Thursday, September 1, 2011

5 Kingdom Concept

    Introduction to R. H. Whittaker 5 Kingdom Classification:
    One of the earliest scheme of classification in the natural system is the two-kingdom classification proposed by Linnaeus in 1758. Linnaeus placed living organisms into two kingdoms - Plantae and Animalia.  The plant kingdom proposed by Linnaeus and subsequent works of that period comprised of bacteria, fungi, algae, liverworts, masses, ferns, conifers and flowering plants.   The animal kingdom proposed by Linnaeus and others included the unicellular and multicellular animals.   The two-kingdom classification has certain demerits, particularly with reference to lower forms of life which show common features with both plants and animals.   In 1858 Earnt Haeckel proposed the three-kingdom classification in which apart from Plantae and Animalia, a new kingdom Protista was created to include all the unicellular organisms, both from plants and animals.   The advent of electron microscope helped scientists in discovering the existence of two forms of cells in organisms - prokaryotic and eukaryotic.   In 1966, Copeland suggested the creation of a fourth kingdom-Monera to include the prokaryotic organisms like bacteria and blue-green alga. This led to a four kingdom classification. In 1969 Robert H. Whittaker proposed the creation of a new kingdom called Fungi to include the fungi exclusively. This led to a five kingdom classification which is in practice today.
    • Criteria for R. H. Whittaker 5 Kingdom Classification:
      • Complexity of cell structure
      • Complexity of organisms
      • Mode of nutrition
      • Phylogenetic relationship
    • The Five Kingdom Classification by Whittaker:
     The five kingdoms of life are:
    • Kingdom: Monera (prokaryotic organisms)  
    • Kingdom: Protista(primitive eukaryotic organisms)
    • Kingdom: Mycota (exclusively fungi)   
    • Kingdom: Metaphyta or Plantae(advanced eukaryotic plants)  
    • Kingdom: Metazoa or Animalia (all multicellular animals)
    Kingdom Monera
     Living things included in the kingdom Monera are minute and single-celled prokaryotic organisms that lack membrane-bound cell organelles & nucleus. Members of this kingdom are bacteria, cyanobacteria or blue-green algae and spirochetes. Some members of the same organism join together to form chains. Cyanobacteria is a type of organism, which is intermediate between algae and bacteria. Their mode of nutrition is by absorbing food through the cell wall. In recent times, scientists have further divided the kingdom Monera into Eubacteria and Archaebacteria. The former refers to true bacteria, whereas the latter encompasses bacteria-like organisms that are adapted to extreme environmental conditions like hot springs and volcanic vents. 
    Kingdom Protista
    Protista includes single-celled eukaryotic organisms, which contain membrane-bound cell organelles. It includes organisms that are neither plants nor animals. In simpler terms, the living things classified under Protista are unusual and diverse forms, which cannot be grouped in any of the four remaining kingdoms. For example, the simplest organisms on Earth, amoeba (a protozoan) and giant sea kelp belong to this kingdom. The members of Protista obtain nutrition by absorption, ingestion and photosynthesis. 

    Kingdom Fungi
    Fungi are group of multi-cellular, eukaryotic, non-motile organisms that form hyphae and mycelium. Members belonging to this kingdom lack chlorophyll, hence they are differentiated from plants. The type of organisms classified under Fungi include molds, yeasts, mildews, smuts and mushrooms. Their size may range from small microscopic yeasts to large mushrooms. Fungi derive their nutrients by absorption from dead and decaying organic materials. 
    Kingdom Plantae
    Kingdom Plantae encompasses multi-cellular, eukaryotic, non-motile living things. The type of organisms included in this kingdom are algae, mosses, ferns, flowering and non-flowering plants. These organisms contain the photosynthetic pigment, called chlorophyll. Hence, they synthesize their own food by means of photosynthesis, which takes place in the presence of carbon dioxide, water and sunlight. 
    Kingdom Animalia
    Animalia are group of multi-cellular, eukaryotic and motile living things. Members belonging to Animalia are insects, worms, fish, reptiles, amphibians, birds and mammals. They cannot synthesize food and their mode of nutrition is by ingesting food. They can feed either on plants or other living things.
    Merits and Demerits of R. H. Whittaker 5 Kingdom Classification:
    • Merits of R. H. Whittaker 5 Kingdom Classification
    This system of classification looks more scientific and natural because of the following considerations:
    1. It is largely the most accepted system of modern classification mainly because of the phylogenetic placing of different groups of living organisms.
    2. Separation of prokaryotes into an independent kingdom is justifiable because they differ from all other organisms in their general organization.
    3. Grouping of all unicellular eukaryotes under the kingdom Protista has solved many problems, particularly related to the position of organisms like Euglena.
    4. Elevation of the group fungi to the status of a kingdom is justifiable since fungi totally differ from other primitive eukaryotes like algae and protozoans.
    5. The kingdoms plantae and animalia are now more homogeneous groups than they were in the two kingdom classification as it shows the phylogeny of different life styles.
    6. The five-kingdom classification gives a clear indication of cellular organization and modes of nutrition, the characters which appeared very early in the evolution of life.

    • Demerits of R. H. Whittaker 5 Kingdom Classification:
    The five-kingdom classification has certain drawbacks also, particularly with reference to the lower forms of life.
    1. The kingdoms Monera and Protista include diverse, heterogeneous forms of life. In both the kingdoms there are photosynthetic (autotrophic) as well as non-photosynthetic (heterotrophic) organisms. Both the kingdoms include organisms which have cells with cell wall as well as without cell wall.
    2. None of the three higher kingdoms include a single ancestor of all its forms.
    3. Multicellular lines have originated from protistans several times.
    4. Unicellular green algae like Volvox and Chlamydomonas have not been included under Protista because of their resemblance to other green algae.
    5. Slime moulds differ totally from other members of Protista in their general organization.
    6. Viruses have not been given proper place in this system of classification.

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