NCERT Solutions for Class 11 Biology Chapter 1: The Living World

Concept. The living world is astonishingly diverse — about 1.7–1.8 million species are already known and described, and new organisms are continuously being identified as new (and even old) areas are explored. Studying each of these organisms one by one is practically impossible. Classification is the process by which anything is grouped into convenient categories based on some easily observable characters, and it is the tool that makes the study of this enormous variety possible.

Reasons why organisms are classified (each point explained in full):

1. To make the study of millions of organisms possible. Because it is nearly impossible to study every living organism separately, we devise a means to study them in groups. Once an organism is placed in a group, everything known about the group (its characters, habitat, life cycle) applies to the organism, so we need not start from zero each time.
2. To group organisms into convenient categories on observable characters. The moment we use terms such as “dogs”, “cats”, “insects”, “mammals”, “wheat” or “rice”, we automatically associate certain characters with that group — think of “mammals” and you picture animals with external ears and body hair; think of “wheat” and you picture wheat plants and not rice. These categories (taxa) are exactly what classification creates.
3. To help identification and naming. Accurate description and placement of an organism makes identification possible, and identification is a prerequisite for nomenclature — giving every organism one universally accepted scientific name (binomial, e.g., Mangifera indica) under ICBN/ICZN, instead of confusing local names that change from place to place.
4. To reveal similarities, differences and relationships. Knowing the characters of an individual or group helps identify similarities and dissimilarities among organisms of the same and of different kinds. Classification shows that present-day organisms share similarities both horizontally and vertically, i.e., all organisms are related to each other and to all that ever lived — the basis of systematics, which also considers evolutionary relationships.
5. To know where an organism “stands” in the living world. Every organism occupies a place or position in the system of classification (species → genus → family → order → class → phylum/division → kingdom), so its relatives and its rank are known immediately.
6. For practical use of bio-resources. According to the NCERT summary, taxonomic studies of plants and animals are useful in agriculture, forestry and industry and for knowing our bio-resources and their diversity. Historically, the earliest classifications themselves arose from human needs — they were based on the uses of organisms for food, clothing and shelter.
7. To fit newly discovered organisms. Because new organisms keep being discovered, an orderly scheme of categories lets a new organism be placed in an existing slot (or a new slot to be created) without confusion.

Extra depth. Notice that the same logic is used in daily life: a library arranges thousands of books subject-wise and shelf-wise so that any book can be found quickly. A biologist’s classification is the “library catalogue” of life. It also explains why the process never really ends — as more organisms and more characters become known, categories are refined (the topic of Question 2).

Conclusion: living organisms are classified because their huge, ever-growing diversity cannot be studied individually; grouping them into taxa on easily observable characters makes study, identification, naming and comparison possible, reveals their relationships, and helps us use and conserve our bio-resources.

Concept. A classification system is a human-made scheme for arranging organisms. It is based on the characters we know and the tools we have at a given time, and it must also be able to accommodate every organism we find. Because knowledge in biology keeps growing, classification systems are never final — they are revised repeatedly to give a truer picture of the living world.

1. New organisms are being discovered continuously. NCERT states that as we explore new areas, and even old ones, new organisms are continuously identified. Each newly described species must be placed in the hierarchy, and sometimes it does not fit the existing categories, forcing taxonomists to modify or create taxa.
2. The basis (criteria) of classification has changed over time. The earliest classifications were based on the “uses” of organisms (food, clothing, shelter). Later, external morphology was used; today external and internal structure, structure of the cell, developmental process and ecological information form the basis of modern taxonomic studies. When the criteria change, the groupings that result also change.
3. More characters become available with better tools and deeper study. As cell structure and development began to be studied alongside visible form, organisms that looked alike were sometimes found to be quite different (and vice versa), so their positions had to be revised. (Later chapters show how this led from Linnaeus’ simple two-kingdom scheme to Whittaker’s five-kingdom scheme.)
4. Systematics takes evolutionary relationships into account. Classification today tries to reflect how organisms are related through descent. Whenever new evidence about relationships appears, the arrangement of taxa is altered to mirror it — the aim is a natural, not merely convenient, grouping.
5. Taxonomic categories are meant to be distinct biological entities, not merely morphological aggregates. If studies show that a group is not a truly distinct entity, its status (rank) must change. NCERT also notes that taxonomists keep developing sub-categories in the hierarchy for a more sound and scientific placement of taxa.
6. Difficulty increases at higher categories. The higher the category, the fewer the common characters, and the greater the difficulty of determining relationships — so classification at higher levels is the most debated and the most frequently revised.

