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Our team of subject expert teachers has prepared and reviewed the NCERT Solutions for Class 8 Science Chapter 2 Microorganisms: Friend and Foe are given here will help you to prepare well and score good numbers in exams.
Microorganisms: Friend and Foe NCERT Solutions for Class 8 Science Chapter 2
Class 8 Science Chapter 2 Microorganisms: Friend and Foe Exercise Questions and Answers
Question 1.
Fill in the blanks.
(a) Microorganisms can be seen with the help of a _________.
Answer:
Microscope
(b) Blue-green algae fix __________ directly from air to enhance fertility of soil.
Answer:
Nitrogen
(c) Alcohol is produced with the help of __________.
Answer:
Yeast
(d) Cholera is caused by _________.
Answer:
Bacteria
Question 2.
Tick the correct answer.
(a) Yeast is used in the production of
1. Sugar
2. Alcohol
3. Hydrochloric acid
4. Oxygen
Answer:
2. Alcohol
(b) The following is an antibiotic
1. Sodium bicarbonate
2. Streptomycin
3. Alcohol
4. Yeast
Answer:
2. Streptomycin
(c) Carrier of malaria-causing protozoan is
1. Female Anopheles mosquito
2. Cockroach
3. Housefly
4. Butterfly
Answer:
1. Female Anopheles mosquito
(d) The most common carrier of communicable diseases is
1. Ant
2. Housefly
3. Dragonfly
4. Spider
Answer:
2. Housefly
(e) The bread or idli dough rises because of
1. Heat
2. Grinding
3. Growth of yeast cells
4. Kneading
Answer:
3. Growth of yeast cells
(f) The process of conversion of sugar into alcohol is called
1. Nitrogen fixation
2. Moulding
3. Fermentation
4. Infection
Answer:
3. Fermentation
Question 3.
Match the organisms in column A with their action in column B.
A | B |
Bacteria | Fixing nitrogen |
Rhizobium | Setting of curd |
Lactobacillus | Baking of bread |
Yeast | Causing malaria |
A protozoan | Causing cholera |
A virus | Causing AIDS |
Answer:
A | B |
Bacteria | Causing cholera |
Rhizobium | Fixing nitrogen |
Lactobacillus | Setting of curd |
Yeast | Baking of bread |
A protozoan | Causing malaria |
A virus | Causing AIDS |
Question 4.
Can microorganisms be seen with the naked eye? If not, how can they be seen?
Answer:
The microorganisms are so small in size that they cannot be seen with the naked eye. Some of these, such as the fungus that grows on bread, can be seen with a magnifying glass. Others cannot be seen without the help of a microscope. That is why these are called microorganisms or microbes.
Question 5.
What are the major groups of microorganisms?
Answer:
The major groups of microorganisms are bacteria, fungi, protozoa, and some algae.
Question 6.
Name the microorganisms which can fix atmospheric nitrogen in the soil.
Answer:
Rhizobium, Clostridium, and Azotobactor.
Question 7.
Write 10 lines on the usefulness of microorganisms in our lives.
Answer:
The usefulness of microorganisms is as follows :
- Lactobacillus, a bacterium is useful in the formation of curd.
- Microorganisms like bacteria and yeast are also involved in the making of breads, cheese, etc.
- They are also used in cleaning up of the environment.
- Bacteria are also helpful in agriculture. They increase soil fertility by fixing nitrogen.
- Microorganisms like yeast are used for the production of alcohol since ages.
- They are used to make vaccines.
- They are used to make antibiotics.
- Microorganisms are used to make manures.
- Microorganisms help in dumping by decomposing plant and animal waste.
- About 50% of atmospheric carbon dioxide is fixed by microorganisms (algae).
Question 8.
Write a short paragraph on the harmful effects of microorganisms.
Answer:
Microorganisms which cause diseases in men, animals, and plants, are called pathogens. Pathogens or germs enter a healthy body through air, water, contaminated food, and an infected person by direct or indirect contact or by a carrier. Common ailments like cold, influenza (flu), coughs, polio, chickenpox are caused by viruses. Foot and mouth diseases of cattle are also caused by viruses. Typhoid, tuberculosis (TB) is caused by bacteria.
Anthrax a dangerous human and cattle disease is also caused by bacteria. Disease like dysentery and malaria are caused by protozoa. Ringworm is caused by fungi. Some microorganisms release toxic substances when they grow in food. Citrus canker is a bacterial disease that affects trees of citrus fruits and is spread by air. Yellow vein mosaic disease is caused by insects in bhindi. Rust of wheat is a fungi disease spread by air.
Question 9.
