Classification of Living Organisms Notes for Competitive Exams

There are millions of kinds of living organisms on Earth, and scientists need an orderly method to study them. The science of classifying organisms is called taxonomy. In the General Science section of SSC, railways, defence, police and state-level examinations, questions come from the five-kingdom system, the plant kingdom, the animal kingdom and the characteristics of vertebrate groups. These are scoring topics, because the facts are fixed and easy to learn with tables. This note explains the classification of living organisms in simple language, with examples, features and quick-revision points.

Why Do We Classify?

Classification makes the study of the huge variety of life easier, shows the relationships between organisms and helps us to understand how life has evolved. The main basis of classification is the structure of the cell (prokaryotic or eukaryotic), the number of cells (unicellular or multicellular), the mode of nutrition (autotrophic or heterotrophic) and body organisation. The Swedish scientist Carl Linnaeus is called the father of modern taxonomy. He gave the system of binomial nomenclature, in which each organism has a two-part scientific name: the genus name (starting with a capital letter) and the species name (small letter), written in italics, such as Homo sapiens for human beings and Mangifera indica for mango.

The Hierarchy of Classification

The groups go from the largest to the smallest: Kingdom, Phylum (Division in plants), Class, Order, Family, Genus and Species. A useful memory sentence is "King Philip Came Over For Good Soup". The species is the basic unit of classification: a group of organisms which can breed among themselves and produce fertile offspring.

The Five Kingdom Classification

R. H. Whittaker proposed the five kingdom system in 1969. It divides the organisms on the basis of cell type, body organisation, nutrition and reproduction.

Viruses are not included in the five kingdoms, because they are non-cellular and show the characteristics of life only inside a host cell. Later, Carl Woese proposed the three domains of life: Bacteria, Archaea and Eukarya. Lichens are an example of symbiosis between an alga and a fungus, where the alga makes food and the fungus gives protection and water.

The Plant Kingdom

The first three groups (algae, bryophytes and pteridophytes) do not produce seeds and are called cryptogams, and the last two (gymnosperms and angiosperms) are the phanerogams, which produce seeds. Angiosperms are divided into monocots (one cotyledon, parallel veins, fibrous roots, for example, wheat and rice) and dicots (two cotyledons, net veins, tap root, for example, pea and mango).

The Animal Kingdom

The animals are classified by their body plan: symmetry, presence of a body cavity, segmentation and the presence of a notochord. The animals without a backbone are called invertebrates, and those with a backbone are vertebrates (a part of the phylum Chordata).

Among arthropods, insects have three pairs of legs and the body in three parts (head, thorax and abdomen), while spiders are arachnids with four pairs of legs. Butterflies show complete metamorphosis (egg, larva, pupa and adult), and grasshoppers show incomplete metamorphosis (egg, nymph and adult).

Classes of Vertebrates

The egg-laying mammals, the platypus and the echidna, are exceptions. The blue whale is the largest animal on Earth, the ostrich is the largest living bird, and the cheetah is the fastest land animal. Bats are the only mammals that can truly fly.

Modes of Nutrition

Organisms get their food in different ways, and this is one basis of classification. Autotrophs, like green plants and some bacteria, make their own food, mainly by photosynthesis. Heterotrophs depend on other organisms for food. Among heterotrophs, herbivores eat plants, carnivores eat animals, omnivores eat both, saprophytes (like fungi) feed on dead and decaying matter, and parasites (like tapeworm and Cuscuta) live on or inside a host and take food from it. Some plants, such as the pitcher plant and Venus flytrap, are insectivorous and trap insects to get nitrogen, even though they also perform photosynthesis. This variety shows why a single feature is not enough to group organisms.

Rules for Writing Scientific Names

• The name has two words: the first is the genus (capital letter) and the second is the species (small letter).
• In print, the name is written in italics; when handwritten, both words are underlined separately.
• The name of the scientist who named the species may be added in the end, such as Oryza sativa Linn.
• Examples: Panthera tigris (tiger), Panthera leo (lion), Azadirachta indica (neem) and Ocimum sanctum (tulsi). Notice that the tiger and the lion belong to the same genus.

