CBSE Class 8 Science Microorganisms Assignment Set A

A. Fill in the blanks

1.Microrganisms may be unicellular or ___
2. __ bacteria help to make curd from milk.
3. Disease causing microorganisms are called ___.
4.Ammonium salt in the soil is converted first into nitrites by____ bacteria.
5. The ability of one's body to resist a disease is called ___.
6. Salting draws out ___from food.
7. Nitrates in the soil are converted by __ bacteria into nitrogen gas.
8. The process of preventing food spoilage by chemical or physical methods is called food___.
Answers…
1. multicellular       5. immunity
2.lactobacillus        6. water(moisture)
3.pathogens           7. pseudomonas
4.nitrosomonas       8. Preservation

ANSWER THE FOLLOWING QUESTIONS

B.Very short questions.

Q.1 Name any two antibiotics.
Answer. Streptomycin 2. Tetracycline

Q.2 Give the names of any two communicable diseases.
Answer. Common cold and chicken pox

Q.3 Name any two common diseases each that occur in plants and animals.
Answer. Plant disease – Citrus canker and Rust
Animal disease – Anthrax and bird flu

Q.4 Name two microbial diseases that spread through air.
Answer. Tuberculosis and Pneumonia

Q.5 What are microorganisms?
Answer. The organisms that cannot be seen with the naked eye and can only be observed through a microscope are called microorganisms.

Q.6 Name any disease caused by virus in humans.
Answer. Smallpox

C. Short Questions

Q.1 What is pasteurization?
Answer. Pasteurization is a process for preservation of milk. In this method, milk is heated at about 70°C for 15 to 30 seconds to kill the bacteria present in it and cooled quickly to 10º C to prevent the remaining bacteria from growing.

Q.2 How do legiuminous plants increase the soil fertility?
Answer. Rhizobium bacteria present in the root nodules of leguminous plants absorb nitrogen gas from the atmosphere and convert it into simple nitrogenous compound called nitrates. After harvesting , when the roots decompose, the nitrates present in the root nodules mix with the soil and enrich it with nitrogenous compounds and hence increase the soil fertility.

Q.3 What do you mean by nitrifying bacteria?
Answer. The bacterias that change nitrite into nitrate from ammonia or ammonium in nitrogen cycle are called nitrifying bacteria. Nitrosomonas, Nitrobacter are the nitrifying bacteria found in soil.

Q.4 How does female Anopheles mosquito spread malaria.
Answer. The female Anopheles mosquito sucks the blood of human beings. While sucking blood, it transmits plasmodium into human blood. Plasmodium causes malaria

Q.5 How do antibiotics work?
Answer. Antibiotics fight bacterial infections either by killing bacteria or slowing and suspending its growth. They do this by: attacking the wall or coating surrounding bacteria. Or by interfering with bacteria reproduction.

Q.6 (a) How do viruses differ from other microorganisms?
(b) What is meant by fermentation?
Answer. (a) 1. Viruses are microscopic organisms, generally very smaller than bacteria. They are parasitic in nature, i.e. they can thrive and reproduce only on the host cell like animals, bacteria and plants etc.
2.Viruses are non-living in nature until they come in contact with the living cells. Most of the viruses are infectious in nature. Whereas some of the other microorganisms are useful in nature. Viruses can multiply very rapidly than the microorganisms.
(b) The process of conversion of sugar into alcohol by the action of yeast is known as fermentation.

Q.7 What are communicable diseases? Name any two diseases each caused by viruses and bacteria.
Answer. Microbial diseases that are spread from an infected person to other healthy person through air, water, food, physical contact or by animals are called communicable diseases.
Two diseases caused by
Viruses - measles and AIDS
Bacteria-typhoid and cholera

Q.8 What is vaccine? How does it work?
Answer. A small dose of dead and weakened disease causing microbes used to stimulate immune response in the body is called vaccine.
When a vaccine is introduce in the body of a healthy person, it acts like a antigen . The body of the person produces antibodies against these introduced microbes. These antibodies remain in the body and protect him/her against future infections of the same microbe.

