Classification of organisms overview: Two kingdom, Three kingdom, Four kingdom and Five kingdom classification systems, Practice Problems and FAQs

You know that we are living in a world which has a great biodiversity. Millions of species already exist in this world and still, there are species yet to be discovered and to be added to this count. We need to name these organisms to identify them easily. Have you ever thought about how to keep a track of them and align them?

Fig: Biodiversity

Several people attempt different ways to classify these plants and animals. Do you think there are only two kinds of organisms? That means plants and animals. Well, the answer is no. You might have heard of microscopic organisms like bacteria. They also need to be classified. Now the question is, what should be the basis of this classification? Can you think of some? Yes, it can be morphological characteristics, their cell structure, cell organisation, mode of nutrition, etc.

Let us understand about different classification systems given by different scientists, their pros and cons and the basis of classification in this article.

Table of contents

• Two kingdom classification
• Three kingdom classification
• Four kingdom classification
• Five kingdom classification
• Six kingdom classification
• Practice Problems
• FAQuestion

Two kingdom classification

The two kingdom classification by Carolus Linneaus divided the entire living world into two kingdoms as follows:

• Kingdom Plantae

• Kingdom Animalia.

Fig: Carolus Linneaus

Kingdom Plantae

It includes bacteria, photosynthetic plants and non photosynthetic fungi. They are kept in this kingdom based on the following characteristics.

• Presence of cell wall
• Occurrence of inorganic crystals and central vacuole in the cell
• The mode of nutrition is the absorptive type.
• They show unlimited growth.
• They have starch as reserve food.
• Sense organs are absent.
• They lack nervous system and excretory system.
• They are able to synthesise their own food.
• They don't show locomotion.
• They possess branches without a proper shape.

Kingdom Animalia

It includes unicellular protozoans like Amoeba and multicellular metazoans like lions. They are kept in the particular kingdom based on the following characteristics.

• They lack a cell wall.
• They show locomotion.
• They have glycogen as their reserve food.
• They have a definite shape.
• They lack branches.
• They depend on other organisms for their food as they have no chlorophyll.
• They possess sense organs.
• They have a nervous system and excretory system.
• They show a holozoic mode of nutrition.
• They show limited growth.

Demerits of two kingdom classification

The following are the demerits of the two kingdom classification.

• Prokaryotes and eukaryotes were placed together.
• Unicellular and multicellular organisms were not separately classified.
• Photosynthetic plants and non-photosynthetic fungi were not separately classified.
• Euglena was placed in the kingdom Animalia. It has locomotory movements like animals but it has chlorophyll like plants. It shows holozoic and autotrophic modes of nutrition.
• Lichens were not classified properly.

Three kingdom classification

Ernst Haekel proposed the three kingdom classification system. He classified the organisms based on their morphological complexities, mode of nutrition, tissue system and division of labour. He placed bacteria, fungi, protozoa and other microorganisms into a new and separate kingdom, i.e., Protista.

Three kingdom classification includes the following kingdoms:

• Kingdom Protista
• Kingdom Plantae
• Kingdom Animalia

Fig: Ernst Haeckel

Demerits of three kingdom classification

This system places chlorophyll containing prokaryotic and eukaryotic organisms together in the same kingdom, Plantae.

Four kingdom classification

Herbert Copeland proposed the four kingdom classification system. He suggested a new kingdom, i.e., Monera in which all prokaryotic organisms were accommodated. This was possible as the electron microscopic studies revealed that the bacteria and related organisms have a different nuclear structure compared to other organisms. It included the following kingdoms:

• Kingdom Monera
• Kingdom Protista
• Kingdom Plantae
• Kingdom Animalia

Fig: Herbert Copeland

Demerits of four kingdom classification

The major drawback of this classification system is that fungi are not placed properly. They were kept along with the plants in the kingdom Plantae.

Five kingdom classification

In 1969, Robert H. Wittaker proposed a five kingdom classification system to place all organisms properly. He established the following kingdoms in his classification system:

• Kingdom Monera - It includes unicellular prokaryotic organisms. Examples include has bacteria, cyanobacteria and Archaea
• Kingdom Protista - It includes unicellular eukaryotic organisms. Examples include Amoeba, Hydra etc.
• Kingdom Fungi or Mycota - It includes multicellular eukaryotic saprophytic organisms. Yeast is an exception here, as it is unicellular. Examples include mushroom, toadstool etc.
• Kingdom Plantae or Metaphyta - It includes multicellular eukaryotic producer organisms. Examples include plants.
• Kingdom Animalia or Metazoa - It includes multicellular eukaryotic consumer organisms. Examples include animals like lions, tigers etc.

Fig: Robert H. Whittaker

Basis of five kingdom classification

Whittaker’s basis of classification includes the following:

Fig: Five kingdom classification

Merits of five kingdom classification

The following are the main merits of five kingdom classification.

It placed prokaryotes under a separate kingdom as Monera.

Unicellular and multicellular organisms were placed separately.

Non-photosynthetic fungi and photosynthetic plants were placed separately.

Autotrophs and heterotrophs were separately placed.

Phylogenetic relationships between organisms were considered.

Demerits of five kingdom classification

The main demerits of the five kingdom classification are listed below.

