Five Kingdom Classification

Robert Harding Whittaker (1920-1980) was an American ecologist. In 1969, he divided living organisms into 5 groups.

For this classification, Whittaker considered the following criteria:

1. Complexity of cell structure

Prokaryotic and eukaryotic.

2. Complexity of organisms

Unicellular or multicellular.

3. Mode of nutrition

Plants: Autotrophic, photosynthetic.

Fungi: Saprophytic (absorption from dead organisms).

Animals: Heterotrophic and ingestive.

4. Lifestyle

Plants: Producers.

Animals: Consumers.

Fungi: Decomposers.

5. Phylogenetic relationship

From prokaryotic to eukaryotic, unicellular to multicellular.

Five-kingdom system of classification

Linnaeus proposed a classification system by arranging organisms into taxonomic groups at different levels according to the characteristics they have.

1. Two kingdom classifications: Carolus Linnaeus in 1758 classified living organisms into two groups, plants and animals.
2. Five kingdom classification: H. Whittaker in 1959 further classified the organisms into five kingdoms: Kingdom Monera, Kingdom Protista, Kingdom Fungi, Kingdom Plantae, and Kingdom Animalia.

Ernst Haeckel (1894), Robert Whittaker (1959), and Carl Woese (1977) have tried to classify all living organisms into broad categories, called kingdoms.

Further classification is done by naming the subgroups at various levels, as given in the following scheme:

The five kingdoms and their key characteristics are given below:

1. Monera: These are prokaryotes, which means nuclear materials are not membrane-bound in them. They may or may not have a cell wall. They can be autotrophic or heterotrophic. All organisms of this kingdom are unicellular. For example: bacteria, blue-green algae (cyanobacteria), and mycoplasma.
2. Protista: These are eukaryotes and unicellular. Some organisms use cilia or flagella for locomotion. They can be autotrophic or heterotrophic. For example: Plants like unicellular algae and diatoms; animals like protozoans (Amoeba, Paramecium, and euglena)
3. Fungi: These are eukaryotic organisms with cell walls made up of chitin. They do not perform photosynthesis (heterotrophs). They may be unicellular (yeast) or filamentous (most fungi). They feed on decaying organic materials. Such a mode of nutrition is called saprophytic. Some fungi live in a symbiotic relationship with other organisms (lichens), while some are parasites as well. For example: mushrooms (Agaricus), green mould (Penicillium), and smut (Aspergillus).
4. Plantae: These are multicellular and autotrophs. The presence of chlorophyll is a distinct characteristic of plants, because of which they are capable of taking out photosynthesis. The cell wall is present.
5. Animalia: These are eukaryotic, multicellular, and heterotrophic organisms. The cell wall is absent.