NCERT Exemplar Solution for Class 8 Science (Biology ...

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NCERT Exemplar Solution for Class 8

Science (Biology)

Chapter 2 - Microorganisms: Friends and Foe

Multiple Choice Questions

1. Which of the following reproduces only inside a host cell?

(a) Bacteria

(b) Virus

(c) Amoeba

(d) Fungus

Ans: (b) Virus

Bacteria, protists, and fungi can reproduce by cell division without the requirement

of a host body. In contrast, viruses are considered both living and non-living since

they act like non-living organisms outside the host but reproduce inside the host,

thus behaving like living organisms.

2. A disease in human beings caused by a virus is

(a) typhoid

(b) influenza

(c) dysentery

(d) cholera

Ans: (b) influenza

Influenza (also known as the flu) is a viral infection caused by the influenza virus.

Diphtheria, cholera, and typhoid are all bacterial infections caused by Vibrio

cholerae, Corynebacterium diphtheriae, and Salmonella typhi, respectively.

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3. Pathogenic microorganisms present in host cells are killed by medicines

called

(a) pain killer

(b) antibodies

(c) antibiotics

(d) vaccines

Ans: (c) antibiotics

Antibiotics are chemical compounds that can slow or stop the growth of pathogenic

bacteria. They inhibit the bacteria from reproducing inside the host's body.

Consumption of antibiotics kills the microorganisms that cause infection in host

bodies.

4. The two microorganisms which live in symbiotic association in lichens are

(a) fungus and protozoa

(b) alga and bacteria

(c) bacteria and protozoa

(d) alga and fungus

Ans: (d) alga and fungus

Lichens are symbiotic relationships between fungi (mycobionts) and photosynthetic

partners (photobionts), which might be algae or bacteria (blue-green algae). Algae

are autotrophic as they have chlorophyll and perform photosynthesis. Fungi are

heterotrophic as they are dependent on others for their nutrient requirements. Algae

provide food to the fungus. In return, the fungus provides water, minerals, and

shelter to the algae.

5. The gas released during the preparation of bread is

(a) oxygen

(b) carbon dioxide



(c) nitrogen

(d) sulphur dioxide

Ans: (b) carbon dioxide

Carbon dioxide gas is released due to the fermentation process conducted by yeast

cells. Yeast converts sugars present in flour into carbon dioxide and ethanol (ethyl

alcohol). Alcohol evaporates due to the heat of the oven, whereas carbon dioxide gas

is trapped in the dough, which is released during the preparation of bread, which

makes the bread dough fluffy.

6. The disease caused by a protozoan and spread by an insect is __________.

(a) dengue

(b) malaria

(c) polio

(d) measles

Ans: (b) malaria

Malaria is a mosquito-borne infectious disease caused by a plasmodium (protozoa)

parasite. The primary host is mosquitoes (female Anopheles) and the secondary or

intermediate host is humans. Dengue, Polio and Measles are all viral diseases that

are caused by the dengue virus, poliovirus, and rubeola virus, respectively.

7. Paheli dug two pits, A and B, in her garden. In pit A, she put a polythene bag

packed with some agricultural waste. In pit B, she dumped the same kind of a

waste but without packing it in a polythene bag. She then covered both the pits

with soil. What did she observe after a month?

(a) Waste in pit A degraded faster than that in pit B.

(b) Waste in pit B degraded faster than that in pit A.

(c) Waste in both pits degraded almost equally.

(d) Waste in both pits did not degrade at all.



Ans: (b) Waste in pit B degraded faster than that in pit A.

Pit B's waste degraded faster than Pit A's because it was held open, and as a result,

it absorbed oxygen far faster than Pit A. Oxygen helps in the decomposition of

organic waste by microbes. The plastic in pit A prevented the proper flow of air. As

a result, microbial degradation did not occur in pit A.

VERY SHORT ANSWER QUESTIONS

8. Unscramble the jumbled words underlined in the following statements.

(a) Cells of our body produce a santiidobe to fight pathogens.

Ans: Antibodies

Antibodies are proteins that are produced by our immune system in response to

disease-causing microbes and remain in the body to fight against further infection

caused by the same pathogens.

(b) Curbossulite is an air-borne disease caused by a bacterium.

Ans: Tuberculosis

Tuberculosis (TB), a potentially serious infectious bacterial disease that mainly

affects the lungs, is caused by a bacterium Mycobacterium tuberculosis.

(c) Xanrhat is a dangerous bacterial disease?

Ans: Anthrax

Anthrax is a serious bacterial disease caused by Bacillus anthracis, a spore-forming

bacteria.

