Microorganisms: Friend and Foe (Class 8 Science)

Introduction

Microorganisms are very tiny living organisms that cannot be seen with the naked eye. They can only be seen with the help of a microscope. Microorganisms are found everywhere around us — in air, water, soil, and even inside the bodies of plants and animals.

Some microorganisms are useful (friends) because they help in making food, medicines, and maintaining the balance of nature. However, some microorganisms are harmful (foes) because they cause diseases in humans, animals, and plants.

Thus, microorganisms play both positive and negative roles in our lives.

What are Microorganisms?

Microorganisms are microscopic organisms that are too small to be seen without a microscope.

Examples include:

Bacteria
Fungi
Protozoa
Algae
Viruses

Microorganisms can survive in extreme conditions such as hot springs, deep oceans, deserts, and even inside living organisms.

Types of Microorganisms

Microorganisms are classified into different groups.

1. Bacteria

Bacteria are single-celled microorganisms. They are found in many places such as soil, water, air, and inside living organisms.

Examples:

Lactobacillus
Rhizobium
E. coli

Some bacteria are useful while others cause diseases.

2. Fungi

Fungi are microorganisms that grow on dead and decaying matter.

Examples:

Yeast
Bread mould
Mushroom

Yeast is used in baking and fermentation processes.

3. Protozoa

Protozoa are single-celled organisms that live in water and moist environments.

Examples:

Amoeba
Paramecium
Plasmodium

Some protozoa cause diseases such as malaria.

4. Algae

Algae are simple plant-like microorganisms that live in water and can produce their own food through photosynthesis.

Examples:

Chlamydomonas
Spirogyra

5. Viruses

Viruses are extremely tiny microorganisms. They can only reproduce inside the cells of living organisms.

Examples:

Virus causing common cold
Polio virus
Influenza virus

Viruses become active only when they enter a living cell.

Friendly Microorganisms

Many microorganisms are beneficial to humans and the environment.

1. Making Food

Microorganisms help in preparing many food items.

Examples:

Lactobacillus converts milk into curd.
Yeast is used to make bread, cakes, and pastries.
Bacteria help in making cheese and yogurt.

2. Industrial Production

Microorganisms are used in industries to produce substances such as:

Alcohol
Vinegar
Organic acids
Enzymes

Yeast is commonly used in alcohol production through fermentation.

3. Medicines

Microorganisms are used to produce medicines called antibiotics.

Examples:

Penicillin
Streptomycin
Tetracycline

Antibiotics help kill or stop the growth of harmful bacteria.

Microorganisms are also used to produce vaccines, which protect us from diseases.

4. Agriculture

Some microorganisms help improve soil fertility.

Example:

Rhizobium bacteria live in the root nodules of leguminous plants such as peas and beans. They fix nitrogen from the atmosphere and convert it into a form that plants can use.

This process increases soil fertility.

5. Cleaning the Environment

Microorganisms help in decomposing dead plants and animals into simpler substances.

This process:

Recycles nutrients in nature
Maintains environmental balance
Improves soil fertility

Harmful Microorganisms

Some microorganisms cause diseases and spoil food.

1. Disease Causing Microorganisms

Microorganisms that cause diseases are called pathogens.

Examples of human diseases:

Disease	Causing Microorganism
Cholera	Bacteria
Tuberculosis	Bacteria
Malaria	Protozoa
Common Cold	Virus
Measles	Virus

These diseases can spread through air, water, food, or insect bites.

2. Diseases in Animals

Microorganisms also cause diseases in animals.

Examples:

Anthrax in cattle (caused by bacteria)
Foot and mouth disease in animals (caused by virus)

These diseases reduce animal productivity.

3. Diseases in Plants

Microorganisms can damage crops and reduce agricultural production.

Examples:

Citrus canker in plants (bacteria)
Rust of wheat (fungus)
Yellow vein mosaic of bhindi (virus)

Farmers use pesticides and other methods to control these diseases.

Food Spoilage by Microorganisms

Microorganisms grow on food and spoil it.

Spoiled food may develop:

Bad smell
Change in color
Sour taste
Fungus growth

Eating spoiled food can cause food poisoning.

Food Preservation

Food preservation is the method of preventing food from spoilage.

