Lakhmir Singh Manjit Kaur Class 8 Science 2nd Chapter Micro Organisms : Friend And Foe Solutions

Lakhmir Singh and Manjit Kaur Science solution: Micro Organisms : Friend And Foe Chapter 2. Here you get easy solutions of Lakhmir Singh and Manjit Kaur Science solution Chapter 2. Here we give Chapter 2 all solution of class 8. Its help you to complete your homework.

Board – CBSE

Text Book – SCIENCE

Class – 8

Chapter – 2


(1) Microscope.

(2) Viruses

(3) Bacteria, viruses, Protozoa, and some other Fungi and Algae.

(4) Cholera and Typhoid.

(5) Influenza and chickenpox.

(6) Dysentery and Malaria.

(7) Ringworm and athlete’s foot.

(8) Lactobacilli.

(9) Yeast.

(10) Penicillin and streptomycin.

(11) Penicillium.

(12) Cholera, Polio, Smallpox and Typhoid.

(13) Edward Jenner.

(14) Alexander Fleming.

(15) Nitrogen Fixation.

(16) Rhizobium.

(17) Mosquito, Housefly.

(18) Typhoid, Cholera.

(19) Mosquito.

(20) Mosquito.

(21) Mosquito

(22) Malaria, Dengue

(23) Plasmodium parasite

(24) Common cold

(25) Malaria

(26) Cholera

(27) (a) Virus (b) Bacteria

(28) Spinach and Cauliflower

(29) Fish, Meat       \

(30) Jams and Jellies

(31) Fish and Meat

(32) Sugar.

(33) Vinegar.

(34) Aamla, Raw mango

(35) Sodium meta bisulphate and sodium sorbate.

(36) Rhizobium.

(37) Leguminous.

(38) Rhizobium

(39) Beans and Peas.

(40) Fill in the blanks

(a)  Alcohol is produced with the help of :- YEAST

(b) Blue-green algae fix NITROGEN directly from the air to enhance fertility of soil.

(c) Micro-organisms can be seen with help of a MICROSCOPE

(d)  Cholera is caused by BACTERIA

(e) Common salt has been used to preserve FISH and MEAT for ages.

(f) The food material which is preserved by pasteurization is MILK

(g) As a result of nitrogen cycle, the percentage of nitrogen in the atmosphere remains more or less CONSTANT


(41) Ans: Viruses are much smaller than bacteria and also viruses don’t show most of the characteristics of living things so that’s why viruses differ from other micro-organisms.

(42) Ans: Those organisms which are too small to be seen without a microscope are called micro-organisms.

(43) Ans: No, micro-organisms cannot be seen with the naked eye. They can be seen by Microscope.

(44) Ans:  (a) When Flies sit on feces, garbage, and other waste where they pick up bacterial, helminthes, and protozoan infections to the hair in the legs or another body parts of housefly. And when this housefly now sits on our uncovered food, our food contaminated with disease-causing micro-organisms.

(b) The spreading of diseases by houseflies can be prevented in the following ways:

i> The food should always be kept covered so that flies cannot sit on it

ii> We should avoid eating uncovered food items from the road-side stalls.

(45) Ans: (a) When a female Anopheles mosquito bites a person suffering from malaria disease, it sucks the blood of that person which contains malarial microbes then when this mosquito bites a healthy person to suck blood, it transfer the malarial microbes into his blood stream along with saliva. By receiving this blood, the healthy person also gets malaria diseases.

(b) We can prevent the spreading of diseases like malaria and dengue caused by mosquitoes in the following ways:

i> The windows and doors of the house should have fine iron wire mesh so that mosquitoes cannot enter the house.

ii> Insecticides should be sprayed in house periodically to kill mosquitoes.

iii> Oil should be sprayed on the surface of water in dirty water drains to kill the larvae of mosquitoes.

(46) Ans. (a) The process of conversation of sugar into alcohol by the action of yeast is called fermentation. Fermentation was discovered by Louis Pasteur in 1857.