(d) Yeasts are used in the wine industry because of their property of

meronettinaf.

Ans: Fermentation

Fermentation is the process by which yeast converts sugars (such as glucose) into

alcohol and releases carbon dioxide gas.

9. Suggest a suitable word for each of the following statements.



(a) Chemicals are added to food to prevent the growth of microorganisms.

Ans: Preservatives

Chemicals added to food are known as preservatives. These preservatives prevent

microbial growth without altering the taste or appearance of food. These

preservatives are usually added to jams, jellies, ketchup, etc.

(b) Nitrogen-fixing microorganisms present in the root nodules of legumes.

Ans: Rhizobium

Rhizobium are the nitrogen-fixing bacteria, present in the root nodules of

leguminous plants such as clover, alfalfa, beans, peas, and soybeans. Rhizobium

fixes the atmospheric nitrogen into that form (i.e. nitrates, nitrates) which is usable

for plants.

(c) The agent spreads pathogens from one place to another.

Ans: Carrier/Vector

Vector, also called a carrier, helps to transmit pathogens from one organism to

another organism.

(d) Chemicals which kill or stop the growth of pathogens.

Ans: Antibiotics

Antibiotics are chemical compounds that can slow or stop the growth of pathogenic

bacteria. They inhibit the bacteria from reproducing inside the host's body.

Consumption of antibiotics kills the microorganisms that cause infection in host

bodies.

10. Match the names of scientists given in Column A with the discovery made

by them given in Column B.

Column A Column B

(a) Louis Pasteur (i) Penicillin

(b) Robert Koch (ii) Anthrax bacterium

(c) Edward Jenner (iii) Fermentation



(d) Alexander Fleming (iv) Smallpox vaccine

(v) Typhoid

Ans: The correct matching for the given questions is as follows-

Column A Column B

(a) Louis Pasteur (iii) Fermentation

(b) Robert Koch (ii) Anthrax bacterium

(c) Edward Jenner (iv) Smallpox vaccine

(d) Alexander Fleming (i) Penicillin

11. Name one commercial use of yeast.

Ans: Yeast is the microorganism that is used for the large-scale production of

alcohol.

12. Name the process in yeast that converts sugars into alcohol.

Ans: Fermentation is the process by which yeast converts sugars (such as glucose)

into alcohol and releases carbon dioxide gas.

13. In the soil, which nutrient is enriched by blue-green algae (cyanobacteria)?

Ans: In the soil, nitrogen gets enriched by nitrogen-fixing bacteria called

cyanobacteria or blue-green algae. They convert atmospheric nitrogen to nitrites or

nitrates that can be absorbed by the plants.

14. Why should we avoid standing close to a tuberculosis patient while he/she is

coughing?

Ans: Tuberculosis (TB), a potentially serious infectious bacterial disease that mainly

affects the lungs, is caused by the bacterium Mycobacterium tuberculosis. A patient

who is affected by tuberculosis may transfer the bacteria while coughing because it's



an air-borne disease and spreads from person to person through tiny droplets released

into the air via coughs and sneezes. Any healthy body may suffer from tuberculosis

disease after coming into contact with this bacterium. That's why one should avoid

standing near any tuberculosis patient.

15. Polio drops are not given to children suffering from diarrhoea. Why?

Ans: Polio (poliomyelitis) is a life-threatening disease caused by the poliovirus. The

Polio drop, which is an oral vaccine, is not given to children suffering from diarrhoea

because it may be excreted from the body as a result of frequent motion. Diarrhoea

is a disease that causes frequent motions in the human body. As a result, the polio

drop won't be effective in fighting against the poliovirus due to diarrhoea.

16. Paheli watched her grandmother making a mango pickle. After she bottled

the pickle, her grandmother poured oil on top of the pickle before closing the

lid. Paheli wanted to know why oil was poured. Can you help her understand

why?

Ans: The oil poured on the tip of pickles while bottling forms a layer of protection

between the air and the pickle from the microorganisms present in the air. The use

of oil (or chemical preservatives) prevents the spoilage of pickles because bacteria

cannot survive in such an environment. This increases the life of the pickle and

doesn't let the bacteria destroy the pickle by attacking it.

SHORT ANSWER QUESTIONS

17. Match the microorganisms given in Column A to the group to which they

belong in Column B.

Column A Column B

(a) Lactobacillus (i) Algae

(b) Aspergillus (ii) Protozoa

(c) Spirogyra (iii) Fungi

(d) Paramecium (iv) Bacteria



Ans:

Column A Column B

(a) Lactobacillus (iv) Bacteria (form curd from milk)

(b) Aspergillus (iii) Fungi (helps to make Penicillin)

(c) Spirogyra (i) Algae (Spiral in shape, filamentous green, often found

in water bodies.