Extra depth. The change in classification is a sign of a healthy science, not a weakness. The scientific names (binomials) are fixed by international codes (ICBN, ICZN) and so are stable, but the placement of a species in genus, family, order etc. may change when new evidence arrives.

Conclusion: classification systems change because new organisms keep being discovered, the criteria used for grouping have progressed from mere “uses” to structure, cell structure, development and ecology, and systematics aims to mirror evolutionary relationships — every new piece of evidence can therefore alter the arrangement.

Concept. Classification means grouping into convenient categories on the basis of easily observable characters. The same principle used for plants and animals can be applied to any set of objects or people — what changes is the criterion we select. Biologically, all people belong to one species, Homo sapiens, so this question is an exercise in applying the principles of classification, not in biological taxonomy.

Possible criteria (a good scheme uses characters that are easy to observe, consistent and unambiguous):

1. Place / role in my daily life: family members, relatives, neighbours, classmates, teachers, shopkeepers, doctors, bus conductors. (This resembles the earliest classification of organisms, which was based on their uses.)
2. Occupation or profession: students, teachers, doctors, farmers, drivers, traders, engineers.
3. Age group: children, teenagers, adults, senior citizens.
4. Language spoken or region of residence: Odia-, Hindi-, English-speaking; people of my village / town / city.
5. Interests and hobbies: sports persons, musicians, readers, artists, gardeners.
6. Observable physical features: height (tall/short), use of spectacles, hair type — useful only for quick identification, not for meaningful grouping.
7. Level of familiarity: close friends, acquaintances, people I meet only occasionally.

Building a hierarchy (like taxonomic categories). A hierarchy can be built just as in biology: People I meet → at school / at home / in the neighbourhood → (at school) students / teachers / staff → (students) classes → sections → individual friends. As we go down, the members share more and more characters, exactly as in taxonomic categories.

Extra depth. Whatever criterion is used must be applied consistently to everybody, otherwise the groups overlap and confuse — the same reason why taxonomists fix criteria such as structure, cell structure, development and ecology. Note also that characters like skin colour or place of origin have no taxonomic value — every human being belongs to the same species, Homo sapiens.

Conclusion: people can be classified by role, occupation, age, language, interests or familiarity, and arranged into a hierarchy; the key requirement, as in taxonomy, is a clear, observable and consistently applied criterion.

Concept. Identification means describing an organism correctly so that we know exactly what organism a name is attached to. A population is a group of individuals of the same kind living in an area. Without correct identification, neither naming (nomenclature) nor placement in a classification is possible — so identification is the first, most basic step of taxonomy.

What we learn from identification:

1. The kind of organism it is — whether it is a known species or a new, undescribed one (new organisms are continuously being identified in new and even old areas).
2. Its correct scientific name — once an organism is identified, it can be given or matched with its unique binomial (ICBN/ICZN), so that description of any organism lets people anywhere in the world arrive at the same name.
3. Its characters and its similarities / dissimilarities with others — identifying individuals tells us what they share with other individuals of the same kind and how they differ from other kinds. For example, potato and brinjal are different species but share the genus Solanum.
4. Its position in the taxonomic hierarchy — species, genus, family, order, class, phylum / division and kingdom, and therefore its closest relatives (e.g., lion, tiger and leopard in Panthera; Panthera + Felis in Felidae).
5. Diversity within and among populations — identifying all the individuals of a population as one species, and noting the variation among them, helps us count the number of species in an area, i.e., its biodiversity, and shows which species are common or restricted.
6. Evolutionary and ecological relationships — identification of a group by its characters (including ecological information) helps in understanding how organisms are related to one another and to the environment.
7. Practical value — the NCERT summary says taxonomic studies are useful in agriculture, forestry, industry and in knowing our bio-resources and their diversity (for example, identifying crop species and their wild relatives, or timber-yielding trees). It also forms the base for conservation of biodiversity.

