NCERT Solutions for Class 11 Biology Chapter 2: Biological Classification

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

    Ans. Biological classification systems have undergone several changes over time. The sequential discussion is listed below:

    (a) Aristotle first of all classified plants as herbs, shrubs, and trees. Animals, on the other hand, were classified on the basis of presence or absence of red blood cells. This system of classification failed to classify all the known organisms.

    (b) Linnaeus gave a two-kingdom system of classification. It consists of kingdom Plantae and kingdom Animalia. However, this system did not differentiate between
    unicellular and multicellular organisms and between eukaryotes and prokaryotes. Therefore, there were large numbers of organisms that could not be classified under
    the two kingdoms.

    (c) After that, Ernst Haeckel, then, separated unicellular eukaryotic organisms into separate kingdom named Protista and hence, gave three kingdom classification.

    (d) Copeland, then separated all prokaryotic organisms into a separate kingdom named Monera and hence, Four-Kingdom systems of classification came into existence.

    (e) Finally, R.H. Whittaker proposed a five kingdom system of classification in 1969. He separated achlorophyllous, decomposer organisms into a separate kingdom named Fungi. According to this system, there are following five kingdoms:

    1. Monera

    2. Protista

    3. Fungi

    4. Plantae

    5. Animalia

    2. State two economically important uses of:

    (a) heterotrophic bacteria

    (b) archaebacteria

    Ans. (a) Curd and antibiotics are made by using heterotrophic bacteria named Lactobacillus and Actinomycetes respectively.

    (b) Methanogens are archeabacteria, responsible for production of biogas which can be used as fuel. The archaebacteria which live in extreme conditions also give us a clue about the beginning of life on earth.

    3. What is the nature of cell-walls in diatoms?

    Ans. The nature of cell walls in diatoms is like thin overlapping shells fitted into each other just like a soap box. When the diatoms die, the silica in their cell walls gets deposited in to form of diatomaceous earth. This diatomaceous earth is very soft and quite inert. It is used in filtration of oils, sugars, and for other industrial purposes.
    The process of cell wall construction of diatoms is known as Frustule.

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

    Ans. Algal bloom: Algal bloom refers to an increase in the population of algae or blue-green algae in water, resulting in discoloration of the water body. This causes an increase in the biological oxygen demand (BOD), resulting in the death of fish and other aquatic animals.

    Red-tides: Red tides are caused by red dinoflagellates (Gonyaulax) that multiply rapidly. Due to their large numbers, the sea appears red in color. They release large amounts of toxins into the water that can cause the deaths of a large number of fish.

    5. How are viroids different from viruses?

    Ans. Viroids are different from viruses in the following ways:

    1. Viroids are free RNA molecules of low molecular weight without any protein coat called a capsid, i.e., the capsid is absent, while viruses can have either RNA or DNA
      molecules encapsulated in a protein coat called a capsid, i.e., the capsid is present.
    2. Viroids are smaller in size than viruses.
    3. Viroids infect only plants, whereas viruses infect all types of organisms.

    6. Describe briefly the four major groups of Protozoa.

    Ans. Kingdom Protozoa consists of following four major groups:

    (a) Amoeboid protozoans: The amoeboid protozoans live in freshwater, sea water or in moist soil. They produce pseudopodia for locomotion and for capturing food. The marine forms have silica shells on their surface. Some of them are parasites, e.g., Entamoeba sps.

    (b) Flagellated protozoans: They are either free-living or parasitic. Flagella are present for locomotion. Many of them are parasites, e.g., Trypanosoma sps.

    (c) Ciliated protozoans: They are aquatic. Cilia are present for locomotion. A cavity (gullet) is present which opens to the outside of the cell surface. The coordinated movement of cilia facilitates the entry of food-laden water into the gullet. Example: Paramoecium sps.

    (d) Sporozoans: The sporozoans have an infectious spore-like stage in their life cycle. Example: Plasmodium sps.

