NCERT Solutions: Microorganisms- Friend and Foe

The CBSE Class 8 Science Chapter 2 - Microorganisms: Friend and Foe is an essential topic in the field of microbiology, which aims to teach students about the different types of microorganisms, their role in the ecosystem, and their impact on human life. Let's have a look at NCERT Solutions of the chapter.

Exercises

Q1. Fill in the blanks:
(a) Microorganisms can be seen with the help of a microscope.

Microscope: A microscope is a scientific instrument used to magnify objects that are too small to be seen with the naked eye.

Microscope

(b) Blue-green algae fix nitrogen directly from the air to enhance the fertility of the soil.

Blue-green algae: 

• Blue-green algae are a type of microorganism known as cyanobacteria. 
• They have the unique ability to convert atmospheric nitrogen gas into a usable form that plants can absorb, a process called nitrogen fixation.

Blue-green algae

(c) Alcohol is produced with the help of yeast.

Yeast: 

• Yeast, a type of microorganism, is crucial in the commercial production of alcohol and wine.
• It carries out fermentation, where sugars are converted into alcohol and carbon dioxide.
• Yeast is grown on natural sugars present in various sources such as grains (barley, wheat, rice) and crushed fruit juices.
• During fermentation, the yeast breaks down these natural sugars, producing alcohol as the primary product.

(d) Cholera is caused by bacteria.

Bacteria: 

• Bacteria are single-celled microorganisms that can cause various diseases in humans, animals, and plants. 
• Cholera is a severe diarrheal illness caused by a specific bacterium called Vibrio cholerae.

Q2. Tick the correct answer:
(a) Yeast is used in the production of 
(i) Sugar
(ii) Alcohol
(iii) Hydrochloric acid
(iv) Oxygen
Ans: (ii)
Explanation: Yeast carries out fermentation, a process in which it converts sugars into alcohol and carbon dioxide. This property is used in breweries and in production of alcoholic beverages such as beer and wine. Yeast does not produce hydrochloric acid or oxygen, and it does not itself become sugar.

Yeast causes dough to rise

(b) The following is an antibiotic
(i) Sodium bicarbonate
(ii) Streptomycin
(iii) Alcohol
(iv) Yeast
Ans: (ii)
Explanation: Streptomycin is an antibiotic produced from certain microorganisms and used to kill or stop the growth of bacteria. Antibiotics are medicines that act against bacterial infections; sodium bicarbonate is a chemical used as an antacid, alcohol is a disinfectant but not an antibiotic, and yeast is a microorganism used in fermentation.

(c) Carrier of malaria-causing protozoan is
(i) Female Anopheles mosquito
(ii) Cockroach
(iii) Housefly
(iv) Butterfly
Ans: (i)
Explanation: The female Anopheles mosquito transmits the Plasmodium protozoan when it bites a human. The protozoan enters the bloodstream and causes malaria. Cockroaches, houseflies and butterflies are not the vectors for malaria.

Mosquito

(d) The most common carrier of communicable diseases is
(i) Ant
(ii) Housefly
(iii) Dragonfly
(iv) Spider
Ans: (ii)
Explanation: Houseflies visit decaying matter, garbage and faeces and pick up disease-causing microbes on their legs and body. When they land on food or utensils, they transfer these microbes and can spread many communicable diseases to humans. Other insects listed are not as common as carriers.

(e) The bread or idli dough rises because of
(i) Heat
(ii) Grinding
(iii) Growth of yeast cells
(iv) Kneading
Ans: (iii)
Explanation: When yeast is added to dough, it ferments the sugars present and grows, producing carbon dioxide gas. The gas forms bubbles that get trapped in the dough, causing it to expand and rise. Heat during baking helps set the risen structure but the primary cause of rising is yeast activity.

(f) The process of conversion of sugar into alcohol is called
(i) Nitrogen fixation
(ii) Molding
(iii) Fermentation
(iv) Infection
Ans: (iii)
Explanation: Fermentation is a biological process where microorganisms such as yeast break down sugars to produce alcohol and carbon dioxide in the absence of oxygen. This process is used in brewing, winemaking and some food preparations.

Q3. Match the organisms in Column I with their action in Column II.

Ans: 

• (i) Bacteria - (e) Causing Cholera
  Bacteria are tiny organisms that can cause diseases. Cholera is a severe illness that is caused by a specific bacterium called Vibrio cholerae. 
• (ii) Rhizobium - (a) Fixing Nitrogen
  Rhizobium is a helpful type of bacteria that lives in the roots of certain plants. It has a special ability to take nitrogen from the air and convert it into a form that plants can use. 
  
