classification

• Since the dawn of civilisation, there have been many attempts to classify living organisms
• Aristotle was the earliest to attempt a more scientific basis for classification
• Aristotle's classification


  • Used simple morphological characters to classify plants into trees, shrubs and herbs
  • Divided animals into two groups, those which had red blood and those that did not
• In Linnaeus' time a Two Kingdom system of classification with Plantae and Animalia kingdoms was developed
• The two kingdom classification used for a long time was found inadequate
• A need was felt for including other characteristics like cell structure, nature of wall, mode of nutrition, habitat, methods of reproduction, evolutionary relationships, etc. in classification
• Classification systems for the living organisms have undergone several changes over the time
• R.H. Whittaker (1969) proposed a Five Kingdom Classification
• Five Kingdom Classification


  • Kingdoms: Monera, Protista, Fungi, Plantae, Animalia
  • Main criteria: cell structure, body organisation, mode of nutrition, reproduction and phylogenetic relationships
• The three-domain system has also been proposed that divides the Kingdom Monera into two domains
• Earlier classification systems included bacteria, blue green algae, fungi, mosses, ferns, gymnosperms and the angiosperms under 'Plants'
• The character that unified the 'Plant' kingdom was that all the organisms included had a cell wall in their cells
• This placed together groups which widely differed in other characteristics
• It brought together the prokaryotic bacteria and the blue green algae (cyanobacteria) with other groups which were eukaryotic
• It also grouped together the unicellular organisms and the multicellular ones
• The classification did not differentiate between the heterotrophic group - fungi, and the autotrophic green plants, though they also showed a characteristic difference in their walls composition
• When such characteristics were considered, the fungi were placed in a separate kingdom - Kingdom Fungi
• All prokaryotic organisms were grouped together under Kingdom Monera and the unicellular eukaryotic organisms were placed in Kingdom Protista
• Kingdom Protista has brought together Chlamydomonas, Chlorella (earlier placed in Algae within Plants and both having cell walls) with Paramoecium and Amoeba (which were earlier placed in the animal kingdom which lack cell wall)
• Over time, an attempt has been made to evolve a classification system which reflects not only the morphological, physiological and reproductive similarities, but is also phylogenetic, i.e., is based on evolutionary relationships
• Bacteria


  The sole members of the Kingdom Monera
• Bacterial shapes


  • Coccus (spherical)
  • Bacillus (rod-shaped)
  • Vibrium (comma-shaped)
  • Spirillum (spiral)
• Bacteria


  • They are very complex in behaviour
  • They show the most extensive metabolic diversity
  • Some are autotrophic (photosynthetic or chemosynthetic)
  • Majority are heterotrophic
• Archaebacteria


  Bacteria that live in harsh habitats like extreme salty areas, hot springs, and marshy areas
• Eubacteria


  True bacteria, characterised by the presence of a rigid cell wall and flagella (if motile)
• Types of eubacteria


  • Cyanobacteria (photosynthetic autotrophs)
  • Chemosynthetic autotrophic bacteria
  • Heterotrophic bacteria
• Bacterial reproduction


  1. Fission

  2. Spore formation

  3. Primitive type of DNA transfer (sexual reproduction)
• Mycoplasma


  Organisms that completely lack a cell wall, the smallest living cells known, can survive without oxygen, many are pathogenic
• Protista


  Kingdom of single-celled eukaryotes, includes Chrysophytes, Dinoflagellates, Euglenoids, Slime moulds and Protozoans
• Members of Protista are primarily aquatic
• Protists reproduce asexually and sexually by a process involving cell fusion and zygote formation
• Chrysophytes


  Group including diatoms and golden algae, found in fresh water and marine environments, mostly photosynthetic, microscopic and float passively in water currents (plankton)
• Diatoms


  • Cell walls form two thin overlapping shells embedded with silica, indestructible cell walls have left behind large deposits in their habitat over billions of years
• Protista


  A kingdom that includes Chrysophytes, Dinoflagellates, Euglenoids, Slime moulds and Protozoans
• Protista


  • Primarily aquatic members
  • Eukaryotic cells with well-defined nucleus and membrane-bound organelles
  • Some have flagella or cilia
  • Reproduce asexually and sexually
• Chrysophytes


  A group that includes diatoms and golden algae
• Chrysophytes


  • Diatoms
  • Golden algae (desmids)
• Chrysophytes


  • Found in fresh water and marine environments
  • Microscopic and float passively in water currents (plankton)
  • Mostly photosynthetic
  • Diatoms have silica cell walls that accumulate as 'diatomaceous earth'
  • Diatoms are chief producers in oceans
• Dinoflagellates


  Mostly marine and photosynthetic organisms
• Dinoflagellates


  • Cell wall has stiff cellulose plates
  • Have two flagella, one longitudinal and one transverse
  • Red dinoflagellates (e.g. Gonyaulax) can cause 'red tides' and release toxins