(d) Paramecium (ii) Protozoa (Unicellular organisms, a protozoan, often

found in water bodies).

18. Classify the following into friendly and harmful microorganisms. Yeast,

malarial parasite, Lactobacillus, bread mold, Rhizobium, Bacillus anthracis.

Friendly Harmful

________ ________

________ ________

________ ________

Ans: Friendly microorganisms- Microorganisms which support human life are

known as friendly or useful microorganisms.

Yeast- Yeasts are commercially used in the food industry. It helps in the preparation

of bread loaf, and alcoholic beverages such as wine and beer.

Lactobacillus- Lactobacillus (Lactic Acid Bacteria) helps to make curd from milk.

Rhizobium- Rhizobium is a nitrogen-fixing bacteria present in the root nodules of

leguminous plants and helps in fixing atmospheric nitrogen into nitrates and nitrites

for plants use.

Harmful microorganisms- These kinds of organisms don't help human life and

cause several diseases in humans, plants, and animals, sometimes leading to even

death.

Malarial parasite- Malaria parasite present in the saliva of female Anopheles

mosquito causes malaria by entering in host bodies.



Bacillus anthracis- Bacillus anthracis, a rod-shaped bacterium, causes the deadly

disease anthrax in livestock and occasionally in humans.

Bread mould- Bread mold (Rhizopus) spoils the bread and when this spoiled bread

is consumed it can cause food poisoning.

19. While returning from the school, Boojho ate chaat from a street hawker.

When he reached home, he felt ill and of stomach ache and fell ill. What could

be the reason?

Ans: This could be the result of chaat that might have been contaminated with

bacteria, viruses, parasites, or toxins (pathogenic microbes). Often, street foods get

contaminated by pathogenic or disease-causing microorganisms. The unhygienic

conditions near the shop attract flies and other microbial organisms, which help

pathogens grow. Sometimes the utensils that are used for serving food are also

contaminated by pathogens. When someone eats these contaminated foods, he/she

will suffer from some deadly and painful diseases like dysentery, cholera, diarrhoea,

etc.

20. What will happen to 'pooris' and 'unused kneaded flour ' if they are left in

the open for a day or two?

Ans: The "unused kneaded flour," if kept open for a long time in warm conditions,

may get contaminated by microbes which may cause fermentation of the dough.

Thus, these microbes may spoil the taste and/or texture of food. But it's not

necessarily true for pooris as they are deep-fried in oil, which may kill the microbes.

As a result, they may last longer than the kneaded flour.

21. (a) Name two diseases that are caused by viruses.

Ans: Chicken Pox and Influenza are two viral diseases caused by the Varicella-

zoster virus and Myxovirus influenzae virus, respectively.

(b) Write one important characteristic of a virus.

Ans: A virus remains in a dead condition, but as soon as it comes into contact with

a living organism, it becomes alive. That's why viruses are known as the connecting



links between living and non-living things. They have no effect until they enter a

host cell.

LONG ANSWER QUESTIONS

22. Observe Fig.2.1 and answer the questions that follow

(a) Write the name of the disease.

Ans: The name of the given plant is okra (lady’s finger) and the disease is yellow

vein mosaic of lady’s finger, which is characterized by alternating yellow and green

patches and chlorosis in the veins of the leaves.

(b) Name the causative agent of this disease?

Ans: The causative agent of the disease is the yellow vein mosaic virus.

(c) How does the disease spread from one plant to another?

Ans: The disease spreads from one plant to another by carriers which are flies and

other insects. They sit on any diseased plant and then transfer it to other plants.

(d) Name any two plant diseases and the microbes that cause them.



Ans: Citrus canker is caused by bacterium-Xanthomonas axopodia.

Rust of Wheat is caused by Puccinia triticina which is a fungal disease.

23. How do vaccines work?

Ans: Vaccines contain disease-specific microbial strains that have been weakened

or killed. When a vaccine is introduced into a healthy body, either orally or through

injection, the body fights against the infection by producing suitable antibodies

against the pathogen. These antibodies remain in the body, and the body also

remembers how to fight when the same microbe enters the body again.

24. Observe the set-up given in Fig. 2.2 and answer the following questions.

(a) What happens to the sugar solution in A?

Ans: Yeast causes fermentation and converts the sugar into alcohol (ethyl alcohol

or ethanol) and carbon dioxide gas is released.

(b) Which gas is released in A?



Ans: During the process of fermentation, carbon-di-oxide gas is released from flask

A.

