Different classification systems recognize two to six kingdoms. For centuries the living organisms have been classified into two kingdoms. Plants and animals. Plants can prepare their own food from simple inorganic material and store energy (autotrophs). While animals cannot synthesize their own food from simple inorganic material. Animals depend for their food either on autotrophs or on decaying organic matter (heterotrophs).
Bacteria were present in the plants. Many biologists found this system satisfactory. While others found it unworkable for many unicellular organisms like euglena. Because euglena have both plant like (presence of chlorophyll) and animal like (lack of cell wall) characters. Also because it ignores the difference between prokaryotic and eukaryotic cells.
In 1866 Ernst Haeckel proposed a third kingdom Protista to accommodate euglena like organisms and bacteria. In 1937, E-Chatton suggested differentiating terms procariotique (from greek pro, meaning before, and karyon, meaning nucleus) used to describe bacteria and blue-green algae. The term eu-cariotique (from greek eu, mean true) to describe animal and plant cells.
Some biologists also disagree about the classification of fungi. Such as bread mold, yeast and mushrooms, which resemble plants in many ways but are not autotrophs.
Fungi are special forms of heterotrophs. Fungi obtain energy and structural material by breaking down (decomposing) and absorbing food substances from the surroundings. And possess chitin as a major structural component in their cell walls.
Five Kingdoms Classification
A relatively recent system of classification, Robert whittaker (1969) had proposed the five kingdom system. This system of classification shown in Fig 5.1 is based on three different levels of cellular organization. This organization associated with three principal modes of nutrition – photosynthesis, absorption and ingestion. The five kingdoms proposed
- The prodaryotic unicellular organisms (Monera) such as bacteria
- The eukaryotic predominantly unicellular organisms (Protista) such as euglena and amoeba,
- The eukaryotic multicellular autotrophs (Plantae)
- The eukaryotic multicellular reducers (fungi) for example mushrooms and
- The eukaryotic multicellular consumers (Animalia).
Plants are autotrophic in nutritional mode, making their own food by photosynthesis, such as mosses, ferns, flowering plants. Fungi are heterotrophic organisms that are absorptive in their nutritional mode. Most fungi are decomposers that live on organic matter, secrete digestive enzymes, and absorb small organic molecules that result from digestion.
Animals live mostly by ingesting food and digesting it within specialized cavities. They lack cellulose and show movements for example birds and reptiles. In five kingdom classification, all eukaryotes that did not fit the definition of plants, fungi or animalia were included in protista. Most protists are unicellular forms. But this kingdom also includes relatively simple multicellular organisms. That are believed to be direct descendants of unicellular protists.
Lynn Margulis and Karlene Schwartz (1988) modified five kingdom classification of Whittaker, by considering cellular organization, mode of nutrition, cytology, genetics and organelles of symbiotic origin (mitochondria, chloroplast). These five kingdoms are prodaryotae (monera), protoctista (protists), plantae, animalia and fungi (fig 5.1).
