NCERT Solutions for Class 11 Biology Chapter 2: Biological Classification (NCERT 2026–27)
These Class 11 Biology Chapter 2 solutions cover Biological Classification in full — every numbered question from the NCERT “Exercises” reproduced verbatim and answered in clear, exam-ready prose. The chapter traces classification from Aristotle and Linnaeus to R. H. Whittaker’s Five Kingdom Classification (Monera, Protista, Fungi, Plantae and Animalia), and explains the acellular agents — viruses, viroids, prions and the symbiotic lichens — left outside it.
Chapter 2, Biological Classification, explains how living organisms have been grouped over time. Aristotle made the earliest scientific attempt using morphology; Linnaeus formalised a two-kingdom system (Plantae and Animalia), which failed to separate prokaryotes from eukaryotes and autotrophs from heterotrophs. To resolve these problems, R. H. Whittaker (1969) proposed the Five Kingdom Classification — Monera, Protista, Fungi, Plantae and Animalia — based on cell structure, body organisation, mode of nutrition, reproduction and phylogenetic relationships. The chapter details the bacteria of Monera (archaebacteria and eubacteria), the single-celled eukaryotes of Protista (chrysophytes, dinoflagellates, euglenoids, slime moulds and protozoans), the four classes of Fungi (phycomycetes, ascomycetes, basidiomycetes, deuteromycetes), and finally the acellular viruses, viroids and prions, plus lichens.
Key Concepts & Definitions
Five Kingdom Classification: proposed by R. H. Whittaker (1969) — Monera, Protista, Fungi, Plantae and Animalia, based on cell structure, body organisation, mode of nutrition, reproduction and phylogeny.
Kingdom Monera: all prokaryotes (bacteria); includes archaebacteria (extreme habitats) and eubacteria (true bacteria, including cyanobacteria).
Kingdom Protista: all single-celled eukaryotes — chrysophytes, dinoflagellates, euglenoids, slime moulds and protozoans.
Kingdom Fungi: heterotrophic, chitin-walled organisms; mostly saprophytic, parasitic or symbiotic; four classes — phycomycetes, ascomycetes, basidiomycetes, deuteromycetes.
Heterocysts: specialised cells in some cyanobacteria (Nostoc, Anabaena) that fix atmospheric nitrogen.
Lichen: a symbiotic (mutualistic) association of an alga (phycobiont, autotrophic) and a fungus (mycobiont, heterotrophic); a sensitive pollution indicator.
Acellular agents: viruses (nucleoprotein, RNA or DNA), viroids (free, low-molecular-weight RNA without a protein coat) and prions (abnormally folded infectious proteins).
NCERT Exercises — Solutions
All questions below are reproduced verbatim from the NCERT textbook “Exercises”; the answers are original and written in CBSE exam-ready style.
1. Discuss how classification systems have undergone several changes over a period of time?
ANSWERClassification has evolved continuously as our understanding of organisms deepened. Aristotle made the earliest scientific attempt, using simple morphological characters to sort plants into trees, shrubs and herbs, and animals into those with red blood and those without.In Linnaeus’ time a two-kingdom system (Plantae and Animalia) was developed. However, it could not distinguish prokaryotes from eukaryotes, unicellular from multicellular forms, or photosynthetic from non-photosynthetic organisms, and many organisms fitted neither kingdom — so it proved inadequate.As more characters were considered — cell structure, nature of the cell wall, mode of nutrition, habitat, reproduction and evolutionary relationships — newer systems arose, culminating in Whittaker’s five kingdom classification (1969). Later, a three-domain system split Monera into two domains, giving a six-kingdom arrangement. Such changes will continue in the future as we better understand characteristics and phylogenetic (evolutionary) relationships among organisms.
2. State two economically important uses of:
(a) heterotrophic bacteria(b) archaebacteria
ANSWER(a) Heterotrophic bacteria: (i) They are used to make curd from milk (e.g. Lactobacillus) and in production of antibiotics. (ii) Many fix nitrogen in the roots of legumes, enriching soil fertility, and act as important decomposers that recycle nutrients.(b) Archaebacteria: (i) Methanogens live in the gut of ruminants such as cows and buffaloes and produce methane (biogas) from their dung, used as fuel. (ii) Archaebacteria survive extreme habitats and are useful in biogas production and in industrial/sewage-treatment processes where ordinary microbes cannot survive.
