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Chapter 2 Microorganisms: Friends and Foe 
 
Microorganisms – The organisms that cannot be seen with our naked eyes. They can only be seen with the help 
of an instrument called microscope. 
 
 
 
 
 
 
Fig. A microscope 
Categories of microbes: 
1. Bacteria 
2. Fungi 
3. Protozoa 
4. Algae  
Bacteria 
 
 
 
 
Rod shaped bacteria        Spiral shaped bacteria 
 
Algae 
 
 
 
 
 
 
 
Chlamydomonas    Blue green algae  
 
Page 2


Chapter 2 Microorganisms: Friends and Foe 
 
Microorganisms – The organisms that cannot be seen with our naked eyes. They can only be seen with the help 
of an instrument called microscope. 
 
 
 
 
 
 
Fig. A microscope 
Categories of microbes: 
1. Bacteria 
2. Fungi 
3. Protozoa 
4. Algae  
Bacteria 
 
 
 
 
Rod shaped bacteria        Spiral shaped bacteria 
 
Algae 
 
 
 
 
 
 
 
Chlamydomonas    Blue green algae  
 
 
Virus 
Virus is another category of microorganism but is kept separately than other microorganism. Viruses are living 
only inside the body or cells of other organisms. They are dead outside, in the environment. 
Viruses do not have their own cellular machinery and use the machinery of the host cells to divide or replicate.  
 
 
 
  
 
   
 
                        Virus 
Microorganism as friends 
Curd and Bread 
Lactobacillus  is used in the making of curd. The curd that is put in the milk contain Lactobacillus, which 
multiplies in the mild and turn it to curd. 
Yeast is used to convert dough into bread. It releases carbon dioxide in form of bobbles, which convert the 
dough into porous bread. 
Alcohol 
Yeast is used to produce alcohol. The process of conversion of sugars from wheat, barley, grapes and rye into 
alcohol by yeast is called fermentation. It was discovered by Louis Pasteur in 1857. 
Page 3


Chapter 2 Microorganisms: Friends and Foe 
 
Microorganisms – The organisms that cannot be seen with our naked eyes. They can only be seen with the help 
of an instrument called microscope. 
 
 
 
 
 
 
Fig. A microscope 
Categories of microbes: 
1. Bacteria 
2. Fungi 
3. Protozoa 
4. Algae  
Bacteria 
 
 
 
 
Rod shaped bacteria        Spiral shaped bacteria 
 
Algae 
 
 
 
 
 
 
 
Chlamydomonas    Blue green algae  
 
 
Virus 
Virus is another category of microorganism but is kept separately than other microorganism. Viruses are living 
only inside the body or cells of other organisms. They are dead outside, in the environment. 
Viruses do not have their own cellular machinery and use the machinery of the host cells to divide or replicate.  
 
 
 
  
 
   
 
                        Virus 
Microorganism as friends 
Curd and Bread 
Lactobacillus  is used in the making of curd. The curd that is put in the milk contain Lactobacillus, which 
multiplies in the mild and turn it to curd. 
Yeast is used to convert dough into bread. It releases carbon dioxide in form of bobbles, which convert the 
dough into porous bread. 
Alcohol 
Yeast is used to produce alcohol. The process of conversion of sugars from wheat, barley, grapes and rye into 
alcohol by yeast is called fermentation. It was discovered by Louis Pasteur in 1857. 
Antibiotics 
Antibiotics are chemicals produced by microorganisms to stop the growth of other microorganisms. These 
antibiotics are used to prevent the growth of some harmful microbes in plants and animals. E.g. Streptomycin, 
tetracycline etc. Alexander Fleming discovered the first antibiotic, Penicillin. 
Vaccines 
Vaccines are dead or weakened microorganism that are given to a healthy person to prevent the onset of the 
disease.  
The body prepares antibodies against the week microbe, which are stored inside the body and kill the 
microorganism when it actually infects a person. 
Antibodies are proteins that are produced by the body to kill any foreign pathogen infecting the body. 
Edward Jenner first discovered vaccine in 1798 for smallpox. 
Increasing the soil fertility 
Some microorganism can increase the fertility of the soil by fixing the atmospheric nitrogen. Plants or animals 
cannot utilize the atmospheric nitrogen. It is converted into certain chemicals (nitrates and nitrites), which are 
used by plants to produce proteins. 
Microorganisms that can fix atmospheric nitrogen into nitrogenous compounds are called nitrogen-fixing 
bacteria. E.g. Rhizobium and Blue Green algae. Nitrogen fixing bacteria are present in the root nodules of 
leguminous plants such as pulses. 
Clean the environment 
Microorganisms can decompose dead and decaying matter into simple substances that can be used by the plants 
and animals. This helps to remove waste and clean the environment. 
 