  Rhizobium
• (iii) Lactobacillus - (b) Setting of curd
  Lactobacillus is a type of bacteria used in the process of making curd. When we add curd starter containing lactobacillus to milk, it helps in fermenting the milk. This fermentation process turns the milk into curd by converting the milk sugar into lactic acid.
• (iv) Yeast - (c) Baking of bread
  Yeast is a type of microorganism that is used in baking bread. When we add yeast to the dough, it produces tiny bubbles of gas called carbon dioxide through a process called fermentation. These bubbles make the dough rise and become fluffy when we bake it.
• (v) A protozoan - (d) Causing Malaria
  A protozoan is a type of tiny organism. Some protozoans can cause diseases. Malaria is a disease that is caused by a protozoan parasite.
• (vi) A Virus - (f) Causing AIDS
  A virus is a tiny organism that can cause diseases. The Human Immunodeficiency Virus (HIV) is a virus that can weaken a person's immune system. It can lead to a disease called Acquired Immunodeficiency Syndrome (AIDS), which makes it harder for the body to fight off infections and diseases.
• (g) Producing antibodies
  Antibodies are produced by the immune system of animals and humans, not by the organisms listed in Column A. They help to fight infections by recognising and neutralising pathogens.

Q4. Can microorganisms be seen with the naked eye? If not, how can they be seen?
Ans: Microorganisms cannot be seen with the naked eye because they are extremely small. Most of them can be observed only with the help of a microscope, which magnifies tiny objects so their shape and structure become visible. A few larger kinds, such as some fungi that grow on bread, can be seen without a microscope or with a magnifying glass.

Q5. What are the major groups of microorganisms?
Ans: Microorganisms are classified into five major groups:

1. Bacteria
2. Fungi
3. Protozoa
4. Algae
5. Viruses

Q6. Name the microorganisms which can fix atmospheric nitrogen in the soil.
Ans: Microorganisms that fix atmospheric nitrogen include Rhizobium (found in the root nodules of leguminous plants), free-living bacteria such as Azotobacter, and certain cyanobacteria (blue-green algae). These organisms convert nitrogen gas from the air into forms that plants can use, improving soil fertility.

Blue-green algae

Q7. Write 10 lines on the usefulness of microorganisms in our lives.
Ans: Microorganisms are useful in many ways:

• Alcohol and Vinegar Production: Yeast is used in the large-scale production of alcohol, wine, and acetic acid (vinegar) through fermentation.
• Curd Formation: The bacterium Lactobacillus helps in converting milk into curd by fermenting milk sugar into lactic acid.

Curd

• Baking Industry: Yeast produces carbon dioxide during fermentation, which makes bread and other baked goods rise and become soft.
• Antibiotics: Many antibiotics, such as streptomycin, tetracycline and erythromycin, are produced using microorganisms; these medicines help to treat bacterial infections.
• Soil Fertility: Microorganisms like Rhizobium and blue-green algae fix atmospheric nitrogen and enrich the soil.
• Nitrogen Cycle: Certain bacteria convert nitrogen compounds back into nitrogen gas, helping to maintain the nitrogen cycle in nature.
• Waste Decomposition: Microorganisms decompose dead plants, animals and organic waste, recycling nutrients and cleaning the environment.
• Digestion: Beneficial microbes in our gut help digest food and produce some useful vitamins.
• Scientific Research and Industry: Microorganisms are used in biotechnology, genetic studies and industrial processes to produce enzymes, hormones and other useful products.

Q8. Write a short paragraph on the harms caused by microorganisms.
Ans: Microorganisms can be harmful in several ways. Many are pathogens that cause diseases in humans, animals and plants - for example, bacteria that cause cholera or tuberculosis, viruses that cause the common cold or influenza, and protozoans that cause malaria. They can also spoil food, clothing and leather and cause food poisoning if contaminated food is eaten. Some microorganisms damage crops, reducing agricultural yield, and certain types can produce harmful toxins. Because of these effects, hygiene, sanitation and proper food handling are important to reduce harm from microbes.

Q9. What are antibiotics? What precautions must be taken while taking antibiotics?
Ans: Antibiotics are medicines used to treat infections caused by bacteria. They either kill bacteria or stop them from growing, helping the body's immune system to clear the infection. Antibiotics are not effective against viral infections such as the common cold or most cases of flu.

• They work by either killing bacteria or inhibiting their growth and reproduction. 
• Antibiotics are effective against bacterial infections such as strep throat, pneumonia, and urinary tract infections, but they do not work against viral infections like the flu or common cold. 
• Common examples of antibiotics include penicillin, amoxicillin and tetracycline.

Antibiotics

Precautions for taking Antibiotics:

• Follow Prescription Guidelines: Always take antibiotics exactly as prescribed by your healthcare provider, including the dose and the timing.

• Complete the Course: Finish the entire prescribed course even if you feel better earlier. Stopping antibiotic treatment too soon can allow some bacteria to survive and become resistant.

• Inform Your Doctor: Tell your doctor about any other medicines, vitamins or herbals you are taking so they can check for harmful interactions.

• Watch for Side Effects: Be aware of possible side effects such as rashes, stomach upset or diarrhoea. Seek medical advice if severe reactions occur.

• Do Not Share or Reuse: Do not use leftover antibiotics or share them with others. An antibiotic suited to one infection may be ineffective or unsafe for another.