Biological Classification of Living Organisms

BIOLOGICAL CLASSIFICATION

•  Two kingdom classification includes Plantae and Animalia - proposed by Linnaeus
• Advanced and most popular system of classification - Five Kingdom Classification proposed by R.H. Whittaker (1969)
• Five Kingdom Classification as proposed by R.H. Whittaker (1969):
  1. Monera,
  2. Protista,
  3. Fungi,
  4. Plantae and
  5. Animalia.       
  
  
  
  
• The main criteria for classification used by Whittaker:
  1. Cell Structure
  2. Thallus Organization
  3. Mode of Nutrition
  4. Reproduction and
  5. Phylogenetic Relationships 
  
  

                                       Kingdom Monera

  Monera is a Kingdom that contains unicellular organisms without a nucleus ( i.e. a prokaryotic cell organisation ) such as bacteria.
  
  

  Bacteria are grouped under four categories based on their shape:

  1. The spherical Coccus (pl.: cocci),

  2. The rod-shaped Bacillus (pl.: bacilli),

  3. The comma-shaped Vibrium (pl.: vibrio) and

  4. The spiral Spirillum (pl.: spirilla)

  
  

  Though the bacterial structure is very simple, they are very complex in behaviour. Compared to many other organisms, bacteria as a group show the most extensive metabolic diversity. Some of the bacteria are autotrophic, i.e., they synthesise their own food from inorganic substrates. They may be photosynthetic autotrophic or chemosynthetic autotrophic. The vast majority of bacteria are heterotrophs, i.e., they do not synthesise their own food but depend on other organisms or on dead organic matter for food.
  

  Archaebacteria
  
  

  These bacteria are special since they live in some of the most harsh habitats and named according to their habitats as follows:
  

  1. Halophiles live in extreme salty areas,

  2. Thermoacidophiles live in hot springs and

  3. Methanogens live in marshy areas
  
  

  Archaebacteria differ from other bacteria in having a different cell wall structure and this feature is responsible for their survival in extreme conditions. Methanogens are present in the guts of several ruminant animals such as cows and buffaloes and they are responsible for the production of methane (biogas) from the dung of these animals.
  

  Eubacteria
  
  

  There are thousands of different eubacteria or ‘true bacteria’. They are characterised by the presence of a rigid cell wall, and if motile, a flagellum. The cyanobacteria (also referred to as blue-green algae) have chlorophyll a similar to green plants and are photosynthetic autotrophs. The cyanobacteria are unicellular, colonial or filamentous, marine or terrestrial algae. The colonies are generally surrounded by gelatinous sheath. They often form blooms in polluted water bodies. Some of these organisms can fix atmospheric nitrogen in specialised cells called heterocysts, e.g., Nostoc and Anabaena. Chemosynthetic autotrophic bacteria oxidise various inorganic substances such as nitrates, nitrites and ammonia and use the released energy for their ATP production. They play a great role in recycling nutrients like nitrogen, phosphorous, iron and sulphur.
  

  Heterotrophic bacteria are the most abundant in nature. The majority are important decomposers. Many of them have a significant impact on human affairs. They are helpful in making curd from milk, production of antibiotics, fixing nitrogen in legume roots, etc. Some are pathogens causing damage to human beings, crops, farm animals and pets. Cholera, typhoid, tetanus, citrus canker are well known diseases caused by different bacteria.
  

  Bacteria reproduce mainly by fission. Sometimes, under unfavourable conditions, they produce spores. They also reproduce by a sort of sexual reproduction by adopting a primitive type of DNA transfer from one bacterium to the other.
  

  The Mycoplasmas are organisms that completely lack a cell wall. They are the smallest living cells known and can survive without oxygen. Many mycoplasma are pathogenic in animals and plants.