Common methods include:

1. Drying

Removing moisture from food prevents microbial growth.

Example: drying grains, fruits, and fish.

2. Refrigeration

Low temperature slows down microbial growth.

Example: storing food in refrigerators.

3. Salting

Salt removes water from food and prevents microbial growth.

Example: preservation of pickles and fish.

4. Sugaring

High sugar concentration prevents microbial growth.

Example: jams and jellies.

5. Use of Chemical Preservatives

Certain chemicals prevent microbial growth.

Examples:

Sodium benzoate
Sodium metabisulphite

6. Pasteurization

Pasteurization is the process of heating milk to kill harmful microorganisms.

It was discovered by Louis Pasteur.

Steps of pasteurization:

Milk is heated to about 70°C for 15–30 seconds.
Then it is rapidly cooled.
The milk is stored safely.

This process kills harmful bacteria without affecting the taste of milk.

Nitrogen Cycle

Nitrogen is an essential nutrient for plant growth.

Although nitrogen is abundant in the atmosphere, plants cannot use it directly.

Microorganisms help convert atmospheric nitrogen into usable forms through the nitrogen cycle.

Important microorganisms involved:

Rhizobium
Nitrogen-fixing bacteria
Blue-green algae

These microorganisms convert nitrogen into nitrates and nitrites that plants can absorb.

Conclusion

Microorganisms are tiny living organisms found everywhere in nature. They play an important role in our daily life. Some microorganisms are beneficial because they help in making food, medicines, and maintaining soil fertility. They also help clean the environment by decomposing waste materials.

However, some microorganisms are harmful and cause diseases in humans, animals, and plants. They can also spoil food.

Understanding microorganisms helps us use their benefits and protect ourselves from their harmful effects. Therefore, microorganisms are rightly called “Friends and Foes.”

Microorganisms: Friend and Foe — Detailed Explanation (Class 8 Science)

Microorganisms are among the smallest living things on Earth, yet they play a very important role in our daily life and in nature. Although they are invisible to the naked eye, their activities affect food production, health, agriculture, and the environment. Because some microorganisms help humans while others cause diseases and damage, they are called “Friends and Foes.”

This chapter explains what microorganisms are, their types, their useful roles, their harmful effects, and how we can control harmful microorganisms.

1. What are Microorganisms?

Microorganisms are very small living organisms that cannot be seen without a microscope. They are also called microbes.

They are found everywhere:

In soil
In water
In the air
Inside plants and animals
On our skin
Even in extreme environments like hot springs and polar regions

Some microorganisms live independently, while others live inside other organisms as parasites.

Microorganisms reproduce very quickly. Under suitable conditions such as warmth, moisture, and nutrients, they multiply rapidly.

2. Major Groups of Microorganisms

Microorganisms are classified into several groups based on their structure and characteristics.

(a) Bacteria

Bacteria are single-celled organisms. They are among the most common microorganisms on Earth.

Characteristics:

Very small and simple in structure
Can survive in different environments
Some are useful while others cause diseases

Examples of useful bacteria:

Lactobacillus – converts milk into curd
Rhizobium – fixes nitrogen in soil

Examples of harmful bacteria:

Bacteria causing cholera
Bacteria causing tuberculosis

(b) Fungi

Fungi are microorganisms that grow on dead and decaying organic matter. They obtain nutrients by breaking down organic substances.

Examples:

Yeast
Bread mould
Mushroom

Uses:

Yeast helps in fermentation for making bread and alcohol.

Some fungi also spoil food and cause plant diseases.

(c) Protozoa

Protozoa are single-celled organisms that live mainly in water and moist environments.

Examples:

Amoeba
Paramecium
Plasmodium

Some protozoa are harmless, but others cause diseases.

Example:

Plasmodium causes malaria in humans.

(d) Algae

Algae are plant-like microorganisms that contain chlorophyll and can perform photosynthesis.

They mostly live in water bodies such as ponds and lakes.

Examples:

Chlamydomonas
Spirogyra

Some algae are useful because they produce oxygen and help maintain the ecological balance.

(e) Viruses

Viruses are extremely tiny microorganisms. They are smaller than bacteria and can only reproduce inside the cells of living organisms.

