NCERT Solutions for Class 11 Biology Chapter 2: Biological Classification

Concept. A classification system is a human-made arrangement of organisms into groups. Since it depends on the characters we know and use, it changes whenever our knowledge or the criteria change. Biological classification has therefore moved through several stages — from use-based grouping, to morphology-based grouping, to systems based on cell structure, nutrition, reproduction and evolutionary (phylogenetic) relationships.

Stage 1 — Instinctive, use-based classification. Since the dawn of civilisation, people classified organisms not on scientific criteria but out of a need to use them — for food, shelter and clothing. This was practical and unscientific.

Stage 2 — Aristotle. Aristotle was the earliest to attempt a more scientific basis. He used simple morphological characters to classify plants into trees, shrubs and herbs, and divided animals into two groups — those with red blood and those without. Limitation: only external, easily visible characters were used.

Stage 3 — Two Kingdom system (Linnaeus’ time). A system with Plantae and Animalia included all plants and animals respectively. It was easy to understand, but it did not distinguish between eukaryotes and prokaryotes, unicellular and multicellular organisms, and photosynthetic (green algae) and non-photosynthetic (fungi) organisms. A large number of organisms (bacteria, fungi, unicellular eukaryotes) fell into neither category, so the system was found inadequate. Bacteria, blue-green algae, fungi, mosses, ferns, gymnosperms and angiosperms were all lumped as “plants” merely because they had a cell wall.

Stage 4 — Five Kingdom Classification (R.H. Whittaker, 1969). Besides gross morphology, a need was felt to include cell structure, nature of wall, mode of nutrition, habitat, methods of reproduction and evolutionary relationships. Whittaker used cell structure, body organisation, mode of nutrition, reproduction and phylogenetic relationships and proposed five kingdoms: Monera, Protista, Fungi, Plantae, Animalia. Consequences of the changed criteria: (a) all prokaryotes → Monera; (b) unicellular eukaryotes → Protista (which now brings Chlamydomonas and Chlorella, earlier in algae under plants, together with Paramoecium and Amoeba, earlier in animals); (c) fungi (heterotrophic, chitin in the wall) separated from green plants (autotrophic, cellulose wall) → Fungi.

Stage 5 — Three-domain system. It divides Kingdom Monera into two domains (Archaebacteria and Eubacteria), leaving all the eukaryotic kingdoms in the third domain, giving a six-kingdom classification; it is studied in higher classes.

Extra depth. Notice that the plant and animal kingdoms have been constant under all systems, but the understanding of what groups belong to them has changed, and so has the number and nature of the other kingdoms. NCERT adds that changes will continue as our understanding of characteristics and evolutionary relationships improves — the goal is a system that reflects morphological, physiological and reproductive similarities and is also phylogenetic.

Conclusion: classification changed from use-based grouping, through Aristotle’s morphology and Linnaeus’ two kingdoms, to Whittaker’s five-kingdom system based on cell structure, body organisation, nutrition, reproduction and phylogeny (and later three domains), because new criteria and new knowledge revealed that earlier groupings mixed unlike organisms.

Concept. Bacteria, the sole members of Kingdom Monera, are the most abundant micro-organisms and show the most extensive metabolic diversity of any group. The vast majority are heterotrophs (depend on other organisms or dead organic matter), while archaebacteria are a special group living in harsh habitats such as extreme salty areas (halophiles), hot springs (thermoacidophiles) and marshy areas (methanogens), distinguished by a different cell wall structure.

(a) Economically important uses of heterotrophic bacteria (NCERT names the following):

1. Making curd from milk. Lactic-acid bacteria (e.g., Lactobacillus) convert milk sugar (lactose) into lactic acid; the acid coagulates milk proteins and gives curd its sour taste. Curd is more nutritious than milk (it also supplies vitamin B₁₂), and the same process is used on a commercial scale to make curd, cheese and yoghurt.
2. Production of antibiotics. Certain heterotrophic bacteria (for example, species of Streptomyces, the source of streptomycin) form antibiotics that kill or inhibit disease-causing micro-organisms — a huge pharmaceutical and economic industry.
3. Fixing nitrogen in legume roots. Symbiotic bacteria (e.g., Rhizobium) in the root nodules of pulses and other legumes convert atmospheric nitrogen into usable forms, enriching the soil and reducing the need for costly nitrogenous fertilisers.
4. Decomposers. The majority of heterotrophic bacteria are important decomposers: they break down dead organic matter, recycle nutrients and clean the environment; they are exploited in sewage treatment and compost making.

