Biological Classification

Biology · Class 11

Lesson 12 of 12 · 16 min

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Must-know facts

25 facts

  1. 1Aristotle: plants as trees, shrubs and herbs; animals with and without red blood.
  2. 2Linnaeus: two kingdoms, Plantae and Animalia.
  3. 3R.H. Whittaker (1969): five kingdoms Monera, Protista, Fungi, Plantae, Animalia.
  4. 4Whittaker's criteria: cell structure, body organisation, mode of nutrition, reproduction, phylogenetic relationships.
  5. 5Monera wall: non-cellulosic (polysaccharide + amino acid); Fungi wall: chitin; Plantae wall: cellulose; Animalia: no wall.
  6. 6Three-domain system splits Monera into two domains, giving six kingdoms.
  7. 7Bacterial shapes: coccus, bacillus, vibrium (comma), spirillum (spiral).
  8. 8Archaebacteria: halophiles, thermoacidophiles, methanogens; different wall structure; methanogens make biogas in ruminant gut.
  9. 9Nostoc and Anabaena fix nitrogen in heterocysts.
  10. 10Mycoplasma: no wall, smallest living cells, can live without oxygen.
  11. 11Diatoms: soap-box silica walls, diatomaceous earth, chief producers of the oceans.
  12. 12Gonyaulax causes red tides; dinoflagellates have cellulose plates and two flagella.
  13. 13Euglena: pellicle instead of wall, two flagella, pigments same as higher plants.
  14. 14Slime mould spores have true walls and are dispersed by air.
  15. 15Protozoan examples: Amoeba and Entamoeba (amoeboid), Trypanosoma (flagellated, sleeping sickness), Paramoecium (ciliated), Plasmodium (sporozoan, malaria).
  16. 16Fungal walls: chitin and polysaccharides; coenocytic hyphae have no septa.
  17. 17Sexual cycle of fungi: plasmogamy, karyogamy, meiosis; dikaryon (n + n) in ascomycetes and basidiomycetes.
  18. 18Phycomycetes: aseptate coenocytic mycelium, zygospore; Mucor, Rhizopus, Albugo.
  19. 19Ascomycetes: conidia exogenous, ascospores endogenous in asci; Penicillium, yeast, Aspergillus, Claviceps, Neurospora.
  20. 20Basidiomycetes: four exogenous basidiospores on the basidium; Agaricus, Ustilago, Puccinia.
  21. 21Deuteromycetes: only asexual stage known; Alternaria, Colletotrichum, Trichoderma.
  22. 22Partly heterotrophic plants: bladderwort, Venus fly trap (insectivorous), Cuscuta (parasite).
  23. 23Ivanowsky 1892, Beijerinck 1898 (Contagium vivum fluidum), Stanley 1935 (crystallised virus).
  24. 24A virus has RNA or DNA, never both; bacteriophages usually double-stranded DNA; plant viruses usually single-stranded RNA.
  25. 25Viroid: free low-molecular-weight RNA, no protein coat, potato spindle tuber, Diener 1971. Prions: misfolded protein, BSE and CJD. Lichen: phycobiont + mycobiont, pollution indicator.

Common traps

Where marks are lost

Placing Chlamydomonas and Chlorella in Plantae, or Amoeba and Paramoecium in Animalia, under the five kingdom system.

All four are unicellular eukaryotes, so Whittaker puts all of them in Protista.

Calling the cyanobacteria 'algae' and putting them in Protista or Plantae.

Blue-green algae are prokaryotes, so they are eubacteria in Kingdom Monera.

Saying archaebacteria survive extremes because of their cell membrane or spores.

NCERT links their survival to a cell wall structure that differs from other bacteria.

Writing that Euglena has a cellulose cell wall.

Euglenoids have no cell wall; they have a flexible, protein-rich pellicle. Dinoflagellates are the ones with cellulose plates.

Listing yeast as a bacterium or a protist because it is unicellular.

Yeast is a fungus, an ascomycete (Saccharomyces); unicellular fungi still belong to Kingdom Fungi.

Thinking the dikaryon is found in every fungus.

The n + n stage occurs in ascomycetes and basidiomycetes; in some fungi fusion gives diploid cells at once.

Swapping endogenous and exogenous spores between ascomycetes and basidiomycetes.

Ascospores form inside the ascus (endogenous); basidiospores form outside on the basidium (exogenous). Conidia are exogenous.

Saying a virus can contain both DNA and RNA.

A virus carries either RNA or DNA, never both.

Crediting Beijerinck with crystallising viruses.

Stanley (1935) crystallised viruses; Beijerinck (1898) named the virus and called the fluid Contagium vivum fluidum; Ivanowsky (1892) showed it passed bacteria-proof filters.

Describing a viroid as a small virus with a protein coat.

A viroid is free RNA with no protein coat. Prions are the protein-only agents.

Calling the fungal partner of a lichen the phycobiont.

Phyco = alga (phycobiont, autotrophic); myco = fungus (mycobiont, heterotrophic).

Key terms

24 terms

Five kingdom classification
Whittaker's 1969 system: Monera, Protista, Fungi, Plantae and Animalia.
Three-domain system
A proposed system that splits Monera into two domains and places all eukaryotes in a third, giving six kingdoms.
Archaebacteria
Bacteria of extreme habitats (halophiles, thermoacidophiles, methanogens) with a distinctive cell wall structure.
Heterocyst
A specialised cell in some cyanobacteria, such as Nostoc and Anabaena, where atmospheric nitrogen is fixed.
Mycoplasma
Wall-less prokaryotes; the smallest living cells known, able to survive without oxygen.
Diatomaceous earth
Gritty deposits of silica-rich diatom walls, used for polishing and for filtering oils and syrups.
Red tide
Reddening of the sea by rapid multiplication of red dinoflagellates such as Gonyaulax, whose toxins can kill fish.
Pellicle
The protein-rich flexible layer that euglenoids have in place of a cell wall.
Plasmodium (slime mould)
The spreading aggregation formed by a slime mould in favourable conditions.
Pseudopodia
Temporary projections ('false feet') used by amoeboid protozoans to move and capture prey.
Hypha and mycelium
A hypha is a single thread of a fungal body; the network of hyphae is the mycelium.
Coenocytic hypha
A hypha without cross walls, a continuous tube of cytoplasm with many nuclei.
Plasmogamy
Fusion of the protoplasms of two gametes or cells, the first step of the fungal sexual cycle.
Karyogamy
Fusion of two nuclei, the second step of the fungal sexual cycle.
Dikaryon
A cell with two separate haploid nuclei (n + n), found between plasmogamy and karyogamy in ascomycetes and basidiomycetes.
Ascus and ascocarp
The ascus is the sac in which ascospores form; ascocarps are the fruiting bodies that hold the asci.
Basidium and basidiocarp
The basidium bears four basidiospores on its outside; basidiocarps are the fruiting bodies that hold the basidia.
Deuteromycetes
The imperfect fungi, whose sexual stage is unknown; they reproduce only by conidia.
Alternation of generations
The alternation of a diploid sporophytic phase and a haploid gametophytic phase in plant life cycles.
Capsid and capsomeres
The capsid is the protein coat of a virus; capsomeres are the small subunits that build it.
Bacteriophage
A virus that infects bacteria, usually with double-stranded DNA.
Viroid
An infectious agent of free, low-molecular-weight RNA without a protein coat, e.g. the cause of potato spindle tuber disease.
Prion
An infectious agent made of abnormally folded protein that causes neurological diseases such as BSE and CJD.
Phycobiont and mycobiont
The algal (autotrophic) and fungal (heterotrophic) partners of a lichen.
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