WHAT IS CLASSIFICATION?
- Classification, in biology, refers to the establishment of a hierarchical system of categories on the basis of assumed natural relationships among organisms.
- The science of biological classification is referred to as Taxonomy
- Scientists at various times, have made efforts to group living organisms into different categories.
- At first, biologists classified organisms into plants and animals.
- Modern studies like genetics made it possible to unravel evolutionary relationships to a much greater extent.
- Ernst Haeckel, Robert Whittaker, and Carl Woese are some biologists who gave a broader system of classification. However, the most significant contribution was made by Robert Whittaker by coining the five kingdom classification.
NEED OF CLASSIFICATION
- The earth is estimated to harbor about 8.7 million species. Therefore, it is very difficult to remember and study individual species.
- In order to aid in the study of various species, they are grouped into specific categories based on shared features.
ADVANTAGES OF CLASSIFICATION
- Classification allows us to understand diversity better.
- It helps in identification and understanding the diversity of living organisms.
- Classification helps us to learn about features, similarities and differences among various groups of plants and animals.
- It enables us to understand how complex organisms evolve from simpler organisms.
- To apprehend and study the features, similarities and differences between various living organisms, they are grouped into different categories.
- Classification helps us to deal with a vast diversity of living forms.
- It is essential to understand the inter-relationships between different groups of organisms.
- Classification forms a foundation for the development of other biological sciences.
FIVE KINGDOM CLASSIFICATION
- The five kingdom system of classification is the most common way of grouping living organisms.
- All the organisms in the world are classified into five groups (kingdoms) based on modest distinctive characteristics.
- Five- Kingdom System of Classification was proposed by Robert H. Whittaker in 1969.
- This system is based on the work of previous biologists such as Carolus Linnaeus.
BASIS OF FIVE KINGDOM CLASSIFICATION
- In this classification system, organisms are broadly divided into five kingdoms, based on certain characters such as;
- The structure of cell
- Mode of nutrition
- source of nutrition
- Body organization
- Mode of Reproduction
SALIENT FEATURES OF FIVE KINGDOM CLASSIFICATION
- Bacteria and archaea are grouped into Kingdom Monera
- Algae and protozoa are grouped into Kingdom Protista
- Fungi are placed in the Kingdom Fungi.
- Plants are included in the Kingdom Plantae.
- Animals belong to Kingdom Animalia.
- Kingdom Monera consists of unicellular organisms with a prokaryotic cell organization.
- These include;
- These include;
- Organisms involved in this kingdom are microscopic (1 micron to few microns in length)
- Organisms included in Monera are the simplest organisms. They are unicellular and prokaryotic (they lack a well-defined nucleus and membrane-bound organelles i.e. endoplasmic reticulum, mitochondria etc.).
- Most of these organisms are bound by a rigid cell wall.
- The nutrition methods vary from species to species i.e. they can either be heterotrophs or chemo-autotrophs.
- These organisms are important decomposers.
- Some of these organisms can live in extreme environmental conditions.
The kingdom Protista includes unicellular nucleated microscopic organisms.
- They are Unicellular in organization and microscopic in nature.
- These Organisms are eukaryotic meaning that they possess a well-defined nucleus and membrane-bound organelles such as mitochondria, E.R., Golgi apparatus, etc.
- In many protists, flagella or cilia are present for locomotion.
- These organisms are found in aquatic and damp terrestrial habitats.
- Organisms of kingdom Protista have a diversified mode of nutrition. Most of them are autotrophic (producer), some are holozoic while some are parasitic.
- A few organisms are symbiotic and some are decomposers.
- Cell wall is found in some of these organism, such as in phytoplankton (unicellular algae) while it is not found in some other protozoans, such as in Amoeba and Paramecium.
- These Organisms can reproduce both by asexual and sexual mode of reproduction.
Fungi is a group of eukaryotic micro-organisms that include organisms such as yeasts, moulds and mushrooms.
- Fungi are eukaryotic, multicellular organisms. They are microscopic in size.
- They consist of a heterotrophic mode of nutrition.
- Fungi are saprobes or saprophytes i.e. fungi live on dead organic matter.
- Fungi which live on other organisms are called parasitic fungi.
- Fungi secrete digestive enzymes through their cell wall into the immediate environment where complete organic matter is made soluble and absorbed as solution.
- Fungi are usually non motile but shows protoplasmic flow in mycelium.
- They reproduce by both asexual and sexual means.
- Plastids are not found in the cells of fungi.
- Their cell wall is composed of chitin.
- They also act as decomposers by decomposing complex organic substances of dead mater.
- Fungi includes moulds, mushrooms, puff balls and bracket fungi.
- Yeasts form a group of exceptional fungi.
- Fungi cause many diseases in animals and plants. For example, ringworm (a disease of human skin).
all the plants are grouped into kingdom Plantae. They are eukaryotic, multicellular organisms and are autotrophic in nature.
- They are multicellular and eukaryotic organisms
- The outermost covering in a plant cell is the cell wall, which is composed of cellulose. Cell membrane is present inner to the cell wall.
- Majority of the plants are autotrophic and make their own food by the process of photosynthesis. This process involves the conversion of light energy into chemical energy and takes place in the photosynthetic pigments present in plastids.
- Some plants are heterotrophic in nature and live as parasites such as Cuscuta.
- Plants store their food in the form of starch.
- They are primarily non motile and anchored to a substrate.
- They reproduce by both asexual and sexual methods.
All animals are placed in the Kingdom Animalia. It consists of eukaryotic multicellular heterotrophic consumers.
- Kingdom Animalia includes multi-cellular eukaryotes. They are also called metazoans.
- They are heterotrophic in nature and rely on external sources for nutrition.
- The outermost covering of animal cell is formed by the cell membrane.
- They exhibit muscular contractibility thus can move and show locomotion.
- Many of them are parasitic and symbionts while a few are commensals.
- Animals are ecological consumers.
- They possess nervous system for coordination.
- Animals reproduce by means of sexual reproduction.
Drawbacks of this classification
Although the five-kingdom classification of Whittaker was a revolutionary step in the field of taxonomy, it fell short in explaining the following;
- Similar organisms are kept away from each other. For example, unicellular and multi cellular algae. Unicellular algae are placed in kingdom Protista, whereas multicellular algae are kept in kingdom Plantae.
- Dissimilar organisms should not be classified in the same group. However, there is diversity in kingdom Protista.
- All organisms other than those in kingdom Monera are polyphyletic in origin.
- The five kingdom classification fails to classify viruses.