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Samacheer Kalvi 8th Science Books Microorganisms

Science : Chapter 16 : Microorganisms


I. Choose the best answer.

1. Microorganisms are measured in _______.

a) cm 

b) mm 

c) micron 

d) meter.

[Answer: (c) micron]

2. _______ shows both living and nonliving characteristics.

a) Protozoa

 b) Virus

c) Bacteria

d) Fungi

[Answer: (b) virus]

3. ______ is a prokaryotic microorganisms.

a) Virus 

b) Algae 

c) Fungi 

d) Bacteria

[Answer: (d) Bacteria]

4. Based on shape, the bacteria are classified into _______ types.

a) two 

b) three 

c) four 

d) five

[Answer: (c) four]

5. Common cold in human is caused by ______.

a) plasmodium

b) influenza

c) vibrio cholera

d) aphthovirus

 [Answer: (b) Influenza]

II. Fill in the blanks.

1. Penicillin is prepared from a mould called penicillium.

2. Prions are the infectious protein particles.

 3. The infecting virus particle found outside the host cell is virion.

4. Microorganism can be seen with the help of a microscope.

5. Bacteria, which has a flagellum at one end is classified as monotrichous.

III. State true or false. If false, correct the statement.

1. Disease causing microorganisms are called pathogens. [Answer: True.]

2. Female anopheles mosquito is a carrier of dengue virus. [Answer: False.]

Correct statement: Female Anopheles mosquito is a carrier of malaria parasite.

3. Chicken pox is a communicable disease. [Answer: True.]

4. Citrus canker is transmitted by insects. [Answer: False.]

Correct statement: Citrus cranker is transmitted by air and water.

5. Yeast is used in the large scale production of alcohol. [Answer: True.]

IV. Match the following.

1. Nitrogen fixing bacteria – Vaccine

2. Tuberculosis – Prion

3. Kuru – Lactobacillus acidophilus

4. Probiotics – Bacteria

5. Edward Jenner – Rhizobium

[Answer: (1 – e, 2 – d, 3 – b, 4 – c, 5 – a)]

1. Nitrogen fixing bacteria (e) Rhizobium

2. Tuberculosis (d) Bacteria

3. Kuru – (b) Prion

4. Probiotics (c) Lactobacillus acidophilus

5. Edward Jenner – (a) Vaccine 

V. Answer the following questions.

Mark the correct one as:

a. If both assertion and reason are true and reason is the correct explanation of assertion.

b. If both assertion and reason are true and reason is not the correct explanation of assertion.

c. If assertion is true but reason is false.

d. If both assertion and reason are false.

1. Assertion: Malaria is caused by Protozoa.

Reason: The disease is transmitted by mosquito.

[Answer: (a) Both Assertion and Reason are true and Reason is the correct explanation of Assertion]

2. Assertion: Algae are heterotrophic.

Reason: They do not have chlorophyll.

[Answer: (d) Both Assertion and Reason are false]

VI. Answer very briefly.

1. Write the name of any nitrogen fixing bacteria.

Answer: Rhizobium.

2. Name the bacteria used in the production of vinegar.

Answer: Acetobacter aceti.

3. Write the names of any three protozoans


(i) Amoeba

(ii) Plasmodium

(iii) Paramecium.

4. Who discovered penicillin?

Answer: Alexander Flemming.

5. Which diseases can be prevented by vaccination?

Answer: Small pox, polio, measles, mumps, rubella, tuberculosis.

VII. Answer briefly.

1. Write the four types of bacteria, based on their shape.


Bacteria are described according to the shape of their cells. They are :

(i) Bacilli – Rod shaped bacteria.

(ii) Spirilla – Spiral shaped bacteria.

(iii) Cocci – Spherical or ball shaped bacteria.

(iv) Vibrio – Comma shaped bacteria.

2. What are antibiotics?

Answer: Antibiotic is a substance produced by living organisms which is toxic for other organisms.

Ex : Penicillin.

3. What are pathogens?

Answer: Disease causing organisms are called pathogens.

Ex : The bacterium Mycobacterium tuberculosis causes tuberculosis in human beings.

4. How disease causing microorganisms enter into human beings?

Answer: The disease causing microbes are called pathogens. They enter into the body through

(i) Cuts and wounds in the skin, mouth or nose.

(ii) Some pathogens are found in air and enter into a healthy person by breathing. When a patient sneezes, droplets containing microbes spread in air.

Ex : Tuberculosis, Flu.