(b) Economically important uses of archaebacteria:

1. Biogas (methane) production. Methanogens present in the gut of ruminant animals such as cows and buffaloes are responsible for the production of methane (biogas) from the dung of these animals. Biogas is a clean, renewable fuel for cooking and lighting in rural areas, and the slurry left over is a good manure.
2. Sewage / waste treatment. The anaerobic digestion of sludge in sewage-treatment plants and biogas digesters is carried out by methanogens, producing methane while reducing organic waste. (Standard extra: heat-stable enzymes from thermophilic archaebacteria, e.g., Pyrococcus furiosus DNA polymerase used in PCR, have industrial uses because they work at extreme temperatures.)

Extra depth. The same group can be both useful and harmful: heterotrophic bacteria also cause cholera, typhoid, tetanus and citrus canker. The different cell-wall structure of archaebacteria is what lets them live where very few other organisms can.

Conclusion: heterotrophic bacteria are economically important in making curd and antibiotics and in fixing nitrogen in legume roots, while archaebacteria (methanogens) produce methane as biogas from the dung of ruminant animals.

Concept. Diatoms belong to Chrysophytes of Kingdom Protista (along with golden algae or desmids). They are microscopic, found in fresh water and marine environments, float passively in water currents (plankton) and are mostly photosynthetic — the chief “producers” in the oceans.

Nature of the diatom cell wall:

1. Two overlapping shells. The cell wall forms two thin overlapping shells which fit together as in a soap box — the larger shell is the lid and the smaller the base.
2. Embedded with silica. The walls are impregnated with silica, making them rigid, glassy and decorated with fine patterns.
3. Indestructible. Because silica does not decay, the walls are indestructible; they remain intact after the cell dies.
4. Diatomaceous earth. Diatoms have thus left behind large amounts of cell-wall deposits in their habitat; this accumulation over billions of years is referred to as “diatomaceous earth”.
5. Economic uses. Being gritty, this soil is used in polishing and in filtration of oils and syrups.

Extra depth. The silica wall protects the delicate cell, yet its two-piece design allows growth and cell division. The abundance of diatoms makes them the chief producers of the oceans — the base of marine food chains — and their indestructible walls are a rare case where an organism’s body part leaves a huge geological record.

Conclusion: diatom cell walls are made of two thin overlapping, soap-box-like shells embedded with silica, which makes them indestructible and leads to the accumulation of diatomaceous earth used in polishing and in filtering oils and syrups.

Concept. Both terms describe the explosive multiplication of microscopic photosynthetic organisms in water, usually in nutrient-rich or polluted conditions. The first is associated with cyanobacteria and algae in general; the second with red dinoflagellates of Kingdom Protista.

Algal bloom. An algal bloom is the rapid, excessive growth of algae (including cyanobacteria or blue-green algae) in a water body, forming a visible coloured layer or scum on the surface. NCERT states that cyanobacteria “often form blooms in polluted water bodies”. Features and consequences:

• Occurs when water is rich in nutrients such as nitrates and phosphates (e.g., sewage and fertiliser run-off) — eutrophication.
• The dense mat cuts off sunlight to submerged plants; when the algae die, bacterial decomposition uses up dissolved oxygen, killing fishes and other aquatic animals.
• Some cyanobacterial blooms release toxins that make water unfit for drinking and harm animals and humans.

Red tide. Red tides are caused by red dinoflagellates (example: Gonyaulax), which are mostly marine and photosynthetic. They undergo such rapid multiplication that they make the sea appear red. The colour depends on the main pigments in their cells (dinoflagellates appear yellow, green, brown, blue or red). Toxins released by such large numbers may even kill other marine animals such as fishes; the toxins may also accumulate in shellfish and affect humans who eat them.

Extra depth. Both phenomena show how a normally useful producer can become harmful when its numbers explode; they also link back to NCERT’s statement that cyanobacteria are indicators of polluted water.

Conclusion: an algal bloom is the excessive growth of algae/cyanobacteria in nutrient-rich or polluted water, and a red tide is the rapid multiplication of red dinoflagellates such as Gonyaulax that makes the sea appear red and whose toxins can kill fishes and other marine animals.

Concept. Both viruses and viroids are acellular, non-cellular infectious agents that are not included in Whittaker’s five kingdoms because they lack a cell structure and are obligate parasites that replicate only inside a host. However, they differ sharply in their structure and chemistry.