(c) What changes will you observe in B when the released gas passes through

it?

Ans: As soon as the carbon-di-oxide gas passes through the flask B, the lime water

present in it turns milky.

25. Observe Fig.2.3 and answer the following questions.

(a) Name the microorganism and the group to which it belongs.

Ans: The given picture is of bread mold, which is also known as Rhizopus.

Rhizopus/ bread mold belongs to the fungi group.

(b) Name the food item on which the organism grows.

Ans: The organism Rhizopus grows on stale and moist bread.

(c) Does it grow well in dry or in moist conditions?

Ans: The organism grows well in moist conditions.



(d) Is it safe to eat infected bread?

Ans: No, it's not safe to eat infected bread because the harmful toxins produced by

bread mold can spoil the bread and its consumption can lead to food poisoning in

the human body.

26. Give reasons for the following.

(a) Fresh milk is boiled before consumption while processed milk stored in

packets can be consumed without boiling.

Ans: Boiling fresh milk is necessary to kill the harmful or disease-causing

microorganisms present in it. On the other hand, processed milk is stored in packets

and is safe to consume directly because it has been pasteurized. Pasteurized milk is

boiled at 70 degrees Celsius and then cooled rapidly. As a result of pasteurization,

all the microorganisms die and the milk becomes consumable.

(b) Raw vegetables and fruits are kept in refrigerators whereas jams and

pickles can be kept outside.

Ans: The microorganisms present in the open atmosphere easily spoil the vegetable

by attacking it. Refrigerators work at low temperatures, which inhibit the growth of

microorganisms. On the other hand, jams and pickles contain preservatives such as

sugar and salt, which save them from the attack of microorganisms and give them a

long life.

(c) Farmers prefer to grow beans and peas in nitrogen-deficient soils.

Ans: Leguminous plants such as peas and beans have nitrogen-fixing bacteria called

Rhizobium. Rhizobium present in the root nodules of leguminous plants helps in

fixing atmospheric nitrogen and transforming it into nitrates and nitrites for plants'

use. After harvesting, the roots are left in the soil, which results in the enrichment of

the soil with nitrogen and increases its fertility. Therefore, farmers prefer to grow

peas and beans in nitrogen-deficient soil.

(d) Mosquitoes can be controlled by preventing stagnation of water though they

do not live in water. Why?

Ans: Water stagnation is the process in which clean water gets collected in a small

area. Although mosquitoes live on land, they lay eggs in clean water, and the

stagnant water provides a breeding ground for them. Their larvae grow in clean



water. Therefore, by preventing stagnation of water, we can prevent mosquitoes

from breeding (mating). As a result, mosquitoes can be controlled.

27. How can we prevent the following diseases?

(a) Cholera

Ans: Cholera- Cholera is a bacterial disease caused by a bacterium called Vibrio

Cholerae. A person can get cholera by drinking water or eating food contaminated

with the bacterium Vibrio Cholerae. We can prevent cholera by maintaining personal

hygiene and good sanitation, drinking boiled water, and eating properly cooked food.

(b) Typhoid

Ans: Typhoid- Typhoid is a bacterial disease caused by the bacterium Salmonella

Typhi. A person can get infected when they ingest the typhoid bacterium by any

means, such as food, water, etc. To prevent this disease, one should drink boiled or

filtered water and eat properly cooked hygienic food.

(c) Hepatitis

Ans: Hepatitis- Hepatitis is a viral disease caused by the hepatitis virus. The

symptoms of this disease appear after two weeks of infection. To prevent this

disease, infants should be given vaccinations. Apart from this, proper sanitation

practices and hygiene can prevent this disease.

28. Complete the following cycle given as Fig. 2.4 by filling the blanks (Al, (b),

(c), (d).



Ans: The above cycle is the Nitrogen cycle, a biogeochemical cycle that occurs in

the environment. The Nitrogen which is present in the atmosphere cannot be

consumed directly by trees and plants. There are some bacteria and blue-green algae

(Cyanobacteria) present in the soil which fixes atmospheric nitrogen and transforms

it into nitrates and nitrites for plant use. Below are the necessary missing components

which help in fixing atmospheric nitrogen are: -

(a) Nitrogen is fixed by lightning.

(b) Blue-green algae (cyanobacteria) and nitrogen-fixing bacteria fix the

atmospheric nitrogen.

(c) Death and decaying organisms excrete nitrogenous waste.

(d) Bacteria transform compounds of nitrogen into a gaseous state by the process of

denitrification.


NCERT Exemplar Solution for Class 8 Science (Biology ...

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