NCERT Book Class 8 Science Microorganisms Friend And Foe

MIICROORGANIISMS :: FRIIEND AND FOE

You have seen several kinds of plants and animals. However, there are other living organisms around us which we cannot see with eyes alone. These are called microorganisms or microbes. For example, you might have observed that during rainy season moist bread gets spoilt and its surface gets covered with greyish white patches. Observe these patches through a magnifying glass. You will see tiny, black rounded structures. Do you know what these structures are and where did these come from?

2.1 Microorganisms

These observations show that water and soil are full of tiny organisms, though not all of them fall into the category of microbes. Thes microorganisms or microbes are so small in size that they cannot be seen with the unaided eye. Some of these, such as the fungus that grows on bread, can be seen with a magnifying glass. Others cannot be seen without the help of a microscope. That is why these are called microorganisms or microbes. Microorganisms are classified into four major groups. These groups are bacteria, fungi, protozoa and some algae. Some of these common microorganisms are shown in Figs. 2.1 - 2.4. Viruses are also microscopic. They, however, reproduce only inside the cells of the host organism, which may be a bacterium, plant or animal. Some of the viruses are shown in Fig. 2.5. Common ailments like cold, influenza (flu) and most coughs are caused by viruses. Serious diseases like polio and chicken pox are also caused by viruses. Diseases like dysentery and malaria are caused by protozoans whereas typhoid and tuberculosis (TB) are bacterial diseases. You have learnt about some of these microorganisms in Classes VI and VII.

Friendly Microorganisms Microorganisms are used for various purposes. They are used in thepreparation of curd, bread and cake. 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. An important ingredient of rava (sooji ) idlis and bhaturas is curd. Can you guess why? Activity 2.3

Take ½ kg flour (atta or maida), add some sugar and mix with Fig. 2.5 : Viruses I saw that my mother added a little curd to warm milk to set curd for the next day. I wonder why! They are also used in cleaning up of the environment. For example, the organic wastes (vegetable peels, remains of animals, faeces, etc.) are broken down into harmless and usable substances by bacteria. Recall that bacteria are also used in the preparation of medicines. In agriculture they are used to increase soil fertility by fixing nitrogen. Making of Curd and Bread You have learnt in Class VII that milk is turned into curd by bacteria. 2.2 Where do Microorganisms Live?

Microorganisms may be single-celled like bacteria, some algae and protozoa, or multicellular, such as algae and fungi. They can survive under all types of environment, ranging from ice cold climate to hot springs and deserts to marshy lands. They are also found inside the bodies of animals including humans. Some microorganisms grow on other organisms while others exist freely. Microorganisms like amoeba can live alone, while fungi and bacteria may live in colonies.

2.3 Microorganisms and Us

Microorganisms play an important role in our lives. Some of them are beneficial in many ways whereas some others are harmful and cause diseases. Let us study about them in detail. Yeast reproduces rapidly and produces carbon dioxide during respiration. Bubbles of the gas fill the dough and increase its volume (Fig. 2.6). This is the basis of the use of yeast in the baking industry for making breads, pastries and cakes.

Commercial Use of Microorganisms Microorganisms are used for the large scale production of alcohol, wine and acetic acid (vinegar). Yeast is used for commercial production of alcohol and wine. For this purpose yeast is grown on natural sugars present in grains like barley, wheat, rice and crushed fruit juices, etc.

Question. Anusha observed some green patches on an orange peel she had thrown into the dustbin three days earlier. This green patch is most likely to be
(a) A kind of bacteria
(b) A kind of algae
(c) A kind of fungus
(d) A kind of virus

Answer : C

Question. Animals are made up of many atoms. What happens to the atoms after an animal has died?
(a) The atoms stop moving
(b) The atoms recycle back into the environment.
(c) The atoms split into simpler parts and then combine to form other atoms.
(d) The atoms no longer exist once the animal has decomposed

Answer : B

Question. Anthrax is a type of fever caused by bacteria in all warm-blooded animals. From the list below, choose the one animal that cannot be affected by anthrax.
(a) cow
(b) Goat 
(c) Polar bear
(d) crocodile

Answer : D

Question. Osmansagar lake near Hyderabad, is a source of drinking water for the city. If the water is tested for bacterial content and found to have an elevated <i>E. coli</i> count, this might mean that
(a) the water is safe to drink.
(b) the food chain has been extended to the microscopic level.
(c) sewage may be entering the water.
(d) the oxygen level is below normal.

Answer : C

Question. The figure below shows the concentration of oxygen, the number of bacteria and the number of fish in a river over a distance of 30 km measured from a point P. (P is a point upstream from a source of pollution.) At what place has the river got polluted?

(a) A
(b) B
(c) C
(d) D

Answer : C

Question. Which of the following diseases is spread by both contaminated food and contaminated water?
(a) Common cold
(b) Chicken pox
(c) Rabies
(d) Typhoid

Answer : D

Question. A carpenter cut his finger while using a slightly rusted chisel. Which injection should he take?
(a) Anti rabies
(b) Anti tetanus
(c) BCG
(d) None of the above

Answer : B

Question. BIOLOGICAL CONTROL is term used to describe the use of living organisms, such as predators, parasitoids, and pathogens, to control pest insects, weeds or diseases. which among the following is an example of a biological control?
(a) a cat to kill mice at home
(b) pesticides to kill insects
(c) medicine to poison food of the pest
(d) Scarecrows to scare away birds

Answer : A

Question. Which of these precautions is NOT relevant to prevent malaria?
(a) Pouring a layer of oil on water puddles.
(b) Keeping all water containers covered.
(c) Covering ourselves properly while sleeping.
(d) Washing vegetables before cooking them.

Answer : D

Question. Which of these is made with the help of bacteria?
(a) curd (dahi in Hindi)
(b) cream
(c) soap
(d) cooking oil

Answer : A

Question. Do animals suffer from diseases like malaria and typhoid?
(a) Yes, exactly like human beings.
(b) Yes, but the diseases are different.
(c) Domestic animals do, not wild animals.
(d) No, animals do not suffer from diseases.

Answer : B

Question. Polio is caused by
(a) Virus
(b) Fungi
(c) Bacteria
(d) Protozoa

Answer : A
 

Exercises

1. Fill in the blanks:

(a) Microorganisms can be seen with the help of a ____________.

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

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

(d) Cholera is caused by __________.

2. Tick the correct answer:

(a) Yeast is used in the production of

(i) sugar (ii) alcohol (iii) hydrochloric acid (iv) oxygen

(b) The following is an antibiotic

(i) Sodium bicarbonate (ii) Streptomycin (iii) Alcohol (iv) Yeast

(c) Carrier of malaria-causing protozoan is

(i) female Anopheles mosquito (ii) cockroach

(iii) housefly (iv) butterfly

(d) The most common carrier of communicable diseases is

(i) ant (ii) housefly (iii) dragonfly (iv) spider

(e) The bread or idli dough rises because of

(i) heat (ii) grinding (iii) growth of yeast cells (iv) kneading

(f) The process of conversion of sugar into alcohol is called

(i) nitrogen fixation (ii) moulding (iii) fermentation (iv) infection

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

4. What are the major groups of microorganisms?

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

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

7. Write a short paragraph on the harms caused by microorganisms.

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

Please refer to attached file for NCERT Class 8 Science Microorganisms -Friend And Foe