(b) Yeast converts sugar into alcohol during fermentation.

(47) Ans: (a) Micro-organisms presents in the soil can fixnitrogen gas from the atmosphere to from nitrogen compounds. These nitrogen compounds mix with the soil and increase the soil fertility.

(b) Micro-organisms decompose the harmful and smelly dead remains of plants and animals and animal wastes (like faeces, dung, urine, etc.) into harmless materials.In this process they clean the environment.

(48) Ans: Antibiotics are used to treat many diseases in humans.

Some of the precautions to be observed in the use antibiotics are as follows:

1> Antibiotics  should be taken only on the advice of a qualified doctor.

2>  A person must complete the ‘full course’ of antibiotics prescribed by the doctor.

3> Antibiotics should not be taken unnecessarily. Antibiotics taken unnecessarily may kill the useful bacteria in the body and harm us.

(49) Ans: Antibiotics are not effective against ‘common cold’ and ‘flu’ because they are caused by viruses.

(50) Ans: The full form of HIV is ‘Human Immunodeficiency Virus’.

The disease caused by HIV is AIDS (Acquired Immune Deficiency Syndrome)

(51) Ans: Milk is turned into curd by bacteria. Curd is made from milk in following steps:

1> Milk contains a sugar called lactose. Lactobacilli bacteria convert the lactose sugar into lactic acid.

2> This lactic acid then converts milk into curd.

3> This lactic acid then converts into curd.

(52) Ans:  Yeast is used to makes the dough rise.

When yeast is mixed in dough for making bread, the yeast is mixed in dough for making bread, the yeast reproduces rapidly and gives out carbon dioxide gas during respiration. The bubbles of carbon dioxide gas fill the dough and increase its volume. The holes in the bread are due to the bubbles of carbon dioxide the baking process. This process makes the bread lighty, soft and spongy.

(53) Ans: The disease caused due to the presence of a large number of micro-organisms in the food, or due to the presence of toxic substances in food formed by the action of micro-organisms is called micro-organisms.

How is food poisoning caused:-

Food poisoning caused serious illness and even death. The micro-organisms which cause food poisoning come in to food from the air, dirty hands, unclean food containers, houseflies, cockroaches, insects etc.

(54) Ans. (a) The process in which the food materials are given a suitable physical or chemical treatment to prevent their spoilage is called food preservation.

Five method of preserving food are:

1> Sun-drying

2> Heating.

3> Cooling.

4> Addition of common salt.

5> Addition of sugar

(b) We can preserved cook food by Deep Freezing.

(55) (a) Why should we not let water collect anywhere in the neighborhood ?

Ans: Update Shortly.

(b) 1> by virus: Foot and Mouth disease

2> by bacteria: Anthrax
3> by Fungus:  Aspergillosis.

(56) Ans: Rhizobium bacteria lives in the root nodules of leguminous plants.


Rhizobium bacteria mainly lives in the root of leguminous plants, they increase the fertility of soil.

(57) Ans: (a) Lactobacillus bacteria and Rhizobium bacteria.

(b) Common Cold and  Human Immunodeficiency Virus.

(c) Amoeba, Paramecium

(d) Chlamydomonas, Spirogyra

(e) Penicillium, Aspergillus

(58) Ans:  The beneficial effects of micro-organisms in our lives are as follows:

  • Micro-organisms are utilized in the making curd, bread and cake.
  • Micro-organisms are used in the production of alcohol, wine amd acetic acid (vinegar)
  • Micro-organisms are used in the preparation of medicines (or drugs) called antibiotics.
  • Micro-organisms are used in agriculture to increase the fertility of soil by fixing atmospheric nitrogen gas.