3. What is the nature of cell-walls in diatoms?
ANSWERIn diatoms the cell wall forms two thin overlapping shells that fit together like a soap box. The walls are embedded with silica, which makes them hard and practically indestructible. Because of this, the cell-wall remains of diatoms accumulate over billions of years as “diatomaceous earth”, which, being gritty, is used in polishing and in the filtration of oils and syrups.
4. Find out what do the terms ‘algal bloom’ and ‘red-tides’ signify.
ANSWERAlgal bloom: a rapid, excessive multiplication of algae (often cyanobacteria/blue-green algae) in a water body, frequently in polluted or nutrient-rich water. The blooms colour the water, may release toxins, deplete oxygen and deteriorate water quality.Red-tides: a phenomenon caused by very rapid multiplication of red dinoflagellates such as Gonyaulax, which makes the sea appear red. The toxins released by these huge numbers can even kill other marine animals such as fishes.
5. How are viroids different from viruses?
ANSWERA virus is a nucleoprotein: its genetic material (either RNA or DNA, never both) is enclosed in a protein coat (capsid). A viroid, discovered by T. O. Diener in 1971, is a much smaller infectious agent that is a free RNA and lacks the protein coat found in viruses.Furthermore, the RNA of a viroid is of low molecular weight. Viroids caused the potato spindle tuber disease, while viruses cause diseases such as mumps, small pox, herpes, influenza and AIDS.
6. Describe briefly the four major groups of Protozoa.
ANSWERAll protozoans are heterotrophs that live as predators or parasites and are regarded as primitive relatives of animals. The four major groups are:(i) Amoeboid protozoans: live in fresh water, sea water or moist soil; move and capture prey using pseudopodia (false feet), as in Amoeba. Marine forms have silica shells; Entamoeba is a parasite.(ii) Flagellated protozoans: free-living or parasitic, bearing flagella. Parasitic forms cause diseases such as sleeping sickness, e.g. Trypanosoma.(iii) Ciliated protozoans: aquatic and actively moving owing to thousands of cilia; they have a gullet (cavity) into which coordinated ciliary beating steers food-laden water, e.g. Paramoecium.(iv) Sporozoans: diverse organisms with an infectious spore-like stage in the life cycle; the most notorious is Plasmodium, the malarial parasite.
7. Plants are autotrophic. Can you think of some plants that are partially heterotrophic?
ANSWERYes. Some plants are partially heterotrophic. Insectivorous plants such as Bladderwort and Venus fly trap trap and digest insects to obtain nitrogen, even though they still photosynthesise.Cuscuta (dodder) is a parasite that derives nutrition from its host plant. These plants are therefore not fully autotrophic.
8. What do the terms phycobiont and mycobiont signify?
ANSWERBoth terms refer to the two partners that make up a lichen, a symbiotic association of an alga and a fungus. The phycobiont is the algal component, which is autotrophic and prepares food (by photosynthesis) for the partnership.The mycobiont is the fungal component, which is heterotrophic; it provides shelter and absorbs mineral nutrients and water for its partner. Their association is so close that a lichen appears to be a single organism.
9. Give a comparative account of the classes of Kingdom Fungi under the following:
(i) mode of nutrition(ii) mode of reproduction
ANSWERThe four classes of Kingdom Fungi differ as follows:
Class
(i) Mode of nutrition
(ii) Mode of reproduction
Phycomycetes
Saprophytic on decaying wood / obligate parasites on plants (aquatic habitats).
Asexual by zoospores (motile) or aplanospores (non-motile) in sporangia; sexual by zygospore formed by fusion of isogamous, anisogamous or oogamous gametes.
Ascomycetes
Saprophytic, decomposers, parasitic or coprophilous (growing on dung).
Asexual by conidia produced exogenously on conidiophores; sexual by ascospores produced endogenously in sac-like asci, borne in fruiting bodies (ascocarps).
Basidiomycetes
Grow in soil, on logs and tree stumps, or as parasites in plants (e.g. rusts, smuts).
Asexual spores generally absent; vegetative reproduction by fragmentation. Sex organs absent; plasmogamy by fusion of vegetative cells, forming basidiospores on basidia in basidiocarps.
Deuteromycetes
Saprophytes, parasites, and largely decomposers of litter helping in mineral cycling.
Reproduce only by asexual spores (conidia); sexual stage unknown (‘imperfect fungi’).
