8th cbse

1. CHAPTER - 2
MICROORGANISMS : FRIEND
AND FOE
CLASS :- VIII
SUBJECT :- SCIENCE

2. 1) Microorganisms (Microbes) :-
i) Microorganisms are very small organisms which cannot be seen
with the unaided eye. They can be seen only with a magnifying glass
or microscope.
ii) Microorganisms may be unicellular or multicellular.
iii) Microorganisms may exist alone or in colonies.
iv) Microorganisms are found in all kinds of environment like ice cold
climate, hot springs, deserts, marshy lands etc. They are also found
inside the bodies of other organisms.

3. 2) Types of microorganisms :-
There are four main types of microorganisms. They are :- bacteria,
fungi, algae and protozoans.
Viruses are also considered as microorganisms. They are different
from other microorganisms because they reproduce only in the body of
host organisms like bacteria, plants or animals.
Eg :- Algae – Chlamydomonas, Spirogyra etc.
Fungi – Bread mould, Penicillium, Aspergilles etc.
Protozoans – Amoeba, Paramaecium etc.

4. Virus

5. 3) Friendly microorganisms (Useful microorganisms) :-
i) Microrganisms like bacteria decomposes organic waste into manure
and increases the fertility of the soil.
ii) Microorganisms help in fixing atmospheric nitrogen into nitrogen
compounds in the soil and increases the fertility of the soil.
iii) Microorganisms are used for making curd from milk, for making
cheese, pickles, bread, cakes, pastries, alcohol, wine, vinegar (acetic
acid) etc.
iv) Microorganisms are used for making medicines.
a) Making curd from milk :- The bacterium called Lactobacillus
reproduces in milk and helps to convert milk into curd.
b) Making bread :- The fungus called yeast reproduces in flour dough
and produces carbon dioxide during respiration which makes the dough
soft and helps in making bread, cakes, biscuits, pastries etc.
c) Making alcohol :- The fungus called yeast reproduces in sugar solution
and converts it into alcohol. This process is called fermentation.
d) Making medicines :- Some bacteria and fungi are used to make
medicines which kill or stops the growth of disease causing
microrganisms. Such medicines are called antibiotics. Eg :-
streptomycin, tetracycline, erythromycin etc.
Vaccines are dead or weakened microbes introduced into the body to
produce antibodies. These antibodies protect the body from disease
causing microbes. Diseases like polio, cholera, typhoid, small pox,
hepatitis etc. can be prevented by taking vaccines.

6. 4) Harmful microorganisms :-
i) Some microorganisms cause diseases in plants and animals. Such
disease causing microorganisms are called pathogens.
ii) Some microorganisms grow in food substances and produce toxic
substances and makes the food poisonous. Food poisoning causes
illness and even death.
iii) Some microorganisms spoil materials like clothing, leather, wood etc.
5) Disease causing microorganisms in Humans :-
Disease causing microorganisms enter our body through air, water,
food, contact or insects. These microorganisms are called pathogens.
Diseases which can spread from an infected person to a healthy
person are called communicable diseases.
Eg :- cholera, common cold, chicken pox, tuberculosis etc.
Some insects and animals act as carriers of disease causing microbes.
Eg :- House fly, Female anopheles mosquito (malaria), Female aedes
mosquito (dengue fever) etc.

7. Female anopheles mosquito Aedes mosquito

8. 6) Disease causing microorganisms in plants :-
Microorganisms cause diseases in plants like rice, wheat, potato,
sugarcane, apple, orange etc. These diseases reduce crop yield. They
can be controlled by using chemicals which kill the microbes.

9. Some common human diseases caused by microorganisms:-
Human disease Causative Mode of
microorganism transmission
Chicken pox Virus Air / Contact
Polio Virus Air / Water
Measles Virus Air
Hepatitis – B Virus Water
Tuberculosis Bacteria Air
Typhoid Bacteria Water
Cholera Bacteria Water / Food
Malaria Protozoa Mosquito
Some common plant diseases caused by microorganisms :-
Plant disease Causative Mode of
microorganism transmission
Citrus canker Bacteria Air
Rust of wheat Fungi Air / Seed
Yellow mosaic of
Okra (Lady’s finger) Virus Water
bhindi

10. 7) Food preservation :-
Food is spoilt by microorganisms. Spoilt food has bad smell and bad
taste and causes food poisoning. Food can be preserved by protecting
it from microorganisms. Food can be preserved by different methods.
They are :-
i) Chemical method :- Food like pickles can be preserved by using
chemicals like salts and edible oils. Jams and squashes can be
preserved by using sodium benzoate, sodium metabisulphite. These
chemicals are called preservatives.
ii) By using common salt :- Food items like fish, meat, amla, raw
mangoes, tamarind etc can be preserved by using common salt. It
prevents the growth of microbes.
iii) By using sugar :- Food items like jams, jellies, squashes etc. can be
preserved by using sugar solution. It prevents the growth of microbes.
iv) By using oil and vinegar :- Food items like pickles, vegetables, fish,
meat etc. are preserved by using oil and vinegar. It prevents the growth
of microbes.
v) By heat and cold treatments :- Heating food items kills microbes.
Similarly storing food items at low temperatures prevents growth of
microbes.

11. Pasteurisation :- The process of heating milk to about 70oC for 15 to
30 seconds and then suddenly chilling it to prevent the growth of
microbes is called pasteurisation. This process was discovered by
Louis Pasteur.
vi) By storage and packing :- Many food items are stored in air tight
containers to protect them from microbes.

12. 8) Nitrogen cycle :-
ATMOSPHERIC
NITROGEN
LIGHTNING FIXES
NITROGEN
NITROGEN FIXING BACTERIA
AND BLUE GREEN ALGAE
FIX ATMOSPHERIC NITROGEN
COMPOUNDS OF
NITROGEN IN THE SOIL
UPTAKE BY
PLANTS
ANIMAL EATS
PLANTS
NITROGENOUS
WASTE FROM
EXCRETION
AND DEATH
BACTERIA TURN
COMPOUNDS OF
NITROGEN INTO
GASEOUS NITROGEN

13. Nitrogen cycle :-
The nitrogen in the atmosphere is converted into nitrogen
compounds in the soil by nitrogen fixing bacteria and blue green
algae. Lightning also converted into nitrogen compounds in the soil.
The nitrogen compounds in the soil is used by plants for the synthesis
of proteins and other compounds. Animals feeding on plants get these
proteins and other compounds. When plants and animals die, bacteria
and fungi in the soil converts the nitrogenous waste into nitrogen
compounds in the soil which are again use by plants. Some other
bacteria converts some nitrogen compounds in the soil into nitrogen
gas which goes back into the atmosphere. Hence the nitrogen in the
atmosphere almost remains constant.