Biological Classification Class 11 Biology NCERT Solutions

Step-by-step solution

1. 1The thread running through the whole history is that each system is built on the criteria available at its own time, and each is abandoned when better criteria appear. The systems to name are the two-kingdom, the five-kingdom, the two-empire and the three-domain arrangements, in that order.
2. 2The first attempt was the two-kingdom system of Linnaeus, which divided all life into Plantae and Animalia. It worked only while a unicellular organism was regarded as a plant, and it broke as soon as a bacterium such as Paramoecium, with a contractile vacuole and definite cilia, had to be placed. Its failing categories were that it did not separate the prokaryotes from the eukaryotes, and that it did not separate the fungi from the plants, since the fungi were placed among the plants.
3. 3Haeckel then proposed a third kingdom, Protista, in 1866, to hold all unicellular eukaryotes, and that single change fixed the fungi error, since with the Protista separated the fungi could be recognised as a group of their own.
4. 4The four-kingdom system of Copeland, in 1956, separated the prokaryotes into the kingdom Monera. That separation is the important one, because it is the first system to recognise the fundamental divide in living matter between a cell with a membrane-bound nucleus and a cell without one.
5. 5In 1957 Whittaker proposed the five-kingdom system, adding the kingdom Fungi to the four of Copeland and dividing the eukaryotes by mode of nutrition, so Monera, Protista, Fungi, Plantae and Animalia each rests on a clear criterion: the first two on cell type, Fungi and Plantae on nutrition, and Animalia on cell type plus nutrition and organisation. His system was the first to separate bacteria from all other organisms on a cell-type basis and to give fungi their own kingdom.
6. 6The problem left by the five-kingdom system was that it still put all very different bacteria in Monera together, so in 1990 Woese and his colleagues studied base-pair sequence in 16S rRNA and proposed three domains, Bacteria, Archaea and Eukarya, which separated the prokaryotes into two very different groups. Woese's method is the clue to the whole change: the classification stopped being a matter of opinion and became a matter of measurement.

Step-by-step solution

1. 1(a) Heterotrophic bacteria, which include the free-living nitrogen-fixing bacteria, are used in agriculture as biofertilisers, because the Rhizobium living in the root nodules of legumes fixes atmospheric nitrogen and enriches the soil, and Azotobacter and the cyanobacteria do the same as free-living forms.
2. 2(a) They are also used in the preparation of curd and yoghurt, because the lactic acid bacteria Lactobacillus ferments the milk sugars, and in the production of vinegar, because Acetobacter aceti carries out the conversion of ethanol to acetic acid.
3. 3(a) The third group of uses is industrial and medical. Several produce antibiotics such as streptomycin from Streptomyces, they are used in sewage treatment and in the tanning of leather, and the recombinant Lactobacillus, with the human insulin gene inserted, is used in the large-scale production of human insulin.
4. 4(b) Archaebacteria live in some of the most harsh habitats on earth, and the chapter names four. Halophiles live in extremely salty areas such as the salt lakes, thermoacidophiles live in very hot acidic springs, methanogens live in the gut of several ruminant animals such as cows and buffaloes and in marshy areas, and in the face of this the chapter names the osmolyte and the branched-chain lipid that let them survive, which is why their membranes do not rupture at those extremes.
5. 5(b) The two uses to state are the methanogens as the source of biogas, since Methanobacterium breaks down the dung of the ruminants anaerobically to produce methane, which is a clean fuel; and the halophiles of the salt lakes as the source of the enzyme halophilease, whose catalytic activity is unaffected by salt and which is used in the manufacture of high-fidelity DNA in biotechnology.
6. 6(a) There is also a third and a fourth archaeal use worth knowing: the gas vesicles of Halobacterium are used as internal organelles in cloning vectors, because they are cheap and easy to isolate, and the archaebacterial DNA polymerase is the enzyme of choice in DNA sequencing because it is thermostable and so survives the repeated heating steps a thermal cycler imposes.

Step-by-step solution

1. 1The answer is a two-part wall made of two very different materials, and both parts have to be named.
2. 2The inner layer is a thin, siliceous or silica wall. The cell wall of a diatom is made chiefly of silica, in the form of hydrated silicon dioxide, and the cells of the diatoms are unique in this respect among the protists.
3. 3The outer layer is organic. The inner layer of silica is enclosed by a thin organic layer, commonly a carbohydrate, and it is this organic layer that gives the diatom its golden-brown colour, so the colour is not the chlorophyll itself.
4. 4Two consequences follow and both are examinable. Because the wall is siliceous it is indestructible, and the remains of diatoms accumulate over geological time to give diatomaceous earth, which is used in filtration, polishing and as a mild abrasive. And because the wall is in two concentric parts with a girdle where the two overlap, the cell is girdled, which is why the group is named Bacillariophyceae, or the diatoms.
5. 5The wall is also the reason the two forms of diatom are distinguished. Pennate diatoms are bilaterally symmetrical with a feather-like outline, and centric diatoms are radially symmetrical.

