The living organisms have been classified into two kingdoms for centuries i.e., plants and animals. However, the biologist found this system unsatisfactory and unworkable as many unicellular organisms displayed traits of both plant and animal. This system was also unable to support the structural and functional differences of a variety of living organisms and does not support the evolutionary evidence.
Whittaker's system of classification
In 1969, Robert Whittaker proposed a more sophisticated system known as the five-kingdom system of classification or Whittaker's System of Classification. Margulis and Schwartz in 1988 modified the 5 kingdom classification of Whittaker. In this classification system, the living organisms are divided into five different kingdoms based on their
- Modes of nutrition
- Cell structure complexity
- Evolutionary relationship
- Cell organization
- Reproduction
The five kingdoms of Whittaker's system of classification are:
- Kingdom Monera
- Kingdom Protista
- Kingdom Fungi
- Kingdom Animalia
- Kingdom Plantae
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| Whittaker's System of Classification |
1. Kingdom Monera
The organisms in this kingdom are prokaryotic unicellular. They lack a nucleus and membrane-bounded organelles. Some of the organisms have cell walls while some are without any cell wall.
They may be autotrophic (prepare their food) or heterotrophic (obtain food from another source). Examples are bacteria, cyanobacteria, and archaea.
2. Kingdom Protista
They are eukaryotic unicellular organisms. They have a nucleus and membrane-bounded organelles. Their mode of nutrition might be autotrophic or heterotrophic.
The organisms in this kingdom are the simplest form of eukaryotes. The mode of transport of the organisms might be flagella, cilia, or pseudopodia. Euglena, paramecium, and amoeba are some examples.
3. Kingdom Fungi
This kingdom includes multicellular heterotrophic eukaryotic organisms. They are decomposers and obtain their nutrients from decaying organic matter by secreting digestive enzymes.
The cells are organized into filaments and the secret digestive help in the penetration and break down of food. Their cell wall contains chitin as a strengthening substance. Examples of fungi include yeast, Penicillium, mushrooms, and molds.
4. Kingdom Animalia
The multicellular eukaryotic organisms are placed in this kingdom. Their mode of nutrition is heterotrophic and cannot prepare their food. They lack a cell wall and have complex body structures organized into organs and organ systems.
The organisms in this kingdom are highly organized and evolved. They require oxygen for living and reproduce sexually. This kingdom is divided into many phyla, classes, and families. Some examples are birds, reptiles, fish, insects, dinosaurs, and humans.
5. Kingdom Plantae
The multicellular eukaryotic organisms are present in kingdom Plantae. They are autotrophic and can prepare their food by the photosynthesis process. They have cellulose in their cell wall. A special pigment called chlorophyll is present in their cells to capture sunlight.
All plants are included in this kingdom. This kingdom is also divided into further phyla and classes. Flowering plants, mosses, ferns, trees are examples of kingdom Plantae.
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