CHAPTER - 2
MICROORGANISMS : FRIEND
AND FOE
Microorganisms (Microbes) :• Very small organisms which cannot be
seen with the naked eye.
• may be unicellular or multicellular
• may exist alone like amoeba or in
colonies like Fungi & Bacteria
• 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.
Types of microorganisms :•
•
•
•
Bacteria
Fungi :
Bread mould, Penicillium,
Algae :
Chlamydomonas, Spirogyra
Protozoan : Amoeba, Paramaecium
Virus
They are different from other microorganisms
as they reproduce only in the body of host
organisms like bacteria, plants or animals.
Friendly microorganisms
• 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 :- Yeast is used for commercial production of
alcohol & wine.
» The fungus called yeast reproduces in sugar solution
and converts it into alcohol. This process is called
fermentation.
» Louis Pasteur discovered fermentation in 1857.
• 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.
Increasing Soil Fertility
Some Bacteria and Blue Green Algae are able
to fix nitrogen from the atmosphere to enrich
soil with nitrogen and increase its fertility.
These microbes are called Biological Nitrogen
Fixers.
Cleaning the Environment
Micro organisms decompose dead organic
waste of plants and animals and convert them
into simple substances which are again used by
other plants and animals.
Harmful microorganisms
i) Some micro organisms cause diseases in plants and
animals. Such disease causing microorganisms are
called pathogens.
ii) Some micro organisms 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.
Pathogens
• Disease causing microorganisms.
• Enter our body through air, water, food, contact or
insects.
• 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.
Female anopheles mosquito
Aedes mosquito
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.
Some common human diseases caused by microorganisms:Human disease
Chicken pox
Polio
Measles
Hepatitis – B
Tuberculosis
Typhoid
Cholera
Malaria
Causative
microorganism
Virus
Virus
Virus
Virus
Bacteria
Bacteria
Bacteria
Protozoa
Mode of
transmission
Air / Contact
Air / Water
Air
Water
Air
Water
Water / Food
Mosquito
Some common plant diseases caused by microorganisms :Plant disease
Citrus canker
Rust of wheat
Yellow mosaic of
Okra (Lady’s finger)
bhindi
Causative
microorganism
Bacteria
Fungi
Virus
Mode of
transmission
Air
Air / Seed
Water
Some common plant diseases caused by
microorganisms
Food Poisoning
• Could be due to consumption of food spoilt
by micro organisms
• So it is important to preserve food.
Food preservation
• Chemical method :
– Pickles can be preserved by using chemicals like
salts and edible oils.
– Jams and squashes can be preserved by using
sodium benzoate, sodium meta bisulphate
• These chemicals are called preservatives.
Salting
• Food items like fish, meat, amla, raw
mangoes, tamarind etc can be
preserved by using common salt.
• It prevents the growth of microbes
By using sugar
• Food items like jams, jellies, squashes etc. can be
preserved by using sugar solution.
• It reduces the moisture content which inhibits the
growth of bacteria.
By using Oil & Vinegar
• Prevents spoilage of pickles because bacteria
cannot live in such an environment.
• Vegetable, Fruit, Fish and meat are often preserved
by this method.
heat and cold treatments
• Heating food items kills microbes.
• Storing food items at low temperatures
prevents growth of microbes.
storage and packing
• Many food items are stored in air tight
containers to protect them from
microbes.
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
• This process was discovered by Louis
Pasteur.
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.
8) Nitrogen cycle :ATMOSPHERIC
NITROGEN
LIGHTNING FIXES
NITROGEN
NITROGEN FIXING BACTERIA
AND BLUE GREEN ALGAE
FIX ATMOSPHERIC NITROGEN
BACTERIA TURN
COMPOUNDS OF
NITROGEN INTO
GASEOUS NITROGEN
ANIMAL EATS
PLANTS
UPTAKE BY
PLANTS
NITROGENOUS
WASTE FROM
EXCRETION
AND DEATH
COMPOUNDS OF
NITROGEN IN THE SOIL
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.