Living Organisms and Their Classification

Living Organisms and Their Classification

What is Living?

• The word 'living' cannot be defined.

• Living organisms mostly use solar energy.

There are certain characters by which living can be distinguished from non-living.

Characteristics of Living Organisms

1. Growth

• An increase in the number of cells or mass is called growth.

2. Reproduction

• Living organisms produce young ones of their same kind.

3. Metabolism

• Chemical reaction occurring inside a living cell.

4. Response to Stimuli

• Living things have the ability to sense the condition of their surroundings and respond to these stimuli.

• When we touch leaves of the “Touch me not” plant, they close; these movements are called seismonastic movements.

Origin of Life

Life on earth originated by chemosynthesis was proved in the laboratory by the scientist Miller. J.B.S. Haldane suggested that life must have developed from simple inorganic molecules.

Classification of Organism

There are millions of organisms. It is impossible to study each individual separately.

Classification means to categorise organisms into different groups. The study of an individual of a group gives us the idea of the rest of the members of that group.

Linnaeus divides all organisms into two kingdoms – Plantae and Animalia – in his book “Systema Naturae”. The foundation of the modern classification system was laid in the line of classification systems started by Linnaeus. Therefore, Linnaeus is called ‘Father of Taxonomy’.

Due to the disputed position of organism like bacteria, viruses, fungi and euglena, there is a need for reconsideration of the system of classification. The book ‘Genera Plantarum’ was written by Bentham and Hooker.

Five Kingdom Classification

Five-kingdom classification was proposed in 1969 by R.H. Whittaker.

The criteria for classifying organism into five kingdoms are complexity of cell structure, complexity of body of organism, mode of nutrition, lifestyle and phylogenetic relationship.

1. Monera: It includes all prokaryotic organisms like bacteria.

• Bacterial structures are very simple, but they show very complex behaviour.

• They may be photosynthetic autotrophic or chemosynthetic autotrophic.

• The vast majority of bacteria are heterotrophic.

• Archaebacteria and Eubacteria are groups of bacteria.

• Archaebacteria are found in most harsh conditions, like hot springs, marshy areas and extreme salty areas.

• Eubacteria is also known as true bacteria.

• Cyanobacteria is also known as blue-green algae, having chlorophyll ‘a’ similar to green plants.

• They are unicellular, colonial or filamentous and found in both fresh water and marine water.

• Nostoc and Anabaena can fix atmospheric nitrogen due to the presence of specialised cell heterocysts.

• Chemosynthetic bacteria play a great role in recycling elements like nitrogen, phosphorous, iron & sulphur.

• Heterotrophic bacteria are decomposers.

• They are helpful for humans, like making curd, antibiotics, fixing nitrogen, etc.

• Some are pathogens causing disease in humans, plants, farm animals and pets.

• Bacteria reproduce mainly by fission.

• The mycoplasma are the smallest organisms that lack a cell wall. They can survive without oxygen.

• Plasmids are small, circular, double-stranded, self-replicating extra-chromosomal DNA commonly found in prokaryotes.

2. Protista: All single-celled eukaryotes are placed under Protista.

• This group includes diatoms, dinoflagellates, euglenoids, slime moulds and protozoa.

• Dinoflagellates are mostly marine and photosynthetic.

• Slime moulds are saprophytic protists.

• Protozoa are heterotrophs.

• Protozoa are grouped as:

  • Amoeboid protozoans (Amoeba)

  • Flagellated protozoans (Trypanosoma)

  • Ciliated protozoans (Paramoecium)

  • Sporozoans (Plasmodium)

• Euglena have both heterotrophic and autotrophic nutrition.

• Euglena is a connecting link between plants and animals.

3. Fungi: This kingdom includes non-green plants having heterotrophic nutrition.

• Most fungi are saprophytes, obtaining their nutrition from dead and decaying organic matter. Ex. mushrooms, toadstools.

• Yeasts are unicellular fungi used in making bread and beer. They reproduce asexually by budding.

• Some fungi are parasitic, causing disease in plants and animals.

• Aspergillus, Mucor, Rhizopus, etc., are examples of fungi.

• Except for yeasts, fungi are filamentous and consist of long, slender, thread-like structures called hyphae.

• Some fungi live as symbionts in association with algae known as lichens and with the root of a higher plant called mycorrhiza.

• Some fungi are sources of antibiotics, like Penicillium.

4. Plantae

• This kingdom includes all plants except algae, diatoms, fungi and members of Monera and Protista.

5. Animalia

• Almost all animals come under this kingdom except protozoa.

Binomial Nomenclature:

• There was the need for uniform informational naming of organisms. In biology every organism is represented by two proper names. The first name is the genus, always started with a capital letter, and the second name is the species, started with a small letter.

  • For example, the scientific name of a human is Homo sapiens. 'Homo' is the name of the genus, whose one species is 'sapiens'.

• Panchanan Maheshwari popularised the use of embryological characters in taxonomy.

• The botanist Joseph Dalton Hooker is credited with creating one of the first widely used natural systems of plant classification.

• The basic unit of classification is species.

• Highest unit of classification in a kingdom.

• Phylogenetic systems of classification are based on evolutionary relationships between organisms and are widely acceptable at the present day.

Scientific Names of Some Organisms

Final Thoughts

Living organisms show unique characteristics such as growth, reproduction, metabolism, and response to stimuli. Life is believed to have originated through chemical evolution, supported by Miller’s experiment and Haldane’s theory.

To study the vast diversity of life, organisms are classified into five kingdoms — Monera, Protista, Fungi, Plantae, and Animalia — based on cell structure, nutrition, and organisation. Linnaeus, known as the Father of Taxonomy, introduced the system of binomial nomenclature, giving each organism a scientific name like Homo sapiens for humans.

This systematic classification helps us understand relationships among species and the unity of life in nature.