SolveItClass 11 · NCERT

NCERT Solutions · Class 11 Biology Biological Classification

12 questions · 12 still being checked

Exercises 2.1–2.12

  1. Exercise 2.1

    Discuss how classification systems have undergone several changes over a period of time?

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    Classification systems changed because the criteria used for classifying changed — from gross morphology alone to cell structure, nature of the cell wall, mode of nutrition, habitat, methods of reproduction and evolutionary relationships.
    Aristotle made the earliest scientific attempt: he used simple morphological characters to classify plants into trees, shrubs and herbs, and divided animals into those with red blood and those without.
    In Linnaeus' time a Two Kingdom system — Plantae and Animalia — included all plants and all animals.
    This system failed because it did not distinguish eukaryotes from prokaryotes, unicellular from multicellular organisms, or photosynthetic (green algae) from non-photosynthetic (fungi) forms; a large number of organisms fell into neither category.
    The single character unifying the old 'Plants' was the cell wall. That put prokaryotic bacteria and blue-green algae with eukaryotes, unicellular Chlamydomonas with multicellular Spirogyra, and heterotrophic fungi (chitin wall) with autotrophic green plants (cellulose wall).
    R.H. Whittaker ($\displaystyle 1969$) proposed the Five Kingdom Classification — Monera, Protista, Fungi, Plantae and Animalia — using cell structure, body organisation, mode of nutrition, reproduction and phylogenetic relationships.
    Under it, fungi were given a separate kingdom, all prokaryotes were grouped in Monera, and unicellular eukaryotes were placed in Protista.
    Protista therefore brought together Chlamydomonas and Chlorella (earlier algae, with walls) with Amoeba and Paramoecium (earlier animals, without walls) — organisms previously kept in different kingdoms.
    A three-domain system has since been proposed, which divides Monera into two domains and leaves the remaining eukaryotic kingdoms in the third, giving a six kingdom classification.
    Such changes will continue as our understanding of characteristics and evolutionary relationships improves; the aim is a system that is phylogenetic (based on evolutionary relationships), not merely morphological.

  2. Exercise 2.2

    State two economically important uses of:
    (a)
    heterotrophic bacteria
    (b)
    archaebacteria

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    (a) Heterotrophic bacteria
    Making curd from milk.
    Production of antibiotics.
    (Also) fixing nitrogen in the roots of legumes, and acting as important decomposers.
    (b) Archaebacteria
    Methanogens are present in the gut of ruminant animals such as cows and buffaloes, and they are responsible for the production of methane (biogas) from the dung of these animals.
    This is the only economically important use of archaebacteria that the chapter gives. Its other archaebacteria — halophiles of extremely salty areas and thermoacidophiles of hot springs — are described only for the harsh habitats they survive in, not for any use.

  3. Exercise 2.3

    What is the nature of cell-walls in diatoms?

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    The cell wall of a diatom forms two thin overlapping shells which fit together like a soap box.
    The walls are embedded with silica, and are therefore indestructible.
    Because they do not decay, diatoms have left behind large cell-wall deposits in their habitat; this accumulation over billions of years is called 'diatomaceous earth'.
    Being gritty, this soil is used in polishing and in the filtration of oils and syrups.

  4. Exercise 2.4

    Find out what do the terms ‘algal bloom’ and ‘red-tides’ signify.

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    Algal bloom
    The rapid, excessive growth of algae in a water body, forming a visible mass on the water.
    Cyanobacteria (blue-green algae) often form such blooms in polluted water bodies, so a bloom signifies water pollution.
    Red tide
    The reddening of sea water caused by red dinoflagellates — for example *Gonyaulax* — multiplying so rapidly that they make the sea appear red.
    Toxins released by such large numbers may kill other marine animals such as fishes.

  5. Exercise 2.5

    How are viroids different from viruses?