Microorganisms as Foe 
 
Pathogens – Microorganisms that cause diseases are known as pathogens. 
Communicable disease – Disease that can spread from a diseased person to a healthy person through air, 
water, food or contact. E.g. Common cold, Chicken pox, tuberculosis etc. 
Non-Communicable disease – Disease that cannot spread from a diseased person to a healthy person through 
air, water, food or contact. E.g. AIDS, Night Blindness, Scurvy etc. 
Carriers – Animals or insects that carry disease-causing microorganisms from a diseased person to a healthy 
person are called carriers. E.g., Female Anopheles mosquito carries the parasite of malaria. Aedes carries the 
dengue virus. 
 
 
 
Page 4


Chapter 2 Microorganisms: Friends and Foe 
 
Microorganisms – The organisms that cannot be seen with our naked eyes. They can only be seen with the help 
of an instrument called microscope. 
 
 
 
 
 
 
Fig. A microscope 
Categories of microbes: 
1. Bacteria 
2. Fungi 
3. Protozoa 
4. Algae  
Bacteria 
 
 
 
 
Rod shaped bacteria        Spiral shaped bacteria 
 
Algae 
 
 
 
 
 
 
 
Chlamydomonas    Blue green algae  
 
 
Virus 
Virus is another category of microorganism but is kept separately than other microorganism. Viruses are living 
only inside the body or cells of other organisms. They are dead outside, in the environment. 
Viruses do not have their own cellular machinery and use the machinery of the host cells to divide or replicate.  
 
 
 
  
 
   
 
                        Virus 
Microorganism as friends 
Curd and Bread 
Lactobacillus  is used in the making of curd. The curd that is put in the milk contain Lactobacillus, which 
multiplies in the mild and turn it to curd. 
Yeast is used to convert dough into bread. It releases carbon dioxide in form of bobbles, which convert the 
dough into porous bread. 
Alcohol 
Yeast is used to produce alcohol. The process of conversion of sugars from wheat, barley, grapes and rye into 
alcohol by yeast is called fermentation. It was discovered by Louis Pasteur in 1857. 
Antibiotics 
Antibiotics are chemicals produced by microorganisms to stop the growth of other microorganisms. These 
antibiotics are used to prevent the growth of some harmful microbes in plants and animals. E.g. Streptomycin, 
tetracycline etc. Alexander Fleming discovered the first antibiotic, Penicillin. 
Vaccines 
Vaccines are dead or weakened microorganism that are given to a healthy person to prevent the onset of the 
disease.  
The body prepares antibodies against the week microbe, which are stored inside the body and kill the 
microorganism when it actually infects a person. 
Antibodies are proteins that are produced by the body to kill any foreign pathogen infecting the body. 
Edward Jenner first discovered vaccine in 1798 for smallpox. 
Increasing the soil fertility 
Some microorganism can increase the fertility of the soil by fixing the atmospheric nitrogen. Plants or animals 
cannot utilize the atmospheric nitrogen. It is converted into certain chemicals (nitrates and nitrites), which are 
used by plants to produce proteins. 
Microorganisms that can fix atmospheric nitrogen into nitrogenous compounds are called nitrogen-fixing 
bacteria. E.g. Rhizobium and Blue Green algae. Nitrogen fixing bacteria are present in the root nodules of 
leguminous plants such as pulses. 
Clean the environment 
Microorganisms can decompose dead and decaying matter into simple substances that can be used by the plants 
and animals. This helps to remove waste and clean the environment. 
 