(iii) Some pathogens enter into our body through food and water Contaminated with the pathogen.

Ex: Cholera.

(iv) Pathogens also enter through blood transfusion from a infected to healthy person.

Ex : AIDS 

5. Why microorganisms are essential for agriculture?


Role of microbes in agriculture :

(i) Natural fertilizer:

Microbes like bacteria, fungi act as decomposers and break down the dead and degradable waste of plants and animals. During this process nutrients are released into the soil and the soil becomes fertile. This compost is called natural fertilizer.

(ii) Nitrogen fixation :

Microbes can fix atmospheric gaseous nitrogen as nitrate salts in the soil.

Ex: Rhizobium (root nodule bacteria in leguminous plants). Free living bacteria, cyanobacteria (Nostoc).

(iii) Biocontrol agents :

Microbes act as natural biocontrol agents and protect crops from pests.

Ex : Baculoviruses attack insects which harm the plants.

VIII. Answer in detail.

1. Write a short note on bacteria and its structure.


(i) Bacteria are single-celled prokaryotes (cells without nuclei).

(ii) Bacteria are grouped under the kingdom Monera. The study of Bacteria is called Bacteriology.

(iii) Bacteria are of two types based on respiration

(1) Aerobic bacteria (requires oxygen).

(2) Anaerobic bacteria (Does not requires oxygen).

(iv) A bacterium has an outer covering known as the cell wall. Nuclear material is represented by a nucleoid without nuclear membrane.

(v) An extra chromosomal DNA called plasmid is present in the cytoplasm.

(vi) Protein synthesis is carried out by 70S ribosomes. Other cell organelles (mitochondria, Golgi body endoplasmic reticulum etc.,) are absent. Flagella aids in locomotion.

(vii) Bacteria are described according to the shape of their cells. They are :

(1) Bacilli – Rod shape bacteria

(2) Spirilla – Spiral shaped bacteria

(3) Cocci – Spherical or ball shaped bacteria

(4) Vibrio – Comma shaped bacteria

Bacteria are also classified according to number and arrangement of flagella as follows ;

(i) Monotrichous – Single flagella at one end. EgVibrio cholera

(ii) Lophotrichous – Tuft of flagella at one end. EgPseudomonas.

(iii) Amphitrichous – Tuft of flagella at both ends. Eg. Rhodospirillum rubrum.

(iv) Peritrichous – Flagella all around. Eg. E.coli.

(v) Atrichous – Without any flagella. Eg. Corynebacterium diptherae.

2. How microorganisms are useful in the field of medicine?


Role of microbes in medicine :

We obtain antibiotics and vaccines from microbes.

(i) Antibiotics : It is a substance produced by living organisms which is toxic to other organisms.

The antibiotic penicillin got from the fungus Penicillium chrysogenum is used to treat diseases like tetanus, diphtheria. Streptomycin got from streptomyces bacteria is used to cure bacterial infections like plague.

(ii) Vaccines: They are prepared from dead or weakened microbes. When a vaccine is injected into the body of a patient, it produces antibodies to fight the germs. These antibodies protect the body from infections in future.

Ex : MMR vaccine for measles.

3. Write a short note on common human diseases caused by microorganisms.


4. How can we improve the beneficial bacterial count in human beings?

Answer: We can improve the beneficial bacterial count in human beings by adopting the following methods :

(i) Intake of fibre rich foods.

(ii) Intake of seasonal fruits and vegetables.

(iii) Intake of plenty of fermented foods with live microbes. Eat more of prebiotic foods.

(iv) Intake of whole grains.

(v) Prefer plant based diet.

(vi) Avoid artificial sweeteners.

5. Write a short note on probiotics.

Answer: Probiotics: Probiotics are live food supplements used in yoghurt and other fermented milk products. Eg. Lactobacillus acidophilus and Bifidobacterium bifidum. These bacteria improve the microbial spectrum in the gut and thus contribute to the following effects:

(i) Decrease the risk of colon cancer

(ii) Decrease cholesterol absorption

(iii) Prevent diarrheal diseases by increasing the immunity power.

Student Activities

Activity 1

Take one or two drops of butter milk on a slide and spread it. Heat the slide slightly on a lamp ( 3 – 4 seconds). Add a few drops of crystal violet and leave it for 30 to 60 seconds. Then wash the slide gently with water. Observe the slide under the compound microscope.

Activity 2

Take one or two drops of hay (In tamil, vaikol) decoction on a slide and observe it under the microscope.

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