The Living World

Living organisms show organisation, metabolism, growth, reproduction, and responsiveness to the environment. No single property alone is sufficient to define life — it must be identified through a combination of features.

• Metabolism is the sum of all chemical reactions inside a cell; it is the most inclusive defining property of life.
• Growth in living organisms occurs from inside (intrinsic); non-living things may grow from outside (extrinsic).
• Consciousness means the ability to sense and respond to environmental stimuli.
• Reproduction is not universal — mules are sterile, yet they are living organisms.

An estimated 1.7–1.8 million species have been named so far, out of millions yet to be discovered. To avoid confusion caused by regional common names, organisms are given universal scientific names.

• Binomial nomenclature (Carolus Linnaeus) gives each species a two-part name: Genus species.
• Scientific names are Latinised and written in italics when printed, or underlined when handwritten.
• The genus name starts with a capital letter; the species name with a lowercase letter.
• The author's name is often abbreviated at the end — e.g., Mangifera indica Linn.

Classification arranges organisms into a nested hierarchy from most inclusive to least inclusive. Each rank is a taxon (plural: taxa).

As we move up the hierarchy, the number of similar characters decreases while the number of organisms included increases.

A taxon is a specific group of organisms at any level of classification (e.g., Felidae, Mammalia). A category is the rank itself (e.g., Family, Class) — an abstract level in the hierarchy.

Species is the basic unit of classification. It includes organisms that are structurally similar and capable of interbreeding under natural conditions to produce fertile offspring.

• Tiger (Panthera tigris) and Lion (Panthera leo) are in the same genus but different species.
• All domestic cats belong to Felis domesticus.

Taxonomical aids help scientists identify, preserve, and study organisms systematically.

• Keys — identification tools based on contrasting (dichotomous) characters. Each step presents two mutually exclusive options.
• Herbaria — institutions storing dried and pressed plant specimens mounted on sheets for reference.
• Museums — store preserved specimens of animals or biological material in fluid or as mounted specimens.
• Zoological parks (zoos) — living specimens; facilitate study of habits and behaviour in near-natural settings.
• Botanical gardens — living plant collections; Kew Gardens (UK) is the most famous. Indian Botanical Garden, Kolkata, houses the great banyan tree.
• Monographs — detailed published information about a single taxon.
• Catalogues — alphabetical listing of taxa with related information.
• Flora — accounts of all plants found in an area, including their identification keys.