Viruses. A virus is a nucleoprotein — a core of genetic material (either RNA or DNA, never both) surrounded by a protein coat (capsid) made of capsomeres arranged in helical or polyhedral forms. Viruses show an inert crystalline structure outside a living cell (W.M. Stanley, 1935) and are obligate parasites. The name “virus” was given by M.W. Beijerinck (1898) after the tobacco mosaic work of D. Ivanowsky (1892).

Viroids. T.O. Diener (1971) discovered viroids as infectious agents smaller than viruses that caused potato spindle tuber disease. A viroid is a free RNA of low molecular weight that lacks the protein coat found in viruses — hence the name viroid (virus-like).

Extra depth. Both are inert outside a host and obligate in nature, but because viroids have no capsid, their infectivity depends on the RNA alone; this made their discovery one of the landmarks that extended the idea of what an infectious agent can be. A third, still simpler agent, the prion, is an abnormally folded protein with no nucleic acid at all (BSE, CJD).

Conclusion: viruses are nucleoproteins with an RNA or DNA core enclosed in a protein capsid, whereas viroids are smaller, consist only of low-molecular-weight free RNA and lack a protein coat (as in potato spindle tuber disease).

Concept. Protozoans (Kingdom Protista) are all heterotrophs and live as predators or parasites. They are believed to be primitive relatives of animals. Being protists, each is a single-celled eukaryote with a well-defined nucleus and membrane-bound organelles. They are classified on the basis of their locomotory organs into four major groups:

1. Amoeboid protozoans. They live in fresh water, sea water or moist soil. They move and capture prey by putting out pseudopodia (false feet), as in Amoeba. Marine forms have silica shells on their surface. Some, such as Entamoeba, are parasites (causing amoebic dysentery in humans).

2. Flagellated protozoans. The members are either free-living or parasitic and possess flagella for locomotion. The parasitic forms cause diseases such as sleeping sickness. Example: Trypanosoma.

3. Ciliated protozoans. They are aquatic, actively moving organisms because of the presence of thousands of cilia. They have a cavity, the gullet, that opens to the outside of the cell surface. The coordinated movement of rows of cilia causes the water laden with food to be steered into the gullet. Example: Paramoecium (Fig. 2.4d).

4. Sporozoans. This includes diverse organisms that have an infectious spore-like stage in their life cycle. The most notorious is Plasmodium (the malarial parasite) which causes malaria, a disease which has a staggering effect on the human population.

Extra depth. Protozoans are placed with other protists because they are unicellular eukaryotes; however, their heterotrophic, predatory or parasitic way of life is animal-like, so NCERT calls them primitive relatives of animals. Many important human diseases — malaria, sleeping sickness, amoebic dysentery — are protozoan.

Conclusion: the four groups of protozoans are amoeboid (pseudopodia, Amoeba), flagellated (flagella, Trypanosoma), ciliated (cilia and gullet, Paramoecium) and sporozoans (spore-like infectious stage, Plasmodium), all being heterotrophic predators or parasites.

Concept. Kingdom Plantae includes all eukaryotic chlorophyll-containing organisms, and most of them are autotrophs — they make their own food by photosynthesis. However, NCERT notes that a few members are partially heterotrophic, such as insectivorous plants and parasites. These plants still have chlorophyll, but they supplement their nutrition from other sources.

1. Insectivorous plants. They are green, photosynthetic plants that grow in soils poor in nitrogen. To make up for the deficiency, they trap and digest insects and absorb the nitrogenous compounds. NCERT examples:

• Bladderwort (Utricularia) — an aquatic plant with small bladder-like traps; tiny aquatic animals touching the trigger hairs are sucked in by a sudden inflow of water.
• Venus fly trap (Dionaea) — the two lobes of the leaf snap shut when insects touch the sensitive hairs; enzymes then digest the prey.
• (Additional examples from other chapters — Nepenthes (pitcher plant) and Drosera (sundew).)

2. Parasitic plants. These plants depend on a host plant for part or all of their nutrition. NCERT example: Cuscuta (dodder) — a leafless, yellowish, thread-like twiner that coils around its host and develops haustoria which penetrate the host tissue to absorb water and food. (Other well-known examples: Loranthus and mistletoe, which are partial stem parasites, having chlorophyll and absorbing water and minerals from the host.)

Extra depth. These plants show that autotrophy and heterotrophy are not always sharply separated — they are classified as plants because they have chloroplasts and a cellulose cell wall and show alternation of generations, yet they are only partially autotrophic.

Conclusion: insectivorous plants such as bladderwort and Venus fly trap, and parasitic plants such as Cuscuta, are partially heterotrophic: they photosynthesise (or parasitise) but also obtain nutrients from animals or host plants.

