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Class 8th-Science: Chapter 2- Microorganisms Friend and Foe


The living organisms which are so small that they cannot be seen with naked eyes are called as microorganisms or microbes.

And they can only be seen with the help of microscope.

Types of Microorganisms

1. Bacteria 

Example: Spiral shaped bacteria, Rod shaped bacteria

2. Fungi

Example: Bread mould, Penicillium, Aspergillus

3. Protozoa

Example: Amoeba, Paramecium

4. Algae 

Example: Chlamydomonas, Spirogyra


(i) Viruses are also microorganisms.

(ii) But they are quite different from other microorganisms.

(iii) They reproduce only inside the cells of the host organism, which may be a bacterium, plant or animal.

Where do Microorganisms Live?


They can survive under all types of environment.

They are also foun inside the bodies of animals including humans.

How many Microorganisms?

(i)Microorganisms are too many to count.

(ii) They are very small. But they make up the largest number of living organisms on the planet.

(iii) It’s not millions, billions, or trillions. There are trillions of trillions of trillions of microbes around the Earth. Maybe more.

(iv) Ther are more viruses than stars in the universe.

How do Microorganisms Live?

(i) A protozoan like an amoeba might spent its whole life alone, cruising through the water.

(ii) While fungi and bacteria may live in colonies.

(iii) Microorganisms may be single-celled like bacteria, some algae and protozoa, or multicellular, such as algae and fungi.

(iv) Some microorganisms grow on other organisms while others exist freely.

(v) They can help us or hurt us.

(vi) Microorganisms were the first one to set foot on Earth 3500 million years ago.

(vii) Humans —-> 0.3 million years ago.

(viii) Microbes have always existed, scientists did not always know they were there.

(ix) It took one invention to change the way we see the world of microbes – the microscope.

(x) It 1673, Anton von Leeuwenhoek made the fist microscope.

Use of Microorganisms

1. Making of curd, bread and cake

   i  Microorganisms are used in the preparation of curd, bread and cake.

   ii  They are also used for the large scale production of alcohol, wine and acetic               acid (vinegar).

   iii  They are also used in cleaning up of the environment.

   iv  They are also used in the preparation of medicines.

   v  In agriculture, they are used to increase soil fertility by fixing nitrogen.  

2. Commercial Use of Microorganisms

   Yeast(Alcohol, wine, acetic acid.) Fermentation

3. Medicinal Use of Microorganisms

   i  Antibiotics 

   ii  Vaccination

   iii Antibodies

4. Cleaning the Environment

Microorganisms decompose dead organic waste of plants and animals  converting 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.

5. Increasing Soil Fertility

Some bacteria are able to fix nitrogen from the atmosphere to enrich soil with nitrogen and increase its fertility. These microbes are commonly called biological nitrogen fixers.

Harmful Microorganisms


Some of the micoorgenisms cause diseases in human beings, plants and animals. Such disease-causing microorganisms are called pathogens.

For example: Coronavirus, dengue virus.

Some microorganisms also spoil food, clothing and leather

Harms caused by Microorganisms

1.  Disease – causing Microorganisms in Humans

2.  Disease – causing Microorganisms in Animals

3.  Disease – causing Microorganisms in Plants 

4.  Food Poisoning

1.  Disease – causing Microorganisms in Humans

Pathogens enter our body through the air we breathe, the water we drink or the food we eat.

They can also get transmitted by direct contact with an infected person or carried through an amimal.

Communicable Diseases:

Microbial diseases that can spread from an infacted person to a healthy person through air, water, food or physical contact are called communicable diseases.

For Example:  Cholera, common cold, chicken pox and tuberculosis.

When a person suffering from common cold sneezes, fine droplets of moisture carrying thousands of viruses are spread in the air. The virus may enter the body of a healthy person while breathing.

Q. Then how do you prevent the spread of communicable diseases?

Ans.  We should keep a handkerchief on the nose and mouth while sneezing. It is better to keep a distance from infected persons.


Those living organisms that carry disease-causing microbes are called carriers.

For Example:  


Female Anopheles Mosquito – It carries the parasite of malaria.

Female Aedes Mosquito – It acts as a carrier of dengue virus.

The flies sit on the garbage and animal excreta. Pathogens stick to their bodies.  when these flies sit on uncovered food they transfer the pathogens to the food. Thus, flies act as carriers.

So, it is advisable to always keep food covered. Avoid consuming uncovered items of food.

Q.  Why does the teacher keep telling us not to let water collect anywhere in the neighbourhood?

Ans. All mosquitoes breed in water. Hence, one should not let water collect anywhere, in coolers, tyres, flower pot etc.

   By keeping the surroundings clean and dry we can prevent mosquitoes from breeding.

Table 2.1: Some Common Human Diseases caused by Microorganisms

Human DiseaseCausative MicroorganismMode of TransmissionPreventive measures (General)
TuberculosisMeaslesChicken PoxPolioBacteriaVirusVirusVirusAirAirAir/ContactAir/WaterKeep the patient in complete isolation. Keep the personal belongings of the patient away from those of the others. Vaccination to be given at suitable age.
CholeraTyphoidBacteriaBacteriaWater/FoodWaterMaintain personal hygiene and good sanitary habits. Consume properly cooked food and boiled drinking water. Vaccination.
Hepatitis BVirusWaterDrink boiled drinking water. Vaccination.
MalariaprotozoaMosquitouse mosquito net and repellents. Spray insecticides and control breeding of mosquitoes by not allowing water to collect in the surroundings.

2.  Disease – causing Micrrorganisms in Animals

Anthrax is a dangerous human and cattle disease caused by a bacterium. 

   Food and mouth disease of cattle is caused by a virus.

