2. Microorganisms : Friend And Foe Science class 8 in English Medium ncert book solutions Text Book Exercise

Text Book Exercise 

Q1. Fill in the blanks:

(a) Microorganisms can be seen with the help of a ____________.

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

(c) Alcohol is produced with the help of __________.

(d) Cholera is caused by __________.

Ans :     

(i) Microscope

(ii) Nitrogen

(iii) Yeast

(iv) bacterial

Q2. Tick the correct answer:

(a) Yeast is used in the production of

(i)  sugar    

(ii) alcohol               

(iii) hydrochloric acid                          

(iv) oxygen

Ans: (ii) alcohol               

(b) The following is an antibiotic

(i)  Sodium bicarbonate

(ii) Streptomycin

(iii) Alcohol

(iv) Yeast

Ans:(ii) Streptomycin

(c) Carrier of malaria-causing protozoan is

(i)  emale Anopheles mosquito

(ii) cockroach

(iii) housefly

(iv) butterfly

Ans: (i) emale Anopheles mosquito

(d)  The most common carrier of communicable diseases is

(i)  ant

(ii)  housefly

(iii) dragonfly

(iv) spider

Ans: (ii)housefly

(e) The bread or idli dough rises because of

(i) heat

(ii) grinding

(iii) growth of yeast cells

(iv) kneading

Ans: (iii) growth of yeast cells

(f) The process of conversion of sugar into alcohol is called

(i) nitrogen fixation

(ii) moulding

(iii) fermentation

(iv) infection

Ans: (iii) fermentation

Q3. Match the organisms in Column I with their action in Column II.

    Column I                                     Column II

(i) Bacteria                                (a) Fixing Nitrogen

(ii) Rhizobium                           (b) Setting of curd

(iii) Lactobacillus                     (c) Baking of bread

(iv) Yeast                                   (d) Causing Malaria

(v) A protozoan                         (e) Causing Cholera

(vi) A Virus                                (f) Causing AIDS

                                                   (g) Producing antibodies

Ans: 

(i) - (e)

(ii) - (a) 

(iii) - (b)

(iv) - (c)

(v) - (d)

(vi) - (f)

Que: Can microorganisms be seen with the naked eye? If not, how can they be seen?

Ans: No, we cannot see microorganism with unaided eyes. They can be seen with the help of a microscope.

Que: What are the major groups of microorganisms?

Ans: The major groups of microorganisms are bacteria, fungi, protozoa and some algae. 

Que: Name the microorganisms which can fix atmospheric nitrogen in the soil?

Ans: They are Rhizobium, Azobacter, blue-green algae, etc.

Que: Write 10 lines on the usefulness of microorganisms in our lives?

Ans:

(i) They are used in winemaking, baking, pickling, and other food making processes.

(ii) Alcoholic fermentation by yeast is widely used in the preparation of wine and bread. A bacterium Lactobacillus, promotes the formation of curd. 

(iii) Yeast reproduces rapidly and produces carbon dioxide during respiration. Bubbles of the gas fill the doughand increases its volume.

(iv) Microbes are used to reduce pollution. For example, decomposers such as bacteria and fungi break down dead bodies and excreta to form inorganic compounds, which can be absorbed by plants. 

(v) They are used to increase the soil fertility by fixing the atmospheric nitrogen with the help of bacterium Rhizobium and some other blue-green algae. 

(vi) Microbes also play an important role in the preparation of medicines.

(vii) Antibiotics are chemicals produced by micro-organisms to kill bacteria. Streptomycin, for example, is an antibiotic. 

(viii) Certain microbes are also used in the biological treatment of sewage and industrial effluents.

(ix) Antibiotics are used to control many plant diseases.

(x) Antibiotics are mixed with the feed of livestock and poultry to check microbial infection in animals.

Que: Write a short paragraph on the harms caused by microorganisms?

Ans:

Harmful effects of micro-organisms:

(i) Micro-organisms cause diseases in animals. For example, in humans, bacteria cause diseases such as tuberculosis, cholera, typhoid, etc. 

(ii) In cattle, the foot and mouth disease is caused by a virus. 

(iii)  Also, several microbes cause diseases in plants. For example, the productivity of wheat, orange, apple, etc. is reduced due to microbial diseases in plants. 

(iv) Certain microbes, on entering into our body, produce toxic substances. This leads to food poisoning. Some micro-organisms such as fungus spoil our food. For example, bread when left unused under moist conditions gets spoilt by fungus, producing a white cotton-like growth on the bread.

Que: What are antibiotics? What precautions must be taken while taking antibiotics?

Ans: Antibiotics are medicines produced by certain micro-organisms to kill other disease-causing micro-organisms. These medicines are commonly obtained from bacteria and fungi. Streptomycin, tetracycline, penicillin, etc. are common antibiotics.

Precautions to be taken while using antibiotics:

(i) Antibiotics should be taken under the supervision of a well qualified doctor.

(ii) Course (intake) of antibiotics should be completed as per the prescription given by the doctor.

(iii) Antibiotics should be taken in the right amount and at the right time. A wrong dose of antibiotic makes the drug ineffective. Also, excessive consumption of drugs may kill the useful bacteria present in our body.