Concept. Lichens are symbiotic associations (mutually useful associations) between algae and fungi. So close is the association that, if one saw a lichen in nature, one would never imagine that two different organisms live in it. The two partners have special names.

Phycobiont — the algal component of a lichen (phyco = alga, biont = living partner). It is autotrophic: it carries out photosynthesis and prepares food for the fungus.

Mycobiont — the fungal component of a lichen (myco = fungus). It is heterotrophic: it provides shelter to the alga and absorbs mineral nutrients and water for its partner.

Extra depth. Lichens are very good pollution indicators — they do not grow in polluted areas (they are especially sensitive to sulphur dioxide). They are not included in the five-kingdom classification of Whittaker, along with viruses, viroids and prions.

Conclusion: phycobiont is the algal partner (autotrophic, makes food) and mycobiont is the fungal partner (heterotrophic, gives shelter and absorbs water and minerals) in the symbiotic association called a lichen.

Concept. Fungi are a unique kingdom of heterotrophic organisms with bodies made of thread-like hyphae forming a mycelium (except unicellular yeasts); their cell wall is of chitin and polysaccharides. The morphology of the mycelium, the mode of spore formation and the fruiting bodies form the basis for dividing the kingdom into four classes: Phycomycetes, Ascomycetes, Basidiomycetes and Deuteromycetes.

(i) Mode of nutrition. All fungi are heterotrophic — they absorb soluble organic matter from the substrate. Most are saprophytes (on dead substrates); those on living plants/animals are parasites; some live as symbionts (lichens, mycorrhiza). Class-wise:

• Phycomycetes — found in aquatic habitats and on decaying wood in moist, damp places (saprophytic) or as obligate parasites on plants (e.g., Albugo on mustard); Mucor and Rhizopus (bread mould) are saprophytes.
• Ascomycetes — saprophytic, decomposers, parasitic or coprophilous (growing on dung); e.g., Penicillium, Aspergillus, Neurospora; morels and truffles are edible.
• Basidiomycetes — grow in soil, on logs and tree stumps (saprophytes) and in living plant bodies as parasites (rusts and smuts); mushrooms, bracket fungi, puffballs.
• Deuteromycetes — saprophytes or parasites; a large number are decomposers of litter and help in mineral cycling (e.g., Alternaria, Colletotrichum, Trichoderma).

(ii) Mode of reproduction. Fungi reproduce by vegetative means (fragmentation, fission, budding), asexually (conidia, sporangiospores, zoospores) and sexually (oospores, ascospores, basidiospores) in fruiting bodies. The sexual cycle has three steps: plasmogamy (fusion of protoplasms), karyogamy (fusion of two nuclei) and meiosis in the zygote giving haploid spores; ascomycetes and basidiomycetes have an intervening dikaryotic (n + n) stage.

Extra depth. The names tell you how the sexual spores are borne: asco- (sac) → ascospores inside asci; basidio- (small base/club) → basidiospores outside on the basidium. Deuteromycetes are called imperfect fungi because their sexual (perfect) stage is not known — once it is found they are transferred to Ascomycetes or Basidiomycetes.

Conclusion: all fungi are heterotrophic (saprophytic, parasitic or symbiotic); Phycomycetes have aseptate mycelium with zoospores/aplanospores and zygospores, Ascomycetes form conidia and ascospores in asci, Basidiomycetes lack asexual spores and form basidiospores on basidia, and Deuteromycetes reproduce only by conidia.

Concept. Euglenoids are single-celled eukaryotic protists (Kingdom Protista) that show a remarkable mixture of plant-like and animal-like features — a good example of why the boundaries of Protista are not well defined. The typical example is Euglena (Fig. 2.4b).

Characteristic features (all from NCERT, with explanation):

1. Habitat. The majority are fresh-water organisms found in stagnant water.
2. Pellicle instead of a cell wall. They have no cell wall; instead they have a protein-rich layer called the pellicle which makes the body flexible (they can change shape).
3. Two flagella. They have two flagella — one short and one long — for locomotion.
4. Mixotrophic nutrition. They are photosynthetic in the presence of sunlight, but when deprived of sunlight they behave like heterotrophs by predating on other smaller organisms.
5. Plant-like pigments. The pigments of euglenoids are identical to those present in higher plants (chlorophylls a and b, carotenoids) held in chloroplasts.
6. Eukaryotic organisation. Being protists, they have a well-defined nucleus and other membrane-bound organelles (as in all Protista), and they reproduce asexually (by longitudinal binary fission).