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    A viroid is a free RNA with no protein coat; a virus is a nucleoprotein — its genetic material is enclosed in a protein coat (capsid) made of capsomeres.
    A viroid is smaller than a virus, and its RNA is of low molecular weight.
    A virus may have either RNA or DNA (never both), single or double stranded; a viroid has only RNA.
    Viroids were discovered by T.O. Diener ($\displaystyle 1971$) as the cause of potato spindle tuber disease.

  6. Exercise 2.6

    Describe briefly the four major groups of Protozoa.

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    All protozoans are heterotrophs living as predators or parasites, and are believed to be primitive relatives of animals. The four groups are:(i) Amoeboid protozoans
    Live in fresh water, sea water or moist soil.
    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.
    (ii) Flagellated protozoans
    Members are either free-living or parasitic, and bear flagella.
    The parasitic forms cause diseases such as sleeping sickness, e.g. Trypanosoma.
    (iii) Ciliated protozoans
    Aquatic and actively moving because of thousands of cilia.
    Possess a cavity, the gullet, opening to the outside of the cell surface; coordinated movement of rows of cilia steers water laden with food into the gullet.
    Example: Paramoecium.
    (iv) Sporozoans
    Diverse organisms having an infectious spore-like stage in the life cycle.
    The most notorious is *Plasmodium* (malarial parasite), which causes malaria.

  7. Exercise 2.7

    Plants are autotrophic. Can you think of some plants that are partially heterotrophic?

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    Yes — insectivorous plants and parasitic plants are partially heterotrophic.
    Bladderwort and Venus fly trap are insectivorous: they contain chlorophyll and photosynthesise, but also trap and digest insects for nutrition.
    *Cuscuta* is a parasite, drawing its nutrition from the host plant.
    They are still placed in Kingdom Plantae because they are eukaryotic and chlorophyll-containing; only a few members are partially heterotrophic.

  8. Exercise 2.8

    What do the terms phycobiont and mycobiont signify?

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    They are the two partners of a lichen — a symbiotic, i.e. mutually useful, association between an alga and a fungus.
    Phycobiont = the algal component, which is autotrophic and prepares food for the fungus.
    Mycobiont = the fungal component, which is heterotrophic and provides shelter and absorbs mineral nutrients and water for its partner.
    So close is the association that a lichen seen in nature never looks like two different organisms.
    Lichens are very good pollution indicators — they do not grow in polluted areas.

  9. Exercise 2.9

    Give a comparative account of the classes of Kingdom Fungi under the following:
    (i)
    mode of nutrition
    (ii)
    mode of reproduction

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    (i) Mode of nutrition
    Phycomycetes — found in aquatic habitats and on decaying wood in moist and damp places (saprophytic), or as obligate parasites on plants, e.g. Albugo on mustard.
    Ascomycetes — saprophytic, decomposers, parasitic, or coprophilous (growing on dung).
    Basidiomycetes — grow in soil, on logs and tree stumps (saprophytic) and in living plant bodies as parasites, e.g. rusts and smuts.
    Deuteromycetes — saprophytes or parasites; a large number are decomposers of litter and help in mineral cycling.
    (ii) Mode of reproduction
    Phycomycetes — asexual by zoospores (motile) or aplanospores (non-motile), produced endogenously in a sporangium; sexual reproduction by fusion of two gametes, which may be isogamous, anisogamous or oogamous, forming a zygospore. Mycelium is aseptate and coenocytic.
    Ascomycetes — asexual by conidia, produced exogenously on special mycelium called conidiophores; sexual by ascospores produced endogenously in sac-like asci, which are arranged in fruiting bodies called ascocarps. Mycelium branched and septate.
    Basidiomycetes — asexual spores are generally not found; vegetative reproduction by fragmentation is common. Sex organs are absent; plasmogamy occurs by fusion of two vegetative or somatic cells of different strains, giving a dikaryotic structure that forms a basidium. Karyogamy and meiosis in the basidium produce four basidiospores exogenously, and the basidia are arranged in basidiocarps.
    Deuteromycetes — reproduce only by asexual spores called conidia; the sexual (perfect) stage is unknown, which is why they are called imperfect fungi. Mycelium is septate and branched. Once the sexual stage is discovered they are moved to Ascomycetes or Basidiomycetes.

