Solved examples for class 8 science : Chapter 2

    Microorganisms: Friend and Foe

    I. Multiple Choice Questions (MCQs).
    Q.1. The plant like organisms having a cell wall but lacking chlorophyll, are

    a) viruses
    b) bacteria
    c) fungi
    d) algae
    Answer: c) fungi

    Q.2. Which of the following is an antibiotic?
    a) Tetracycline
    b) Rhizobium
    c) Yeast
    d) Lactobacillus
    Answer. a) Tetracycline
    Q.3. A carrier of disease dengue are
    a) Anopheles mosquitoes
    b) Aedes mosquitoes
    c) Flies
    d) Fine droplets
    Answer. (b) Aedes mosquitoes

    Q.4. The microorganism which causes food poisoning is
    a) Penicillium
    b) Rhizobium
    c) Yeast
    d) Staphylococci
    Answer. d) Staphylococci

    Q.5. A bacterium used to convert free atmospheric nitrogen into usable forms for the plants, is
    a) Nostoc
    b) Anabena
    c) Rhizobium
    d) None of these

    Answer. c) Rhizobium

    II. Fill in the blanks by selecting appropriate words from the given list :

    List: Oscillatoria, Antibiotics, proteins, binary, carrier

    Q.1. All organisms including plants and animals are good …………………………….of the microorganisms.
    Q.2. ……………………. algae produces toxins in water, making it unfit for drinking.

    Q.3. Bacteria reproduce by the process of …………………..fission.

    Q.4. …………………………are chemicals which help or stop the growth of disease causing microorganisms.

    Q.5. Nitrates are absorbed by plants and converted into…………………….. .

    Answer
    1- carrier,
    2- Oscillatoria,
    3- binary,
    4- Antibiotics,
    5- proteins

    III. Match the items of Column A with those of Column B.


    Answer. 1-e, 2-a, 3-d, 4-b, 5-c,

    IV. Write ‘T’ for the true and ‘F’ for the false statements.

    Q.1. There are many bacteria which are very beneficial to us in our day to day life.
    Q.2. Moulds, yeast and mushroom are some of the common protozoa.
    Q.3. Fungi reproduce either by budding or by spore formation.
    Q.4. Algae cannot synthesise their own food, so they are heterotrophic.
    Q.5. Food material has a tendency of getting spoiled by microorganisms.

    Answer. 1-T, 2-F, 3-,T 4-F, 5-T

    V. Very Short Answer Questions.

    Q.1. Who observed microorganisms for the first time?
    Answer: Microorganisms were observed for the first time by Leeuwenhoek.
    Q.2. Name three viral diseases.
    Answer: Common cold, Measles and Hepatitis
    Q.3. Name the bacteria which reside in the soil and also have the capability to fix nitrogen.
    Answer: Rhizobium, Azotobacter and Clostridium
    Q.4. Name the fungi which reproduce by budding and spore formation respectively.
    Answer: Yeast reproduces by budding whereas moulds and mushrooms reproduce by spore formation.
    Q.5. Name the algae which are an important source of iodine, sodium and potassium.
    Answer: Brown algae such as kelps are an important source of iodine, sodium and potassium.

    VI. Short Answer Questions.

    Q.1. What do you understand by antibiotics?
    Answer: Antibiotics are chemicals which help or stop the growth of disease causing microorganisms. Some of the antibiotics produced from bacteria are streptomycin, aureomycin, tetracycline, chloromycetin, etc.

    Q.2. How is fungus largely employed in the bakeries and breweries?

    Answer: Yeast is the fungus which is largely employed in the bakeries and breweries.
    Yeast undergoes anaerobic respiration known as fermentation. During the process of fermentation, yeast converts glucose into alcohol and CO2. The CO2 released causes the dough to rise in case of bread and idlis making them fluffy.

    Q.3. What do you mean by communicable diseases?

    Answer: The diseases which can be transferred from an infected person to a healthy person through air, water, food or physical contact are known as communicable diseases. For example, viral disease such as common cold is easily transferred by sneezing.

    Q.4. What are viruses? Discuss some harmful effects of viruses.
    Answer: Viruses are microscopic organisms even smaller than bacteria. They can be both living and non-living.