Extra depth. Identification is closely linked with the four basic processes of taxonomy: characterisation → identification → classification → nomenclature. Careful characterisation is what makes accurate identification possible.

Conclusion: identification of individuals and populations tells us what organisms they are, their correct names, their characters and relationships, their place in the taxonomic hierarchy and the extent of biodiversity — knowledge that is applied in agriculture, forestry, industry and conservation.

Concept. Scientific names are given according to the binomial nomenclature proposed by Carolus Linnaeus and governed by ICBN (plants) and ICZN (animals). Each name has two components — the generic name (genus) and the specific epithet — and must follow universal rules of writing.

Answer: the correctly written name is Mangifera indica.

Why Mangifera Indica is wrong: Rule 4 of nomenclature states that the first word (genus) starts with a capital letter but the specific epithet starts with a small letter. In “Mangifera Indica” the epithet indica has been written with a capital I, which violates this rule. The name Mangifera indica follows it, and is also italicised (or, when handwritten, both words are separately underlined) to show its Latin origin.

All the universal rules (revision):

1. Biological names are generally in Latin and written in italics; they are Latinised or derived from Latin irrespective of their origin.
2. The first word denotes the genus, the second the specific epithet.
3. When handwritten, both words are separately underlined; when printed, they are in italics.
4. Genus begins with a capital letter; the specific epithet begins with a small letter.
5. The author’s name (abbreviated) is written after the specific epithet, e.g., Mangifera indica Linn. — indicating that the species was first described by Linnaeus.

Extra depth. Notice how the binomial makes a relationship visible: Solanum tuberosum and Solanum melongena share the genus name, telling us at once that potato and brinjal are related. Also, names like Mangifera indica are unique worldwide, so a mango is never confused with any other organism whatever the local language calls it.

Conclusion: Mangifera indica is the correct name because the genus is capitalised and the specific epithet is in small letters, with both words italicised (or underlined separately when handwritten), exactly as the rules of binomial nomenclature require.

Concept. To study millions of organisms, biologists group them into convenient categories based on characters. Each such category is a unit of classification and represents a rank in the taxonomic hierarchy. The scientific term for these categories is taxa (singular: taxon); the word is derived from the idea of an arrangement into groups (taxonomy).

Definition. A taxon (pl. taxa) is a taxonomic group or category of any rank — a unit of classification representing a particular level in the taxonomic hierarchy, containing organisms that share certain common characters. NCERT stresses that taxa can indicate categories at very different levels: “Plants” form a taxon, and “wheat” is also a taxon; “animals”, “mammals” and “dogs” are all taxa, but at different levels — a dog is a mammal and mammals are animals.

Features of a taxon:

• It is a distinct biological entity, not merely a morphological aggregate of organisms.
• It has a definite rank / category (species, genus, family, order, class, phylum/division, kingdom) — the lower the taxon, the more characters its members share; the higher, the fewer.
• All the taxonomic categories together make up the taxonomic hierarchy.

Extra depth — category vs taxon. Strictly speaking, a category is the rank itself (for example “genus”), while a taxon is an actual group placed at that rank (for example Panthera is a taxon of the rank genus; Felidae is a taxon of the rank family). NCERT uses the word “taxon” for each such unit of classification.

Conclusion: a taxon is a unit of classification of any rank in the taxonomic hierarchy; groups such as Mangifera indica, Panthera, Felidae, Carnivora, Mammalia, Chordata and Animalia are taxa at the levels of species, genus, family, order, class, phylum and kingdom respectively.