Characteristics:

Inactive outside living cells
Active only inside host cells

Examples of viral diseases:

Common cold
Influenza
Measles
Polio

Viruses infect humans, animals, and plants.

3. Useful Microorganisms (Friends)

Many microorganisms provide important benefits to humans, agriculture, and the environment.

(a) Microorganisms in Food Production

Microorganisms help in preparing many food products.

Examples:

Curd formation
Lactobacillus bacteria convert milk sugar (lactose) into lactic acid, which turns milk into curd.
Bread making
Yeast produces carbon dioxide gas during fermentation, making the dough rise and become soft.
Cheese and yogurt production
Special bacteria are used to prepare these foods.

(b) Microorganisms in Industry

Industries use microorganisms to produce various useful substances.

Examples:

Alcohol production
Vinegar production
Organic acids
Enzymes used in food processing

Fermentation is a process in which microorganisms break down sugars to produce alcohol and other substances.

(c) Microorganisms in Medicine

Microorganisms help in producing antibiotics.

Antibiotics are medicines that kill or stop the growth of harmful bacteria.

Examples:

Penicillin
Streptomycin
Tetracycline

Vaccines are also prepared using microorganisms. Vaccines protect us from diseases by helping our immune system recognize harmful microbes.

Example vaccines:

Polio vaccine
Measles vaccine

(d) Microorganisms in Agriculture

Microorganisms help improve soil fertility.

Example:

Rhizobium bacteria live in the root nodules of leguminous plants such as peas, beans, and gram. They convert nitrogen from the air into compounds that plants can use.

This process is called nitrogen fixation.

As a result, soil fertility increases.

(e) Microorganisms as Decomposers

Microorganisms break down dead plants and animals into simpler substances.

This process is called decomposition.

Benefits:

Recycles nutrients in the soil
Keeps the environment clean
Maintains ecological balance

4. Harmful Microorganisms (Foes)

Although many microorganisms are useful, some are harmful.

(a) Disease-Causing Microorganisms

Microorganisms that cause diseases are called pathogens.

Diseases in Humans

Examples:

Disease	Microorganism
Cholera	Bacteria
Tuberculosis	Bacteria
Malaria	Protozoa
Measles	Virus
Polio	Virus

These diseases spread through:

Air
Contaminated water
Food
Insect bites

(b) Diseases in Animals

Some microorganisms infect animals.

Examples:

Anthrax in cattle (bacteria)
Foot and mouth disease (virus)

These diseases reduce livestock productivity.

(c) Diseases in Plants

Microorganisms can destroy crops and reduce food production.

Examples:

Plant Disease	Cause
Citrus canker	Bacteria
Rust of wheat	Fungus
Yellow vein mosaic	Virus

Farmers use pesticides to control these diseases.

5. Food Spoilage

Microorganisms grow rapidly on food under warm and moist conditions. They break down food substances and make them unfit to eat.

Signs of food spoilage:

Bad smell
Change in colour
Sour taste
Growth of fungus

Eating spoiled food can lead to food poisoning.

6. Food Preservation

Food preservation prevents microorganisms from growing on food.

Common methods:

Drying

Removing moisture prevents microbial growth.

Refrigeration

Low temperature slows down microbial growth.

Salting

Salt removes water from food.

Sugaring

High sugar concentration prevents microbial growth.

Pickling

Oil and vinegar prevent microbial growth.

Chemical Preservatives

Examples:

Sodium benzoate
Sodium metabisulphite

7. Pasteurization

Pasteurization is a method used to kill harmful microorganisms in milk.

Process:

Milk is heated to about 70°C for 15–30 seconds.
It is quickly cooled.
It is stored safely.

This process was discovered by Louis Pasteur.

Pasteurization makes milk safe to drink without destroying nutrients.

8. Nitrogen Cycle

Nitrogen is essential for plant growth. However, plants cannot use nitrogen directly from the atmosphere.

Microorganisms help convert nitrogen into usable forms through the nitrogen cycle.

Important microorganisms involved:

Nitrogen-fixing bacteria
Rhizobium
Blue-green algae

These microorganisms convert nitrogen into nitrates and nitrites that plants can absorb.