(59) Ans: The method of pasteurization for the preservation of milk as follows:

First the milk is heated to about 70c for 15 to 30 seconds to kill most of the bacteria present in it. next, this hot milk is cooled very quickly to a low temperature to prevent any remaining bacteria from growing further. And then this milk is stored in cold. Pasteurized milk can be consumed without boiling because it is free from micro-organisms.

(60) Ans: 1> ‘Rust of wheat’

2> ‘Yellow vein mosaic of bhindi (or okra)’

3> ‘Citrus canker’

(61) Ans: 1> Female Anopheles mosquito spread the parasite of malaria.

2>  Female Aedes mosquito spread the virus of dengue.

(62) Ans. 1> Raw mangoes, lemons are two fruits which can be preserved in the form of pickles.

2> Orange , Strawberry are two fruits which can be preserved in the form of jams.

(63) Ans: 1> fungi , 2> Bacteria , 3> Virus

(64) Ans: Two animal diseases caused by micro-organisms are:  Foot and mouth diseases and Anthrax.

Two plant  diseases caused by micro-organisms are:  Rust of wheat and Citrus canker.

(65) Ans:


(66) Ans: Communicable diseases means those microbial diseases which can spread from an infected person to a healthy person through air, water, or physical contact etc.  In communicable diseases, the disease-causing germs get transmitted from a human being.  For example ‘common cold’ spreads by breathing air containing micro-organisms.

Name of the two communicable diseases are: 1> Common cold and 2> Cholera

(b) The communicable diseases can occur and spread in following ways:

  • by breathing of air containing micro-organisms,
  • by taking infected food or water
  • through insect bites (such as mosquito bites)
  • by sharing infected injections needles, and
  • by physical contact with an infected person (or by using articles of an person such as towel, clothes, bed, utensils, etc.

(67) Ans: (a)

1> Tuberculosis:

  • Causative micro-organisms: Bacteria
  • Mode of Transmission: Air

2> Measles

  • Causative micro-organisms: Virus
  • Mode of Transmission: Air


  • Causative micro-organisms: Virus
  • Mode of Transmission: Air/ Contact

4> Cholera:

  • Causative micro-organisms: Bacteria
  • Mode of Transmission: Water/Food


  • Causative micro-organisms: Bacteria
  • Mode of Transmission: Water

(b)  Some of the methods for preventing the occurrence and spreading of communicable diseases are given below:

  • A person suffering from cold should always cover his mouth and nose with a handkerchief while sneezing, so that micro-organisms do not get into air.
  • we should keep our food covered to protect it from infected by flies.
  • We should make sure that only disposable syringes and needles are used for giving us injections.
  • Some of a diseases can be prevented by vaccination at a proper time.

(68) Ans: (a) A vaccine is a special kind of preparation which provides immunity against a particular diseases.

When the vaccine containing dead or alive micro-organisms is entered into the body of a healthy person orally or by injection, the body of that person responds by producing some substance called ‘antibodies’ in its blood. Some of the antibodies remain in the blood of the person for a very long time and fight against the same micro-organisms and kill them. Thus, a Vaccine develops the immunity from a diseases.

(b) For good health and provides a strong immunity, all children should be vaccinated at proper age.

(69) Ans: The process of converting nitrogen gas of atmosphere (or air) into compounds of nitrogen (which can be used by the plants) is called nitrogen fixation.

Two ways that nitrogen gas of the atmosphere can be “fixed” in nature to get nitrogen compounds in the soil are:

  • Rhizobium bacteria have symbiotic relationship with leguminous plants. Thus some nitrogen-fixing bacteria live freely in the soil and other bacteria live in the root nodules of leguminous plants.
  • When lighting takes place in the sky during thunderstorm, a high temperature is produced in the atmosphere. At this high temperature, nitrogen gas of air combines with oxygen gas of air to form nitrogen compounds.


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  1. Thanks for the solution

    1. You welcome Aniket

  2. Thanks very much


  4. Divyanshu kumar.

    this is very useful to the students,because in this,solutions are very easy to understand.I like this solution very very very much. Thanks.

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