Microorganisms : Friend and Foe Class 8 NCERT Notes

Microorganisms :Friend and foe CBSE class 8


Organisms which are too small to see with naked eyes and can be seen only with the help of microscope are called as Microorganisms.

Where are microorganisms found???

Microorganism can be found anywhere on earth.
They are found in

  • Soil
  • Water
  • Air
  • Decaying substances 
  • They are also found in Harsh condition like hottest desert, in coolest icy polar region.
  • Microorganisms can anyone survive volcanic eruption.
  • In marshland
  • Found in salt water
  • Outside the body of living organism
  • And even inside the body of living organism
How microorganisms can survive in Harsh condition??
  • Microorganisms can survive in Harsh condition.
  • They form a hard outer covering called cyst around themselves.
  • Microorganisms forms cyst covering when conditions are not favorable.
  • The survivor remaining inactive within the cyst until conditions are favorable again.
Types of microorganisms
  • Microorganisms are diverse in size, shape, preference of Nutrition and habitat.
  • Microorganism maybe unicellular (single celled ) or multicellular( many celled).
Microorganisms can be categorized into 5 groups
  • bacteria (singular: bacterium)
  • algae (singular:  alga)
  • protozoa (singular: protozoan)
  • fungi (singular: fungus)
  • viruses( singular: virus).


Bacterial cell

Structure of Bacterial cell: –

Shape of bacteria

Habitat of bacteria

Mode of nutrition in bacteria /  How bacteria feed themselves??

Reproduction in bacteria

Functions of Bacteria……..


Algae are simple plant like organisms having cell walls and chlorophyll within the cells. They have ability to conduct photosynthesis.

Structure of algae
  • Algae are unicellular or multicellular.
  • Some algal cell join each other to form filaments and some have branches or fronds.
  • Both unicellular and multicellular algae live as individual or in colonies.

Algae is being found in different forms

  • Unicellular forms
  • Multicellular forms
  • A colonial form- volvox

Unicellular forms:-

Examples of unicellular Algae are as follows—


  • It is a green single Cellular alga.
  • They are generally oval, spherical or peared shape.
  • Flagella is used for locomotion.
Habitat of algae
  • Algae live mostly in an aquatic environment floating on the water. Many times they are found in ponds, lakes and seawater.
  • Some algae leave on bark of tree and some on moist soil and rocks.
  • Some Algae are also found in snow and some in hot spring.
Mode of nutrition in Algae
  • Algae are considered autotrophs as all of them have chlorophyll in it.
  • Most Algal groups are considered photoautotrophs, they depend entirely upon their photosynthetic Apparatus for their food production, using sunlight as a source of energy and CO2 as a source of carbon to produce their own food.
  • Many of the algae have a symbiotic relationship with plants and animals.
Types of Algae

On the basis of the pigment found cell they are classified into following groups

Blue green Algae: –
  • They have blue pigment along with chlorophyll.
  • Nucleus is not present in blue green algae. They are more like bacteria. And therefore they are called as Cyanobacteria.
  • They live in freshwater
Green algae: –
  • This contains mainly chlorophyll.
  • They live in freshwater

Brown algae: –

  • Contents brown pigment in addition to chlorophyll.
  • Brown algae generally marine.
  • Many seaweeds are brown algae, which are almost 50 to 60-meter long.
Red algae: –
  • Contains red pigments besides chlorophyll.
  • Red algae are generally marine.
Uses of Algae
  • Algae are the major source of food for fishes, prawns and many aquatic animals.
  • Sea Algae is eaten in many countries. Most of the oxygen in the atmosphere is released by algae.
  • Red algae yield algin, substance used to thicken cosmetics and food items like ice cream, jelly coma chocolates.
  • Agar a solidifying substance obtained from red algae used in microbiological research.
  • Algae also consumes huge amount of carbon dioxide which is good for our ecosystem.

Disadvantages of Algae

Algal blooms can be toxic and it harms aquatic ecosystem and human health.


Singular: fungus

Characteristic features of fungi

  • Fungi are unicellular or multicellular and filamentous.
  • All fungi are made up of fine threads called hyphae.
  • Fungi are plant like organism. They have cell wall.
  • They lack chlorophyll and  that’s why do not perform the process of photosynthesis.
  • Store food in the form of glycogen instead of starch.
    Examples of  fungi are  yeast, moulds such as  Penicillium,  bread mould and aspergillus,  mushrooms  etc.

Study of fungi is called mycology.

Habitat of  fungi

  • Fungi are present everywhere  and in any habitat.
  • They are found in deserts, cold regions, sea waters, fresh  water, in soil, on plants and animals, human skin  etc.
  • They grow best in dark warm moist places.
Mode of nutrition in fungi
  • Fungi look like plant but they cannot make their own food.
  • Fungi are heterotrophic i.e.  they depend upon another organism for their food.
  • Fungi generally follow saprophytic or parasitic mode of nutrition.
  • Saprophytic fungi obtain their nutrients from non-living organic matter, usually dead and decaying plants and animals matter by absorbing soluble organic compounds.
  • Parasitic fungi live in or on other organism and get their nutrients from their host. It generally causes disease or illness to the host.
Mode of reproduction in fungi

The Reproduction of fungi can be either sexual or Asexual.

In asexual mode of reproduction fungi reproduce by

  • Fragmentation
  • Budding
  • Produces spores.

The majority of fungi reproduces asexually.

