MICROORGANISMS - FRIEND AND FOE

🦠 Notes

🔬 What are Microorganisms?

Very tiny living organisms, not visible to the naked eye
Seen using a microscope
Found everywhere: air, water, soil, inside our body

Types of Microorganisms:

Bacteria
Fungi
Protozoa
Algae
Viruses (active only inside host cells)

😊 Useful Microorganisms (Friends)

🍞 In Food Production

Lactobacillus → converts milk into curd
Yeast → used in baking bread and cakes
Used in making cheese and vinegar

💊 In Medicine

Used to make antibiotics
Penicillium → produces penicillin
Help in vaccine production

🌱 In Agriculture

Rhizobium → fixes nitrogen in soil
Improves soil fertility

😈 Harmful Microorganisms (Foes)

🤒 Diseases in Humans

Disease	Microorganism	Spread Through
Cholera	Bacteria	Contaminated water
Tuberculosis	Bacteria	Air
Malaria	Protozoa	Mosquito
Measles	Virus	Air

💉 Vaccination

Protects us from diseases
Vaccines contain weakened microbes
Help the body develop immunity

💊 Antibiotics

Medicines that kill bacteria
Example: Penicillin
Should be taken only on doctor’s advice

🧂 Food Preservation Methods

Chemical Methods

Salt, sugar, oil, vinegar

Physical Methods

Refrigeration
Boiling
Drying

Pasteurization

Heating milk to kill microbes
Discovered by Louis Pasteur

🔄 Nitrogen Cycle

Nitrogen is essential for plant growth
Microorganisms convert nitrogen into usable forms

Nitrogen fixation
Nitrification
Assimilation
Denitrification

⚠️ Key Points to Remember

Not all microorganisms are harmful
Some are useful and essential
Maintain hygiene to prevent diseases
Proper food storage is important

Question 1

What is a Microorganism?

Ans:- A microorganism is a living organism that is very minute. It can only be seen with the aid of a microscope.

Question 2

What are the habitat of microorganisms ?

Ans:- (i) Microorganisms are present everywhere in air, soil, water, foodstuffs, sewage waste as well as inside animal bodies.
(ii) They can survive in all types of environment even in extremely harsh environmental conditions like hot springs, desert soil, saline water, ice - cold water and marshy lands.
(iii) They are also present in dead and decaying organic matter.
(iv)Some of them can live alone while others in colonies.

Question 3

What are the major groups of microorganisms ?

Ans:- (i) Bacteria (ii) Fungi (iii) Protozoa (iv) Algae (v) Viruses

Question 4

What are bacteria ?

Ans:- Bacteria are single celled organisms, found in a wide range of habitats ranging from glaciers to deserts and hot springs. examples - Lactobacillus ( curd bacteria ).Bacteria are generally heterotroph but some bacteria also show autotrophic nutrition.

Question 5

What are fungi ?

Ans:- Fungi are multicellular, heterotrophic organisms, lack of chlorophyll and are generally found in colonies.
Exception - Yeast is unicellular in nature e,g., Penicillium, Aspergillus.

Question 6

What are protozoa?

Ans:- Protozoa are unicellular , heterotrophic microorganisms. They are usually found in moist and aquatic habitats. Examples- Amoeba, Paramecium.

Question 7

What are algae ?

Ans:- These are unicellular or multicellular eukaryotic autotrophic organisms, contain chlorophyll pigment and carry out photosynthesis. Cell wall of algae is mainly made up of cellulose. Examples - Chlamydomonas, Spirogyra, Chlorella.

Question 8

What are viruses ?

Ans:- Viruses are acellular structures because they are not built up of cells.They are much smaller than cells. These are ultramicroscopic organisms, which require cells of host organisms to reproduce.
Examples - Influenza virus, polio virus, AIDS etc.,. Structure of the virus consists of nucleic acid and a protein coat. nucleic acid can either be DNA or RNA.

Question 9

How the microorganisms are useful in the formation of food ?

Ans:- (a) Formation of curd (dahi) :-Making of curd is a process in which the lactose (milk sugar) of the milk is converted into lactic acid by the action of certain bacteria (mainly Lactobacillus). This curd is further used in the preparation of idli - dosa and bhaturas.
(b) In bread making :- Yeast is added to the uncooked dough to make the dough "rise". The dough rises due to the production of carbon dioxide( because of respiration of yeast) in the dough. The bread is then baked. Heat of baking drives off the carbon dioxide, making the bread porous and light.
(c) Add flavor :- Some fungi (Aspergillus and Penicillium) are added to cheese to give a special flavor.

Question 10

What are the commercial use of microorganisms ?

Ans:- In industry the wine and the beer (brewery) industries are important industries which make use of the fungus yeast.

Yeast is grown on natural sugars present in fruit juices and grains like barley and rice. Yeast brings about conversion of sugars into alcohol and carbon dioxide.

Question 11

What are the agricultural use of microorganisms? ?

Ans:- All plants and animals need nitrogen, the gas that makes up 78% of the air. Some bacteria and Blue green algae are the only organisms that can take nitrogen directly from air.

These microbes convert atmospheric nitrogen into compounds of nitrogen and so called as biological nitrogen fixers. Thus , they are very useful to agricultural, as they enrich the soil with nitrogen.

Question 12

What are the medical use of microorganism ?

Ans:- Microbes are being employed for the production of both antibiotics and vaccines.

Antibiotics: These are produced by a number of microorganisms and inhibit the growth of other microorganisms even at very low concentrations. Fungi and bacteria are important microbes producing a variety of antibiotics.

Antibiotics obtained from bacteria:
Streptomycin, aureomycin and chloromycin.

Antibiotics obtained from fungi :
Penicillin and Griseofulvin. These antibiotics are used to cure a variety of diseases in humans, plants and animals.

Question 13

How the microorganisms are helpful in cleaning the environment ?

Ans:- Microorganisms help clean the environment by decomposing dead matter, recycling nutrients, breaking down pollutants, treating sewage, and converting waste into compost.

🌱 Nitrogen Cycle – Class 8 Science

🔬 What is Nitrogen Cycle?

The nitrogen cycle is the natural process by which nitrogen circulates between the atmosphere, soil, plants, animals, and microorganisms. It converts nitrogen into different usable forms for living organisms.

📊 Nitrogen Cycle Diagram

🔄 Steps of Nitrogen Cycle

1. Nitrogen Fixation:
Nitrogen gas (N₂) from the atmosphere is converted into ammonia (NH₃) by bacteria like Rhizobium.

2. Nitrification:
Ammonia is converted into nitrites and then into nitrates by soil bacteria.

3. Assimilation:
Plants absorb nitrates and use them to grow. Animals get nitrogen by eating plants.

4. Ammonification:
Dead plants and animals decompose, releasing ammonia back into the soil.

5. Denitrification:
Bacteria convert nitrates into nitrogen gas, returning it to the atmosphere.

🌍 Importance
Essential for plant growth
Helps in making proteins and DNA
Maintains nitrogen balance in nature

⚡ Short Summary

Nitrogen gas → converted by bacteria → used by plants → passed to animals → returned to soil → back to atmosphere.

MICROORGANISMS - FRIEND AND FOE, Class 8