Microorganisms: Friend and Foe Question Answers: NCERT Class 8 Science

(a) Microscope

Microorganisms can be seen with the help of microscope because they are very small in size. We cannot see the microorganisms with the naked eyes.

(b) Nitrogen

To increase in the fertility of the soil, the composition of nitrogen should be rich in soil. Blue green algae fix the nitrogen from environment and increases the fertility of the soil.

(c) Yeast

The process of fermentation of alcohol with the help of yeast which is known as fermentation process. With the help of yeast, the carbon dioxide gas production occurs.

(d) Bacteria

Cholera is a water- born bacterial disease. It spreads by impure water.

(a) (ii) Alcohol

Yeast is used in the production of alcohol. This process is known as fermentation.

(b) (ii) Streptomycin

Streptomycin is an antibiotic, it is used to relieve the pain and kills other disease causing microorganisms.

(c) (i) Female Anopheles mosquito

Malaria is caused by a mosquito. The carrier of Malaria causing protozoan is Female Anopheles mosquito.

(d) (ii) Housefly

The most common carrier of communicable disease is housefly. It spreads the disease causing microorganisms.

(e) (iii) Growth of yeast cells

This process is called as fermentation. The dough rises due to the increased growth of yeast cells.

(f) (iii) Fermentation 

The conversion of sugar into alcohol with the help of yeast cells is called as fermentation.

Column I                                               Column II

(i) Bacteria                                           (e) Causing Cholera

(ii) Rhizobium                                      (a) Fixing Nitrogen

(iii) Lactobacillus                                  (b) Setting of curd

(iv) Yeast                                              (c) Baking of bread

(v) A protozoan                                    (d) Causing Malaria

(vi) A Virus                                            (f) Causing AIDS

• Cholera is a water born disease which is spread by drinking impure water.
• In soil, some bacteria are present like Rhizobium which fixes the nitrogen and increases the fertility of the soil.
• Setting of curd is occur due to the presence of Lacto bacillus bacteria. It converts milk into curd.
• Baking of bread is occurs in the presence of yeast. Growth of yeast cell increases the size of the dough.
• Malaria is caused by a protozoan. The carrier of malaria caused protozoan is female Anopheles mosquito.
• AIDS is caused by a virus. This disease is caused by the sexual intercourse. 

No, microorganisms cannot be seen with the naked eyes because they are too small. They can be seen only under the microscope.

There are five major groups of microorganisms which are as follows:

1. Bacteria: These are single celled organisms which are harmful as they causes diseases and some of them may be useful as they help. Based on their shape bacteria are of four types:

• Bacillus (rod shape)
• Coccus (spherical shape)
• Spirillum (spiral shape)
• Vibrio (comma shape)

1. Fungi: These are non- green plants which consumes their food from dead and decay organic matter. And, they are heterotrophs.
2. Protozoa: It is a group of single- celled microorganisms that have animal like characteristics. For example: Amoeba and Paramecium.  
3. Algae: It is large group of simple plant like organisms. But they do not contain roots, stem or leaves. They can be unicellular or multicellular.
4. Viruses: These are smallest microorganisms which reproduce only inside the host body. Outside the host body it behaves like non- living things.  

1. Rhizobium bacteria
2. Blue- green algae

This Rhizobium bacteria and certain blue- green algae (Anabaena, Nostoc) present in the soil can fix atmospheric nitrogen and convert it into the usable nitrogenous compounds.

Microorganisms are useful for the human beings and the plants in the following ways:

1. Rhizobium bacteria increase fertility of soil by fixing free nitrogen of atmosphere.
2. Lacto bacillus bacteria helps in making the curd of the milk.
3. Mushrooms are edible fungi.
4. Antibiotics like penicillin are made from fungi.
5. Yeast is used in the preparation of the alcohol, wine and beer etc.
6. Some bacteria are used for tanning of leather.
7. Bacteria and fungi are decomposers and acts as natural cleansing agents.
8. Blue- green algae increase the fertility of the soil.
9. Yeasts are useful in making idli, dosa, processed cheese, pizza dough etc.
10. Microorganisms make compost manure.

Microorganisms are harmful for human beings in various ways. They causes diseases in human beings, plants and animals too. Following are the harmful effects of microorganisms that have been shown below in points:

1. They cause a large variety of diseases in plants, animals and human beings.
2. In humans, bacteria causes diseases like cholera, typhoid, tuberculosis etc.
3. In cattle, the foot and mouth diseases are caused by viruses.
4. Several microorganisms cause diseases in plants as well. For example: tikka disease in groundnuts, late blight of potato etc.
5. Food substances like sauce, jams, pickles etc. are spoiled by microorganisms.
6. Many fungi can decay wood.
7. Microorganisms can damage clothes, leather etc.
8. Some microbes secrete toxins which leads to food poisoning.

Antibiotics: Those substances which are obtained from certain microorganisms are called antibiotics. It is a medicine which is used for destroying bacteria and curing infections. Antibiotics are taken in the form of tablets, capsules or injections which are used to kill or stop the growth of pathogens.

For example: Penicillin, tetracycline etc.

Precautions to be taken while taking antibiotics:

1. Antibiotics should be taken in the proper doze only on the advice of a doctor.
2. Course prescribed by the doctor should be complete.
3. Over dose of antibiotics should be avoided as they can destroy useful microorganisms in body.
4. Do not take antibiotics in healthy condition.