Conclusion

Microorganisms are tiny living organisms that exist everywhere in nature. They have both beneficial and harmful effects. Useful microorganisms help in food production, medicine manufacturing, agriculture, and environmental cleaning. Harmful microorganisms cause diseases and spoil food.

Understanding microorganisms helps us use their benefits and protect ourselves from their harmful effects. Therefore, microorganisms play a vital role in maintaining life on Earth and are rightly called “Friends and Foes.”

30 Important MCQs

Chapter: Microorganisms – Friend and Foe (Class 8 Science)

1.

Microorganisms can be seen only with the help of a:
A. Telescope
B. Microscope
C. Magnifying glass
D. Binocular
Answer: B. Microscope

2.

Which of the following is NOT a microorganism?
A. Bacteria
B. Virus
C. Mushroom
D. Dog
Answer: D. Dog

3.

Which microorganism converts milk into curd?
A. Rhizobium
B. Lactobacillus
C. Yeast
D. Amoeba
Answer: B. Lactobacillus

4.

Yeast is used in the production of:
A. Oxygen
B. Alcohol
C. Nitrogen
D. Sugar
Answer: B. Alcohol

5.

Which of the following microorganisms causes malaria?
A. Virus
B. Bacteria
C. Protozoa
D. Fungi
Answer: C. Protozoa

6.

The microorganism that causes cholera is:
A. Bacteria
B. Virus
C. Fungi
D. Algae
Answer: A. Bacteria

7.

Which microorganism helps in nitrogen fixation?
A. Yeast
B. Rhizobium
C. Amoeba
D. Virus
Answer: B. Rhizobium

8.

Microorganisms that cause diseases are called:
A. Antibiotics
B. Pathogens
C. Vaccines
D. Preservatives
Answer: B. Pathogens

9.

Which microorganism causes the common cold?
A. Bacteria
B. Virus
C. Protozoa
D. Algae
Answer: B. Virus

10.

The process of converting sugar into alcohol is called:
A. Fermentation
B. Pasteurization
C. Respiration
D. Photosynthesis
Answer: A. Fermentation

11.

Which scientist discovered pasteurization?
A. Isaac Newton
B. Louis Pasteur
C. Albert Einstein
D. Charles Darwin
Answer: B. Louis Pasteur

12.

Antibiotics are used to kill:
A. Harmful bacteria
B. Plants
C. Animals
D. Viruses
Answer: A. Harmful bacteria

13.

Penicillin is an example of:
A. Fertilizer
B. Antibiotic
C. Vaccine
D. Vitamin
Answer: B. Antibiotic

14.

Which microorganism is used in baking bread?
A. Yeast
B. Bacteria
C. Virus
D. Algae
Answer: A. Yeast

15.

Which of the following is a viral disease?
A. Cholera
B. Tuberculosis
C. Measles
D. Malaria
Answer: C. Measles

16.

The microorganism that causes tuberculosis is:
A. Bacteria
B. Virus
C. Protozoa
D. Fungi
Answer: A. Bacteria

17.

Food spoilage occurs due to the growth of:
A. Plants
B. Microorganisms
C. Animals
D. Rocks
Answer: B. Microorganisms

18.

The process of heating milk to kill harmful microorganisms is called:
A. Fermentation
B. Pasteurization
C. Filtration
D. Condensation
Answer: B. Pasteurization

19.

Which method of food preservation uses salt?
A. Salting
B. Refrigeration
C. Drying
D. Boiling
Answer: A. Salting

20.

Which method of preservation uses low temperature?
A. Drying
B. Refrigeration
C. Sugaring
D. Pickling
Answer: B. Refrigeration

21.

Which microorganism causes rust of wheat?
A. Virus
B. Bacteria
C. Fungus
D. Protozoa
Answer: C. Fungus

22.

Which of the following is a protozoan disease?
A. Cholera
B. Malaria
C. Measles
D. Polio
Answer: B. Malaria

23.

Microorganisms help in decomposition of:
A. Living plants
B. Dead plants and animals
C. Stones
D. Metals
Answer: B. Dead plants and animals

24.

Which of the following is NOT a food preservation method?
A. Refrigeration
B. Salting
C. Heating
D. Painting
Answer: D. Painting

25.