10. What are the characteristic features of Euglenoids?
ANSWEREuglenoids are mostly fresh-water organisms found in stagnant water. Instead of a cell wall they have a protein-rich layer called the pellicle that makes the body flexible. They bear two flagella, one short and one long.They are mixotrophic: photosynthetic in sunlight, but when deprived of light they behave like heterotrophs, preying on smaller organisms. Interestingly, their photosynthetic pigments are identical to those of higher plants. A common example is Euglena.
11. Give a brief account of viruses with respect to their structure and nature of genetic material. Also name four common viral diseases.
ANSWERStructure: Viruses are non-cellular, obligate parasites that are inert (crystalline) outside their specific host cell. A virus is a nucleoprotein — its genetic material is protected by a protein coat called the capsid, made of small subunits called capsomeres arranged in helical or polyhedral forms.Nature of genetic material: Viruses contain either RNA or DNA — never both, and the genetic material is infectious. In general, plant viruses have single-stranded RNA; animal viruses have single- or double-stranded RNA or double-stranded DNA; and bacteriophages (bacterial viruses) are usually double-stranded DNA viruses.Four common viral diseases: mumps, small pox, herpes and influenza (AIDS is also viral).
12. Organise a discussion in your class on the topic – Are viruses living or non-living?
ANSWERViruses show features of both living and non-living things, which is why they are debated. Non-living traits: outside a host they are inert, crystalline particles; they can be crystallised like a chemical (as shown by W. M. Stanley, 1935); they have no cellular structure, no metabolism and cannot reproduce on their own.Living traits: inside a host cell they take over the cell’s machinery to replicate (multiply), possess genetic material (RNA or DNA) that is infectious and can mutate, and show host specificity. Conclusion of the discussion: viruses are best regarded as a bridge between the living and non-living — living when inside a host, non-living when outside it. (A classroom discussion activity; the above gives the points students should raise on both sides.)
Extra Practice Questions
Short Answer Type Questions
Q1. Who proposed the Five Kingdom Classification and on what main criteria?
ANSWERR. H. Whittaker (1969) proposed it. The main criteria were cell structure, body organisation, mode of nutrition, reproduction and phylogenetic relationships.
Q2. Differentiate between archaebacteria and eubacteria.
ANSWERArchaebacteria live in extreme habitats (halophiles, thermoacidophiles, methanogens) and have a different cell-wall structure that helps them survive harsh conditions. Eubacteria are ‘true bacteria’ with a rigid cell wall (and a flagellum if motile), and include the photosynthetic cyanobacteria.
Q3. What are heterocysts? Give two examples of organisms that bear them.
ANSWERHeterocysts are specialised cells in some cyanobacteria that fix atmospheric nitrogen. Examples: Nostoc and Anabaena.
Q4. Why are diatoms called the chief ‘producers’ in the oceans?
ANSWERDiatoms are photosynthetic chrysophytes that float as plankton in vast numbers in marine water. By making their own food through photosynthesis, they form the base of aquatic food chains, hence they are the chief producers in the oceans.
Q5. What is dikaryophase in fungi?
ANSWERIn ascomycetes and basidiomycetes, after plasmogamy an intervening stage occurs in which each cell carries two nuclei (n + n) without their immediate fusion. This condition is called a dikaryon and the phase is the dikaryophase; later the parental nuclei fuse to give diploid cells.
Long Answer Type Questions
Q1. Explain why the two-kingdom classification was found inadequate.
ANSWERLinnaeus’ two-kingdom system divided all organisms into Plantae and Animalia, grouping organisms only by gross morphology, chiefly the presence or absence of a cell wall. It could not distinguish prokaryotes from eukaryotes (bacteria and cyanobacteria were placed with eukaryotic algae), nor unicellular from multicellular forms (e.g. Chlamydomonas with Spirogyra). It also failed to separate photosynthetic green algae from non-photosynthetic fungi, ignoring the difference that fungi have chitin walls while green plants have cellulose walls. Moreover, a large number of organisms (such as Euglena, slime moulds) fitted neither kingdom. Because so many characters — cell structure, wall composition, nutrition, habitat and reproduction — were ignored, the system became inadequate, paving the way for the five kingdom classification.
Q2. Describe the steps of the sexual cycle in fungi.
ANSWERThe sexual cycle in fungi involves three steps. (i) Plasmogamy: fusion of the protoplasms of two motile or non-motile gametes. (ii) Karyogamy: fusion of the two nuclei brought together by plasmogamy. (iii) Meiosis: reduction division in the zygote, producing haploid spores. When a fungus reproduces sexually, two haploid hyphae of compatible mating types come together and fuse. In some fungi this immediately produces diploid (2n) cells, but in ascomycetes and basidiomycetes an intervening dikaryotic stage (n + n) occurs before the nuclei fuse. The fungi then form fruiting bodies in which reduction division leads to haploid spores.