Step-by-step solution

1. 1Both terms describe a visible consequence of the same thing, an explosive growth of plankton, so begin with the shared cause and then give the two names their own separate meaning.
2. 2The shared cause is nutrient enrichment of a water body. Algal bloom is the rapid multiplication of phytoplankton in a water body that has been enriched with nutrients, especially nitrates and phosphates, which are carried into rivers and lakes by agricultural runoff and industrial effluent.
3. 3Algal bloom is also called a water bloom, and the algae that cause it are dinoflagellates. The bloom appears as a visible scum or a discolouration on the surface, and the important consequence is that the algae consume so much of the dissolved oxygen that the fish and other aquatic animals are killed; some of the dinoflagellates are also brilliantly bioluminescent.
4. 4Red tides are caused by red dinoflagellates, chiefly the genus Gonyaulax, multiplying explosively so that the sea itself takes a red colour, which is the literal meaning of the term. The red colour comes from the pigment in the dinoflagellate's photosynthetic chromoplast.
5. 5The serious consequence of a red tide is toxicity rather than oxygen depletion. The dinoflagellates produce potent neurotoxins such as saxitoxin, and these pass along the food chain, so eating the contaminated shellfish causes paralytic shellfish poisoning, and in severe cases the toxin can kill humans as well.
6. 6Both terms are worth keeping distinct in the exam: an algal bloom is named for its effect on the water and kills by deoxygenation, whereas a red tide is named for its colour and kills by toxin carried through the food chain.

Step-by-step solution

1. 1The difference is a difference in what the entity is made of, and a difference in how it is replicated, so answer both.
2. 2A virus has a protein coat. A viroid is a naked, circular, single-stranded RNA molecule with no protein coat at all, and it is much smaller than a virus, being a fraction of its size. The protein coat of a virus is built from capsid protein, and the viral genome is either DNA or RNA.
3. 3The consequence for replication is a difference in who copies the molecule. A virus needs a host cell's machinery to replicate, but the replication of a viroid does not depend on any protein, because the RNA of a viroid is replicated by an RNA polymerase that the viroid itself encodes. This absence of any protein component is what allows viroids to be infectious on their own.
4. 4A third difference is the range of hosts. Viroids are found only in plants, where they cause disease, so they do not infect animals, whereas viruses have a wide host range covering bacteria, plants and animals alike.
5. 5The practical measure of the difference is therefore the protein: a virus is a nucleoprotein with a coat, a viroid is RNA alone, and that single missing coat changes the size, the replication mechanism and the host range.

Step-by-step solution

1. 1The four groups are separated by their mode of nutrition and locomotion, so use those two features as the headings rather than trying to name groups vaguely.
2. 2The Amoeboids, or Sarcodina, move by pseudopodia, produced by streaming of the cytoplasm, and they feed by holozoic nutrition, engulfing food whole. Examples are Amoeba and Entamoeba, and Entamoeba histolytica is the pathogen of amoebic dysentery.
3. 3The flagellates, or Mastigophora, move by flagella and may be autotrophic, as in the photosynthetic dinoflagellates and Euglena, or heterotrophic. Examples are Giardia, which causes giardiasis, and Trypanosoma, which causes sleeping sickness.
4. 4The ciliates, or Ciliophora, move by cilia and feed by holozoically, and the group is uniquely characterised by the presence on the cell of a differentiated organisation, the ciliary apparatus, used in locomotion and in feeding. The single large macronucleus that regulates the everyday functions, and one or more small micronuclei concerned with reproduction and sexual reproduction, are the features to name. Examples are Paramecium, Opalina and Plasmodium.
5. 5The Sporozoans, or Apicomplexana, move by gliding, and they are all parasites. The apical complex of structures, including the apical ring, is the diagnostic feature, and there is no locomotory organ of the usual kind. Their life cycle is distinctive, because a sporozoan passes through several distinct stages, including a motile form and a spore-forming form, and often needs two different hosts to complete it. Examples are Plasmodium, the malaria parasite, and the gregarines and the coccidia.

