American taxonomist Robert HOT. Whittaker proposed the 5-kingdom system of classification of the living organisms in the year 1969. This system was based on three criteria
(a) Cellular organization: the cells may be prokaryotic or eukaryotic
(b) Body organization – the organisms with unicellular and simple structure to multi cellular complex body.
(c) Mode of nutrition from autotrophic to heterotrophic and absorptive. Ecological role of the organisms was also taken into account in classifying the world of living beings. This system classifies the organisms into five kingdoms namely Monera, Protista, Mycota (Fungi), Plantae and animalia.
Micro-organisms were spread in the Kingdoms – Monera, Protista and Mycota. Fungi were given the status of a separate kingdom above the protists. According to this system the primitive prokaryotic organisms under Monera gave rise to unicellular, Protists. The kingdoms of mycota, plantae and animalia developed in three independent lines. The characteristic features of the kingdoms are given in the following.
I. KINGDOM MONERA:
Example Bacteria, Cyanobacteria (Blue green algae)
1. Unicellular organisms with no nucleus, nuclear membrane comprise this kingdom. Besides all the membrane bound organelles such as mitochondria chloroplasts, lysosomes etc. are absent from the monera cell. Photosynthetic organisms have pigments enclosed by ingrowths of the plasma membrane.
2. The only cytoplasm organelles that occur in these organisms are ribosome.
3. The major component of the cell wall is peptidoglycan.
4. Flagellated ones have single thread like structures. The major component of it is flagellin with no 2+9 organization.
5. The organisms under this group show variation in nutrition. These may be autotrophic or heterotrophic. Many heterotrophic ones are important in causing decay and decomposition.
6. Sexual reproduction very rarely occurs in these organisms. Mostly organisms reproduce vegetative or asexually. Sometimes gene recombination may occur.
7. Only a single naked chromosome is present in the cell. DNA component in it is circular with no chromosomal proteins.
Example – Protozoans, Euglenoids
1. The organisms are unicellular eukaryotes.
2. These are predominantly aquatic.
3. Cell wall when present is constituted mainly of cellulose.
4. All membrane bound organelles are present.
5. Flagella when present show 2+9 arrangement.
6. Nutrition in these organisms is very often photosynthetic (autotrophic)
7. Asexual and sexual reproduction occurs.
8. The DNA component associated with basic protein is called histones.
KINGDOM – MYCOTA OR FUNGI;
Example – Aspergillus, Penicillium, Saccharomyces (Yeast)
1. These are uni or multi cellular – achlorophyllous, spore bearing thallophytes.
2. Major component of cell wall may be cellulose or chitin or both.
3. When multi cellular, thread like structures branched in various patter called mycelia (singular – mycelium) constitute the thallus. Individual branches of it are called hypha (plural-hyphae).
4. These are heterotrophic, may be saprophytic or parasitic.
5. Mode of nutrition is absorptive which means the enzymes secreted by hyphae digest complex nutrients of the host. The simplified nutrients then absorbed by these organisms.
6. The reproduction may be vegetative, asexual and sexual. Asexual reproduction Lakes place under favorable conditions.
7. The reserve food materials are glycogen.
IV. KINGDOM – PLANTAR
1. It includes multi cellular, eukaryotic and autotrophic organisms.
2. The major component of the cell wall is cellulose.
3. The cells contain one or two large vacuoles and no structure like Centrosome.
4. These organisms are also called producers.
5. The reserve food material is generally starch.
6. Growth in higher plants is always terminal.
7. The trees generally grow being fixed to the soil.
8. Reproduction may be vegetative, asexual or sexual.
(Example: – Multi cellular algae, bryophytes, pteridophytes, phanerogams.)
V. KINGDOM – ANIMALIA
1. All the heterotrophic multi cellular organisms are under this kingdom.
2. Higher animals show organ organization.
3. Cell wall is lacking
4. Large number of small valoules with centrosome present in their cells.
5. Animals mostly show locomotion.
6. Reproduction occurs in higher animals by sexual method.
7. Reserve food material is glycogen.
8. Growth occurs equally in all parts of the body.
1. This system of classification is based on origin and evolution of the organisms, hence, phylogenetic one.
2. Since prokaryotes arc distinctly different from all other organisms, they were given the status of a kingdom.
3. Likewise, Fungi are organisms with cell wall but the composition of walls and their physiological and nutritional properties are distinctly different. Hence, the organisms were kept in a separate kingdom.
1. There were various complexities in this classification. For example, type of organism such as, algae was spread among the protista, Mod and plantae.
2. All groups are polyphyletic and not monophyletic in origin.
3. Protista is an artificial and heterogeneous group. It comprises of walled wall less organisms which may be photosynthetic or non photosynthetic
4. No place was assigned to for the viruses.
In all these systems of classification, bacteria and viruses have posed problem in assigning them to a specific position, hence, their status in living world is dealt separately.