Biological 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 was found inadequate as a large number of organisms did not fall into either category
• Besides gross morphology, a need was also felt for including other characteristics like cell structure, nature of wall, mode of nutrition, habitat, methods of reproduction, evolutionary relationships, etc.
• Classification systems for the living organisms have undergone several changes over time
• Though plant and animal kingdoms have been a constant under all different systems, the understanding of what groups/organisms be included under these kingdoms have been changing
• The number and nature of other kingdoms have also been understood differently by different scientists over time
• Five Kingdom Classification


  Kingdoms defined by R.H. Whittaker in 1969: Monera, Protista, Fungi, Plantae and Animalia
• Criteria used by Whittaker for classification


  • Cell structure
  • Body organisation
  • Mode of nutrition
  • Reproduction
  • Phylogenetic relationships
• The three-domain system has also been proposed that divides the Kingdom Monera into two domains, leaving the remaining eukaryotic kingdoms in the third domain and thereby a six kingdom classification
• 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, like the prokaryotic bacteria and the blue green algae with the eukaryotic organisms
• 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 organisms which, in earlier classifications, were placed in different kingdoms
• Kingdom Protista


  Grouping of primarily unicellular eukaryotic organisms that share a common ancestry, based on molecular evidence
• Eukaryotic

  Cells have a true nucleus and other membrane-bound organelles
• Unicellular


  Organisms consist of a single cell, although some form multicellular colonies or simple multicellular organisms
• Molecular evidence


  Modern molecular techniques, such as DNA sequencing, have shown that protists share a common ancestry and are therefore grouped together in the same kingdom
• Flagellates

  Unicellular organisms that move by whipping their flagella, or hair-like structures, through the water
• Algae

  Primarily photosynthetic protists, ranging from tiny, single-celled organisms to large, multicellular seaweeds
• Photosynthesis


  Process used by some organisms to convert light energy into chemical energy in the form of organic compounds, such as glucose
• Chlorophyll


  Pigment found in chloroplasts that absorbs light energy and converts it into chemical energy
• Chloroplasts


  Structures found in the cells of photosynthetic organisms that contain chlorophyll and are the site of photosynthesis
• Organic compounds


  Chemical compounds that contain carbon and are typically produced by living organisms
• Light-dependent reactions


  First stage of photosynthesis that occurs in the presence of light and involves the conversion of light energy into chemical energy in the form of ATP and NADPH
• Light-independent reactions


  Second stage of photosynthesis, also known as the Calvin cycle, that occurs in the absence of light and involves the use of ATP and NADPH to convert carbon dioxide into glucose and other organic compounds
• ATP


  Adenosine triphosphate, a high-energy molecule used to store and release energy in cells
• NADPH


  Nicotinamide adenine dinucleotide phosphate, a high-energy molecule used to store and release energy in cells
• Glucose


  Simple sugar that is a common product of photosynthesis and serves as a source of energy for cells