The Living World

1. Chapter at a Glance

The living world is enormously diverse: scientists have described about 1.7–1.8 million species, and many more are still unknown. To study them, biologists need to know what makes something "living", how to name organisms so that everyone understands the same name, and how to arrange them into groups. This chapter covers the characteristics of living organisms, biodiversity, the need for classification, the three domains of life, the rules of binomial nomenclature, taxonomic categories (the hierarchy from species to kingdom) and the tools used to study taxonomy, such as herbaria, botanical gardens, museums and zoos. NEET asks one question from here in most years, usually on nomenclature rules or the taxonomic hierarchy. The chapter is short, but it sets up the vocabulary for the whole of Unit 1 (Diversity in the Living World), so learn the terms carefully.

2. What is "Living"?

When we try to define "living", we usually list features that living organisms show. However, some of these features are also seen in non-living things, and some living organisms lack them. A feature is called a defining property only if it is shown by all living organisms and by no non-living object.

• Growth: increase in mass and number of cells. In multicellular organisms, growth happens by cell division. In plants, cell division continues throughout life (indeterminate growth); in most animals it continues only up to a certain age, after which cell division only replaces lost cells. Unicellular organisms grow by cell division as well, so for them growth and reproduction are the same process. In living things growth is internal (from within). Non-living things like crystals and mountains can also "grow", but by accumulation of material on the surface (external growth). So growth alone does not define life.
• Reproduction: producing offspring that have features similar to the parents. Sexual reproduction is common in multicellular organisms. Asexual methods are also widespread: fungi multiply by millions of asexual spores; yeast and Hydra by budding; filamentous algae, fungi and the protonema of mosses by fragmentation. In Planaria (flatworm), true regeneration occurs — a fragment of the body regenerates the lost part and becomes a new organism. Unicellular organisms like bacteria, unicellular algae and Amoeba reproduce by simply growing and dividing. Reproduction is not a defining property because many organisms cannot reproduce (mules, sterile worker bees, infertile human couples), yet they are clearly alive.
• Metabolism: the sum total of all chemical reactions occurring in the body. All living organisms, from bacteria to humans, constantly make and break chemicals. No non-living object shows metabolism. Metabolic reactions can be demonstrated outside the body in cell-free systems (in a test tube), but an isolated reaction is neither living nor non-living — it is simply a chemical reaction. Metabolism is therefore a defining feature.
• Cellular organisation: all living organisms are made of one or more cells. The cellular organisation of the body is the defining feature of life forms.
• Consciousness: the ability to sense the surroundings and respond to environmental stimuli, which may be physical, chemical or biological. Plants respond to light, water, temperature, other organisms and pollutants; photoperiod affects reproduction in seasonal breeders, both plants and animals. All organisms, from prokaryotes to complex eukaryotes, can sense and respond. Consciousness is the most obvious and technically complicated defining property. Human beings are the only organisms aware of themselves (self-consciousness).

3. Biodiversity and the Need for Classification

• Biodiversity is the number and types of organisms present on Earth. The number of species known and described is about 1.7–1.8 million, and new organisms are still being found, especially in tropical forests and the deep sea.
• Local (common) names vary from place to place and language to language. The same plant may have different names in Hindi, Bengali and Tamil, and the same name may refer to different plants. A standard name used worldwide avoids this confusion.
• Identification is finding the correct name of an organism by comparing it with known organisms; nomenclature is naming it according to internationally accepted rules; classification is the process by which anything is grouped into convenient categories based on some easily observable characters.
• Classification is not a single step: it involves a hierarchy of steps in which each step represents a rank or category. Because organisms are grouped, we can study a few members of a group and learn about the whole group.
• Taxonomy covers characterisation, identification, classification and nomenclature. It is based on external and internal structure, the structure of cells, the development process and the ecological information of organisms.
• Systematics (from Latin systema, systematic arrangement) is broader: it deals with the identification, nomenclature and classification of organisms and takes into account their evolutionary relationships. Linnaeus used Systema Naturae as the title of his publication.

Human beings have always tried to classify organisms by their uses — for food, shelter and clothing. This is the oldest form of classification. Early systems were based on a few external characters (artificial), later systems on natural affinities (natural), and modern systems on evolutionary relationships (phylogenetic). These systems are discussed in detail in the next chapter (Biological Classification).