• Viruses were not placed in any kingdom.
• Archaebacteria and Mycoplasma are placed in the Kingdom Monera in spite of differences between them.
• In this system, the kingdom Monera and Protista contain both walled and wall-less organisms.
• Lichens were not included in his classification.
• Unicellular algae like Chlamydomonas were placed under Kingdom Protista and were separated from other algae placed in Kingdom Plantae.

Six kingdom classification

Carl Woese gave us the 3 domain classification system in 1990. In these 3 domains, 6 kingdoms are included.

Fig: Carl Woese

He suggested a separate kingdom for Archaebacteria. The three-domain system is based on the traditional five kingdom system but it divides the Kingdom Monera into two domains, leaving the remaining eukaryotic kingdoms in the third domain. It separates prokaryotes (Monera) into two groups such as Eubacteria (now bacteria) and Archaebacteria (now archaea).

Three domains

• Domain Archaea - It contains only Kingdom Archaebacteria. They are the primitive prokaryotes. They can survive or live in extreme environments. They are mostly obligate anaerobes. Examples include Methanogens, Thermoacidophiles and Halophiles.
• Domain Bacteria - They contain only Kingdom Eubacteria. They are prokaryotic organisms and lack membrane bound cell organelles. Examples include bacteria and cyanobacteria.
• Domain Eukarya - They contain Kingdoms such as Protista, Fungi or Mycota, Plantae or metaphyta and Animalia or Metazoa. They are eukaryotic organisms with membrane bound cell organelles.

Fig: Six kingdom classification

Practice Problems

Q1. The boundaries of a kingdom named ‘x’ are not very well defined. Identify the kingdom ‘x’.

A. Monera
B. Protista
C. Fungi
D. Plantae

Solution: The boundaries of kingdom Protista are not well defined. Thus, the kingdom ‘x’ is Protista. Hence, option b is correct.

Q2. Match Column I with Column II and identify the correct option.

A. i - a, ii - b, iii - c
B. i - c, ii - b, iii - a
C. i - a, ii - c, iii - b
D. i - c, ii -a, iii - b

Solution: Kingdom Protista shows cellular level of body organisation where a single cell performs all the life processes. Examples include Amoeba. Kingdom Mycota or Fungi shows multicellular level of organisation except unicellular yeast. Kingdom Animalia or Metazoa shows tissue, organ and organ level of organisation. Hence the correct answer is option d. The correct match is as follows:

Q3. ‘Euglena’ shows the properties of both plants and animals. Identify the kingdom under which it is placed in two kingdom classification.

A. Monera
B. Plantae
C. Animalia
D. Protista

Solution: In two kingdom classification, Euglena was placed under kingdom Animalia. Hence, option c is correct.

Q4. In five kingdom classification, organisms were classified on the basis of presence or absence of nuclear membrane. How many kingdoms have the nuclear membrane?

A. 1
B. 2
C. 3
D. 4

Solution: In five kingdom classification, four kingdoms (Protista, Fungi, Plantae and Animalia) out of five have a nuclear membrane. Kingdom Monera lacks a nuclear membrane and contains only prokaryotes like bacteria, cyanobacteria etc. Hence, option d is correct.

Q5. What are the drawbacks of two kingdom classification?
Answer: In two kingdom classification, unicellular and multicellular organisms were placed together. Similarly, prokaryotes and eukaryotes were not separately classified. Photosynthetic plants and non-photosynthetic fungi were also placed together.

Q6. Name the criteria used in five kingdom classification?
Answer: R.H. Whittaker used five criteria for his five kingdom classification as follows:

• Cell type
• Body organisation
• Presence or absence of cell wall
• Mode of nutrition
• Presence of absence of nuclear membrane

Q7. How many kingdoms are included in 3 domain classification?
Answer: In 3 domain classification, 6 kingdoms are included as follows:

• Kingdom Archaebacteria
• Kingdom Eubacteria
• Kingdom Protista
• Kingdom Fungi
• Kingdom Plantae
• Kingdom Animalia

FAQs

Q1. Why have Archaea been kept in a separate domain?
Answer: Archaea differs from bacteria in the composition of cell walls. Their cell wall is made up of glycoconjugates and polysaccharides They differ from bacteria and eukaryotes in the composition of membranes and rRNA types. Because of all these differences they have been kept in a separate domain.

Q2. Euglena considered as a plant or animal?
Answer: Euglena are tiny organisms belonging to the Kingdom Protista. They are placed in the Eukarya Domain. These single-celled organisms possess characteristics of both plant and animal cells.

Q3. Why is yeast considered as a fungus and not bacteria?
Answer: Yeast is a unicellular eukaryote but bacteria are prokaryotes. Yeast belongs to the kingdom Fungi or Mycota. But bacteria belong to the kingdom Monera. Yeast possess membrane-bound organelles in the cell but bacteria lack membrane-bound organelles.

Q4. Virus belongs to any kingdom?
Answer: Viruses are not placed under any kingdoms as they are submicroscopic and do not possess a cell. They are obligate intracellular parasites and replicate only inside the living cells using the host machinery. Viruses have the ability to infect almost all types of life forms including animals, microorganisms (bacteria and archaea) and plants.