NCERT Class VIII Science Solutions: Chapter 2-Microorganisms: Friend and Foe Part 5 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Question 25:

Observe the Fig and answer the following questions.

Result for Microorganism
  1. Name the microorganism and the group to which it belongs.
  2. Name the food item on which the organism grows.
  3. Does it grow well in dry or in moist conditions?
  4. Is it safe to eat infected bread?


  1. Bread mould. It is a fungus.
  2. Moist and stale bread.
  3. It grows well in moist conditions. Dry conditions inhibit its growth.
  4. No, the fungus spoils the bread by producing poisonous substances.

Question 26:

Give reasons for the following.

  1. Fresh milk is boiled before consumption while processed milk stored in packets can be consumed without boiling.
  2. Raw vegetables and fruits are kept in refrigerators whereas jams and pickles can be kept outside.
  3. Farmers prefer to grow beans and peas in nitrogen deficient soils.
  4. Mosquitoes can be controlled by preventing stagnation of water though they do not live in water. Why?


  1. Fresh milk is boiled before consumption to kill the microorganisms in it. But packed milk is pasteurised and does not contain any microorganisms. It can thus be consumed without boiling.
  2. Raw vegetables and fruits get easily infected by microorganisms and get spoilt. They are kept in refrigerator as low temperature inhibits growth of microbes. Jams and pickles contain sugar and salt as preservatives. They do not get infected by microbes easily.
  3. Beans and peas are leguminous plants and have Rhizobium in their root nodules. These bacteria can fix atmospheric nitrogen to enrich the soil with nitrogen and increase its fertility.
  4. Though mosquitoes live on land, their larvae grow in water. If water stagnation is prevented the larvae cannot survive.

Question 27:

How can we prevent the following diseases?

  1. Cholera
  2. Typhoid
  3. Hepatitis A


  1. Cholera: An infectious disease caused by bacteria vibrio cholera. It occurs due to the consumption of contaminated or unhygienic food and water. It can be prevented by maintaining personal hygiene, good sanitation practice, consumption of clean drinking water etc.
  2. Typhoid: An acute illness caused by bacteria salmonella typhi. If occurs by ingestion of typhoid bacterium through food, water, fomite etc. Preventions includes consumption of hygiene and properly cooked meals, filtered or boiled water etc.
  3. Hepatitis A: A disease caused by hepatitis. A virus symptoms of disease appears after 2 weeks of infection. Prevention includes proper vaccination drinking boiled water, wash hands before eating, etc.

Question 28:

Complete the following cycle given as Fig by filling the blanks (a) , (b) , (c) , (d)

Result for Complete the Following Cycle


  1. Lightning fixes nitrogen.
  2. Nitrogen fixing bacteria and blue green algae fix atmospheric nitrogen.
  3. Nitrogenous waste from excretion and death.
  4. Bacteria turn compounds of nitrogen into gaseous nitrogen.

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