Nitrogen is fixed in soil mainly by:
A. Fungi
B. Rhizobium bacteria
C. Virus
D. Yeast
Answer: B. Rhizobium bacteria

26.

Which disease spreads through mosquito bites?
A. Cholera
B. Malaria
C. Tuberculosis
D. Measles
Answer: B. Malaria

27.

Which microorganism is used in making cheese and yogurt?
A. Bacteria
B. Virus
C. Protozoa
D. Algae
Answer: A. Bacteria

28.

The study of microorganisms is called:
A. Biology
B. Microbiology
C. Botany
D. Zoology
Answer: B. Microbiology

29.

Vaccines help in:
A. Preventing diseases
B. Causing diseases
C. Spoiling food
D. Killing plants
Answer: A. Preventing diseases

30.

Which microorganism group includes Amoeba and Paramecium?
A. Protozoa
B. Fungi
C. Algae
D. Bacteria
Answer: A. Protozoa

Case Study Questions

Chapter: Microorganisms – Friend and Foe (Class 8 Science)

Case Study 1: Curd Formation

Rina’s mother boiled milk and allowed it to cool. She then added a small amount of curd to the milk and kept it in a warm place overnight. The next morning the milk had turned into curd.

Questions

Which microorganism helps in converting milk into curd?
A. Yeast
B. Lactobacillus
C. Rhizobium
D. Amoeba
Answer: B. Lactobacillus
What is the process of converting milk into curd called?
Answer: Fermentation.
Why is milk kept in a warm place for curd formation?
Answer: Warm temperature helps bacteria grow and convert milk into curd.

Case Study 2: Spread of Malaria

During the rainy season, stagnant water collected in many places in a village. After some time, many people in the village suffered from fever caused by malaria.

Questions

Which microorganism causes malaria?
A. Virus
B. Bacteria
C. Protozoa
D. Fungi
Answer: C. Protozoa
Name the protozoan that causes malaria.
Answer: Plasmodium.
Which insect spreads malaria?
Answer: Female Anopheles mosquito.

Case Study 3: Bread Making

A baker prepares bread by mixing flour, sugar, and yeast. The dough rises after some time because a gas is produced during fermentation.

Questions

Which microorganism is used in bread making?
A. Yeast
B. Bacteria
C. Virus
D. Algae
Answer: A. Yeast
Which gas causes the bread dough to rise?
Answer: Carbon dioxide.
What is the process called when yeast converts sugar into alcohol and carbon dioxide?
Answer: Fermentation.

Case Study 4: Nitrogen Fixation

A farmer grows pea plants in his field. Scientists explain that special bacteria live in the root nodules of pea plants and help increase soil fertility by converting atmospheric nitrogen into useful compounds.

Questions

Name the bacteria that live in the root nodules of leguminous plants.
A. Lactobacillus
B. Rhizobium
C. Yeast
D. Amoeba
Answer: B. Rhizobium
What is the process of converting nitrogen into usable form called?
Answer: Nitrogen fixation.
Why is nitrogen important for plants?
Answer: It helps in plant growth and formation of proteins.

Case Study 5: Food Preservation

Ravi’s mother prepared mango pickle and stored it in a jar with oil and salt. The pickle remained fresh for many months without spoiling.

Question

Which method of food preservation is used in this case?
A. Refrigeration
B. Drying
C. Pickling
D. Freezing
Answer: C. Pickling
Why do oil and salt help preserve food?
Answer: They prevent the growth of microorganisms.
Name one chemical preservative used in food.
Answer: Sodium benzoate or sodium metabisulphite.

Higher-Level Case Study Questions

Chapter: Microorganisms – Friend and Foe (Class 8 Science)

Case Study 1: Vaccination and Disease Prevention

In a village health camp, doctors vaccinated children against diseases such as polio and measles. They explained that vaccines help the body develop immunity against harmful microorganisms and prevent diseases in the future.

Questions

What is a vaccine?
Answer: A vaccine is a preparation made from weakened or dead microorganisms that helps the body develop immunity against diseases.
Vaccines help in:
A. Causing diseases
B. Preventing diseases
C. Spoiling food
D. Killing plants
Answer: B. Preventing diseases
Name one disease that can be prevented by vaccination.
Answer: Polio / Measles / Hepatitis.
Which type of microorganism causes polio?
Answer: Virus.

