NCERT Solutions for Class 8 Science Chapter 2 Microorganisms Friends and Foe

Q.1. Fill in the blanks

(a) Microorganisms can be seen with the help of a ____________.
(b) Blue green algae fix __________ directly from air and enhance fertility of soil.
(c) Alcohol is produced with the help of __________.
(d) Cholera is caused by __________.

Answer:

(a) Microorganisms can be seen with the help of a Microscope .
(b) Blue green algae fix Nitrogen directly from air and enhance fertility of soil.
(c) Alcohol is produced with the help of Yeast .
(d) Cholera is caused by Bacteria .

Q.2.(a) Tick the correct answer. Yeast is used in the production of (i) sugar

(ii) alcohol

(iii) hydrochloric acid

(iv) oxygen

(a) Yeast is used in the production of (ii) Alcohol

Q.2(b). Tick the correct answer

The following is an antibiotic

(i) Sodium bicarbonate

(ii) Streptomycin

(iii) Alcohol

(iv) Yeast

(b) The following is an antibiotic (ii) Streptomycin

Q.2(c). Tick the correct answer

Carrier of malaria-causing protozoan is

(i) female Anopheles mosquito (ii) cockroach

(iii) housefly (iv) butterfly

(c) Carrier of malaria-causing protozoan is (i) female Anopheles mosquito

Q.2(d). The most common carrier of communicable diseases is

(i) ant (ii) housefly (iii) dragonfly (iv) spider

(d) The most common carrier of communicable diseases is (ii) housefly.

Q.2(e). Tick the correct answer

The bread or idli dough rises because of

(i) heat (ii) grinding (iii) growth of yeast cells (iv) kneading

(e) The bread or idli dough rises because of (iii) The growth of yeast cells.

Q.2(f). Tick the correct answer

The process of conversion of sugar into alcohol is called

(i) nitrogen fixation (ii) moulding (iii) fermentation (iv) infection

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

Q. 3. Match the organisms in Column A with their action in Column B

A B
(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

Q. 4. Can microorganisms be seen with the naked eye? If not, how can they be seen?

Answer:

No, the microorganisms cannot be seen with the human naked eye because they are too small to be seen. microorganisms can be seen with the help of a scientific instrument called microscope.

Example- fungus on bread.

Q. 5. What are the major groups of microorganisms?

Answer:

The major groups of microorganisms are as follows-

(i) Bacteria- unicellular organisms. most common examples of bacteria are Salmonella typhi, Rhizobium, Mycobacterium etc. Typhoid and tuberculosis (TB) are bacterial diseases.

(ii) Fungi- Multicellular organisms which feed on dead and deaying matter. Bread mould is common examples.

(iii) Protozoa- Unicellular organisms wihich are mostly parasitic. Organisms like Amoeba, Plasmodium etc are under this category. Dysentery and malaria are caused by protozoans.

(iv) Virus- Acellular organisms which reproduce only inside the cells of the host organism. These cause diseases like chicken pox and polio.

(v) Algae- They are unicellular or multicellular organism. Spirogyra, Chlamydomonas, Ulva are some of the examples of algae.

Answer :

Microorganisms are useful for us in following ways-

• Microorganisms are used to increase the fertility of the soil by fixing the atmospheric nitrogen with the help of blue-green algae and Rhizobium.
• Antibiotics which are obtained from microorganisms are used to control the diseases.
• There are some bacteria which can turn nitrogen compounds found in the soil into nitrogen gas which is further released into the environment.
• Bacterium Lactobacillus reproduces in milk and promotes the formation of curd.
• Microorganisms especially yeast can be used for the industrial production of alcohol, wine and acetic acid (vinegar).
• Microbes are used to reduce pollution. example- bacteria and fungi break down the dead bodies and excreta to form inorganic compounds which are absorbed by plants
• Yeast reproduces rapidly and produces carbon dioxide during respiration. Bubbles of the gas fill the dough and increase its volume.Thus they have a use in making of bread

Q. 8. Write a short paragraph on the harmful effects of microorganisms.

Answer:

The harmful effects of microorganisms are given below-

• Microorganisms cause diseases in humans as well as animals. For example cholera and TB are caused by bacteria in humans.
• In plants also diseases are caused by microorganisms. For example, citrus canker in plants is caused by bacteria and rust of wheat is caused by fungi.
• Food poisoning is also caused by the microorganisms that spoils our food by producing some toxic substances. For example- when the bread is left in under moist condition gets spoilt by fungus.

Q. 9. What are antibiotics? What precautions must be taken while taking antibiotics?

Answer:

Antibiotics- These are medicines that kill or stop the growth of the disease-causing microorganisms. There are a lot of antibiotics which are produced with the help of bacteria and fungi. Streptomycin, tetracycline and erythromycin are some of the commonly known antibiotics which are made from fungi and bacteria.

Precautions that must be taken while using antibiotics-

• Antibiotics must be taken obly on the prescrition of a doctor.
• must finish the course prescribed by the doctor.
• An antibiotic should be taken in the right amount only. Antibiotics taken unnecessarily may kill beneficial bacteria in the body.

Answer:

Those medicines kill or stop the growth of the disease-causing microorganisms are called antibiotics. Antibiotics are
being produced from bacteria and fungi.

Precaution to be taken while using antibiotics-

• antibiotics should be taken only on the advice of a qualified doctor.
• must finish the course prescribed by the doctor.
• An antibiotic should be taken in the right amount. antibiotics taken unnecessarily may kill beneficial bacteria in the body.