Why Biodiversity Needs Classification

Classification is the first step in the conservation of biodiversity, because we can protect a species only when it is identified and named. Scientists have described about 1.5 to 2 million species so far, but the real number may be much higher. Tropical rainforests and coral reefs hold a large share of species. India is one of the megadiverse countries of the world and has about 7 to 8 per cent of all recorded species. The study of classification also helps in agriculture, medicine and ecology, for example, in finding the wild relatives of crops or the source of a new drug.

Practice Questions With Answers

• Q1. Which kingdom includes bacteria? Answer: Monera.
• Q2. Which scientist gave the binomial nomenclature? Answer: Carl Linnaeus.
• Q3. Which plant group has naked seeds? Answer: Gymnosperms.
• Q4. To which phylum do the earthworm and the leech belong? Answer: Annelida.
• Q5. Which class of vertebrates has a 2-chambered heart? Answer: Pisces (fish).
• Q6. Which animal is an egg-laying mammal? Answer: The platypus.

Memory Tricks

Use short phrases to remember the groups. For plants, think "A-B-P-G-A": Algae, Bryophyta, Pteridophyta, Gymnosperms, Angiosperms - from simple to complex. For vertebrates, think "F-A-R-B-M": Fish, Amphibians, Reptiles, Birds, Mammals, and add the heart chambers 2-3-3-4-4. For the animal phyla, remember the order from simple to complex: Porifera (pores), Cnidaria (stings), Platyhelminthes (flat), Nematoda (round), Annelida (rings), Arthropoda (jointed legs), Mollusca (soft), Echinodermata (spiny) and Chordata (backbone). Revise the tables once every two days and test yourself by covering the right column.

Useful Facts for the Exam

• Bacteria have different shapes: spherical (cocci), rod-shaped (bacilli), comma-shaped (vibrio) and spiral (spirilla).
• Euglena shows the features of both plants (chlorophyll) and animals (movement).
• Whales, dolphins and bats are mammals, not fish or birds.
• The word "cold-blooded" means that the body temperature changes with the surroundings.
• Penicillin, the first antibiotic, was obtained from the fungus Penicillium by Alexander Fleming in 1928.

Continue your preparation with the notes on cell biology, genetics and plant science and human body systems.

Quick Revision

• Five kingdoms by Whittaker (1969): Monera, Protista, Fungi, Plantae, Animalia.
• Binomial nomenclature by Linnaeus; hierarchy from kingdom to species.
• Plant groups: algae, bryophytes, pteridophytes, gymnosperms, angiosperms.
• Arthropoda is the largest phylum; fish have a 2-chambered heart; birds and mammals have 4 chambers.

Frequently Asked Questions (FAQ)

Who proposed the five kingdom classification?

R. H. Whittaker proposed the five kingdom classification in 1969.

Which are the five kingdoms of life?

They are Monera, Protista, Fungi, Plantae and Animalia.

Who is called the father of modern taxonomy?

Carl Linnaeus is called the father of modern taxonomy.

What is binomial nomenclature?

It is a naming system in which each organism has a two-part scientific name made of the genus and the species.

Why are viruses not placed in the five kingdoms?

Viruses are non-cellular and show life only inside a host cell, so they are not placed in the five kingdoms.

Which plant group is called the amphibians of the plant kingdom?

Bryophytes are called the amphibians of the plant kingdom.

What are gymnosperms?

Gymnosperms are plants with naked seeds, not enclosed in fruits, such as pine and Cycas.

Which is the largest phylum of the animal kingdom?

Arthropoda is the largest phylum.

How many pairs of legs do insects have?

Insects have three pairs of legs.

How many chambers are there in the heart of fish, amphibians and birds?

Fish have 2 chambers, amphibians have 3 and birds have 4.

Which mammals lay eggs?

The platypus and the echidna are egg-laying mammals.

What are warm-blooded animals?

Warm-blooded animals, such as birds and mammals, keep a constant body temperature irrespective of the surroundings.