NCERT Book Class 8 Science Chapter 2 Microorganisms : Friend and Foe

MICROORGANISMS : FRIEND AND FOE

Y
ou have seen several kinds of These observations show that water
plants and animals. However, and soil are full of tiny organisms,
there are other living organisms though not all of them fall into the
around us which we normally cannot category of microbes. These
see. These are called microorganisms microorganisms or microbes are so
small in size that they cannot be seen
or microbes. For example, you might
with the unaided eye. Some of these,
have observed that during the rainy
such as the fungus that grows on bread,
season moist bread gets spoilt and its can be seen with a magnifying glass.
surface gets covered with greyish white Others cannot be seen without the help
patches. Observe these patches through of a microscope. That is why these are
a magnifying glass. You will see tiny, called microorganisms or microbes.
black rounded structures. Do you know Microorganisms are classified into
what these structures are and where do four major groups. These groups are
these come from? bacteria, fungi, protozoa and some
a l g a e. Some of these common
2.1 Microorganisms microorganisms are shown in
Figs. 2.1 - 2.4.
Activity 2.1 Viruses are also microscopic but are
Collect some moist soil from the different from other microorganisms.
field in a beaker and add water to They, however, reproduce only inside the
it. After the soil particles have cells of the host organism, which may be
a bacterium, plant or animal. Some of the
settled down, observe a drop of
viruses are shown in Fig. 2.5. Common
water from the beaker under a
ailments like cold, influenza (flu) and
microscope. What do you see ?
most coughs are caused by viruses.
Serious diseases like polio and chicken
Activity 2.2 pox are also caused by viruses.
Diseases like dysentery and malaria
Take a few drops of water from a are caused by protozoa(protozoans)
pond. Spread on a glass slide and whereas typhoid and tuberculosis (TB)
observe through a microscope. are bacterial diseases.
You have lear nt about some of
Do you find tiny organisms moving these microorganisms in Classes VI
around? and VII.