What are antibiotics? What precautions must be taken while taking antibiotics?
Answer:
Antibiotics are medicines which kill or stop the growth of disease-causing microorganisms. The antibiotics are manufactured by growing specific microorganisms and are used to cure a variety of diseases.
It is important to remember that antibiotics should be taken only on the advice of a qualified doctor. Also, you must finish the course prescribed by the doctor. If you take antibiotics when not needed, you help bacteria in your body to develop resistance to them. Next time when you fall ill and need these antibiotics, they would be less effective. Antibiotics, however, are not effective against cold and flu as they are caused by viruses.
Class 8 Science Chapter 2 Microorganisms: Friend and Foe InText Questions and Answers
Question 1.
If moist bread is left undisturbed for 2-3 days, one can observe that its surface gets covered with greyish-white patches. If these patches are seen through a magnifying glass, one can see tiny, black rounded structures. Do you know what these structures are and where do they come from?
Answer:
These structures are made by microorganisms, especially Rhizopus (bread mould) and they do not come from anywhere else but from the surrounding itself.
2.1 Microorganisms
Activity 2.1
Question 1.
What do you see?
Answer:
Tiny organisms (actually micro-organisms present in the soil) are visible under the microscope.
Question 2.
Do you find tiny organisms moving around?
Answer:
Yes, I can see the tiny organisms moving around.
2.3 Microorganisms and Us
Question 3.
I saw that my mother added a little curd to warm milk to set curd for the next day. I wonder why?
Answer:
Curd contains several microorganisms, mainly bacterium, Lactobacillus, which promotes the formation of curd. When little curd {Lactobacillus) is added to milk, the bacteria start growing in milk and convert milk into curd. This process takes some time that is
why milk is left undisturbed for some, hours after adding a little curd to it.
Question 4.
An important ingredient of Rava (sooji), idlis and bhaturas is curd. Can you guess why?
Answer:
I think adding curd promotes the growth of microorganisms especially yeast in this stuff thus increasing their volumes.
Question 5.
Why are children/infants given vaccination?
Answer:
If dead or weakened microbes are introduced in a healthy body, the body fights and kills them by producing suitable antibodies. The antibodies remain in the body and we are protected from that disease-causing microbes. This is called vaccination. The children/infants are vaccinated to protect them from diseases like cholera, tuberculosis, smallpox, hepatitis etc.
Question 6.
Boojho and Paheli wanted to know how does the decomposition of waste material happen?
Answer:
The decomposition of waste materials- is done by microorganisms because micro-organisms decompose dead organic waste of plants and animals converting them into simple substances.
Activity 2.5
Question 1.
Did you notice that in pot B the polythene bags, empty glasses, bottles and broken toy parts did not undergo any such change? Why?
Answer:
They did not undergo any change because microbes could not act on them and convert them into simple substances (manure).
2.4 Harmful Microorganisms
Question 7.
Why does the teacher keep telling us not to let water collect anywhere in the neighborhood?
Answer:
It is because mosquitoes breed in water. Hence, by keeping the surroundings clean and dry, we can prevent mosquitoes from breeding. This will help to avoid the spread of malaria.
Question 8.
Paheli wonders how food can become a ‘poison’.
Answer:
Some microorganisms can spoil the food when they grow on it. A toxic substance is produced in the food by these microbes. These make the food poisonous. Consuming such food can cause food poisoning and serious illness. Even death can occur.
2.5 Food Preservation
Question 9.
How do we preserve cooked food at home?
Answer:
We preserve cooked food by keeping it in refrigerators.
Question 10.
Is spoiling of food a chemical reaction?
Answer:
Yes, it is a chemical reaction, because spoiled food has a bad smell and taste. Its colour changes as well. Besides, spoiled food cannot be converted back into fresh food.
Question 11.
Paheli bought some mangoes but she could not eat them for a few days. Later she found that they were spoilt and rotten. But, she knows that the mango pickles her grandmother makes do not spoil for a long time. She is confused. Why does this happen?
Answer:
The mangoes were acted upon by the microorganisms which spoiled them. But, the mango pickles made by Paheli’s grandmother contain preservatives like salt and oil, which prevent the attack of microbes.
Question 12.
Why does the milk that comes in packets does not spoil? What is pasteurization?
Answer:
The milk that comes in packets does not spoil because it is pasteurized. Pasteurization is a process in which milk is heated first to a temperature of 70°C for 15 seconds and then suddenly is chilled and stored. By doing so, it prevents the growth of microbes. This process was discovered by Louis Pasteur. That is why the process is called Pasteurization.
Read More: Write a short paragraph on the harmful effects of microorganisms