Step-by-step solution

1. 1The term to use is partial heterotrophy, and the mechanism in every case is a plant obtaining carbon from a source other than its own photosynthesis.
2. 2Parasitic plants take their nourishment wholly or partly from another plant, and they are the clearest case. They possess chlorophyll and can photosynthesise, but they obtain water and minerals from the host, and the two families to name are Rafflesiaceae and Loranthaceae, the mistletoes. Rafflesia is worth remembering as the classic example, because it is completely non-green and lacks chlorophyll altogether, so it is heterotrophic in appearance as well as in fact.
3. 3Saprophytes, or saprotrophs, obtain their food from dead decaying organic matter and are therefore not really parasites of a living host. The example to give is the Indian pipe, Monotropa uniflora, which lacks chlorophyll and feeds on decaying matter with the help of a mycorrhizal fungus, and the non-green plants in general.
4. 4Carnivorous or insectivorous plants are the third group, and they illustrate the idea best, because the plant still photosynthesises for itself and eats only to obtain nitrogen. Pitcher plant, Venus flytrap and Drosera are the named examples, and the mechanism is that the insect is trapped, digested, and the released nitrogen, phosphorus and sulphur are absorbed.
5. 5So the three cases to distinguish are: parasitic, drawing on a living host; saprophytic, drawing on dead matter; and carnivorous, a green plant supplementing itself with animal protein. In the first two the plant may have no chlorophyll, but in the third it always has.

Step-by-step solution

1. 1These are the two partners of a lichen, and each name states the partner's role. Both are suffixes meaning living, attached to the group name, so the terms can be read straight from their roots.
2. 2Phycobiont is the algal partner of a lichen. The suffix -biont means an organism living in a particular way, and phyco- refers to algae, so the phycobiont is the photosynthetic half of the association. It supplies the organic food, that is the carbohydrate synthesised by photosynthesis.
3. 3Mycobiont is the fungal partner, since myco- refers to a fungus. The mycobiont supplies the water and the minerals, takes up nutrients from the substrate, and provides the protection and the structural housing for the algal cells.
4. 4The division of labour is what has to be stated in the answer: the algal partner makes the food and the fungal partner supplies water, minerals and shelter, and the association as a whole is capable of photosynthesis while occupying habitats such as bare rock and bark that neither partner could occupy alone.
5. 5One qualification is worth carrying: most lichens are this two-part association, but a few, such as those in the genus Candida and the algal partner of such lichens, are exceptions, and the two organisms are not merely adjacent but physiologically interdependent, with the algal cells living in a protected state inside the fungal tissue.

Step-by-step solution

1. 1This is the long comparison question of the chapter, so structure it as two tables of four classes each. The four classes are Phycomycetes, Ascomycetes, Basidiomycetes and Deuteromycetes, and they differ in exactly the two characters asked for.
2. 2(i) Phycomycetes are saprophytic or parasitic, generally coenocytic, and the mycelium is aseptate and multinucleate, with a non-septate filament. Examples are Rhizopus, the pin mould, and the algal fungal group is a useful pointer that the name means algal fungi.
3. 3(i) Ascomycetes are mostly saprophytic, some parasitic, and the mycelium is septate but unicellular in structure, because the septa separate cells each with a single nucleus. Examples are Aspergillus, Claviceps, Neurospora, and the edible morel and truffle.
4. 4(i) Basidiomycetes are mostly saprophytic, rarely parasitic, and the mycelium is septate and dikaryotic, meaning it has two nuclei per cell. Examples are Agaricus, the mushroom, Ustilago, the smut, and the puffballs, and their fruiting body is a basidiocarp.
5. 5(i) Deuteromycetes are saprophytic or parasitic, and their mycelium is septate and well developed. Examples are Alternaria, Neurospora and Penicillium, and the group is also called the Fungi Imperfecti because no sexual stage was known for them.
6. 6(ii) Phycomycetes reproduce asexually by motile zoospores or aplanospores, and sexually by the fusion of motile gametes, so the sexual reproduction is of the oogamous type and their mycelium is coenocytic.
7. 7(ii) Ascomycetes reproduce asexually by conidia or budding or ascospores, and sexually by the formation of sac-like asci containing ascospores, which is why the sexual spores are carried in an sac. The ascocarps that hold the asci include apothecia, perithecia, cleistothecia and pseudothecia.
8. 8(ii) Basidiomycetes reproduce asexually rarely, by conidia, and sexually by the formation of club-shaped basidia, each bearing four basidiospores externally, and the basidia are borne on a basidiocarp.
9. 9(ii) Deuteromycetes reproduce only asexually, by conidia, and no sexual stage is known for any of them, which is precisely why the name Fungi Imperfecti is used for the group. They produce two types of conidia, namely sporangioconidia and conidioconidia.

