Microorganisms Friend And Foe - Exemplar Solutions 4

CBSE Class 08 Science
NCERT Exemplar Solutions
Microorganisms: Friend and Foe

22. Observe the Fig. 2.1 and answer the questions that follows.
(a) Write the name of the disease.
(b) Name the causative agent of this disease?
(c) How does the disease spread from one plant to another?
(d) Name any two plant diseases and the microbes that cause them.
Ans.(a) Yellow vein mosaic of lady’s finger/okra
(b) Virus
(c) The disease spreads from one plant to another through insects.
(d) (i) Citrus canker caused by bacteria
(ii) Rust of wheat caused by fungus or any other disease

23. How do vaccines work?
Ans.Vaccines contain a suspension of weakened or dead disease- causing microbes of a particular disease. When a vaccine is introduced into a healthy body, it stimulates the immune system to produce antibodiesand kills the disease causing microbes. These antibodies remain in the body and protects it when the microbes enters the body again.

24. Observe the set up given in Fig. 2.2 and answer the following questions.
(a) What happens to the sugar solution in A?
(b) Which gas is released in A?
(c) What changes will you observe in B when the released gas passes through it?
Ans.(a) Yeast causes fermentation converting sugar into alcohol and carbon dioxide and the solution smells like alcohol.
(b) Carbon dioxide gas is released.
(c) Lime water turns milky.

25. Observe the Fig. 2.3 and answer the following questions.
(a) Name the microorganism and the group to which it belongs.
(b) Name the food item on which the organism grows.
(c) Does it grow well in dry or in moist conditions?
(d) Is it safe to eat infected bread?
Ans.(a) Bread mould. It is a fungus.
(b) Moist and stale bread.
(c) It grows well in moist conditions.
(d) No, the fungus spoils the bread by producing poisonous substances.

26. Give reasons for the following.
(a) Fresh milk is boiled before consumption while processed milk stored in packets can be consumed without boiling.
(b) Raw vegetables and fruits are kept in refrigerators whereas jams and pickles can
be kept outside.
(c) Farmers prefer to grow beans and peas in nitrogen deficient soils.
(d) Mosquitoes can be controlled by preventing stagnation of water though they do not live in water. Why?
Ans.(a) 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.
(b) 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.
(c) 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.
(d) Though mosquitoes live on land, their larvae grow in water. If water stagnation is prevented the larvae cannot survive.

27. How can we prevent the following diseases?
(a) Cholera
(b) Typhoid
(c) Hepatitis A
Ans.(a) Cholera: By drinking boiled water, cooking food well, eating covered food and keeping our surroundings clean.
(b) Typhoid: Eating properly cooked food, drinking boiled food, getting vaccinated against the disease.
(c) Hepatitis A: Drinking boiled water, washing hands thoroughly after using rest room and getting vaccinated against the disease.

28. Complete the following cycle given as Fig. 2.4 by filling the blanks (a), (b), (c) (d)
Ans.(a) Lightning fixes nitrogen.
(b) Nitrogen fixing bacteria and blue green algae fix atmospheric nitrogen.
(c) Nitrogenous waste from excretion and death.
(d) Bacteria turn compounds of nitrogen into gaseous nitrogen.