BIOLOGICAL CLASSIFICATION

Earlier systems placed all organisms in Plantae and Animalia. They did not separate prokaryotes from eukaryotes, unicellular from multicellular organisms, or photosynthetic algae from non-photosynthetic fungi.

General characters

Bacteria are the sole members of Monera. They are prokaryotic, occur almost everywhere, and can survive in extreme habitats such as hot springs, deserts, snow and deep oceans.

Nutrition and reproduction

They may be autotrophic (photosynthetic or chemosynthetic) or heterotrophic (saprophytic, parasitic or symbiotic). Bacteria reproduce mainly by binary fission; under unfavourable conditions they form spores. Genetic recombination can occur through conjugation, transformation and transduction.

Archaebacteria and eubacteria

Archaebacteria have a different cell-wall structure and survive harsh habitats: halophiles, thermoacidophiles and methanogens. Methanogens occur in the gut of ruminants and produce methane in biogas plants.

Eubacteria possess a rigid wall and may have flagella. Cyanobacteria are photosynthetic, often in colonies; they can fix atmospheric nitrogen in specialised cells called heterocysts. Mycoplasma lack a cell wall and are the smallest living cells.

Protists are unicellular eukaryotes, primarily aquatic. They have well-defined nuclei and membrane-bound organelles. Some have cilia or flagella for movement, and many reproduce both asexually and sexually.

Chrysophytes

Diatoms and golden algae occur in freshwater and marine habitats. Their cell walls form two thin overlapping shells containing silica; deposits of their dead cells form diatomaceous earth, used in polishing and filtration.

Dinoflagellates

Mostly marine and photosynthetic, with two flagella and cellulose plates. Red dinoflagellates such as Gonyaulax can multiply rapidly, causing red tides; toxins may kill large numbers of fish.

Euglenoids

Freshwater organisms with a protein-rich pellicle instead of a cell wall. They are photosynthetic in light but behave as heterotrophs in darkness. Euglena is the common example.

Slime moulds and protozoans

Slime moulds are saprophytic protists whose aggregation forms a plasmodium. Protozoans are heterotrophs: amoeboid forms use pseudopodia, flagellated forms use flagella, ciliates such as Paramecium use cilia, and sporozoans such as Plasmodium have an infectious spore-like stage.

Fungi are heterotrophic eukaryotes that absorb nutrients. Their cell walls contain chitin. The body is made of thread-like hyphae; a network of hyphae is the mycelium.

Reproduction

Vegetative reproduction occurs by fragmentation, fission and budding. Asexual spores include conidia, sporangiospores and zoospores. Sexual reproduction involves plasmogamy, karyogamy and meiosis; dikaryotic stages occur in many fungi.

Major classes

Phycomycetes: aseptate, coenocytic mycelium; examples Mucor, Rhizopus, Albugo. Ascomycetes: sac fungi with asci and ascospores; Penicillium, Aspergillus, yeast, morels and truffles. Basidiomycetes: mushrooms, bracket fungi and puffballs; basidiospores arise on basidia. Deuteromycetes: imperfect fungi with only asexual/vegetative stages known; Alternaria, Colletotrichum, Trichoderma.

Kingdom Plantae

Plants are multicellular eukaryotes with cellulose cell walls. They are generally autotrophic and photosynthetic. This kingdom includes algae, bryophytes, pteridophytes, gymnosperms and angiosperms.

Kingdom Animalia

Animals are multicellular eukaryotes without cell walls. They are heterotrophic and usually ingest food. They show organ-system level organisation and are classified into major phyla based on body plan and relationships.

Viruses

Viruses are non-cellular and can be crystallised. They are inert outside a host but replicate only inside living cells, using host machinery. A virus has nucleic acid enclosed by a protein coat (capsid); plant viruses usually have single-stranded RNA, bacteriophages usually double-stranded DNA, and animal viruses may contain RNA or DNA.

Ivanowsky recognised the infectious agent in tobacco mosaic disease; Beijerinck called it a virus; W. M. Stanley showed viruses can be crystallised. Viroids are smaller infectious RNA molecules without a protein coat; T. O. Diener discovered them.

Lichens

Lichens are symbiotic associations between algae and fungi. The algal partner (phycobiont) prepares food, while the fungal partner (mycobiont) provides shelter and absorbs minerals and water. Lichens are good pollution indicators because they do not grow in polluted areas.

1. What are the criteria used in Whittaker's five-kingdom classification?
2. Compare archaebacteria and eubacteria.
3. Why are cyanobacteria included in Monera?
4. Describe the four major groups of protozoans.
5. State the nutrition, reproduction and major classes of fungi.
6. Why are viruses placed at the border of living and non-living?
7. What is the significance of lichens as pollution indicators?