Living things are identified and categorised on the basis of their body design in relation to their form and function. So, the classification of life forms is closely related to their evolution whereas living organisms have been classified variously according to different criteria. In the same Whittaker in 1959 classified living organisms into following five kingdoms or broad categories.
1. Kingdom Monera (Prokaryotic bacteria and blue green algae).
2. Kingdom Protista (Unicellular Eukaryotic organisms- protozoans, fungi and algae).
3. Kingdom Fungi (Multinucleate higher fungi).
4. Kingdom Plantae (Multicellular green plants and advanced algae).
5. Kingdom Animalia (Multicellular animals).
In 1982, Margulius and Schwartz revised the five kingdom classification. It includes one prokaryote and four eukaryote kingdoms- the Protocista, the Fungi, the Plantae and the Animalia. This Scheme is widely accepted.
1. Kingdom Monera (Prokaryote)
It is divided in to Archaebacteria (Archae) and Eubacteria (Bacteria). Out of the two Archae is more ancient.
(i) Archaebacteria: Most of them are autotrophs and only a few photosynthesize. They derive their energy for metabolic activities, from the oxidation of chemical energy sources like reduced gases – Ammonia, methane or hydrogen sulphide. In the presence of these chemicals it can manufacture their own amino acids acid proteins.
They are further divided in to three groups: Methanogens (manufacture methane), Thermoacidophiles (Favour extremely hot and acidic environments), and Halophiles (grow in very salty environment).
(ii) Eubacteria: They generally lack membrane-enclosed organelles like nucleus etc. Nucleoid serves as a single chromosome. Electron transport and photosynthesis, takes place on plasma membrane, which sometimes folds inwards into the cell’s interior.
2. Kingdom Protista (Protocista)
This group includes many kinds of unicellular eukaryotic organisms such as unicellular algae, protozoans and unicellular fungi. Some of these organisms use appendages ie hair like structure cilia like in paramecium or whip-like flagellum e.g euglena. Their mode of nutrition can be autotrophic (Unicellular algae, diatoms) or heterotrophic (protozoans).
Examples: Unicellular algae, chlorella, Euglena, Trypanosoma (causes sleeping sickness),Amoeba, Plasmodium, Paramecium, Chlamydomonas etc.
3. Kingdom Fungi
• Non-green plants which are not photosynthetic. They are heterotrophic and eukaryotic organisms. Some fungi are parasites can draw nutrients from living cells of their host- plants like Puccinia, Ustilago etc. Some are decomposers like Penicillium etc and saprophytes derive their nourishment from dead remains of plants and animals.
• Fungus may be unicellular or filamentous. The body of multicellular and filamentous is called mycelium and composed of several thread like structures called hyphae.
• The reserve food is glycogen in them.
Examples: Bread mold, Yeast, sponge, mushroom, rust, smut etc.
4. Kingdom Plantae
• Includes multicellular organisms except relatives of algae.
• They are eukaryotes.
• Cell wall is present.
• Possesses a single large central vacuole bounded by tonoplast (membrane).
• Reserve food for plants in the form of starch and lipids.
• Plastids are present also some have photosynthetic pigments called chloroplasts.
• Autotrophic nutrition.
• Growth of plants is indefinite.
• Irregular body due to presence of branches.
5. Kingdom Animalia
• Animals have wall less eukaryotic cells.
• Heterotrophic nutrition.
• Growth of animals is limited.
• Animals generally possess a definite shape, size and symmetry.
• Most animals are mobile.
• Animals have organisation of cellular, tissue, organs and organ system level.