Five Kingdom Classification of Plants and Animals

Aristotle took the initiative to propose the biological classification of plants and animals by identifying simple morphological features. But Carolus Linnaeus later divided all living organisms into two categories or kingdoms namely Plantae and Animalia.

The two-kingdom classification did not work long as it did not consider all the factors while dividing. As a result, scientists started working on the model again. The five-kingdom classification of plants and animals of 1969 is a proposal by R.H. Whittaker. 

This elaborate division looks at five kingdoms – Monera, Protista, Fungi, Plantae, and Animalia. The division is on the basis of distinctive features of different living organisms. Some of the criteria are the structure of the cell, organization of the body, nutrition and reproduction process, and more.

This is one of the basic topics of biology. Classification is nothing but a system of organizing the organism in different categories according to their similarities and differences. Let’s take a look at each of the kingdom in detail:

Kingdom Monera

This kingdom looks at bacterias at large. These organisms are prokaryotes, they don’t have a well-defined nucleus but bacterias have a cell wall. This cell wall is made up of amino acids and polysaccharides.

They are microscopic in physical nature. They can be heterotrophic and autotrophic. These microorganisms live in extreme conditions and are abundant. 

The nutrition of these organisms can be by synthesizing their own food, getting it from the environment, or by living on other organisms as parasites. Thus they can be autotrophic or heterotrophic. Some well-known bacteria are blue-green algae and mycoplasma.

These bacterias mainly reproduce by fission. These bacterias are further classified in 4 categories according to their shapes. They are – 

1. Coccus – Spherical shaped 
2. Bacillus – Rod-shaped
3. Vibrio – Comma shaped 
4. Spirillum – Spiral shaped

There is another division of bacterias under the Kingdom Monera –

Archaebacteria

• These bacteria are important because of their living condition. 
• They live in the harshest habitats like Salty Areas (halophiles), hot springs (thermoacidophiles), and marshy areas (methanogens). 
• The reason they survive in these extreme conditions is because of their different cell structure. 
• Methanogens especially are present on cows and buffaloes and are the reason behind the production of biogas from their dung. 

Eubacteria

• They are the true bacterias. 
• They have a rigid cell wall and have flagella for movement. 
• These are mostly present in intestines and soil.

Photosynthetic Bacteria

• They have chlorophyll and are photosynthetic autotrophs.
• They are unicellular, colonial, marine, or terrestrial algae. 
• These live in places surrounded by a gelatinous sheath.
• They usually form blooms in polluted water bodies.
• They are capable of fixing atmospheric nitrogen in specialized cells. 

Chemosynthetic Bacteria

• They release many inorganic substances like nitrates, nitrites, and ammonia.
• They use this energy for their Adenosine triphosphate production.
• These can recycle nitrogen, phosphorus, iron, and sulphur.

Heterotrophic Bacteria

• They are abundant in nature and are important decomposers.
• They are the bacterias responsible for making curd, production of antibiotics, and more.
• Cholera, typhoid, tetanus, citrus canker are some diseases caused by these bacteria.

Reproduction in Bacteria

• Fission is the most common reproduction method in Bacteria.
• And in unfavorable conditions, they produce spores.
• Sexual reproduction is possible by transfer from one bacteria to another of a primitive DNA. 

Mycoplasma

• They don’t have or lack a cell wall.
• These are the smallest living cells that survive without oxygen.
• They might be pathogenic in animals and plants.

Kingdom Protista

Eukaryotes are organisms that have cells with a nucleus enclosed inside a nuclear envelope. The organisms under the kingdom Protista are eukaryotes and single-celled. For mobility, they have cilia or flagella.

The process of reproduction can be asexual or sexual and follows cell fusion and zygote formation. The nutrition process can be autotrophic or heterotrophic. These organisms are mainly aquatic in nature. This kingdom is further classified into subcategories. They are – 

Chrysophytes

• This division has diatoms and golden algae and is mostly photosynthetic. 
• Diatoms specifically are found in fresh/marine environments. 
• They are microscopic in nature and float on water currents. 
• The cell wall of these organisms forms two thin overlapping shells. 
• The walls are unbreakable because they are embedded with silica. 
• They are commonly used for polishing, filtration of oils, and syrups. 

Dinoflagellates

• The organisms are primarily aquatic and photosynthetic.
• They look yellow, green, blue, or red according to the main pigments in their cells.
• The cell walls of these organisms have stiff cellulose plates on the outer layer.
• They mostly have two flagella – longitudinally and transversely. 
• Red dinoflagellates often multiply rapidly so that they make the sea appear red.
• If they release large amounts of toxins they may even kill marine animals. 

Euglenoids

• They are mostly present in stagnant water.
• They have a protein-rich layer – pellicle and not a cell wall becoming flexible.
• Also They have two flagella – short and long.
• They are photosynthetic in sunlight and in absence, they are heterotrophs and survive on smaller organisms.

Slime Moulds

• These organisms are saprophytic protists.
• They move along decaying twigs and leave organic material behind.
• They can form plasmodium under suitable conditions that spread over several feet.
• Their spores have true walls and are dispersed by air currents. 
• They can survive under adverse conditions.

