Class 8 Science


Microorganism: An organism which cannot be seen by naked eyes is called microorganism or microbe. We need a microscope to see a microbe.

Types of Microorganisms

  • Bacteria: Bacteria are microscopic organisms which have cell walls but do not have nucleus. Some bacteria are autotrophs but most are heterotrophs. Bacteria are found in different shapes; like rod-shaped, spiral, spherical and comma-shaped. Examples: Rhizobium, E. coli, Pseudomonas, Streptococcus, etc.
  • Algae: Algae are green plants which are simple in structure. A cell of algae contains cell wall and chloroplast. Algae are autotrophs. Some algae are microscopic. Examples: Chlamydomonas, Spirogyra, etc.
  • Fungi: Fungi have cell wall and nucleus in their cells but do not have chloroplast. Fungi are saprotrophs, i.e. they feed on dead remains of plants and animals. Almost all fungi are microscopic, except mushrooms. Examples: yeast, Rhizopus (bread mould), Aspergillus, Penicillium, etc.
  • Protozoa: Protozoa do not have cell wall and chloroplast but they do have nucleus. Amoeba and Plasmodium are examples of protozoa.

Virus: Viruses are also microscopic but they are not considered as living beings. They are considered as a borderline case between living and non-living. A virus behaves as non-living when it is outside a host cell, i.e. a virus does not carry on nutrition, respiration or reproduction when it is outside a host. But once it is inside a host, it behaves like a living being, i.e. it carries on nutrition, respiration and reproduction when it is inside a host. Examples: HIV, Tobacco Mosaic Virus (TMV), etc.

Habitat of Microorganisms: Microorganisms live almost everywhere on the earth. They are found on land, water and in air. They are even found inside and on our body. Some microbes are even found in extremely harsh conditions, e.g. in the crater of volcano and in sulphur springs.

Friendly Microorganisms

  • Lactobacillus is an example of bacteria. Lactobacillus facilitates conversion of milk into curd.
  • Yeast causes fermentation of many food items. The process by which sugar solution changes into alcohol due to anaerobic respiration by microbes is called fermentation.
  • When yeast is added to batter of cake, idli or pakora, there is production of carbon dioxide due to respiration by yeast. Carbon dioxide gas creates bubbles in dough or batter which makes fluffy cakes, breads, idli and dosa.
  • Fermentation also helps in production of alcohol from fruit juice, sugar cane juice or from cereals. This process is utilized to make wine, beer and other alcohol products.


Antibiotic: A substance which kills or stops the growth of bacteria is called antibiotic.

Alexander Fleming discovered the antibiotic Penicillin in 1929. During one of his experiments; Alexander Fleming observed that bacteria did not grow on culture where a fungus (Penicillum notatum) was present. Thus, penicillin was prepared from a fungus Penicillium. Penicillin made it possible to treat many difficult diseases. Most of the modern antibiotics are derivatives of penicillin, e.g. amoxicillin, cefotaxime, cefoperazone, cefuroxime, ceftazidime, etc. Some other examples of antibiotics are; tetracycline, norfloxacin, ciprofloxacin, doxycycline, etc.


Our body has the ability to fight diseases. The defence-mechanism of our body produces antibodies to fight a disease-causing microbe which may enter our body. While doing so, our body remembers to fight any future onslaught of that microbe. This concept was utilized to make vaccines against various diseases.

A vaccine made by a weak or killed strain of a disease-causing microbe. When a vaccine is inoculated in the body, the body prepares antibodies against it. Thus, the body learns and remembers how to fight with that microbe in future. Thus, vaccination helps in preventing against a particular disease.

Edward Jenner discovered small pox vaccine in 1798. He observed that people who were rearing cows did not suffer from small pox. That observation led to the discovery of small pox vaccine.

Vaccines are now available for many diseases; like small pox, tuberculosis, polio, tetanus, diphtheria, whooping cough, hepatitis, etc.

Pulse Polio:

Polio: Polio is a viral disease which damages the nerves in spine. This results in weak muscles and paralysis; especially in legs. Polio is also called infantile paralysis. Polio vaccine can prevent this disease.

Pulse Polio is a large scale programme to eradicate polio from India. This programme is being run by the United Nations; in coordination with the Government of India. All children under five years of age are given polio drops; under this programme. Pulse Polio has been highly successful against eradication of polio.

Increasing Soil Fertility: Blue-green algae fix atmospheric nitrogen in soil. Rhizobium bacteria live in root nodules of leguminous plants. These bacteria also help in nitrogen fixation in soil. Thus, these microbes help in improving soil fertility.

Cleaning the Environment: Many microbes feed on dead remains of plants and animals. Thus, these microbes play the role of decomposers. Thus, microbes help in clearing organic waste from our surroundings. Dead cattle, waste from meat and fish shop, waste from vegetable market, etc. are decomposed because of microbes.