NCERT Solution Class 8 Science Chapter 2 - Microorganisms: Friend and Foe

Rhizobium

a) Microscope

b) Nitrogen

c) Yeast

d) Bacteria- Vibrio cholera

a) (ii) alcohol

b) (ii) Streptomycin

c) (i) female Anopheles mosquito

d) (ii) housefly

e) (iii) growth of yeast cells

f) (iii) fermentation

Microorganisms cannot be seen by the naked eye because they are very small. They can be seen by using a microscope.

The major groups of microorganisms are as follows:

1. Bacteria

2. Fungi

3. Protozoans

4. Algae

5. Viruses

The uses of microorganisms are listed below.

• They are helpful in cleaning up the environment. For example, organic wastes (vegetable peels, remains of animals, faeces, etc.) are broken down into harmless and usable substances by bacteria.

• Curd contains several microorganisms. Of these, the bacterium, Lactobacillus promotes the formation of curd. It multiplies in milk and converts it into curd.

• Bacteria are also involved in the making of cheese, pickles and many other food items.

• Bacteria and yeast are also helpful for the fermentation of rice idlis and dosa batter.

• Yeast reproduces rapidly and produces carbon dioxide during respiration. Bubbles of gas fill up the dough and increase its volume. This is the basis of the use of yeast in the baking industry for making bread, pastries and cakes.

• Microorganisms are used for the large-scale production of alcohol, wine and acetic acid (vinegar).

• Antibiotics are produced from bacteria and fungi.

• Microorganisms are helpful in producing vaccines against disease-causing agents.

• Some bacteria (Fig. 2.7) are able to fix nitrogen from the atmosphere to enrich the soil with nitrogen and increase its fertility.

• Microorganisms decompose dead organic waste of plants and animals to convert them into simple substances. These substances are again used by other plants and animals. Thus, microorganisms can be used to degrade harmful and smelly substances and thereby clean up the environment.

Microorganisms cause diseases in plants and animals. For example, Cholera in humans is caused by bacteria called Vibrio cholera. In animals, the virus causes foot and mouth diseases. Microorganisms spoil food by decomposing the food material. Some microorganisms spoil leather and clothes. Microorganisms are also responsible for food poisoning.

Chemical agents that kill or stop the growth of disease-causing microorganisms are called antibiotics. Antibiotics should be taken only on the advice of a qualified doctor. Also, one must complete the course prescribed by the doctor.