Case Study 2: Food Spoilage

Ravi left cooked rice uncovered at room temperature during summer. After some time, the rice developed a bad smell and became unfit to eat. This happened due to the rapid growth of microorganisms.

Questions

What is the reason for food spoilage in this case?
Answer: Growth of microorganisms such as bacteria and fungi.
Which condition helps microorganisms grow quickly in food?
A. Low temperature
B. Warm temperature
C. Dry environment
D. Lack of oxygen
Answer: B. Warm temperature
Name one method to prevent food spoilage.
Answer: Refrigeration / drying / salting.
What may happen if spoiled food is eaten?
Answer: Food poisoning.

Case Study 3: Pasteurization of Milk

Milk collected from dairy farms contains many microorganisms. To make milk safe for drinking, it is heated to about 70°C for a short time and then quickly cooled before packaging.

Questions

What is this process called?
A. Fermentation
B. Pasteurization
C. Filtration
D. Condensation
Answer: B. Pasteurization
Who discovered pasteurization?
Answer: Louis Pasteur.
Why is pasteurization important?
Answer: It kills harmful microorganisms present in milk.
Does pasteurization destroy the nutrients of milk?
Answer: No, it mainly kills harmful bacteria without affecting nutrients.

Case Study 4: Plant Diseases

A farmer noticed yellow spots on the leaves of his crop plants. The crop yield decreased due to infection caused by microorganisms. Agricultural experts advised him to use proper pesticides and disease-resistant varieties.

Questions

Microorganisms that cause plant diseases are called:
A. Vaccines
B. Pathogens
C. Fertilizers
D. Preservatives
Answer: B. Pathogens
Which microorganism causes rust of wheat?
A. Bacteria
B. Virus
C. Fungus
D. Protozoa
Answer: C. Fungus
Name one plant disease caused by bacteria.
Answer: Citrus canker.
Why do plant diseases reduce crop production?
Answer: They damage plant tissues and affect plant growth.

Case Study 5: Nitrogen Cycle

Nitrogen is an essential nutrient for plants. Although the atmosphere contains a large amount of nitrogen, plants cannot use it directly. Certain bacteria and blue-green algae help convert atmospheric nitrogen into compounds that plants can absorb.

Questions

The process of converting atmospheric nitrogen into usable compounds is called:
A. Fermentation
B. Nitrogen fixation
C. Photosynthesis
D. Respiration
Answer: B. Nitrogen fixation
Which bacteria fix nitrogen in the roots of leguminous plants?
A. Lactobacillus
B. Rhizobium
C. Yeast
D. Amoeba
Answer: B. Rhizobium
Why is nitrogen important for plants?
Answer: It is necessary for growth and protein formation.
Name one leguminous plant that contains Rhizobium bacteria.
Answer: Pea / Bean / Gram.

Short Answer Questions (2 Marks Each)

Chapter: Microorganisms – Friend and Foe (Class 8 Science)

1. What are microorganisms?

Answer:
Microorganisms are very small living organisms that cannot be seen with the naked eye and can only be seen with a microscope.

2. Name four groups of microorganisms.

Answer:

Bacteria
Fungi
Protozoa
Algae
(Also viruses)

3. What is fermentation?

Answer:
Fermentation is the process in which microorganisms like yeast convert sugar into alcohol and carbon dioxide.

4. What are pathogens?

Answer:
Pathogens are disease-causing microorganisms.

5. Name two diseases caused by bacteria.

Answer:

Cholera
Tuberculosis

6. Name two diseases caused by viruses.

Answer:

Polio
Measles

7. What is pasteurization?

Answer:
Pasteurization is the process of heating milk to about 70°C for a short time and then cooling it quickly to kill harmful microorganisms.

8. What are antibiotics?

Answer:
Antibiotics are medicines that kill or stop the growth of harmful bacteria.

Example: Penicillin.

9. How do microorganisms help in agriculture?

Answer:
Microorganisms such as Rhizobium fix nitrogen in the soil and increase soil fertility.

10. What is food preservation?

Answer:
Food preservation is the method of preventing food from spoilage by microorganisms.