4. Three Domains of Life

In 1990, Carl Woese proposed a level of classification above kingdom called the domain. Based mainly on differences in ribosomal RNA (16S rRNA) sequences, he grouped all life into three domains:

5. Binomial Nomenclature

The system of giving every organism a two-part scientific name was introduced by Carolus Linnaeus (the "Father of Taxonomy"). Plant names follow the International Code for Botanical Nomenclature (ICBN); animal names follow the International Code of Zoological Nomenclature (ICZN). These codes make sure that each organism has only one correct name and that the same name is not used for two different organisms. Animals are named first by their genus and then species, in the same way as plants.

1. The name has two components: the generic name (genus) and the specific epithet (species). Example: Mangifera indica (mango) — Mangifera is the genus, indica the specific epithet.
2. Biological names are generally in Latin and written in italics. They are Latinised or derived from Latin irrespective of their origin.
3. When printed, names are in italics. When handwritten, both words are underlined separately to show their Latin origin.
4. The first word (genus) starts with a capital letter; the specific epithet starts with a small letter. Example: Mangifera indica.
5. The name of the author who first described the species appears after the specific epithet, at the end of the biological name, in an abbreviated form. Example: Mangifera indica Linn. — the species was first described by Linnaeus.

6. Taxonomic Categories (Hierarchy)

Classification involves a hierarchy of steps, and each step represents a rank or category. Since the category is part of an overall taxonomic arrangement, it is called a taxonomic category, and all categories together form the taxonomic hierarchy. Each category, referred to as a unit of classification, represents a rank and is commonly termed a taxon (plural taxa). For example, insects form a taxon because they share features such as three pairs of jointed legs; "dogs", "mammals", "wheat" and "plants" are also taxa at different levels.

The obligate categories, from lowest to highest, are: Species → Genus → Family → Order → Class → Phylum (Division in plants) → Kingdom. Intermediate categories such as sub-species, sub-family, super-family or sub-class are used when finer grouping is needed.

• Species: a group of individual organisms with fundamental similarities. Taxonomists recognise a species as distinct from other closely related species by distinct morphological differences. Biologically, members of a species can interbreed and produce fertile offspring. Examples of specific epithets: indica, tuberosum, leo.
• Genus: a group of related species which has more characters in common with each other than with species of other genera. Genera are aggregates of closely related species. Potato and brinjal are in Solanum; lion, tiger and leopard in Panthera. Panthera differs from Felis (cat).
• Family: a group of related genera with still fewer similarities than genus and species. Families are characterised on vegetative and reproductive features of plants. Solanaceae includes Solanum, Petunia and Datura; Felidae includes Panthera and Felis. Cat and dog, however, are in separate families: Felidae and Canidae.
• Order: an assemblage of families which show a few similar characters. Similar characters are fewer than in families. Polymoniales includes Convolvulaceae and Solanaceae (based mainly on floral characters); Carnivora includes Felidae and Canidae.
• Class: includes related orders. Order Primata (monkey, gorilla, gibbon) and order Carnivora (tiger, cat, dog) are both placed in class Mammalia, along with other orders.
• Phylum / Division: classes with a few similar characters. Mammals, birds, reptiles, amphibians and fishes form phylum Chordata because they have a notochord and dorsal hollow neural system. In plants, classes with a few similar characters are placed in a division.
• Kingdom: the highest category. All animals belonging to various phyla are in kingdom Animalia; all plants are in kingdom Plantae.
• Moving up the hierarchy, the number of common characters decreases, the number of organisms included increases, and determining relationships becomes more difficult.

7. Tools for the Study of Taxonomy

Taxonomic studies need correct identification, which requires field and laboratory work. Collected specimens of plants and animals are preserved as the primary source for taxonomic study. Biologists have set up procedures and techniques to store and preserve this information. Some places store specimens (herbaria, museums); others keep living organisms (botanical gardens, zoos); written aids (keys, flora, manuals, monographs, catalogues) help identification.

8. Master Summary Table

9. Mind Map

10. Tips and Tricks

11. Important One-liners

• Binomial nomenclature was proposed by Carolus Linnaeus.
• The basic unit of classification is the species.
• Metabolism is a defining property of living organisms.
• Human beings are the only organisms with self-consciousness.
• Growth in non-living things is external (by accumulation); in living things it is internal.
• True regeneration is seen in Planaria.
• In plants, "division" is used in place of "phylum".
• The three-domain system was proposed by Carl Woese.
• Taxonomic keys are based on contrasting characters called couplets.
• A monograph contains information on any one taxon.
• Potato, brinjal and nightshade belong to the same genus, Solanum.
• The order of man is Primata and of housefly is Diptera.

12. NEET-pattern Questions with Detailed Answers

These questions are original and based on concepts that NEET UG asks again and again.

13. Quick Revision Summary