dav class 8 science chapter 2 full solution ( micro organism friends and foe)
Answers
Exercise Questions
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 __________.
Soln:
a) Microscope
b) Nitrogen
c) Yeast
d) Bacteria- Vibrio cholera
2. Tick the correct answer.
(a) Yeast is used in the production of
(i) sugar (ii) alcohol (iii) hydrochloric acid (iv) oxygen
(b) The following is an antibiotic
(i) Sodium bicarbonate (ii) Streptomycin (iii) Alcohol (iv) Yeast
(c) Carrier of malaria-causing protozoan is
(i) female Anopheles mosquito (ii) cockroach (iii) housefly (iv) butterfly
(d) The most common carrier of communicable diseases is
(i) ant (ii) housefly (iii) dragonfly (iv) spider
(e) The bread or idli dough rises because of
(i) heat (ii) grinding (iii) growth of yeast cells (iv) kneading
(f) The process of conversion of sugar into alcohol is called
(i) nitrogen fixation (ii) moulding (iii) fermentation (iv) infection
Soln:
a) (ii) alcohol
b) (ii) Streptomycin
c) (i) female Anopheles mosquito
d) (ii) housefly
e) (iii) growth of yeast cells
f) (iii) fermentation
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
Sol:-
A B
(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
4. Can microorganisms be seen with the naked eye? If not, how can they be seen?
Soln:
Microorganisms cannot be seen by the naked eye because they are very small. They can be seen by using a microscope.
5. What are the major groups of microorganisms?
Soln:
Major group of microorganisms are as follows
Bacteria
Fungi
Protozoans
Algae
Viruses
6. Name the microorganisms which can fix atmospheric nitrogen in the soil.
Soln:
Rhizobium
7. Write 10 lines on the usefulness of microorganisms in our lives.
Soln:
Uses of microorganisms are listed below
They are helpful in cleaning up of the environment. For example, the organic wastes (vegetable peels, remains of animals, faeces, etc.) are broken down into harmless and usable substances by bacteria.
Curd contains several microorganisms. Of these, the bacterium, Lactobacillus promotes the formation of curd. It multiplies in milk and converts it into curd.
Bacteria are also involved in the making of cheese, pickles and many other food items.
Bacteria and yeast are also helpful for fermentation of rice idlis and dosa batter.
Yeast reproduces rapidly and produces carbon dioxide during respiration. Bubbles of the gas fill the dough and increase its volume. This is the basis of the use of yeast in the baking industry for making breads, pastries and cakes.
Microorganisms are used for the large scale production of alcohol, wine and acetic acid (vinegar).
Antibiotics are being produced from bacteria and fungi.
Microorganisms are helpful in producing vaccines against disease-causing agents.
Some bacteria (Fig. 2.7) are able to fix nitrogen from the atmosphere to enrich the soil with nitrogen and increase its fertility.
Microorganisms decompose dead organic waste of plants and animals to convert them into simple substances. These substances are again used by other plants and animals. Thus, microorganisms can be used to degrade the harmful and smelly substances and thereby clean up the environment.
8. Write a short paragraph on the harmful effects of microorganisms.
Soln:
Microorganisms cause diseases in plants and animals Ex; Cholera in humans is caused by bacteria called Vibrio cholera. In animals, the virus causes foot and mouth diseases. Microorganisms spoil the foods by decomposing the food material. Some microorganisms spoil leather and clothes. Microorganisms are responsible for food poisoning.
9. What are antibiotics? What precautions must be taken while taking antibiotics?
Soln:
Chemical agents that kill or stop the growth of disease-causing microorganisms are called antibiotics. Antibiotics should be taken only on the advice of a qualified doctor. Also, one must complete the course prescribed by the doctor.
Answer:
A. Fill in the blanks.
1. The study of microorganisms is known as ___________.
2. The bread dough rises because of the production of ___________.
3. ___________ are the microorganisms that cause diseases.
4. ___________ and ___________ are two chemical preservatives.
5. ___________ is a fungus that causes food poisoning.
Answer: (1) microbiology (2) carbon dioxide (3) pathogens (4) sodium benzoate, potassium metabisulphite (5) Aspergillus
B. Match the following.
DAV Solutions for Class 8 Science Chapter 2
Match the followings
Answer: (1) c, (2) a, (3) d (4) e (5) b
C. Tick the correct option.
1. The process, that helps milk to last longer but does not kill all microbes present in it, is known as-
Answer: pasteurisation
2. Rust of wheat is caused by-
Answer: fungus
3. Medicines, containing killed or weakened pathogens, and used to prevent infectious diseases, are called-
Answer: vaccines
4. Which of the following reproduces only inside a host cell?
Answer: virus
5. A disease, caused by a virus, and spread by an insect, is-
Answer: dengue
D. Answer the following questions in brief.
1. Why are viruses considered as being at the ‘borderline’ between living and non-living things?
Answer: A virus behaves like a non-living when it is outside a host cell. Once a virus enters a host cell, it starts to behave like a living being. Due to this, viruses are considered as the borderline between living and non-living.
2. What is fermentation? How is this process useful in the food and beverage industries?
Answer: An anaerobic process through which sugars/carbohydrates are converted into alcohol or acids and carbon dioxide by microorganisms is called fermentation.
Fermentation is useful in the food and beverages industries in various ways:
It helps in making tasty dosa.
It helps in making bread and cake.
It helps in making wine
3. ‘In the absence of microorganisms the earth would become a heap of dead plants and animals. Justify this statement.
Answer: When a plant or animal dies, its remains are decomposed by microorganisms. Decomposition is the process through which all constituent materials of a living being are sent back to the environment. Had there been no microorganisms, dead remains of plants and animals would be keeping on accumulating all around us. So, it can be said that in the absence of microorganisms theearth would become a heap of dead plants and animals.
4. Munna observes that the yield of wheat, growing in his field, has reduced in the current year. He gets the soil tested and the report confirms the deficiency of one particular nutrient. Accordingly, he is advised to grow peas after harvesting wheat.
(i) Name the nutrient found insufficient.
Answer: Nitrogen
(ii) How do you think growing peas will help in replenishing the soil?
Answer: Pea plants give shelter to Rhizobium bacteria. These bacteria carry out nitrogen fixation in soil. Thus, pea plants help in replenishing nitrogen in soil.
5. Why does it take less time to prepare curd in summers as compared to preparing it in winters?
Answer: Microorganisms reproduce at a faster rate between 20 to 40°C. They reproduce at a much slower rate when temperature is below this range. Milk is converted into curd by multiplication of lactobacillus. This is why it takes less time to prepare curd in summers as compared to preparing it in winters.