    Harmful Effects of Viruses
    Diseases in human: Viruses cause a number of diseases in human. For example, common cold, influenza, polio, measles, AIDS, etc.

    Diseases in plants: Several plant diseases such as tobacco mosaic of tobacco, maize dwarf mosaic, etc. are caused by viruses.

    Diseases in animals: Foot and mouth disease of cattle, pigs and hoofed animals, canine hepatitis in dogs and cats are all viral diseases.

    Q.5. What do you mean by algae? Discuss some beneficial algae and write some harmful effects of algae.
    Answer: Algae are primitive simple plants containing chlorophyll which help them to synthesize their own food. They are popularly known as sea weeds.

    Beneficial Algae
    • As fertilizer : Blue-green algae such as Nostoc and Anabena have the ability to fix atmospheric nitrogen therefore they are used as fertilizers.
    • Food for aquatic organisms : Algae serve as food for fish and other aquatic animals.
    • Food for humans : Various types of algae are Largely being used as food. Spirulina and Chlorella are rich sources of proteins. Various brown algae and sea weeds such as Porphyra Chordrus are used as food in various countries.
    • Vitamins and minerals : Algae are a rich source of vitamins and minerals. Brown algae such as kelps are an important source of iodine, sodium and potassium.

    Harmful Effects of Algae
    • Algae reproduce very rapidly and form a mat like structure on water bodies such as Lakes. This algal mat creates oxygen deficiency in water and Limits the sunlight making the conditions toxic for other aquatic organisms.
    • Algae such as Oscillatoria produces toxins in water, making the water unfit for drinking.

    Q.6. What do you mean by protozoa? Discuss some beneficial and harmful protozoa. Ans: Protozoa are unicellular organisms having animal like characteristics.

    Beneficial Protozoa
    1. Protozoans form an important Link in the aquatic food chain. They feed on algae and in turn are eaten up by Larger aquatic organisms.
    2. Protozoans Living in the gut of ruminants help in the digestion of cellulose.

    Harmful Protozoa
    1. Diseases in humans : Various diseases in humans are caused by protozoa, for example amoebic dysentery by Entameoba histolytica, sleeping sickness, malaria, leshmaniasis, vaginitis, etc. 2. Diseases in animals : Trichomoniasis in bird Liver, coccidiosis in rabbit and kidney coccidiosis in geese.

    VII. Long Answer Questions.

    Q.1. What do you understand by bacteria? Discuss some beneficial bacteria and also write some harmful effects of bacteria.
    Answer: Habitat : Bacteria are unicellular organisms and are found everywhere around us. They are present in soil, water, air, animals, plants and even in humans.
    Size : Size of bacteria varies from 0.15 µm-1.5 µm.
    Shape : They are found in variety of shapes.
    • Spherical [Cocci]
    • Rod shaped [Bacillus]
    • Spiral shaped [Spirillum]
    • Comma shaped [Vibrio]

    Beneficial Bacteria
    • Making of curd and cheese : Curd contains Lactobacillus which promotes the formation of curd. It multiplies in milk and converts it into curd. Similarly, bacteria also helps in cheese production.

    • Decomposition : Dead and decaying matter is decomposed by bacteria into simple organic substances which can be used by plants and animals.

    • Nitrification : Proteins from dead matter is first converted into ammonia by ammonifying bacteria. Ammonia isthen converted into the atmospheric nitrates useful to plants and animals by nitrification. Nitrosomonas is a good example of nitrifying bacteria.

    • Nitrogen fixation : Rhizobium living in symbiotic association in root nodules of leguminous plants has the ability to fix nitrogen into nitrates. Nitrogen fixation help to increase the soil fertility.

    • Production of antibiotics : Some of the antibiotics produced from bacteria are streptomycin, aureomycin, tetracycline, chloromycetin, etc.

    Harmful Effects of Bacteria
    • Food poisoning : Bacteria such as Staphylococcus and Clostridium botulinum produce toxins in food. Bacteria bring about souring of milk, rotting of meat, fruits and other eatables.
    • Diseases in human : Several diseases such as tuberculosis, cholera, typhoid, dysentery are caused by bacteria.
    • Diseases in plants : Fire blight of apple, pear, crown gall disease and bacterial ringspot of potato are some of the bacterial diseases in plants.
    • Diseases in animals : Bovine disease in cattle, salmonelliosis in chicken and horse are some of the diseases caused in animals by bacteria.

