Chapter 2: Biological Classification (Diversity in the Living World)

(i) The earliest classifications were based on the uses of organisms. (ii) Aristotle used simple morphology: plants into trees, shrubs and herbs; animals into those with and without red blood. (iii) Linnaeus proposed the two-kingdom system (Plantae, Animalia). It put prokaryotes with eukaryotes, unicellular with multicellular and photosynthetic green algae with non-photosynthetic fungi, and many organisms fitted neither kingdom. (iv) As cell structure, wall composition, mode of nutrition, reproduction and evolutionary relationships were considered, R.H. Whittaker (1969) proposed five kingdoms: Monera (prokaryotes), Protista (unicellular eukaryotes), Fungi (heterotrophs with chitin walls), Plantae and Animalia. (v) Later a three-domain (six-kingdom) system split Monera. Systems keep changing as our knowledge of characters and evolutionary relationships improves; the aim is a phylogenetic classification.

(a) Heterotrophic bacteria: curd from milk (Lactobacillus); production of antibiotics; nitrogen fixation in legume roots (Rhizobium); decomposition of organic matter (recycling). (b) Archaebacteria: methanogens in cattle gut and sewage make methane (biogas) from dung and waste; they also help digest cellulose in ruminants (and are used in sewage treatment).

The wall forms two thin overlapping shells that fit together like a soap box. It is embedded with silica, so it is indestructible; over billions of years these walls have piled up as diatomaceous earth, a gritty soil used for polishing and for filtering oils and syrups.

Algal bloom: a sudden dense growth of algae or cyanobacteria (blue-green algae) in water rich in nutrients, as in polluted water bodies; it colours the water, depletes oxygen and can kill fish and other animals. Red tides: red dinoflagellates such as Gonyaulax multiply so fast that the sea looks red; the toxins they release in large numbers may kill fish and other marine animals.

(i) Amoeboid: fresh water, sea water or moist soil; move and capture prey with pseudopodia, e.g. Amoeba; marine forms have silica shells; Entamoeba is a parasite. (ii) Flagellated: free-living or parasitic, with flagella; parasitic forms cause disease, e.g. Trypanosoma (sleeping sickness). (iii) Ciliated: aquatic, actively moving with thousands of cilia; a gullet opens outside; rows of cilia steer food-laden water into it, e.g. Paramoecium. (iv) Sporozoans: have an infectious spore-like stage, e.g. Plasmodium, the malarial parasite.

Insectivorous plants such as bladderwort (Utricularia), Venus fly trap (Dionaea), pitcher plant (Nepenthes) and sundew (Drosera) photosynthesise but trap and digest insects for nitrogen. Cuscuta (dodder) is a parasite on other plants (total stem parasite); Viscum (mistletoe) is a partial parasite.

In a lichen, the phycobiont is the algal partner (autotrophic: it prepares food for both) and the mycobiont is the fungal partner (heterotrophic: it gives shelter and absorbs water and mineral nutrients). Together they form a mutually beneficial (symbiotic) association.

(i) Nutrition: Phycomycetes: saprophytes on decaying wood in damp places, or obligate parasites (Albugo). Ascomycetes: saprophytic, decomposers, parasitic or coprophilous (on dung). Basidiomycetes: saprophytes in soil and on logs, or parasites (rusts, smuts). Deuteromycetes: saprophytes or parasites; many decompose litter. (ii) Reproduction: Phycomycetes: asexual by zoospores or aplanospores (endogenous in sporangia); sexual by zygospores (isogamous, anisogamous or oogamous). Ascomycetes: asexual by conidia on conidiophores; sexual by ascospores in asci inside ascocarps. Basidiomycetes: asexual spores usually absent, fragmentation; no sex organs, somatic fusion → dikaryon → basidium → 4 basidiospores in basidiocarps. Deuteromycetes: only asexual conidia; sexual stage unknown.

Mostly fresh-water, in stagnant water; no cell wall but a protein-rich pellicle that makes the body flexible; two flagella, one short and one long; photosynthetic in sunlight but heterotrophic (predators of small organisms) in the dark; their pigments are identical to those of higher plants; e.g. Euglena.

Viruses are non-cellular, inert crystalline particles outside a host and obligate parasites inside one. A virus is a nucleoprotein: a nucleic acid core inside a protein coat, the capsid, built of capsomeres arranged in helical or polyhedral forms. Genetic material is RNA or DNA, never both, and is infectious: plant viruses usually have ss RNA (e.g. TMV), animal viruses ss or ds RNA or ds DNA, and bacteriophages usually ds DNA. Diseases: mumps, smallpox, herpes, influenza (also AIDS).

Living features: they have genetic material (DNA or RNA) and can replicate and mutate inside a host cell; they are infectious and show heredity. Non-living features: no cell structure, no metabolism or respiration of their own, cannot grow or reproduce outside a host, and can be crystallised (Stanley) like chemicals. So viruses are on the borderline: living inside a host cell and non-living outside it; this is why they are not placed in any of Whittaker's kingdoms.