Concept. The complete taxonomic hierarchy, written in ascending order (from the lowest to the highest category) is: Species → Genus → Family → Order → Class → Phylum (Division in plants) → Kingdom. As we move up, the number of common characters decreases and the number of organisms included increases. A correct sequence must follow this order (it may skip some categories, but it can never reverse any two).

Answer: option (c) — Species, Genus, Order, Phylum. It keeps the true rank order (and, unlike (a), includes the lowest two categories, species and genus, in the right order). Option (b) is definitely wrong because it puts genus below species. Option (a) preserves the relative order of the four categories it lists but jumps directly from species to order; it is therefore only an incomplete series.

Extra depth — why species comes first. A genus is an aggregate of closely related species (e.g., Panthera includes leo, pardus, tigris), a family an aggregate of related genera (Felidae = Panthera + Felis), an order of related families, a class of related orders, a phylum of related classes and a kingdom of all phyla. Hence each higher category contains the lower one, and species — common to both plant and animal kingdoms — is the lowest category.

Mnemonic to memorise the descending order: King Philip Came Over For Good Soup → Kingdom, Phylum, Class, Order, Family, Genus, Species.

Conclusion: the correct ascending sequence among the choices is (c) Species → Genus → Order → Phylum, consistent with the full hierarchy Species → Genus → Family → Order → Class → Phylum/Division → Kingdom; option (b) reverses genus and species and so is wrong.

Concept. Species is the lowest and the most fundamental category of the taxonomic hierarchy — every organism, plant or animal, is placed in a species. NCERT defines it in taxonomic studies as a group of individual organisms with fundamental similarities, distinguishable from other closely related species by distinct morphological differences. Because this question asks us to collect the meanings and to discuss how they apply to higher organisms and bacteria, a full answer covers the concepts in use and their limitations. (The concepts below beyond NCERT’s own definition are standard, well-established ideas in biology, as expected by the “discuss with your teacher” instruction.)

Currently accepted meanings (species concepts):

1. Taxonomic / morphological (typological) species concept — a group of individuals with fundamental similarities, separable from other species on morphological characters. This is the NCERT definition and the one used in practical identification, e.g., Panthera leo vs Panthera tigris.
2. Biological species concept (Ernst Mayr) — a group of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups. (NCERT mentions that Ernst Mayr pioneered the currently accepted definition of a biological species.) Reproductive isolation keeps the gene pool of a species distinct.
3. Evolutionary / phylogenetic species concept — a single lineage of ancestor–descendant populations maintaining its identity from other lineages (or the smallest group sharing a common ancestor and unique derived characters). It is useful for fossils and asexual organisms.
4. Ecological species concept — a species is a group of organisms adapted to a particular ecological niche (set of resources and environmental conditions).
5. Genetic / molecular concept — species distinguished by differences in DNA / protein sequences and genetic similarity among members.

Species in higher plants and animals. Most higher animals and many higher plants reproduce sexually, so members of a species can interbreed and produce fertile offspring, while remaining reproductively isolated from other species. The biological species concept works well here: for example, a horse and a donkey are different species because their hybrid, the mule, is sterile. Morphological differences, chromosome number and behaviour are also used. Limitations: some plant species hybridise freely, and fossils cannot be tested for interbreeding.

Species in bacteria. Bacteria reproduce mainly asexually (binary fission), so the idea of “interbreeding” hardly applies; they also exchange genes across groups by conjugation, transformation and transduction, which blurs boundaries. Bacterial species are therefore defined by a polyphasic approach — morphology, staining and biochemical / physiological properties, and DNA similarity (for example, strains with roughly 70% or more DNA–DNA relatedness and very high 16S rRNA gene similarity are treated as one species).

Extra depth. Whatever the concept, species is the lowest taxonomic category, and the second word of the binomial (the specific epithet) names it. No single definition covers all organisms perfectly — which is exactly why the topic is still being discussed.

Conclusion: species are groups with fundamental similarities, often defined as interbreeding, reproductively isolated populations (Mayr) in sexually reproducing plants and animals, whereas for asexually reproducing bacteria they are defined by phenotypic characters and DNA similarity.