    Q.2. What do you mean by fungi? Discuss some beneficial fungi and also write some harmful effects of fungi.

    Answer: They are plant like organisms having a cell wall but lacking chlorophyll. They cannot synthesize their own food.

    Beneficial Fungi
    • Bakeries and breweries : Yeast is the fungus which is largely employed in the bakeries and breweries.
    • Preparation of cheese : Various species of Penicillium are used for preparing different types of cheese.
    • Organic acids : Various organic acids such as oxalic acid, citric acid, gluconic acid, Lactic acid, etc. are obtained from fungi.
    • Vitamins : Yeast is a good source of Vitamin B complex.
    • Food : Several mushrooms such as Agaricus, Morchella can be eaten as food preparations in several ways. ldli, dosa, dhokla, etc. are fermented food which are prepared using yeasts.

    Harmful Effects of Fungi
    • Spoilage of food : Fungi readily spoil food in moderate temperature and humidity.
    • Spoilage of leather : Leather products are easily spoilt by fungi in high humidity.
    • Diseases in human : A number of diseases such as, ringworm disease, athletes foot, scabies, etc. are caused by fungi in humans.
    • Diseases in plants : Downy mildew of grapes, black rust of wheat, etc. are some of the diseases caused by fungi in plants.
    • Diseases in animals : Fungi are known to cause diseases such as blastomycosis, coccidiomycosis, cryptococcosis in animals.

    Q.3. (a) What do you mean by food preservation? Describe some methods adopted for food preservation.
    (b) Write a short note on nitrogen cycle.

    Answer a) Food material has a tendency of getting spoiled by microorganisms. Therefore, we need to preserve food in order to keep it fresh for a longer time.
    • The simplest method used for preserving cooked food at home is refrigeration.
    • Common salt is used to preserve meat, fish, amla, raw mangoes, tamarind, etc.
    • Jams, jellies and squashes are preserved by sugar. Sugar reduces the moisture content and inhibit the growth of bacteria which spoil food.
    • Oil and vinegar make the environment inhabitable for bacteria. Vegetables, fruit, fish and meat are preserved by this method.
    • Boiling : Boiling food items kills the microbes present in them. Example-milk and water.
    • Pasteurization : It is a method of preserving milk. In this process, the milk is heated to about 70 °C for 15-30 seconds and then suddenly chilled and stored.
    • Chemical preservatives : Chemicals which check microbial growth are known as chemical preservatives. Commonly used for pickles, squashes, ketchups, etc.

    b) The amount of nitrogen remains constant in the atmosphere and this is done by nitrogen cycle.

    Following are the steps of nitrogen cycle :
    1. Nitrogen fixation: Free nitrogen in the atmosphere cannot be utilised by plants and animals as such. It has to be first converted into usable forms. This process of conversion of free atmospheric nitrogen into useful nitrogen compounds is called nitrogen fixation.
    (i) Nitrogen fixation by Rhizobium- Rhizobium, a bacterium lives in symbiotic association in the root nodules of leguminous plants. It gains nutrition from the plants and in turn converts free atmospheric nitrogen into usable forms for the plants.
    (ii) Nitrogen fixation by blue-green algae : Blue-green algae Nostoc and Anabena have the capability to fix free nitrogen into usable forms for the plants.
    (iii) Nitrogen fixation by lightning: During the Lightning, the high temperature is produced. At this high temperature, the nitrogen gas of the atmosphere reacts with the oxygen to form nitrogen oxide. Nitrogen oxide comes to the soil along with the rain water in the form of dilute nitric acid which reacts with the alkalis present in the soil to form nitrates and plant uses these nitrates.
    2. Absorption of nitrates by plants: Nitrates are absorbed by plants and converted into proteins.
    3. Plant proteins get converted into animal proteins when they are eaten up by animals.
    4. Ammonification : Proteins from dead and decaying plants and animals matter are converted into ammonia by ammonifying bacteria.
    5. Nitrification: Ammonia so formed is converted into nitrites and nitrates by nitrifying bacteria.
    6. Nitrates are absorbed by plants.
    7. Denitrification: The nitrates are converted into free nitrogen by denitrifying bacteria.

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