Advantages of fungi in day to day life and it’s usage
  • Fungi are decomposers, they are good for the soil and cleaning up the environment.
  • Many fungi are used for the production of antibiotics. For example Penicillium is a fungus which produces antibiotic call penicillin.
  • All mushrooms, including the ones we eat, a large fungi.
  • Yeast  is another useful fungus which is used in many fermentation  processes.
Disadvantages of fungi
  • Many saprophytic fungi spoil our food, clothes, leather articles etc. example you me have noticed the cottony growth on leather or stale bread. Eating such food can make you ill.
  • Many fungus can cause human diseases.
  • Some fungal species produces toxin that affects plants animals and humans.


  • Protozoa are single-celled microscopic organism
  • Protozoa are heterotrophs i.e they don’t produce their own food. They generally feed on organic matter such as other microorganisms or organic tissues and debris.
  • Protozoa are heterotrophic microorganisms which do not fully plant or animals.

Why protozoa were considered animals?

Protozoa are unicellular microorganisms which do not fully  dissemble plant or animals.
The word Protozoa means “early animals” .
Earlier Protozoa  were consider as animals as they do not have cell wall and they can move.
Apart from few examples of protozoa which contains chlorophyll  are  either saprophytic and parasitic.
the protozoa which contains chlorophyll  lack other plant characters.Habitat of protozoa
protozoa freshwater, saline water, ponds , lakes, dirty water drains, rivers, sea water and also in damp soil.
Some parasitic protozoa so live in the bodies of other organisms including human beings.
Examples of protozoa Giardia, Amoeba, Paramecium, euglena etc.How Protozoa can move and feed themselves??!
The most important characteristic of protozoa is that  they can move from one place to another.
this single cell organism  looks like mass of Protoplasm with no fixed shape. It can move in any direction by extending pseudopodia.
Pseudopodia is also called as false feet. A pseudo podium is formed by the flow  of Protoplasm in the direction of movement.
Pseudopodia helps Amoeba in feeding. It extends to encircle the food particle which is then taken in by the cell and digested.
Amoeba feeds tiny algae and bacteria.Paramoecium
This  slipper shaped protozoan which lives in ponds and ditches  swims by beating it’s Cilia against water.
Movement of Cilia helps to direct food and water into an oral groove in  paramoecium.
Paramecium feeds on small algae and bacteria.
Waste matter of paramoecium is thrown out through an anal pore.
euglena is found in ponds, ditches and other stagnant water bodies.
The body  of euglena is spindle shaped slightly elongated and tapers  at the  top.
One end of euglena has  flagellum  which  they use to move from one place to another. Some euglena has two flagella.
Euglena with chlorophyll normally obtain their food through photosynthesis.
Euglena  which lives in dark condition ingest food particles through a mouth like opening leading to a gullet.
this organism has  characteristics of both plants and animals.
Euglena contains chlorophyll and that’s why most of them autotrophs.
Even euglena has chlorophyll and live as autotroph  it doesn’t come under plant Kingdom because they don’t have cell wall.
Some euglena don’t have chlorophyll and live as saprophytes or parasites.

Advantages of protozoa
Saprophytic protozoans benefit the environment by decomposing organic matter in the soil.
Parasitic protozoans prey on bacteria and algae helps to regulate the population of bacteria and algae.

Disadvantages of protozoa
Many  protozoa are parasites and causes many diseases.
Diseases like dysentery and malaria are caused by protozoa. For example entamoeba  causes amoebic dysentery.
Protozoan  name plasmodium causes  disease malaria inhumans.  Plasmodium is commonly known as malarial parasite.

Viruses are the smallest microorganism can be seen only under an electron microscope.
Characteristics of viruses
viruses do not show any characteristic of living things.
Viruses do not have organised cell structure
Viruses do not respire, feed, grow, excrete, move on their own.
Viruses can only reproduce if they enter in a living cell  of other organism( host cell).
When viruses enter into any living cell it starit behaving like a living thing  by carrying out the process of reproduction.
Outside the living cell dresses behave as non living thing.
Due to all this reasons viruses are considered to be an intermediate form between the living and nonliving.

Structure of virus
Viruses do not have an organisation structure.
Viruses are acellular i.e their body has no cellular substances or nucleus.
Viruses consist of nuclear material coated with proteins.
Viruses typically have one or two strands of DNA or RNA.
The Genetic material of virus are covered with a protective coat  of protein called as CAPSID.

How viruses grow or how viruses reproduce???
Viruses cannot reproduce by themselves.
The virus attaches itself to a specific host cell  or the cell in which it can reproduce.
After attaching to the host cell virus  injects it’s  nuclear material( DNA or RNA) inside the host cell.
The nuclear material of virus  attaches to the nuclear material of the host cell.
The nucleic material of virus take the custody of the manufacturing system of the cell.
The host cell ignore its own need and
switches to making  new viruses.
The host cell sometimes destroyed in the process and many of the new viruses are released to infect other host cells.
Advantages of viruses
Even though viruses has lots of disadvantages but some viruses plays very important role in the ecosystem.
For example the bacteriophages which are also called as bacterial virus please an important role in Marine Ecology and carbon cycling. Bacteriophages in the sewage has the ability to  to kill harmful bacteria  and keep the bacteria level in control.
Scientist are trying to find out  viruses which can be used to kill bacteria, and can be used instead of antibiotics.

Disadvantages of viruses
Viruses  have many disadvantages as they are the agent of  many diseases.
Virus causes diseases too many animals and plants.
Human diseases such as common cold, influenza, measles, chicken pox, smallpox are all caused by viruses.
HIV or  immunodeficiency virus causes AIDS in human beings and some animals.


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