Concept. Each of these is a taxonomic category (rank). Moving upward in the hierarchy — species → genus → family → order → class → phylum → kingdom — the number of common characters decreases while the number of organisms included increases. Study the definitions below along with the NCERT examples.

(i) Phylum. The category above class, formed by grouping related classes that share a few common features. The classes of fishes, amphibians, reptiles, birds and mammals are put in the phylum Chordata because all have a notochord and a dorsal hollow neural system. In plants, classes with a few similar characters are assigned to a higher category called Division (e.g., Angiospermae). Several phyla together make a kingdom (all animal phyla → Kingdom Animalia).

(ii) Class. The category that includes related orders. For example, the order Primata (monkey, gorilla, gibbon) is placed in the class Mammalia along with the order Carnivora (tiger, cat, dog); class Mammalia has other orders also. Related classes are grouped into a phylum (or division in plants); the plant classes Dicotyledonae and Monocotyledonae belong to the division Angiospermae.

(iii) Family. A group of related genera with still fewer similarities than are found in genus and species. Among plants, families are characterised on the basis of both vegetative and reproductive features; for example, genera Solanum, Petunia and Datura are placed in the family Solanaceae. Among animals, genus Panthera (lion, tiger, leopard) is put along with genus Felis (cats) in the family Felidae; cats and dogs show similarities as well as differences, so they belong to different families — Felidae and Canidae.

(iv) Order. The assemblage of related families which exhibit a few similar characters; these similar characters are fewer than those shared by the genera in a family. Order and other higher taxonomic categories are identified on aggregates of characters. Plant families Convolvulaceae and Solanaceae are included in the order Polymoniales (as printed in NCERT), mainly on floral characters; the animal order Carnivora includes the families Felidae and Canidae.

(v) Genus. A group of related species which has more characters in common than species of other genera — genera are aggregates of closely related species. Potato and brinjal are two different species (Solanum tuberosum and S. melongena) of the genus Solanum; lion (Panthera leo), leopard (P. pardus) and tiger (P. tigris) are species of Panthera, which differs from the genus Felis that includes cats.

Conclusion: phylum, class, order, family and genus are successive taxonomic categories, each an aggregate of the category below it — genus (related species), family (related genera), order (related families), class (related orders) and phylum (related classes) — with progressively fewer common characters.

Concept. Classification is not a single-step process; it is a hierarchy of steps in which each step represents a rank or category. The common categories are kingdom, phylum (division in plants), class, order, family, genus and species; species is the lowest category in both plant and animal kingdoms. As we go from species up to kingdom, the number of common characteristics goes on decreasing, and the problem of classification becomes more complex.

Animal example — Man (Homo sapiens). Human beings belong to the species sapiens of the genus Homo. Homo belongs to the family Hominidae, which falls in the order Primata (monkey, gorilla, gibbon and man). Primata is one of the orders of the class Mammalia, and all mammals together with fishes, amphibians, reptiles and birds are placed in the phylum Chordata (notochord + dorsal hollow neural system). All animal phyla belong to the Kingdom Animalia.

Plant example — Mango (Mangifera indica). Mango belongs to the species indica of the genus Mangifera, which is placed in the family Anacardiaceae. This family belongs to the order Sapindales, which comes under the class Dicotyledonae of the division Angiospermae; all plant divisions belong to the Kingdom Plantae.

Extra depth. In the hierarchy each lower rank is included in the next higher rank: Mangifera contains the species indica, Anacardiaceae contains the genus Mangifera (and others), and so on. Wheat, though also in Angiospermae, is placed in the class Monocotyledonae and the family Poaceae — showing how two plants can share a division but differ from the class downward. Taxonomists also develop sub-categories in the hierarchy for more sound and scientific placement.

Conclusion: the taxonomic hierarchy arranges organisms from species up to kingdom — man: Homo sapiens → Homo → Hominidae → Primata → Mammalia → Chordata → Animalia; mango: Mangifera indica → Mangifera → Anacardiaceae → Sapindales → Dicotyledonae → Angiospermae → Plantae.