Living World and Classification of Microbes Class 8 Science Notes Maharashtra Board

Living World and Classification of Microbes Class 8 Science Notes Maharashtra State Board

Biodiversity and Need for Classification
Last year we learned that all the living organisms on Earth have adapted according to geographic regions, food ingestion, defense, etc. While adapting, many differences are observed in the organisms of the same species too. According to the 2011 census, around 87 million species of living organisms are found on the earth- including land and sea. To study such a vast number, it was essential to divide them into groups. So groups and subgroups were created considering the similarities and differences among the living organisms. This process of dividing living organisms into groups and subgroups is called Biological classification.

Robert Harding Whittaker (1920-1980) was an American Ecologist. In 1969 he divided living organisms into 5 groups.

For this classification, Whittaker considered the following criteria:

1. Complexity of Cell Structure: Prokaryotic and Eukaryotic.
2. Complexity of Organisms: Unicellular or Multicellular.
3. Mode of Nutrition:
   • Plants – AutotrophicPhotosynthetic
   • Fungi – Saprophytic- Absorption from dead organisms.
   • Animals – Heterotrophic and ingestive.
4. Life Style:
   • Plants – Producers
   • Animals – Consumers
   • Fungi – Decomposers
5. Phylogenetic Relationship: Prokaryotic to Eukaryotic, unicellular to multicellular.

Kingdom 1: Monera
Activity: Take a small drop of curd or buttermilk on a clean glass slide. Dilute it with a little water. Carefully keep a cover slip. Observe it under the high power of the compound microscope. What did you see? Moving, small rod-like microbes are lactobacilli bacteria. All types of bacteria and blue-green algae are included in the kingdom Monera.

Characteristics:

• All the organisms are unicellular.
• They may be autotrophic or heterotrophic.
• These are prokaryotic cells without distinct nuclei or cell organelles

Kingdom 2: Protista
Activity: Prepare a temporary mount of one drop of pond water on a glass slide. Observe it under the low power and power of the microscope. You will find some motile microbes with irregular shapes. These are amoebae.

Characteristics:

• Protista are single-celled organisms with well-defined nuclei enclosed in a nuclear membrane.
• They have pseudopodia or hair-like cilia or whip-like flagella for locomotion.
• Autotrophs – eg. Euglena, Volvox contain chloroplast.
• Heterotrophs – eg. Amoeba, Paramoecium

Kingdom 3: Fungi
Activity: Take a moist piece of bread or bhakri and keep it in a container with a lid for 2-3 days. After 2-3 days a fine cotton thread-like tuft is found growing on the surface of the bread. Prepare a temporary mount of a few threads from this culture and observe it under the microscope.


Characteristics:

• These are non-green, eukaryotic, heterotrophic organisms.
• Most of them are saprotrophs. They feed upon decaying organic matter.
• Their cell wall is made up of tough and complex sugar called ‘Chitin’.
• Some fungi are thread-like and many nuclei are present in the cytoplasm.
• Examples: Baker’s yeast, Aspergillus (fungi on corn), Penicillium, and Mushrooms.

Though many systems of classification were introduced after Whittaker’s, his five-kingdom system is widely accepted.

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

1. Bacteria (size – 1 µm to 10 µm)

• Unicellular, independent/parasitic organisms. Sometimes many bacteria together form colonies.
• Bacterial cells are prokaryotic with cell walls, but distinct nuclei or cell organelles are absent.
• They reproduce by simple binary fission.
• In favorable conditions, bacteria grow vigorously and can double their number in 20 minutes.

2. Protozoa (size – approximately 200 µm)

• Protozoans are found in soil, freshwater, and seawater. Some are found in the body of other organisms and are pathogenic.
• These are unicellular organisms with eukaryotic cells.
• 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

3. Fungi (size – approximately 10 µm to 100 µm)

• These are found in 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, and Mushroom.

4. Algae (size – approximately 10 µm to 100 µm)

• 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 to the naked eye.

5. Viruses (size – approximately 10 nm to 100 nm)
Generally, viruses are not considered living organisms or they are said to be “Organisms at the edge of living and nonliving.” They are studied under microbiology.

• Viruses are extremely minute, i.e., 10 to 100 times smaller than bacteria, and can be seen only with an electron microscope.
• They are found in the form of independent particles. The 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, produce their proteins with the help of host cells, and create numerous replicas. Then they destroy the host cell and become free. These free viruses again infect new cells.
• Viruses cause many diseases in plants and animals.
  • Human – poliovirus, Influenza virus, HIV-AIDS virus etc.
  • Cattle – picorna virus
  • Plants – Tomato – Wilt virus, Tobacco mosaic virus, etc.
  • Bacteria – Bacteriophage (viruses attack bacteria).

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