1. What is Living? — Defining Life
No single definition of life is universally accepted. Instead, biologists identify a set of defining characteristics shared by all living organisms. An organism is considered alive if it exhibits most or all of these attributes:
| Attribute | Description | Example |
|---|---|---|
| Growth | Increase in mass and number from inside (intrinsic growth). In living organisms, growth is from within — new cells are added internally | A seed growing into a tree; a unicellular organism dividing |
| Reproduction | Ability to produce offspring — ensures continuity of life. Not essential for an individual to survive, but essential for species survival | Amoeba divides by binary fission; humans reproduce sexually |
| Metabolism | Sum total of all chemical reactions occurring in the body — the most defining feature of life. Includes anabolism (synthesis) and catabolism (breakdown) | Photosynthesis, respiration, digestion, protein synthesis |
| Cellular Organisation | All living organisms are made of cells — the basic structural and functional unit of life | Bacteria (unicellular) to elephants (multicellular) |
| Consciousness / Sense and Response | Ability to sense and respond to environmental stimuli — the most defining and universal characteristic according to NCERT | Plant bending towards light (phototropism); humans withdrawing from pain |
| Homeostasis | Ability to maintain a stable internal environment despite external changes | Human body maintaining 37°C body temperature |
Critical NEET Points on Growth and Reproduction
- Growth is NOT a defining property of life — non-living things also grow (mountains grow by accumulation; crystals grow by accretion from outside). But living organisms grow from within (intrinsic growth by cell division).
- Reproduction is NOT an essential feature of all individuals — mules are sterile (cannot reproduce) but are alive. Worker bees do not reproduce but are living organisms.
- Metabolism is the most fundamental and universally accepted criterion of life. Even isolated cells and tissues show metabolism. No non-living object has metabolism.
- Consciousness (awareness of the environment) is the defining property of life per NCERT — even plants are conscious in that they respond to stimuli like light, gravity, and touch.
2. Biodiversity — The Variety of Life
Biodiversity refers to the enormous variety and variability among living organisms. The study of the diversity of life is called taxonomy.
- Approximately 1.7–1.8 million species have been described and named so far.
- Estimates suggest there could be anywhere between 5–50 million species on Earth — the vast majority still undiscovered.
- India is one of the 12 mega-diversity nations in the world.
Need for Classification
With such enormous diversity, a systematic method of identification, nomenclature, and classification is essential:
- To identify, name, and describe organisms precisely.
- To understand relationships among organisms (evolutionary connections).
- To study organisms efficiently — studying one member of a group reveals properties of related members.
- To predict properties of unknown organisms based on their relatives.
- To develop a universal language for communication among biologists worldwide.
3. Taxonomy — The Science of Classification
Taxonomy (Greek: taxis = arrangement; nomos = law) is the branch of biology that deals with identification, nomenclature, and classification of organisms.
Systematics (Latin: systema = systematic arrangement) is the broader discipline that includes taxonomy and also considers evolutionary relationships among organisms. The term was coined by Linnaeus (title of his work: Systema Naturae).
| Term | Definition |
|---|---|
| Identification | Correct description of an organism and recognising it as a known or new species |
| Nomenclature | Process of giving scientifically acceptable names to organisms |
| Classification | Grouping organisms into categories (taxa) based on similarities and differences |
| Taxonomy | Study of principles and practice of identification, nomenclature, and classification |
| Systematics | Study of diversity of organisms and their evolutionary relationships (broader than taxonomy) |
4. Binomial Nomenclature
Binomial Nomenclature — the two-name system for naming organisms — was introduced by Carolus Linnaeus in his work Systema Naturae (1758). It is the universally accepted system today, governed by international codes (ICBN for plants, ICZN for animals).
Rules of Binomial Nomenclature
- Every organism has a two-part scientific name: Generic name (genus) + Specific epithet (species).
- Both names are in Latin (or Latinised) — a dead language that doesn't change.
- When handwritten or typed, the name is underlined (e.g., Homo sapiens).
- When printed, the name is in italics (e.g., Homo sapiens).
- The generic name starts with a capital letter; the specific epithet starts with a small letter.
- The name of the author who first described the species is written after the name (abbreviated): e.g., Mangifera indica Linn. (Linn. = Linnaeus).
- Each species has one and only one valid scientific name (principle of priority — earliest validly published name is accepted).
Examples of Binomial Names
| Common Name | Scientific Name | Genus | Species |
|---|---|---|---|
| Human | Homo sapiens | Homo | sapiens |
| Mango | Mangifera indica | Mangifera | indica |
| Housefly | Musca domestica | Musca | domestica |
| Tiger | Panthera tigris | Panthera | tigris |
| Lion | Panthera leo | Panthera | leo |
| Wheat | Triticum aestivum | Triticum | aestivum |
| Dog | Canis lupus familiaris | Canis | lupus familiaris |
| Onion | Allium cepa | Allium | cepa |
5. Taxonomical Aids
Taxonomical aids are tools and techniques that help in identification, classification, and study of organisms.
Herbarium
A herbarium is a collection of dried, pressed plant specimens mounted on sheets of paper and preserved in a storehouse. Each specimen is labelled with the plant's scientific name, date of collection, collector's name, and location. Herbaria serve as repositories and reference points for taxonomists. The largest herbarium in the world is at Royal Botanic Gardens, Kew (England). In India, the largest herbarium is at the Botanical Survey of India, Howrah.
Botanical Gardens
Living collections of plants maintained for study, conservation, and education. Famous botanical gardens include the Royal Botanic Gardens, Kew (UK), Indian Botanical Garden, Howrah (India), and National Botanical Research Institute, Lucknow.
Zoological Parks (Zoos)
Places where wild animals are maintained under human care. They provide an opportunity to study animals' food habits, behaviour, and reproductive patterns. They also serve as conservation centres for endangered species. Example: Zoological Survey of India maintains records of Indian fauna.
Museums
Collections of preserved specimens of both plants and animals — wet preservation (in formaldehyde/glycerine), dried specimens, skeletons, fossils. Used for reference and research.
Keys
A taxonomic key is a tool used for identification of organisms based on their observable characteristics. The most commonly used is the dichotomous key — at each step, a choice is made between two contrasting characters, leading to the identity of the organism.
Monographs, Flora, Manuals and Catalogues
| Tool | Description |
|---|---|
| Monograph | A comprehensive account of a single taxon (e.g., a family, genus) covering all known species within it |
| Flora | Book/publication containing information about plant species of a particular area with keys for identification |
| Manual | A book containing information for identification of names of species in an area |
| Catalogue | An alphabetical listing of all species known from an area with brief details |
6. Important Contributors to Taxonomy
| Scientist | Contribution | Key Work |
|---|---|---|
| Carolus Linnaeus | Father of Taxonomy; introduced binomial nomenclature; developed hierarchical classification | Systema Naturae (1758); Species Plantarum |
| Aristotle | Father of Biology and Zoology; earliest attempts at classification; divided animals into those with blood (Vertebrates) and without (Invertebrates) | Historia Animalium |
| Theophrastus | Father of Botany; classified plants as trees, shrubs, undershrubs, and herbs | Historia Plantarum |
| Ernst Haeckel | Proposed Kingdom Protista; coined the term "ecology" | Three kingdom classification |
| R.H. Whittaker | Proposed five kingdom classification (1969) — Monera, Protista, Fungi, Plantae, Animalia | Five kingdom system |
