Sunday 3 July 2016

CBSE Class 8 - Science - MICROORGANISMS AND VIRUSES – Friends and Foes! (Q and A)


(Chapter Solutions of textbook Prachi Science for Class 8)

CBSE Class 8 - Science - MICROORGANISMS   AND VIRUSES – Friends and Foes! (Q and A)
Fungal Infected Bread

Q1.  Answer in one word.

1. Name two typical habitats of micro-organisms

Answer: Commonly found everywhere – in the air, in water, inside the bodies of living organisms.

2. Name a disease caused by bacteria

Answer: Tuberculosis, Pneumonia

3. Name the microorganism that is used for obtaining alcohol from sugar.

Answer: Yeast.

4. Name one carrier of disease-causing microorganisms.

Answer: House Fly which causes many diseases like cholera, typhoid, diarrhoea.

5. Where do the external parasites of animals live?

Answer: External parasites like lice, flies, ticks, grubs etc. live on the skins of the animals.

Q2. Short- Answer Questions (10-15 words)

1. Name the four major groups of microorganisms.

       Bacteria: Baccili, Cooci
       Fungi: Prencillium, Bread Mould
       Algae: Chlamydomonas, Nostoc
       Protozoa: Amoeba, Paramecium

2. How do the blue-green algae and symbiotic bacteria convert atmospheric nitrogen into nitrogen compounds?

Answer: Rhizobium bacteria can take atmospheric nitrogen and convert it into nitrates soluble in the soil.

3. Name the modes of transmission of communicable disease.

Direct Transmission: By direct contact, By droplet infection, sharing infected needles, syringes and razors, infected blood transfusion.

Indirect Transmission: Infected food, water or air, Through carriers like the house fly, mosquitoes, rats etc., Through dirty hands.

4. What are the symptoms of food poisoning?

Answer: Headache, Fever, Vomiting, Diarrhoea

5. Name two food preservatives used for preserving fruit juices, Jam and Jelly.

Answer: Sodium Benzoate and Potassium metabisulphite.

Q3 Long answer type Questions (Answer in 20-25 words )    

1. Why is yeast added to the mixture made for making a cake?  Explain.

Answer: Yeast contains enzymes zymase and invertase which ferment the dough and releases carbon dioxide gas. This gas makes the dough rise and fluffy.

2. Write the beneficial action of :
(a) Bacteria 
(b) Fungi 
(c) Algae

(a) Bacteria: Bacterium Lactobacillus help in the formation of curd from milk.
(b) Fungi: Yeast help in preparing bakery products like bread and cakes.
(c) Algae: Agar Agar (used in food preservatives and jellies) is obtained from red algae.

3. How is the disease caused by microorganisms classified? Explain by giving a suitable example?

Answer: Disease causing micro-organisms are called pathogens. Pathogens enter the body of animals or plants, multiply and damage the body cells. Based on the type of pathogens,  diseases are classified as:

(a) Bacteria causing diseases: Pneumonia, Tuberculosis, Leprosy etc.
(b) Fungi causing diseases: Skin diseases, food poisoning.
(c) Protozoa causing diseases: Malaria, Dysentery, Kala-Azar
(d) The virus causing diseases: Chicken Pox, AIDS, Dengue, Common cold.

For example, the virus causing cold spreads through the air when an infected person sneezes. The virus enters the body of a healthy person standing nearby and infects him.

4. What are the advantages of food preservation?

    1. Reduces food wastage.
    2. Increases storage duration of perishable items.
    3. Reduces chances of pathogen infection and getting food toxic.

5. Name the causative microorganisms which causes 
(a) Rust in wheat
(b) Blast in paddy
(c) Yellow – vein mosaic in Bhindi (Okra)

Answer: (a) Fungi (b) Fungi (c) Virus

Q4.  Tick the ODD one out giving reason 

(a) Bacteria , Fungi , Virus , algae , protozoa
(b) Milk , curd , Renin , Lactobacillus
(c) Anaerobic bacteria , Blue – green algae ,Animal dung , Biogas
(d) Skin disease , malaria , dysentery , Giardiasis
(e) Salt , sugar , oil , clostridium botulism , sodium metabisulphite

Answer: (a) Virus (b) Renin (c) Blue Green Algae (d) Skin diseases  (e) clostridium botulism

Q5. Define the following terms 

(a) Fermentation
(b) Food poisoning
(c) Nitrogen cycle
(d) Food preservation
(e) Pasteurization

Answer: (a) Fermentation: The enzymatic break down of large organic molecules into smaller ones in the absence of air is called fermentation. e.g. Yeast is used to fermenting sugar into alcohol.

(b) Food gets spoiled by the action of microbes. Toxins produced by microbes make the food rancid. This is called food poisoning.

(c) Nitrogen Cycle: The circulation of nitrogen in various chemical forms in nature is called Nitrogen Cycle. Nitrogen in the air is fixed by lightning and by the action of Rhizobium bacteria into nitrates. These nitrates are absorbed by plants and animals. When plants and animals die, nitrates are decomposed into nitrogen and is released to air, thus completing the cycle.


1. Why is virus not included in the category of microorganisms?

Answer: Viruses possess characteristics of living and non-living. It does not have well-defined body apparatus like other microbes or living beings have. That's why viruses are not included in the category of micro-organisms.

2. Why does curd set faster in summer than in winter?

Answer: Summer temperature is suitable for microbes to grow fast as compared to low temperature in winter.  Higher growth of bacteria leads to faster curd formation.

3. What is meant by causative microorganisms? Illustrate by giving a few examples?

Answer: Causative microbes, also called pathogen, enter host body, multiply and cause diseases. Fungi, Virus, Bacteria and Protozoans are causative microbes. e.g. On mosquito bite, dengue virus enters the host body and attacks blood cells. It causes fever and severe pain.

4. What are the internal and external parasites? Give one example for each?

Answer: Parasites get their food from the body of the animal and make it weak. Parasites are of two types:

External Parasites: They live on the skin and mainly cause skin diseases.  e.g. Lice, ticks.

Internal Parasites: These are worms which live and grow inside the body of the host. e.g. Tapeworms and roundworms grow in human intestines.

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