  10. Exercise 2.10

    What are the characteristic features of Euglenoids?

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    Majority are fresh water organisms found in stagnant water.
    Instead of a cell wall they have a protein-rich layer called the pellicle, which makes their body flexible.
    They bear two flagella, a short one and a long one.
    They are photosynthetic in the presence of sunlight, but when deprived of sunlight they behave like heterotrophs, predating on other smaller organisms.
    Their pigments are identical to those present in higher plants.
    Being protists, they are single-celled eukaryotes with a well-defined nucleus and membrane-bound organelles.
    Example: *Euglena*.

  11. Exercise 2.11

    Give a brief account of viruses with respect to their structure and nature of genetic material. Also name four common viral diseases.

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    Structure
    Viruses are non-cellular organisms, characterised by an inert crystalline structure outside the living cell; W.M. Stanley ($\displaystyle 1935$) showed they could be crystallised and that the crystals consist largely of proteins.
    They are obligate parasites: once they infect a cell they take over the machinery of the host cell to replicate themselves, killing the host.
    A virus is a nucleoprotein — genetic material surrounded by a protein coat called the capsid, made of small subunits called capsomeres, which protect the nucleic acid.
    The capsomeres are arranged in helical or polyhedral geometric forms.
    A bacteriophage additionally shows a head, collar, sheath and tail fibres.
    They are smaller than bacteria — they pass through bacteria-proof filters (Dmitri Ivanowsky, $\displaystyle 1892$, on tobacco mosaic disease); M.W. Beijerinck ($\displaystyle 1898$) named the infectious fluid Contagium vivum fluidum.
    Nature of genetic material
    Either RNA or DNA — no virus contains both; the genetic material is infectious.
    Viruses that infect plants have single stranded RNA.
    Viruses that infect animals have either single or double stranded RNA, or double stranded DNA.
    Bacteriophages (bacterial viruses) are usually double stranded DNA viruses.
    Four common viral diseases
    Mumps
    Small pox
    Herpes
    Influenza
    (AIDS in humans is also caused by a virus; in plants the symptoms are mosaic formation, leaf rolling and curling, yellowing and vein clearing, dwarfing and stunted growth.)
    NCERT_Solution_Class11_Biology_Ch2_Q2-11

  12. Exercise 2.12

    Organise a discussion in your class on the topic - Are viruses living or non-living?

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    This is a class discussion, so there is no single answer — these are the points the chapter gives for each side.Arguments that viruses are LIVING
    They possess genetic material — DNA or RNA — and that genetic material is infectious.
    They replicate and multiply, producing more of their own kind, once inside a host cell.
    They are obligate parasites, like many undoubtedly living parasites.
    They show host specificity — they are inert outside their specific host cell — and cause disease in that host: mumps, small pox, herpes, influenza and AIDS.
    Arguments that viruses are NON-LIVING
    They are non-cellular — they have no cell structure, and 'living' is usually understood as having a cell structure. This is why Whittaker's five kingdom classification gives them no place.
    Outside the living cell they exist as an inert crystalline structure; W.M. Stanley ($\displaystyle 1935$) crystallised them like a chemical, and the crystals are largely protein.
    They have no machinery of their own — they must take over the host cell's machinery to replicate, and they kill the host in doing so.
    They show no metabolism, growth or response of their own outside a host.
    A fair conclusion for the discussion
    Viruses lie on the borderline between the living and the non-living — behaving as living organisms only inside a host cell, and as inert chemicals outside it. The chapter itself leaves this open, asking the reader: would you call viruses living or non-living?