Extra depth. Because Euglena is both photosynthetic and heterotrophic, it is difficult to place it in either the plant or the animal kingdom of the older two-kingdom system — precisely a reason why Whittaker created the separate Kingdom Protista.

Conclusion: euglenoids are mostly fresh-water protists with a flexible protein-rich pellicle instead of a cell wall, two flagella (short and long), plant-like pigments, photosynthetic nutrition in light and heterotrophic predation in the absence of light.

Concept. Viruses are non-cellular (acellular) organisms that have an inert crystalline structure outside the living cell. Once they infect a cell, they take over the machinery of the host cell to replicate themselves, killing the host. The word virus means venom or poisonous fluid. They are obligate parasites and did not find a place in Whittaker’s five-kingdom classification because they are not considered truly “living” if living means having a cell structure.

Historical landmarks. Dmitri Ivanowsky (1892) recognised certain microbes as the causal organism of the mosaic disease of tobacco; they were smaller than bacteria because they passed through bacteria-proof filters. M.W. Beijerinck (1898) showed that the extract of infected tobacco plants could infect healthy plants; he named the pathogen “virus” and the fluid Contagium vivum fluidum (infectious living fluid). W.M. Stanley (1935) showed that viruses could be crystallised and that the crystals consist largely of protein.

Structure. A virus is a nucleoprotein: it has a core of genetic material (nucleic acid) enclosed in a protein coat called the capsid, which protects the nucleic acid. The capsid is made of small subunits called capsomeres, which are arranged in helical (e.g., TMV) or polyhedral geometric forms. Bacteriophages have a polyhedral head, a collar, a sheath (contractile tail) and tail fibres (Fig. 2.6b).

Nature of genetic material. In addition to protein, viruses contain genetic material that could be either RNA or DNA — no virus contains both. The genetic material is infectious. In general:

Four common viral diseases. Viruses cause mumps, small pox, herpes and influenza; AIDS in humans is also caused by a virus. In plants, symptoms can be mosaic formation, leaf rolling and curling, yellowing and vein clearing, dwarfing and stunted growth. (The common cold and flu mentioned in the chapter are also viral.)

Extra depth. Because the infectious unit is just a nucleoprotein, viruses can be crystallised like a chemical — but once inside a host cell, they behave as living. This dual behaviour is the basis of the “living or non-living” discussion in Question 12.

Conclusion: a virus is a non-cellular nucleoprotein with an RNA or DNA genome (never both) enclosed in a protein capsid of capsomeres; it is an obligate parasite causing diseases such as mumps, small pox, herpes, influenza and AIDS.

Concept. The question asks for a class discussion; the best way to prepare is to list the evidence for and against the view that viruses are living. NCERT itself says viruses “did not find a place in classification since they are not considered truly living, if we understand living as those organisms that have a cell structure” — and then asks the reader, “Would you call viruses living or non-living?”

Points supporting that viruses are NON-LIVING:

1. No cellular organisation — they have no cell membrane, cytoplasm, ribosomes or organelles; they are just a nucleic acid core in a protein capsid.
2. Inert crystalline structure outside the host cell — W.M. Stanley (1935) crystallised them; crystals consist largely of protein; they show no metabolism, growth or response when outside a host.
3. They cannot reproduce independently — they are obligate parasites, replicating only by using the host cell’s machinery.
4. No metabolism of their own — they do not respire, synthesise proteins or produce energy by themselves.

Points supporting that viruses are LIVING:

1. They possess genetic material (DNA or RNA) that is infectious and carries hereditary information.
2. They multiply (replicate) inside a host cell, producing many copies of themselves — reproduction is a hallmark of life.
3. They mutate and evolve — new virus strains (e.g., influenza) arise and adapt to hosts and defences.
4. They respond to the host environment, infecting specific host cells and causing disease; Beijerinck called the fluid Contagium vivum fluidum (infectious living fluid).

Conclusion of the discussion. Viruses are best regarded as borderline entities — they are at the threshold between the living and the non-living: non-living by cell-based definitions, but living in their ability to multiply and evolve inside a host. That is why they are placed outside the five kingdoms.

Extra depth. A useful way to summarise: “a virus is chemical outside, biological inside.” Viroids (free RNA) and prions (infectious proteins) push the boundary even further.

Conclusion: viruses show features of both living (genetic material, replication, mutation) and non-living things (acellular, crystallisable, inert outside the host, no independent metabolism), so they are best considered borderline entities and are therefore kept outside the five-kingdom classification.