Lesson: Microorganisms: Friend and Foe

Question: 1

Which of the following reproduces only inside a host cell?

(a) Bacteria

(b) Virus

(c) Amoeba

(d) Fungus



Question: 2

A disease in human beings caused by virus is _______.

(a) Typhoid

(b) Influenza

(c) Dysentery

(d) Cholera



Question: 3

Pathogenic micro-organisms present in host cells are killed by medicines called:

(a) Pain killer

(b) Antibodies

(c) Antibiotics

(d) Vaccines



Question: 4

The two micro-organisms which live in symbiotic association in lichens are:

(a) Fungus and protozoa

(b) Alga and bacteria

(c) Bacteria and protozoa

(d) Alga and fungus



Question: 5

The gas released during the preparation of bread is:

(a) Oxygen

(b) Carbon dioxide

(c) Nitrogen

(d) Sulphur dioxide



Question: 6

The disease caused by a protozoan and spread by an insect is_________.

(a) Dengue

(b) Malaria

(c) Polio

(d) Measles




Question: 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 waste but without packing it in a polythene bag.

Then she 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.



Question: 8

Unscramble the jumbled words underlined in the following statements:

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

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

(c) Xanrhat is a dangerous bacterial disease.

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


(a) Antibodies

(b) Tuberculosis

(c) Anthrax

(d) Fermentation

Question: 9

Suggest a suitable word for each of the following statements.

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

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

(c) Agent which spreads pathogens from one place to another.

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


(a) Preservatives

(b) Rhizobium

(c) Carrier/vector

(d) Antibiotics

Question: 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) small pox vaccine


(v) Typhoid



(a) (iii)

(b) (ii)

(c) (iv)

(d) (i)

Question: 11

Name one commercial use of yeast.


Baking bread/manufacture of alcoholic drinks

Question: 12

Name the process in yeast that converts sugars into alcohol.



Question: 13

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



Question: 14

Why should we avoid standing close to a tuberculosis patient while he/she is coughing?


The tuberculosis (TB) bacteria spread through the air from a person who is ill with active TB to a healthy person standing close to him. The bacteria are contained in small, airborne droplets created by coughing or sneezing, so a person should avoid close contact with a tuberculosis patient.

Question: 15

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


Symptoms of diarrhoea are frequent motion and dehydration. If polio drops are given during this period, the vaccine/medicine may be excreted out with the motion, rendering the polio drops completely ineffective.

Question: 16

Paheli watched her grandmother making 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?


Pouring oil reduces risk of food borne infections, and decreases microbial spoilage as it prevents bacteria, fungus or mould from spoiling the pickle.


Question: 17

Match the microorganisms given in the 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


(a) (iv)

(b) (iii)

(c) (i)

(d) (ii)

Question: 18

Classify the following into friendly and harmful microorganisms.

Yeast, malarial parasite, Lactobacillus, bread mould, Rhizobium, Bacillus anthracis:
















Malarial parasite


Bread mould


Bacillus anthracis


Question: 19

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

When he reached home, he felt ill and complained of stomach ache and fell ill. What could be the reason?


The probable reasons are that the chaat was contaminated by the pathogenic microbes carried by the houseflies that sit on the roadside food due to unhygienic conditions near the shop or the utensil used for serving could have been contaminated due to not washing it properly. Boojho might be prone to diseases, such as typhoid, dysentery or cholera.

Question: 20

What will happen to ‘pooris’ and ‘unused kneaded flour’ if they are left in the open for a day or two?


The unused kneaded flour contains moisture. So, it gets infected with microbes or yeasts, which live and multiply on it easily, and due to fermentation, ultimately spoils the flour. The ‘pooris’ would remain in relatively better condition because they were deep fried in heated oil which, in the process kills microbes.

Question: 21

(a) Name two diseases that are caused by virus.

(b) Write one important characteristic of virus.


(a) Polio/Chicken Pox/Influenza

(b) Virus can reproduce only inside the cells of a living host.