Long Answer Questions (4–5 Marks Each)

1. Explain the different types of microorganisms.

Answer:
Microorganisms are classified into five main groups:

Bacteria: Single-celled organisms found in soil, water, and air. Some are useful while others cause diseases.
Fungi: Organisms like yeast and mould that grow on dead and decaying matter.
Protozoa: Single-celled organisms found in water, such as Amoeba and Paramecium.
Algae: Plant-like organisms found in water that perform photosynthesis.
Viruses: Extremely small microorganisms that reproduce only inside living cells.

2. Explain the useful roles of microorganisms.

Answer:
Microorganisms are useful in many ways:

Food production: Lactobacillus makes curd and yeast is used in bread making.
Medicine: Microorganisms produce antibiotics such as penicillin.
Agriculture: Rhizobium bacteria fix nitrogen in soil and improve fertility.
Industry: Microorganisms produce alcohol and organic acids.
Decomposition: They break down dead plants and animals and recycle nutrients.

3. Describe the harmful effects of microorganisms.

Answer:
Some microorganisms cause diseases and spoil food.

Human diseases: Cholera, tuberculosis, measles, malaria.
Animal diseases: Anthrax and foot-and-mouth disease.
Plant diseases: Citrus canker, rust of wheat, yellow vein mosaic of bhindi.
Food spoilage: Microorganisms grow on food and make it unfit for consumption.

4. Explain different methods of food preservation.

Answer:
Food preservation prevents the growth of microorganisms.

Common methods include:

Drying: Removing moisture from food.
Refrigeration: Low temperature slows microbial growth.
Salting: Salt prevents growth of microorganisms.
Sugaring: Sugar preserves jams and jellies.
Pickling: Oil and vinegar prevent microbial growth.
Chemical preservatives: Sodium benzoate and sodium metabisulphite.

5. Explain the nitrogen cycle.

Answer:
The nitrogen cycle is the process by which nitrogen is converted into different forms in nature.

Nitrogen gas is present in the atmosphere.
Nitrogen-fixing bacteria such as Rhizobium convert nitrogen into usable compounds.
Plants absorb these compounds from the soil.
Animals obtain nitrogen by eating plants.
When plants and animals die, microorganisms decompose them and return nitrogen to the soil and atmosphere.

The nitrogen cycle helps maintain the balance of nitrogen in nature.

Class 8 Science Test Paper

Chapter: Microorganisms – Friend and Foe
Time: 1 Hour
Maximum Marks: 40

Section A – MCQs (1 × 10 = 10 Marks)

Microorganisms can be seen only with the help of a:
a) Telescope
b) Microscope
c) Binocular
d) Magnifying glass
Which microorganism is used in making bread?
a) Bacteria
b) Virus
c) Yeast
d) Algae
Rhizobium bacteria help plants by:
a) Causing disease
b) Fixing nitrogen in soil
c) Spoiling food
d) Producing oxygen
Which of the following is a viral disease?
a) Tuberculosis
b) Cholera
c) Polio
d) Typhoid
Lactobacillus helps in making:
a) Bread
b) Pickle
c) Curd
d) Vinegar
Antibiotics are used to kill:
a) Harmful bacteria
b) Plants
c) Animals
d) Soil
Which method is used to preserve jams and jellies?
a) Salting
b) Sugaring
c) Drying
d) Refrigeration
Pasteurization is mainly used for:
a) Milk
b) Fruits
c) Vegetables
d) Bread
The process of converting sugar into alcohol by yeast is called:
a) Respiration
b) Fermentation
c) Digestion
d) Photosynthesis
Which microorganism causes malaria?
a) Virus
b) Bacteria
c) Protozoa
d) Algae

Section B – Case Study Questions (2 × 5 = 10 Marks)

Case Study 1

Riya’s mother was making bread at home. She mixed flour, water, sugar and yeast and kept the dough in a warm place. After some time the dough rose and became soft.

Questions:

Which microorganism is used in making bread?
What process causes the dough to rise?
Which gas is released during this process?
What is this process called?
Name another food made using microorganisms.

Case Study 2

Rahul bought milk from the market. His mother boiled the milk and stored it in the refrigerator. She said this helps to keep the milk safe for longer.

Questions: