular (Chlamydomonas with Spirogyra), and ignored that fungi have chitin walls while plants have cellulose walls. Fixing this gave Fungi, Monera and Protista their own kingdoms.

2.2 Kingdom Monera — Bacteria

Bacteria are the sole members of Monera — the most abundant micro-organisms, found everywhere, including extreme habitats (hot springs, deserts, snow, deep oceans); many are parasites. Simple in structure but very complex in behaviour, with the most extensive metabolic diversity of any group.

Four shapes

Nutrition: autotrophic (photosynthetic or chemosynthetic) or — for the vast majority — heterotrophic.

2.1.1 Archaebacteria — extremophiles

Live in the harshest habitats; a different cell-wall structure lets them survive extremes:

• Halophiles — extreme salty areas
• Thermoacidophiles — hot springs
• Methanogens — marshy areas; also in the gut of ruminants (cows, buffaloes), producing methane (biogas) from dung

2.1.2 Eubacteria — 'true bacteria'

• Rigid cell wall; if motile, a flagellum.
• Cyanobacteria (BGA) — have chlorophyll a like green plants; photosynthetic autotrophs; unicellular/colonial/filamentous; colonies in a gelatinous sheath; form blooms in polluted water; some fix atmospheric N₂ in heterocysts — e.g., Nostoc, Anabaena.
• Chemosynthetic autotrophs — oxidise inorganic substances (nitrates, nitrites, ammonia) for ATP; recycle N, P, Fe, S.
• Heterotrophic bacteria — most abundant; major decomposers; useful (curd from milk, antibiotics, N-fixing in legume roots); pathogens cause cholera, typhoid, tetanus, citrus canker.

Reproduction: mainly fission; spores in unfavourable conditions; a primitive DNA transfer (sexual). Mycoplasma — completely lack a cell wall, are the smallest living cells, survive without O₂, and many are pathogenic.

2.3 Kingdom Protista

All single-celled eukaryotes (boundaries not well-defined); primarily aquatic; cells have a well-defined nucleus and other membrane-bound organelles; some have flagella/cilia. They reproduce asexually and sexually (cell fusion + zygote). A link between plants, animals and fungi.

• Chrysophytes (diatoms + golden algae/desmids) — fresh & marine; microscopic plankton; mostly photosynthetic. Diatom walls = two overlapping silica shells (soap-box), indestructible; deposits = 'diatomaceous earth' (polishing, filtration). Diatoms = chief producers in oceans.
• Dinoflagellates — mostly marine, photosynthetic; coloured by pigments; stiff cellulose plates; two flagella (one longitudinal, one transverse). Red ones (Gonyaulax) cause red tides; toxins can kill fishes.
• Euglenoids — fresh water, stagnant; no cell wall — a protein-rich pellicle (flexible); two flagella (short + long); photosynthetic in light, heterotrophic (predatory) in the dark; pigments identical to higher plants. e.g., Euglena.
• Slime moulds — saprophytic; engulf organic matter; form a plasmodium (spreads several feet); in bad conditions form fruiting bodies with spores at tips; spores have true walls, resist for years, dispersed by air.
• Protozoans — all heterotrophs (predators/parasites); primitive relatives of animals. (see table below)

2.4 Kingdom Fungi

A unique kingdom of heterotrophic eukaryotes — moulds, mushrooms, toadstools, yeasts, rusts. Cosmopolitan; prefer warm, humid places.

Structure

• Filamentous (except unicellular yeast); thread-like hyphae; network = mycelium.
• Coenocytic hyphae = continuous, multinucleate, no cross walls; others have septae.
• Cell walls of chitin + polysaccharides.

Nutrition

Saprophytes (dead matter), parasites (living hosts), or symbionts — with algae as lichens, with plant roots as mycorrhiza.

Sexual cycle — three steps

In ascomycetes & basidiomycetes an intervening dikaryotic (n+n) stage occurs — the dikaryophase.

Note: Deuteromycetes are 'imperfect' because only their asexual/vegetative stage is known; once a sexual stage is discovered they are moved to Ascomycetes or Basidiomycetes. They are useful decomposers aiding mineral cycling.

2.5 Plantae & Animalia

Kingdom Plantae

All eukaryotic, chlorophyll-containing organisms; prominent chloroplasts; cell wall mainly cellulose. A few are partially heterotrophic:

• Insectivorous: Bladderwort, Venus flytrap
• Parasite: Cuscuta

Includes algae, bryophytes, pteridophytes, gymnosperms, angiosperms. Life cycle shows alternation of generations — a diploid sporophyte alternating with a haploid gametophyte.

Kingdom Animalia

• Heterotrophic, eukaryotic, multicellular; cells lack a cell wall.
• Depend directly/indirectly on plants; digest food in an internal cavity; store reserves as glycogen or fat; nutrition is holozoic (ingestion).
• Definite growth and shape; higher forms have sensory & neuromotor systems and locomotion.
• Reproduction mostly sexual — copulation followed by embryological development.

2.6 Viruses, Viroids, Prions & Lichens

These are not placed in the five kingdoms.

Viruses

Non-cellular; an inert crystalline structure outside a living cell. On infecting, they hijack the host's machinery to replicate, killing the host. The name 'virus' (= venom/poisonous fluid) was given by Pasteur.

• Obligate parasites; a nucleoprotein; genetic material is infectious.
• Contain RNA or DNA — never both.
• Protein coat = capsid, made of subunits called capsomeres (helical or polyhedral).
• Plant viruses: usually ssRNA; animal viruses: ss/ds RNA or dsDNA; bacteriophages: usually dsDNA.
• Diseases: mumps, smallpox, herpes, influenza, AIDS. Plant symptoms: mosaic, leaf curling/rolling, yellowing, vein clearing, dwarfing, stunted growth.

Viroids

Discovered by T.O. Diener (1971); smaller than viruses; cause potato spindle tuber disease; a free RNA of low molecular weight with no protein coat.

Prions

Infectious agents made of abnormally folded protein, associated with diseases of the nervous system.

Lichens

Symbiotic (mutualistic) associations of algae + fungi:

• Phycobiont = algal partner (autotroph) → makes food.
• Mycobiont = fungal partner (heterotroph) → gives shelter, absorbs water & minerals.

Lichens are excellent pollution indicators — they do not grow in polluted areas.