Microorganisms as Foe 
 
Pathogens – Microorganisms that cause diseases are known as pathogens. 
Communicable disease – Disease that can spread from a diseased person to a healthy person through air, 
water, food or contact. E.g. Common cold, Chicken pox, tuberculosis etc. 
Non-Communicable disease – Disease that cannot spread from a diseased person to a healthy person through 
air, water, food or contact. E.g. AIDS, Night Blindness, Scurvy etc. 
Carriers – Animals or insects that carry disease-causing microorganisms from a diseased person to a healthy 
person are called carriers. E.g., Female Anopheles mosquito carries the parasite of malaria. Aedes carries the 
dengue virus. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Food Poisoning –  
Microorganisms that grow in food and water can infect intestine and causes food poisoning.  
 
Food Preservation 
1. Chemical methods 
Preservatives – Chemicals that are used to stop the growth of microorganisms in food are called 
preservatives. E.g. salts, oil, Sodium benzoate and Sodium metasulphite. 
 
Disease Causative 
Organism 
Mode of 
Transmission 
          Preventive measures 
Plants Disease Microorganisms Mode of Transmission 
Citrus Cancer Bacteria Air 
Rust of Wheat Fungi Air, Seeds 
Mosaic of bhindi Virus Insect 
Page 5


Chapter 2 Microorganisms: Friends and Foe 
 
Microorganisms – The organisms that cannot be seen with our naked eyes. They can only be seen with the help 
of an instrument called microscope. 
 
 
 
 
 
 
Fig. A microscope 
Categories of microbes: 
1. Bacteria 
2. Fungi 
3. Protozoa 
4. Algae  
Bacteria 
 
 
 
 
Rod shaped bacteria        Spiral shaped bacteria 
 
Algae 
 
 
 
 
 
 
 
Chlamydomonas    Blue green algae  
 
 
Virus 
Virus is another category of microorganism but is kept separately than other microorganism. Viruses are living 
only inside the body or cells of other organisms. They are dead outside, in the environment. 
Viruses do not have their own cellular machinery and use the machinery of the host cells to divide or replicate.  
 
 
 
  
 
   
 
                        Virus 
Microorganism as friends 
Curd and Bread 
Lactobacillus  is used in the making of curd. The curd that is put in the milk contain Lactobacillus, which 
multiplies in the mild and turn it to curd. 
Yeast is used to convert dough into bread. It releases carbon dioxide in form of bobbles, which convert the 
dough into porous bread. 
Alcohol 
Yeast is used to produce alcohol. The process of conversion of sugars from wheat, barley, grapes and rye into 
alcohol by yeast is called fermentation. It was discovered by Louis Pasteur in 1857. 
Antibiotics 
Antibiotics are chemicals produced by microorganisms to stop the growth of other microorganisms. These 
antibiotics are used to prevent the growth of some harmful microbes in plants and animals. E.g. Streptomycin, 
tetracycline etc. Alexander Fleming discovered the first antibiotic, Penicillin. 
Vaccines 
Vaccines are dead or weakened microorganism that are given to a healthy person to prevent the onset of the 
disease.  
The body prepares antibodies against the week microbe, which are stored inside the body and kill the 
microorganism when it actually infects a person. 
Antibodies are proteins that are produced by the body to kill any foreign pathogen infecting the body. 
Edward Jenner first discovered vaccine in 1798 for smallpox. 
Increasing the soil fertility 
Some microorganism can increase the fertility of the soil by fixing the atmospheric nitrogen. Plants or animals 
cannot utilize the atmospheric nitrogen. It is converted into certain chemicals (nitrates and nitrites), which are 
used by plants to produce proteins. 
Microorganisms that can fix atmospheric nitrogen into nitrogenous compounds are called nitrogen-fixing 
bacteria. E.g. Rhizobium and Blue Green algae. Nitrogen fixing bacteria are present in the root nodules of 
leguminous plants such as pulses. 
Clean the environment 
Microorganisms can decompose dead and decaying matter into simple substances that can be used by the plants 
and animals. This helps to remove waste and clean the environment. 
 
