LIVING WORLD 2

 Living world of micro-organisms

 Among the living organisms, microorganisms are largest in number. Hence they are classified as follows. 

1. Bacteria (size – 1 mm to 10 mm):

They are unicellular, independent / parasitic organisms. Sometimes many bacteria together form colonies.Bacterial cell is prokaryotic with cell wall, but distinct nucleus or cell organelles are absent.They reproduce by simple binary fission. In favourable conditions, bacteria grow vigorously and can double their number in 20 minutes.

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2. Protozoa (size - approximately 200 mm):

Protozoans are found in soil, fresh water and sea water. Some are found in the body of other organisms and are pathogenic.These are unicellular organisms with eukaryotic cell.There is great variation in cell structure, organs of locomotion and modes of nutrition among protozoans. These organisms reproduce by simple cell division. Eg.- Amoeba, Paramoecium - Free living in dirty water. Entamoeba histolytica - causes amoebiasis. Plasmodium vivax- causes malaria Euglena - autotrophic

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3.Fungi- (size- approximately 10 mm to 100 mm):

These are found on decaying organic matter and dead bodies of plants and animals.These are eukaryotic organisms. Some are unicellular and others are visible with naked eyes.Saprotrophic, absorb their food from decaying organic matter.They reproduce sexually and asexually by cell division or by budding. Eg. Baker's yeast, Candida. 

Mushroom

4. Algae- (size- approximately 10 mm to 100 mm) 

They are aquatic eukaryotic, unicellular, autotrophic organisms. Photosynthesis is carried out with the help of chloroplast present in the cell. Eg. Chlorella, Chlamydomonas very few species of algae are unicellular. Most of them are multicellular and visible with naked eyes.

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5. Viruses-( size- approximately 10 nm to 100 nm):

 Generally, viruses are not considered as living organisms or they are said to be “Organisms at the edge of living and nonliving.” They are studied under microbiology. They are extremely minute i.e. they are 10 to 100 times smaller than bacteria and can be seen only with electron microscope.They are found in the form of independent particles . Virus is a long molecule of DNA (Deoxyribo Nucleic Acid) or RNA (Ribo Nucleic Acid) covered by a protein coat.Viruses survive only in living plant or animal cells and produce their own proteins with help of host cell and create their numerous replica. Then they destroy the host cell and become free. These free viruses again infect new cells. Viruses cause many diseases to plants and animals.

Eg:COVID-19(Corona virus),HIV – AIDS virus etc.