Question: 22

Observe the figure given below and answer the questions that follows.

(a) Write the name of the disease.

(b) Name the causative agent of this disease?

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

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


(a) Yellow vein mosaic of lady’s finger/okra

(b) Virus

(c) The disease spreads from one plant to another through insects (whitefly).

(d)       (i) Citrus canker caused by bacteria  

(ii) Rust of wheat caused by fungus

Question: 23

How do vaccines work?


Vaccines are made up of a weaker version of the bacteria or viruses that are responsible for a disease in a human body. When the vaccine is injected, the body’s immune system detects this weakened or dead version and reacts to it just as it would when a new full-blown infection occurs. It starts making antibodies to fight against the vaccine administered. These antibodies then remain in the body and are ready to react when the actual infection occurs.

Question: 24

Observe the set up given in the figure below and answer the following questions.

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

(b) Which gas is released in A?

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


(a) Yeast causes fermentation and converts sugar into alcohol and carbon dioxide.

(b) Carbon dioxide

(c) Lime water turns into a milky precipitate solution when CO2 gas combines with lime water.

Question: 25

Observe the figure given below and answer the following questions.

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

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

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

(d) Is it safe to eat infected bread?


(a) Bread mould. It is a fungus.

(b) Moist and stale bread.

(c) It grows well in moist conditions.

(d) No, it is not safe to eat infected bread because the fungus spoils the bread by producing toxic substances that cause illness if consumed.

Question: 26

Give reasons for the following.

(a)   Fresh milk is boiled before consumption while processed milk stored in packets can be consumed without boiling.

(b)   Raw vegetables and fruits are kept in refrigerators whereas jams and pickles can be kept outside.

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

(d)   Mosquitoes can be controlled by preventing stagnation of water though they do not live in water. Why?


a)      Pathogens found in raw milk make it unsafe for consumption, so fresh milk is boiled before drinking, in order to kill the microorganisms present in it. On the other hand, the packet milk is pasteurized and for that reason does not have any microorganism, therefore is safe for direct consumption.

b)      Raw vegetables and fruits get easily infected by microorganisms and get spoiled. Hence, they must be kept in refrigerators, as low temperature prevents the growth of microbes. Jams and pickles contain preservatives such as sugar, oil and salt respectively. Hence, they do not get infected by microbes easily.

c)      Beans and peas are leguminous plants and have Rhizobium bacteria in their root nodules. These bacteria can fix atmospheric nitrogen to enrich the soil with nitrogen, thereby increasing its fertility.

d)      Mosquitoes breed on stagnated water. By killing the mosquito larvae and eggs which grow in stagnated water, one can control the growth of mosquitoes.

Question: 27

How can we prevent the following diseases?

(a) Cholera

(b) Typhoid

(c) Hepatitis A


a)      Cholera: It can be prevented by drinking filtered or boiled water, maintaining good sanitation practices, maintaining personal hygiene, and also by keeping food covered to avoid contamination by flies.

b)      Typhoid: It can be prevented by maintaining good hygiene and sanitation practices, that include hand washing, and ensuring that drinking water pipes are kept separated from sewage pipes, avoiding raw and uncooked food, reheating leftover food before eating it and drinking boiled water. Vaccination against the disease also helps to prevent typhoid.

c)      Hepatitis A: It can be prevented by washing hands after using toilet, using proper sanitation methods, avoiding raw and uncooked food, drinking boiled or filtered water. Vaccination against the disease too can prevent hepatitis.

Question: 28

Complete the following cycle given in the figure by filling the blanks (a), (b), (c) and (d).


a)      Lightning fixes nitrogen in the soil

b)      Nitrogen fixing bacteria and blue green algae fix atmospheric nitrogen to the soil

c)      Nitrogenous waste got from decomposition of excreta and dead organisms.

d)      Denitrifying bacteria turn compounds of nitrogen into gaseous nitrogen.