Microorganisms as Foe 
 
Pathogens – Microorganisms that cause diseases are known as pathogens. 
Communicable disease – Disease that can spread from a diseased person to a healthy person through air, 
water, food or contact. E.g. Common cold, Chicken pox, tuberculosis etc. 
Non-Communicable disease – Disease that cannot spread from a diseased person to a healthy person through 
air, water, food or contact. E.g. AIDS, Night Blindness, Scurvy etc. 
Carriers – Animals or insects that carry disease-causing microorganisms from a diseased person to a healthy 
person are called carriers. E.g., Female Anopheles mosquito carries the parasite of malaria. Aedes carries the 
dengue virus. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Food Poisoning –  
Microorganisms that grow in food and water can infect intestine and causes food poisoning.  
 
Food Preservation 
1. Chemical methods 
Preservatives – Chemicals that are used to stop the growth of microorganisms in food are called 
preservatives. E.g. salts, oil, Sodium benzoate and Sodium metasulphite. 
 
Disease Causative 
Organism 
Mode of 
Transmission 
          Preventive measures 
Plants Disease Microorganisms Mode of Transmission 
Citrus Cancer Bacteria Air 
Rust of Wheat Fungi Air, Seeds 
Mosaic of bhindi Virus Insect 
2. Common Salt 
Salt is used to prevent the spoilage of raw mangoes, amla, fish and meat. 
 
3. Sugar 
Jams, and jellies are prevented from spoilage using sugar. Sugar absorbs the moisture and check the 
growth of microorganisms 
 
4. Oil and Vinegar 
Bacteria cannot grow in environment rich in oil and vinegar 
 
5. Heat and cold 
Pasteurization – the heating of milk to 70
0
C for 15 – 30 seconds and then storing it at low temperature in 
a refrigerator is known as pasteurization. 
 
Nitrogen Cycle 
 
Explanation 
Atmospheric nitrogen cannot be used directly by plants and animals to make proteins. Nitrogen fixing bacteria 
and lightening convert atmospheric nitrogen into nitrogenous compounds that are used by plants to make their 
proteins. Animals eat these plants and they prepare their proteins. When plants and animals die, the proteins are 
again converted into nitrogenous compounds by decomposers. These nitrogen compounds are also converted to 
atmospheric nitrogen by some microorganisms. This cycle keeps going on in the atmospheric similar to water 
cycle and maintains the level of nitrogen in soil and atmosphere. 
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FAQs on Short Note: Microorganisms friends and foe - Class 8

1. What are microorganisms?
Ans. Microorganisms are microscopic living organisms that are too small to be seen with the naked eye. They include bacteria, fungi, protozoa, and viruses.
2. How can microorganisms be beneficial to us?
Ans. Microorganisms have various beneficial roles in our lives. They help in the decomposition of organic matter, nutrient recycling, nitrogen fixation, and production of useful substances like antibiotics, enzymes, and fermented foods.
3. What are some examples of harmful microorganisms?
Ans. Harmful microorganisms include disease-causing bacteria, viruses, and fungi. They can cause illnesses such as influenza, tuberculosis, food poisoning, and skin infections.
4. How can we prevent the spread of harmful microorganisms?
Ans. To prevent the spread of harmful microorganisms, it is important to practice good hygiene such as washing hands regularly, covering the mouth and nose while coughing or sneezing, and maintaining cleanliness in the surroundings. Vaccination and proper food handling techniques also play a significant role in preventing the spread of diseases.
5. Can all microorganisms be classified as either friends or foes?
Ans. No, not all microorganisms can be strictly classified as either friends or foes. Some microorganisms can have both beneficial and harmful effects depending on the context. For example, certain bacteria in our gut are helpful for digestion, while the same bacteria in other parts of the body can cause infections.
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