3. Disease – causing Micrrorganisms in Plants

Several microorganisms cause diseases in plants like wheat, rice, potato, sugarcane, orange, apple and others.

      The diseases reduce the yield of crops.

They can be controlled by the use of certain chemicals which kill the microbes.

Table 2.2:  Some Common Plant Diseases caused by Microorganisms

Plant DiseasesMicro-organismMode of TransmissionFigures
Citrus cankerBacteriaAir
Rust of wheatFungiAir, seeds
Yellow vein mosaic of bhindi (Okra)VirusInsect

4.  Food Poisoning

Boojho was invited by his friend to a party and he ate a variety of foodstuff.

  On reaching home he started vomiting. He had to be taken to a hospital.

The doctor said that this condition could be due to food poisoning.

Q.  What is Food Poisoning?

Ans.  The ill effect on health caused by consumption of contaminated food is called food poisoning. And the food gets contaminated due to some toxic secretion produced by microbes.

Food Preservation

Q.  How do we preserve cooked food at home?

Ans.  We preserve cooked food at home by keeping it in refrigerators. Low temperature inhibits the growth of microbes.

Q.  In spoiling of food a chemical reaction?

Ans.  Yes, it is a chemical reaction. Because spoiled food emits had smell and has a bad taste and chenged colour. Besides, spoiled food cannot be converted back into fresh food.

Common Methods used to Preserve Food in Our Homes

1.  Chemical Method

Preservatives:  Preservatives are natural or man-made chemicals that are added to foods to prevent the attack of microbes on them and hence stop them from spoiling.

Examples:  Salts, edible oils, sodium benzoate, sodium metabisulphite etc.

2.  Preservation by Common Salt

Common salt has been used to preserve meat and fish for ages.

Salting is also used to preserve amla, raw mangoes, tamarind etc.

3.  Preservation by Sugar

Jams, jellies and squashes are preserved by sugar.

Sugar reduces the moisture content which inhibits the growth of bacteria which spoil food.

4.  Preservation by Oil and Vinegar

Use of oil and vinegar prevents spoilage of pickles because bacteria cannot live in such an environment.

Oil and vinegar are also used to preserve vegetables, fruits, fish and meat.

Q.  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 pickle her grandmother makes does not spoil for a long time.She is confused. Why does this happen?

Ans.  The mangoes were acted upon by microorganisms which spoiled them. But the mango pickels made by Psheli’s grandmother contain preservetives like salt and oil, which prevent the attack of microbes.

5.  Heat and Cold Treatments

You must have observed your mother boiling milk before it is stored or used. Boiling kills many microorganisms.

Similarly, we keep our food in the refrigerator. Low temperature inhidits the growth of microbes.

Why does the milk that comes in packets not spoil? My mother told me that the milk is ‘pasteurized’. What is pasteurization?

The milk that comes in packets does not spoil because it is pasteurized.

Pasteurization is a process in which the milk is first heated to a temperature of about 700C for 15 to 30 seconds and then suddenly chilled and stored. By doing so, it prevents the growth of microbes. This process was discovered by Louis Pasteur. That’s why it is called pasteurization.

6.  Storage and Packing

These days Dry fruits and even vegetables are sold in sealed air tight packets to prevent the attack of microbes.

Importance of Nitrogen

1.  Nitrogen is very important for survival of all living organisms.

Nitrogen —–> Proteins, Chlorophyll, Nucleic Acids, Vitamins

2.  Nitrogen is one of the essential constituents of proteins, chlorophyll, nucleic acids and vitamins.

3.  Therefore, nitrogen is very important for survival of all living organisms.

4.  The atmospheric nitrogen (N2) cannot be taken directly by plants and animals like they take CO2 and O2 respectively.

5.  This problem is solved by plants for animals.

6.  They synthesize compounds having nitrogen.

7.  And when animal consume plants, nitrogen enters in their body.

8.  But even plants too can not take nitrogen directly from atmosphere.

Certain bacteria and blue green algae present in the soil fix nitrogen from the atmosphere and convert into compounds of nitrogen.

Nitrogen Fixation:

The conversion of atmospheric nitrogen into nitrogenous compounds, by any natural means, especially by some bacteria and blue green algae present in the soil, so that it can be used by plants is called as nitrogen fixation.                                  

Biological Nitrogen Fixers:

The bacteria and blue green algae which can fix nitrogen from the atmosphere to enrich soil with nitrogen and increase its fertility are called biological nitrogen fixers.

Once nitrogen is converted into these usable compounds, it can be utilized by plants from the soil throgh their root system.

Nitrogen is then used for the synthesis of plant proteins and other compounds.

Animals feeding on plants get these proteins and other nitrogen componds.

What we learned till now

1.  Bt if all the nitrogen in the atmosphere is gradually fixed by these microbes and used by plants and animals then it should be depleted…

2.  But it doesn’t happen so, the amount of nitrogen in the atmosphere remains constant.

Reason is Nitrogen Cycle

Nitrogen Cycle

Nitrogen Cycle:

The cyclic flow of atmospheric nitrogen to the living world, where it is utilized by living organisms, and returning it back into the atmosphere by various processes is callled nitrogen cycle.

Three Major Ways

1  by some Bacteria and Blue Green Algae present in the soil.

2  by Rhizobium Bacteria living in Root Nodules of Leguminous Plants.

Exampes of Leguminous Plants are pulses, beans and peas.

3  by Lightning

When plants and animals die, bacteria and fungi present in the soil convert the nitrogenous wastes into nitrogenous compounds to be used by plants again.

Certain other bacteria convert some part of them to nitrogen gaes  which goes back into the atmosphere.

    As a result, the percentage of nitrogen in the atmosphere remains more or less constant.

Chapter 2 Ncert Exercises Question Answers

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