Protozoans

• All of them are heterotrophs.
• They live their entire life as predators or parasites. 
• They might be primitive relatives of animals. 
• There are four types of protozoans –

Amoeboid protozoans

• They live mostly in aquatic conditions like seawater or wet soil. 
• They use false feet or amoeba to capture their prey.
• The aquatic ones have a silica shell on them. Some are even parasites. 

Flagellated protozoans

• They have flagella and live free or as parasites. 
• The parasites in this category can cause sleeping sickness. 
• An example is Trypanosoma.

Ciliated protozoans

• They live mostly in aquatic conditions with large numbers of cilia. 
• They have cavities on the cell surface.
• The movement of cilia allows the entry of water in the surface cavity. 
• An example is Paramoecium. 

Sporozoans

• They have a spore-like stage in their life cycle that is infectious. 
• The plasmodium comes under this category and causes malaria disease. 

Kingdom Fungi

The organisms under kingdom fungi are heterotrophic and eukaryotic. They usually absorb soluble organics from dead substances. Thus they are called saprophytes. They are parasites and depend on other organisms.

Some of them also share a dependent relationship with algae and the term for it is a symbiotic relationship. The reproduction process of fungi follows fragmentation, fission, and budding.

Asexual reproduction is by conidia and sexual reproduction is by oospores.their cell wall is of a complex sugar called chitin. Fungi can survive in air, water, soil, and animals and plants. But they grow well in warm and humid locations.

But yeast is an exception as it is unicellular and filamentous. Their structure is threadlike and long called hyphae. The network of hyphae forms mycelium. There are two types – tubes filled with cytoplasm – coenocytic hyphae and cross-walls or septae.

Some examples are mushrooms, bracket fungi, or puffballs. 

Phycomycetes

• They follow asexual reproduction by zoospores or aplanospores. 
• The production of spores takes place in the sporangium.
• The fusion of two gametes leads to the formation of a zygospore. 
• They may be similar or dissimilar in structure. 
• Some examples are Mucor and Rhizopus. 

Ascomycetes

• They are multicellular and are commonly known as sac-fungi.
• Penicillium and yeast are two examples. 

Basidiomycetes

• They are present in soil, logs, and trees as parasites. 
• Lack asexual spores and follow vegetative propagation. 
• They lack reproductive organs but produce plasmogamy by the fusion of cells. The end result is dikaryotic. 
• Some examples are – mushrooms, rusts, and puffballs. 

Deuteromycetes

• They are the imperfect fungi as they are only capable of asexual reproduction. 

Let’s look at some important difference between Kingdom Monera, Kingdom Protista, and Kingdom Fungi – 

Kingdom Plantae

The organisms under this kingdom are multicellular and eukaryotes. Their cells are formed of cellulose. They use chlorophyll for photosynthesis thus are autotrophs. The insectivorous plants like Venus trap and parasite luke Cuscuta are heterotrophic.

This kingdom includes – algae, bryophytes, pteridophytes, gymnosperms, and angiosperms. They follow the nutrition process of holozoic which is ingestion of food. They are two phases in their life cycle – diploid sporophytic and the haploid gametophyte.

These phases are different in different plant groups. This idea is called alternation of generation. 

Kingdom Animalia

The organism under this kingdom is multicellular eukaryotes without any cell walls. They are heterotrophs and depend on plants for food. Their digestion process is done in the internal cavity 

stores fat. There is ingestion of food inside the body or nutrition is holozoic. They have a fixed growth cycle with a definite shape or size for each phase. They can locomote and follow sexual reproduction between male and female. 

Viruses, Viroids, and Lichens

This is not a part of five kingdom classification but is equally important. The reason is viruses are dead and have no cells. They are non-cellular organisms with a crystalline structure. They act as parasites and can replicate the host after killing them.

Pasteur called the virus a poisonous fluid. They have either RNA or DNA with proteins.  The protein coat is a capsid made up of capsomeres to protect the acids. The arrangement is in a helical form. The virus can cause problems like smallpox and AIDS.

The symptoms in plants are the presence of mosaic, leaf rolling, and yellow leaves with slow growth. 

Viroids are smaller than viruses but cause infection. They are RNA-free and don’t have a protein coat thus are viroid. The RNA is low in molecular weight. They are famous for causing potato spindle tuber disease. Lichens live in a symbiotic relationship.

They are usually with algae or fungi. The algal part is autotrophic while the fungal part is heterotrophic. Algae take care of food production while the fungus manages the minerals and vitamins. It is hard to identify both the organisms in lichens. They are best to find non-polluted areas as they only grow there. 

Conclusion

This article covered the five kingdom classification of plants and animals by R.H. Whittaker. It covered all the five kingdoms with details of every aspect. This is a biology article for UPSC which aspirants can read to understand the basic idea behind the model.

It is the most prominent model used to classify organisms when it comes to research and development. It is easy to understand and even the newcomers of the science field can refer to this.

If you are planning to appear for the UPSC exam then you must give it a read. Because it is basic 10th class biology, aspirants can expect this topic in UPSC Prelims as well.