Science STD 8 Chapter 2: Microorganisms: Friend and Foe - Exercises

Science STD 8 Chapter 2: Microorganisms: Friend and Foe - Exercises

1. Fill in the blanks.

  1. (a) Microorganisms can be seen with the help of a microscope.
  2. (b) Blue green algae fix nitrogen directly from air and enhance fertility of soil.
  3. (c) Alcohol is produced with the help of yeast.
  4. (d) Cholera is caused by bacteria.

2. Tick the correct answer.

  1. (a) Yeast is used in the production of
    1. sugar
    2. alcohol
    3. hydrochloric acid
    4. oxygen
  2. (b) The following is an antibiotic
    1. Sodium bicarbonate
    2. Streptomycin
    3. Alcohol
    4. Yeast
  3. (c) Carrier of malaria-causing protozoan is
    1. female Anopheles mosquito
    2. cockroach
    3. housefly
    4. butterfly
  4. (d) The most common carrier of communicable diseases is
    1. ant
    2. housefly
    3. dragonfly
    4. spider
  5. (e) The bread or idli dough rises because of
    1. heat
    2. grinding
    3. growth of yeast cells
    4. kneading
  6. (f) The process of conversion of sugar into alcohol is called
    1. nitrogen fixation
    2. moulding
    3. fermentation
    4. infection

3. Match the organisms in Column A with their action in Column B.

A B
(i) Bacteria (e) Causing cholera
(ii) Rhizobium (a) Fixing nitrogen
(iii) Lactobacillus (b) Setting of curd
(iv) Yeast (c) Baking of bread
(v) A protozoan (d) Causing malaria
(vi) A virus (f) Causing AIDS

4. Can microorganisms be seen with the naked eye? If not, how can they be seen?

No, microorganisms cannot be seen with the naked eye because they are very small. Some can be seen with a magnifying glass, but most of them require a microscope to be seen. That is why they are called microorganisms or microbes.

5. What are the major groups of microorganisms?

Microorganisms are classified into four major groups: **bacteria**, **fungi**, **protozoa**, and some **algae**. Viruses are also considered microbes, but they are different from other microorganisms because they can only reproduce inside a host organism.

6. Name the microorganisms which can fix atmospheric nitrogen in the soil.

The microorganisms that can fix atmospheric nitrogen in the soil are **Rhizobium bacteria** and some **cyanobacteria** (blue-green algae).

7. Write 10 lines on the usefulness of microorganisms in our lives.

Microorganisms are beneficial to us in many ways:

  1. They are used in the production of food items like curd, bread, and cheese.
  2. Yeast is used commercially to produce alcohol and wine.
  3. They are used to prepare medicines called antibiotics, which kill disease-causing microbes.
  4. They are used to make vaccines that protect humans and animals from various diseases.
  5. Microbes like Rhizobium help in fixing atmospheric nitrogen in the soil, increasing soil fertility.
  6. Decomposers break down dead organic waste, cleaning up the environment.
  7. They are essential for the decomposition of organic waste to produce manure.
  8. Microorganisms are used in the cleaning of organic waste in a wastewater treatment plant.
  9. They are used in the production of nitrogenous compounds in the soil, which are essential for plant growth.
  10. Some microorganisms are even used to produce vitamins.

8. Write a short paragraph on the harmful effects of microorganisms.

Microorganisms can be harmful in many ways. Some of them, known as pathogens, cause diseases in humans, plants, and animals. In humans, they cause diseases like cholera, typhoid, tuberculosis, and malaria. They can also spoil food by producing toxic substances, leading to food poisoning. In plants, microorganisms cause diseases like citrus canker and rust of wheat, which can reduce crop yield. In animals, they cause diseases such as anthrax and foot-and-mouth disease. Some microbes can also spoil our clothing and leather goods, especially during hot and humid weather.

9. What are antibiotics? What precautions must be taken while taking antibiotics?

Antibiotics are medicines that kill or stop the growth of disease-causing microorganisms. They are produced from specific bacteria and fungi. Precautions to be taken while taking antibiotics are:

  • Antibiotics should be taken only on the advice of a qualified doctor.
  • You must complete the entire course prescribed by the doctor.
  • Taking antibiotics unnecessarily or in the wrong dosage can make them less effective in the future.
  • Unnecessary use can also kill the beneficial bacteria in your body.
  • Antibiotics are not effective against diseases caused by viruses, like the common cold and flu.

Suggested Activities and Projects

1. Pull out a gram or bean plant from the field. Observe its roots. You will find round structures called root nodules on the roots. Draw a diagram of the root and show the root nodules.

Outline: This activity allows you to observe the symbiotic relationship between plants and bacteria. The root nodules you observe on the leguminous plant are the home of Rhizobium bacteria, which help fix atmospheric nitrogen and enrich the soil. Drawing a diagram of the root and its nodules will help you understand the concept of nitrogen fixation.

2. Collect the labels from the bottles of jams and jellies. Write down the list of contents printed on the labels.

Outline: This activity is designed to help you identify common food preservatives. By collecting and reading the labels, you would find ingredients like sodium benzoate and sodium metabisulphite listed as preservatives. This shows how these chemicals are used to prevent the spoilage of food items like jams and jellies by microorganisms.

3. Visit a doctor. Find out why antibiotics should not be overused. Prepare a short report.

Outline: This project focuses on understanding the medical and health consequences of overusing antibiotics. A doctor would explain that overuse can lead to antibiotic resistance, making the drugs less effective when they are truly needed. It can also harm beneficial gut bacteria, leading to other health problems. The report would summarise these points, emphasising responsible use of antibiotics.

4. Project: Requirements 2 test tubes, marker pen, sugar, yeast powder, 2 balloons and lime water. Take two test tubes and mark them A and B. Clamp these tubes in a stand and fill them with water leaving some space at the top. Put two spoonfuls of sugar in each of the test tubes. Add a spoonful of yeast in test tube B. Inflate the two balloons incompletely. Now tie the balloons on the mouths of each test tube. Keep them in a warm place, away from sunlight. Watch the setup every day for next 3-4 days. Record your observations and think of an explanation. Now take another test tube filled 1/4 with lime water. Remove the balloon from test tube B in such a manner that gas inside the balloon does not escape. Fit the balloon on the test tube and shake well. Observe and explain.

Outline: This project is a hands-on demonstration of fermentation. In test tube B, the yeast will use the sugar to produce alcohol and carbon dioxide gas through fermentation. This gas will inflate the balloon. In test tube A, no reaction will occur. When the gas from balloon B is passed through lime water, it will turn milky, confirming the presence of carbon dioxide. This experiment shows how microorganisms like yeast are used in processes like fermentation to produce useful products.