Q.9 Microorganisms decompose dead organic waste of plants and animals and help in keeping the environment clean.
(a) What would happen, if microorganisms do not perform this function?
(b) Being a student, how can you contribute in keeping the environment clean? Give two ways.
Answer. (a) If decomposers disappeared from a ecosystem, wastes as well as the remains of the dead organisms would pile up, and producers (plants) would not have enough nutrients.
(b) The two ways through which we can keep the environment clean –
(i) Reduce the usage of your electrical appliances.
(ii) Reduce usage of chemicals and pesticides.

D. LONG QUESTIONS

Q.1 Explain two methods of food preservation. Also, discuss the advantages of food preservation.
Answer. The two methods of food preservation are-
a. Chemical method
b. Preservation by sugars
(a) Chemical method :- Food can be preserved by using food preservatives like sodium benzoate, sodium metabisulphite etc.. Most chemicals preservatives prevent microbial growth by removing oxygen from the food items. Sodium benzoate is used for preserving fruit juices and squashes. Citric acid is used as a preservative in confectioneries.
(b) Preservation method :- Jams, jellies and squashes are preserved for a long time because of the presence of sugar in them. Sugar inhibits the growth of food - spoiling microorganisms by reducing the moisture content of the food.
Advantage of food preservation are –
(i) It decreases the wastage of food by avoiding spoilage.
(ii) It increases the storage period of food materials.
(iii) Nutritional value of the food is retained for a long period.
(iv) It ensures the availability of food at distant places and during the off season.

Q.2 Draw a neat diagram to show nitrogen cycle and explain the process of nitrification and denitrification.
Answer
(1) Nitrification–The conversion of ammonia into nitrates by nitrifying bacteria is called nitrification.
(2) Denitrification–The conversion of nitrates into free nitrogen gas by denitrifying bacteria (pseudomonas) is called denitrification.

Q.3 Describe the principal on which the following methods of food preservation are based:
(1) Boiling (2) Canning (3) Freezing (4) Dehydration
Answer.
1. Boiling. Boiling kills bacteria. So, boiling also helps in the preservation of liquid food like milk. Boiled milk can be preserved for 2-3 days, otherwise it goes sour within 8 to 10 hours.
2.Canning. Certain foodstuffs are kept in airtight time or containers. These containers keep away moisture and thus, help in preserving food for a longer time. Foods like fruits and fishes are boiled, sterilised and put into tins and closed airtight.
3. Freezing. Microbes such as fungi and bacteria cannot survive at very low temperature. So milk, eggs, and vegetables are preserved by keeping them in a refrigerator.
4.Dehydration. Dehydrayion means removal of water which is the key requirement of microbial growth from foodstuffs. So, pulses and grains are well dried and perserved. These are stored in moisture free places or vessels. Sun drying is a traditional method for preserving food.

More Questions

Q.1 Give one example of the following :

i) A unicellular algae
ii) A filamentous algae
iii) Algae forming clusters/colonies
iv) A brown algae
v) The bacteria found in curd
vi) One rod-shaped bacteria
vii) One spherical bacteria
viii) A protozoan which can photosynthesize

Q.2 Match the columns:

i) Entamoeba histolytica               a) Botulism
ii) Giardiasis                               b) Giardia
iii) Trypanosoma                         c) AIDS
iv) HIV                                       d) Curd
v) Yeast & lactobacillus                e) Amoebiasis
vi) Penicillium notatum                f) Sleeping sickness
vii) Streptococcus pneumonia      g) Antibiotic
viii) Clostridium botulinum           h) Pneumonia

Q.3 Write briefly about the following:

i) Diatoms ii) Mildew iii) Cyanobacteria iv) Zooplanktons v) Algal bloom
vi) Spores vii) Anaerobic bacteria viii) Binary fission ix) pasteurization x) Microbiology

Q.4 Answer the following questions:

a) List some methods of food preservation

b) Write two uses and two harms caused by the following :
i) Algae ii) Bacteria iii) Protozoa iv) Fungi

c) Draw well labelled diagram of the following :
i) Euglena ii) A bacterial cell iii) Paramecium

d) How do bacteria reproduce?

e) What is the major difference between bacterial cell and the cells of other organisms?

f) Why is “Virus” thought of as something in between the living and non-living ? How do they multiply within a host cell?

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