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

    Ans. Plant are autotrophic, as they prepare their own food by using photosynthetic reactions. But pitcher plant, venus fly trap and bladderwort are some examples of partially heterotrophic plants. Because these plants can carry out photosynthesis but they eat insects to fulfil their nitrogen requirements, which is an essential component of proteins.

    8. What do the terms phycobiont and mycobiont signify?

    Ans. Lichens are a symbiotic association of fungi and algae. Algal component of lichen is called phycobiont. It prepares food for fungus, while, fungal partner is called mycobiont. It provides shelter and absorbs mineral nutrients for algae.

    9. Give a comparative account of the classes of Kingdom Fungi under the following:

    (a) Mode of nutrition
    (b) Mode of reproduction

    Ans. A comparative account of the classes of Kingdom Fungi is as follows:

    Phycomycetes Ascomycetes Basidiomycetes Deuteromycetes
    (a) Mode of nutrition These are obligate parasites or saprophytes i.e., grow on dead and decaying matter. These are saprophytic or decomposers or parasitic or coprophilous (which grows on dung) These have saprotrophic nutrition. They are decomposers also. Deuteromycetes can be saprophytic or decomposers or parasitic.
    (b) Mode of reproduction
    1. Asexual reproduction takes place through the formation of zoospores (motile) or aplanospores (non-motile).
    2. Sexual reproduction takes place via gametangial copulation.

    3. e.g., : Rhizopus, Albugo, etc.
    1. Asexual reproduction takes place through conidia formation.
    2. Sexual reproduction takes place by ascospore formation.

    3. e.g., : Penicillium, Aspergillus, etc.
    1. Asexual reproduction takes place through fragmentation but asexual spores are absent.
    2. Sexual reproduction occurs via basidiospore formation.

    3. e.g., : Ustilago, Agaricus, etc.
    Mode of reproduction is always asexual through the formation of conidia. Sexual reproduction has not been identified in this group.
    e.g., : Alternaria, Trichoderma, etc.

    10. What are the characteristic features of Euglenoids?

    Ans. Euglenoids (such as Euglena) are unicellular microorganisms, which are grouped into the kingdom Protista and the algal phylum. Some characteristic features of Euglenoids are as follows:

    (a)  They are unicellular protists commonly found in fresh water.

    (b) They bear two flagella on the anterior end of the body.

    (c) A small light sensitive eye spot is present.

    (d) Instead of the cell wall, a protein-rich cell membrane known as pellicle is present.

    (e) They contain photosynthetic pigments such as chlorophyll and can thus prepare their own food. However, in the absence of light, they behave similarly to heterotrophs by capturing other small aquatic organisms.

    (f ) They have both plant and animal-like features, which makes them difficult to classify and hence they are called as connecting link between plants and animals.

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

    Ans. Viruses are sub-microscopic, nucleoprotein particles that can infect all living organisms. A virus consists of genetic material either in the form of RNA or DNA surrounded by a protein coat. The protein coat on the virus is called capsid. It is made up of small subunits called capsomeres. The capsid protects the nucleic acid. The capsomeres are arranged in helical or polyhedral geometric forms. Both RNA and DNA cannot be present in the same virus. Plant infecting viruses usually have single-stranded RNA and animal infecting viruses usually have double-stranded RNA or double-stranded DNA. Bacteriophages (bacterial viruses) usually have double-stranded DNA. Some common diseases caused by viruses are common cold, polio, hepatitis and influenza.

    12. Organise a discussion in your class on the topic – Are viruses living or nonliving?

    Ans. Viruses are microscopic organisms that have characteristics of both living and non-living.

    Living characteristics: Viruses replicate inside the host cell. They are capable of taking over machinery of host cell. They have either DNA or RNA, genetic material which is found in cell of living organism as genetic material.

    Non-living characteristics: Viruses are inactive when outside the cell. They do not have cell structure. Viruses can be crystallised.

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