Q3. Compare viruses, viroids and prions as acellular infectious agents.
ANSWERAll three are acellular agents not included in Whittaker’s five kingdoms. A virus is a nucleoprotein with genetic material (RNA or DNA, never both) enclosed in a protein capsid; it is an obligate parasite, inert outside the host, and causes diseases such as mumps, influenza, herpes and AIDS. A viroid (discovered by T. O. Diener, 1971) is smaller than a virus, consists of a free, low-molecular-weight RNA without any protein coat, and caused potato spindle tuber disease. A prion is an infectious agent made of an abnormally folded protein, similar in size to viruses; prions cause bovine spongiform encephalopathy (mad cow disease) and the variant Creutzfeldt–Jacob disease (CJD) in humans. Thus they differ chiefly in their composition: virus = nucleic acid + protein, viroid = RNA only, prion = protein only.
MCQs
1. The Five Kingdom Classification was proposed by:
(a) Aristotle (b) Linnaeus (c) R. H. Whittaker (d) Ivanowsky
2. The sole members of Kingdom Monera are:
(a) fungi (b) bacteria (c) protozoans (d) algae
3. Methanogens, which produce biogas from cattle dung, are:
For each Assertion–Reason question, choose: (A) Both true and the Reason correctly explains the Assertion; (B) Both true but the Reason is not the correct explanation; (C) Assertion true, Reason false; (D) Assertion false, Reason true.
A-R 1. Assertion: Bacteria are placed in Kingdom Monera.
Reason: Bacteria are prokaryotic organisms lacking a true nuclear membrane.
A-R 2. Assertion: Lichens are good pollution indicators.
Reason: Lichens do not grow in polluted areas.
A-R 3. Assertion: All protozoans are autotrophic.
Reason: Protozoans contain chlorophyll and prepare their own food.
A-R 4. Assertion: Cyanobacteria such as Nostoc can fix atmospheric nitrogen.
Reason: They possess specialised cells called heterocysts.
A-R 5. Assertion: Viruses are considered a link between living and non-living things.
Reason: Viruses can be crystallised yet multiply inside a host cell using its machinery.
Answer key: 1-(A), 2-(A), 3-(D), 4-(A), 5-(A).
Common Mistakes to Avoid
Watch out for these
Confusing archaebacteria with eubacteria — archaebacteria live in extreme habitats and have a different cell-wall structure.
Stating a virus has “both RNA and DNA” — no virus contains both; it is either RNA or DNA.
Mixing up phycobiont (alga) and mycobiont (fungus) in a lichen.
Writing that fungi have a cellulose wall — fungal walls are made of chitin; cellulose walls are in plants.
Calling slime moulds ‘animals’ — they are saprophytic protists.
Forgetting that cyanobacteria are prokaryotes (Monera), not algae of Plantae.
Exam tips for this chapter
Memorise Table 2.1 (characteristics of the five kingdoms) — cell type, cell wall, nuclear membrane, body organisation and mode of nutrition for each kingdom are frequently asked. Always link an example to each group (e.g. Nostoc for nitrogen fixation, Gonyaulax for red tides, Trypanosoma for sleeping sickness, Plasmodium for malaria). For virus/viroid/prion comparisons, remember the one-line rule: virus = nucleic acid + protein, viroid = RNA only, prion = protein only.
Frequently Asked Questions
What is Class 11 Biology Chapter 2 about?
Chapter 2, Biological Classification, explains how organisms have been classified over time, focusing on R. H. Whittaker’s Five Kingdom Classification (Monera, Protista, Fungi, Plantae and Animalia) and the acellular agents — viruses, viroids, prions — and lichens.
How many questions are there in the NCERT exercise of this chapter?
The end-of-chapter “Exercises” section has 12 numbered questions, all reproduced and solved on this page in exam-ready prose.
What are the main criteria of the Five Kingdom Classification?
Whittaker used cell structure, body organisation, mode of nutrition, reproduction and phylogenetic (evolutionary) relationships as the main criteria.
Are these Class 11 Biology Chapter 2 solutions free?
Yes. All solutions are free and follow the official NCERT Biology textbook for session 2026–27.
