Biologists, such as Ernst Haeckel (1894), Robert Whittaker (1969) and Carl Woese (1977) have tried to classify all living organisms into broad categories, called kingdoms. The classification Whittaker proposed has five kingdoms: Monera, Protista, Fungi, Plantae and Animalia, and is widely used. These groups are formed on the basis of their cell structure, mode and source of nutrition and body organisation.
The modification Woese introduced by dividing the Monera into Archaebacteria (or Archaea) and Eubacteria (or Bacteria) is also in use.
Further classification is done by naming the sub-groups at various levels as given in the following scheme:
Phylum (for animals) / Division (for plants)
Thus, by separating organisms on the basis of a hierarchy of characteristics into smaller and smaller groups, we arrive at the basic unit of classification, which is a ‘species’. So what organisms can be said to belong to the same species? Broadly, a species includes all organisms that are similar enough to breed and perpetuate.
The important characteristics of the five kingdoms of Whittaker are as follows:
These organisms do not have a defined nucleus or organelles, nor do any of them show multi-cellular body designs. On the other hand, they show diversity based on many other characteristics. Some of them have cell walls while some do not. Of course, having or not having a cell wall has very different effects on body design here from having or not having a cell wall in multicellular organisms. The mode of nutrition of
organisms in this group can be either by synthesising their own food (autotrophic) or getting it from the environment ( heterotrophic ) . This group includes bacteria, blue-green algae or cyanobacteria, and mycoplasma.
This group includes many kinds of unicellular eukaryotic organisms. Some of these organisms use appendages, such as hair-like cilia or whip-like flagella for moving around. Their mode of nutrition can be autotrophic or heterotrophic. Examples are unicellular algae, diatoms and protozoans .
These are heterotrophic eukaryotic organisms. Some of them use decaying organic material as food and are therefore called saprotrophs. Others require a living protoplasm of a host organism for food. They are called parasites. Many of them have the capacity to become multicellular organisms at certain stages in their lives. They have cell walls made of a tough complex sugar called chitin.
Examples are yeasts, molds and mushrooms.
Some fungal species live in permanent mutually dependent relationships with blue green algae (or cyanobacteria). Such relationships are called symbiotic. These symbiobic life forms are called lichens. We have all seen lichens as the slow-growing large coloured patches on the bark of trees.
These are multicellular eukaryotes with cell walls. They are autotrophs and use chlorophyll for photosynthesis. Thus, all plants are included in this group. Since plants and animals are most visible forms of the diversity of life around us, we will look at the subgroups in this category later.
These include all organisms which are multicellular eukaryotes without cell walls.
They are heterotrophs