Class 8 Science NCERT Solutions for Chapter – 2 Microorganisms: Friend and Foe

Microorganisms: Friend and Foe

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.

AB
BacteriaFixing nitrogen
RhizobiumSetting of curd
LactobacillusBaking of bread
YeastCausing malaria
A protozoanCausing cholera
A virusCausing AIDS

Answer:

AB
BacteriaCausing cholera
RhizobiumFixing nitrogen
LactobacillusSetting of curd
YeastBaking of bread
A protozoanCausing malaria
A virusCausing AIDS

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

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

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:
They are

  • Rhizobium
  • Azobhcter
  • blue- green algae (such as Anabaena and Nostoc), etc.

Question 7.
Write 10 lines on the usefulness of microorganisms in our lives.

Answer:
The usefulness of microorganisms is as follows :

  1. Lactobacillus, a bacterium is useful in the formation of curd.
  2. Microorganisms like bacteria and yeast are also involved in the making of breads, cheese, etc.
  3. They are also used in cleaning up of the environment.
  4. Bacteria are also helpful in agriculture. They increase soil fertility by fixing nitrogen.
  5. Microorganisms like yeast are used for the production of alcohol since ages.
  6. They are used to make vaccines.
  7. They are used to make antibiotics.
  8. Microorganisms are used to make manures.
  9. Microorganisms help in dumping by decomposing plant and animal waste.
  10. About 50% of atmospheric carbon dioxide is fixed by microorganism (algae).

Question 8.
Write a short paragraph on the harmful effects of microorganisnis.

Answer:
The harms caused by microorganisms are as follows :
(1) Many communicable diseases, such as cholera, common cold, chickenpox, tuberculosis, etc., are caused by microorganisms.

(2) Malaria is caused by a microorganism called Plasmodium carried by female Anopheles mosquito.

(3) Female Aedes mosquito acts as carrier of dengue virus.

(4) Anthrax is a dangerous human and cattle disease caused by a bacterium called Bacillus anthracis.

(5) Several microorganisms cause diseases in plants like wheat, rice, potato, sugarcane, orange, apple, etc. and reduce the yield of the crops.

(6) Food poisoning is also caused by microorganisms. They make food poisonous by producing toxic substances in the food.

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

Answer:
The medicines, that kill or stop the growth of the disease-causing microorganisms, are called antibiotics. Streptomycin, tetracycline, erythromycin etc. are some of the commonly known antibiotics which are made from fungi and bacteria.
The precautions to be taken while taking antibiotics are as follows :

  1. These medicines should be taken only on the advice of a qualified doctor.
  2. One must complete the course prescribed by the doctor.
  3. If any body takes antibiotics when not needed, his/her body may develop resistance against that antibiotic.

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 these 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 microorganism specially yeast in these stuffs 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 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 neighbourhood ?

Answer:
It is because, mosquitoes breed in water. Hence, by keeping the surrounding 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 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, does 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.

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