Step-by-step solution

1. 1The definition to lead with is that the Euglenoids are a group of flagellated protists that are so named because they resemble the true flagellate Euglena in body form and behaviour, so they are the master key for the group.
2. 2On nutrition the group is unique among the protists in being both autotrophic and heterotrophic, and the mechanism is worth stating precisely: a Euglenoid has chlorophyll in the same three chlorophylls as the green plants, so it can photosynthesise in light, and in the absence of light it can live heterotrophically on organic matter. This mixed nutrition is the single most examinable feature of the group.
3. 3The pigments and the food storage follow from that. The pigments are chlorophyll a, chlorophyll b and carotene, so the colour is green, and because the group is not a true plant there is no cellulose cell wall, and the food is not stored as starch but as paramylon, a carbohydrate, in the form of shiny granules in the cytoplasm.
4. 4Structurally the group is defined by the pellicle, the proteinaceous layer on the outside of the cell membrane, which is a protein-lipid layer, and the absence of a cell wall. The cell is elongated, spindle-shaped and tapering at both ends, with two flagella of unequal length, so one is short and the other long, and there is a single cup-shaped invagination at the anterior end called the gullet or cytopharynx, which leads to a reservoir, and this gullet also serves for ingestion.
5. 5A contractile vacuole is present and serves osmoregulation, and there is a single eyespot, or stigma, at the base of the longer flagellum which is photosensitive, together with a photoreceptor, and the Euglenoids are therefore able to move towards light, which is a phototactic response. They also have a contractile vacuole and a flexible pellicle, which is why the body changes shape when the animal is squeezed, giving the group its name, since the cells are described as euglena, meaning 'true eye'.

Step-by-step solution

1. 1Structure first, and the key point is that a virus has no cellular structure at all. It is non-cellular, has no cell membrane or cell wall, and has no cytoplasm, ribosomes or other organelles, so it can only reproduce inside a host cell.
2. 2A virus is a nucleoprotein. Its structure is a protein coat called a capsid surrounding a nucleic acid core, and the capsid is made of many subunits called capsomeres arranged in a symmetrical pattern. The capsid together with the nucleic acid is called the nucleocapsid, and the envelope is a lipid membrane, present only in some viruses, which is derived from the host cell and carries glycoprotein spikes.
3. 3On the nature of the genetic material, the viruses are grouped, and the grouping is by the type of nucleic acid. A virus contains either DNA or RNA as its genetic material, never both, which is the single most frequently asked point on this question. The Baltimore classification takes this further, and its four groups based on the type of nucleic acid and its route to replication are double-stranded DNA, single-stranded DNA, double-stranded RNA and single-stranded RNA.
4. 4Both types are found, so the answer is not one or the other. The DNA viruses include the pox viruses of smallpox, and the RNA viruses include the tobacco mosaic virus, the influenza virus, the polio virus and the HIV of AIDS.
5. 5The commonness of the viruses and the damage they do give the diseases to name. The chapter names four: the influenza or flu, which is an influenza virus, the poliomyelitis or polio, a poliovirus, the common cold, and AIDS, caused by the human immunodeficiency virus HIV. The common cold and dengue are also routinely accepted answers, as is COVID-19, caused by SARS-CoV-2.
6. 6Two closing properties complete the account, and they are what separates a virus from a bacterium: viruses have a heritable material of only one type, they lack enzymes of their own for replicating it, and they are obligate parasites. A bacterium, by contrast, has both DNA and RNA, ribosomes of its own, and can live independently.

Step-by-step solution

1. 1The most defensible answer to write is that viruses show living characteristics when inside a host cell and non-living characteristics outside one, and that this is why they are described as obligate parasites or as a connecting link between the living and the non-living. The discussion is built by putting the two sides in two columns and then reaching that conclusion.
2. 2The features that support calling them living are: a virus has a definite structure of a protein coat around a nucleic acid; it contains a heredity material and has the ability to mutate; it can grow and multiply; it has the ability to synthesise proteins inside a host cell; and it carries out cellular divisions, so it can be said to reproduce.
3. 3The features that support calling them non-living are: viruses lack a cell structure, having no cell membrane, cytoplasm or organelles; they cannot grow or multiply on their own; they lack their own metabolic machinery, including ribosomes and the enzymes for replicating their nucleic acid; and they do not respire, they do not show independent metabolism, and they are inert outside a host cell.
4. 4The decisive question is where the virus is. Outside a host cell, a virion is inert, it performs no metabolism and it cannot reproduce, which is non-living behaviour. Inside a host cell, it directs the host's ribosomes and enzymes to make new viral particles, and new virions are produced, which is living behaviour.
5. 5A third point to raise in discussion is that a virus is not a degenerate bacterium that lost its machinery but something genuinely different, because it carries no ribosomes of its own and no machinery for energy production, so it occupies a category the five-kingdom system has no place for, which is why it is placed outside all five kingdoms.
6. 6The conclusion to end on is that the question has no yes or no answer. Life is defined by the cell and by independent metabolism, and since a virus has neither, it is classified as non-living, while the fact that it has a definite structure, a heredity material and the ability to mutate and to multiply inside a host makes it the connecting link between the living and the non-living. A good class discussion should reach that both positions are defensible and state the criterion each uses.