MICROORGANISMS FRIEND AND FOE SCIENCE CLASS 8 CHAPTER 2 BIOLOGY

1. Microorganism
s: Friend and
Foe-BY
NIDHI KAUSHIK
CLASS-8

2. Microbes or microorganisms are
tiny organisms which are so
s m a l l that w e cannot see them
w i t h an unaided eye. Some
microorganisms can be seen w i t h
the help o a magni ying glass
(such as ungus that g r o w s on
bread) w h i l e some can only be
seen when you use a microscope
(such as bacteria and protozoa).
M i cr oor gani sm w er e i r st
observed by Anton Van
Leeuwenhoek.

3. THEREAREFOURMAJORTYPESOFMICROORGANISMS:

4. BACTERIA: THESE ARE SINGLE-CELLED ORGANISMS WITH A RIGID CELL WALL. THEY CAN
ONLY BE SEEN UNDER A MICROSCOPE WHICH ENLARGES IMAGES FROM 100 TO 1000
TIMES.

5. Fungi: These are non-green plants and hence, cannot
make their own ood.
T h e y either live as parasites (deriving nutrition
r o m host organisms, or example, Puccinia which
causes wheat lea r u s t ) o r g r o w on the organic
m a t t e r (such as bread mould).

6. Fungi, l i k e mushrooms, moulds, mildews , and years, are
eukaryotic. I t means that they have a t r u e nucleus.
T h e main components o ungi are:
i. Hyphae: T h e y are thread-like ilaments which
penetrate into substrates, secrete enzymes to break
down nutrients into s m a l l e r molecules, and absorb
them.
ii. Spores are a unit o sexual or asexual reproduction.
T h e y can adapt or dispersal and s u r v i v a l or extended
periods o time in un avourable conditions.

7. A lg a e : These are simple plant- l i k e organisms which are
u s u a l l y aquatic in nature.
T h e y contain a c e l l w a l l and chlorophyll and can make their
own ood by photosynthesis.
A lg a e can be u n i c e l l u l a r or m u l t i c e l l u l a r. Some o the
common examples are diatoms, Chlamydomonas, and seaweed.
Protozoa: Protozoa are u n i c e l l u l a r are organisms. Some o
them live independently w h i l e others live as parasites.
Many o the parasitic protozoans cause diseases in plants,
domestic animals, and human beings. Example o some
protozoans are Amoeba, Plasmodium and Paramecium

8. HOW AR E VI R U SES DI ER ENT
R OM
other microbes?
Viruses are microscopic organisms but they are di erent r o m other
microbes because they reproduce only inside the c e l l s o the host
organism (which can be a plant, animal, or a bacterium).
Viruses a r e much s m a l l e r than bacteria and come in a w i d e variety o shapes and sizes. A
complete v i r u s p a r t i c l e is known as Virion.
Virion consists o a nucleic acid surrounded by 'capsid'. Capsid is a protective coat made o protein.
T h e subunits o this protein c a l l e d 'Capsomeres'. Viruses can be seen only by an electron
microscope as they a r e u l t r a m i c r o s c o p i c in size.
Outside the body o a living organism, they do not show any reaction and hence, can be c r y s t a l l i z e d
and stored l i k e non-living things.

9. WHERE
DO
Microorganisms Live?
Microbes can survive in a l l kinds
o environments – r o m icy cold
climates to hot springs (any kind
o temperature); and deserts to
marshy lands (any humidity
level). Some live independently
w h i l e others live as parasites –
inside the bodies o other
organisms (including animals and
human beings).

10. MICROORGA
NISMS AND
US
SOME
MICROORGANISMS ARE
BENEFICIAL TO US WHILE
OTHERS ARE HARMFUL
AND CAUSE DISEASES.

11. HOW ARE BACTERIA USE U L
TO US?
How is c u r d ormed?Bacteria are help u l because:
I t decomposes organic wastes (such as vegetable peels, animal remains, and aeces etc.).
I t is used in the preparation o medicines.
I t increases soil e r t i l i t y by ixing nitrogen.
I t is used in the setting o c u r d and making cheese, pickles, and other ood items.
How is yeast use u l to us?
What is Fermentation?Yeast is used in the baking industry (to make bread,
pastries, and cakes) because i t helps in ermentation. I t reproduces r a p i d l y and
produces carbon dioxide during respiration. Bubbles o the carbon-dioxide gas i t
produces i l l the spaces in the dough and increases its volume.
I t is also used in the commercial production o alcohol and wine which is done
by growing yeast on n a t u r a l sugars present in r u i t juices and grains like rice,
wheat, and barley.

12. W
AHAT A
NR E
TA NT
IIB
BIOT IC
IS?
OWH
TAT A
IR
CE T HE
SIR
U SES?
Antibiotics are medicines that can k i l l or stop the growth o disease-causing microorganisms. For Example, Penicillin.
Who discovered Pencillin and when?Antibiotics are used to:
Cu r e a var i et y o di seases ( su ch as st r ept om yci n, er yt hr om yci n, and t et r acycl i ne t hat ar e m ade rom bact er ia and u ngi ) ,
Cure microbial in ection in animals (by mixing antibiotics w i t h the eed o livestock and p o u ltr y) , and
Control several plant diseases.
What are antibiotics? What precautions must be taken while taking antibiotics?
Antibiotics are medicines produced by certain micro-organisms to kill other disease-causing
micro-organisms. Precautions to be taken while using antibiotics are:
i. Antibiotics should be taken under the supervision of a well qualified doctor.
ii. Course (intake) of antibiotics should be completed as prescribed by the doctor.
iii. Antibiotics should be taken in the right amount and at the right time. A wrong dose of
antibiotics makes the drug ineffective. Also, excessive consumption of drugs may kill the
useful bacteria present in our body.

13. Name some o the diseases which can be prevented by vaccines
Who discovered vaccine o r smallpox?Some o the diseases that can be prevented by vaccination are:
Cholera,
Hepatitis,
Smallpox, and
T u ber cu l osis.
One can get necessary vaccines r o m nearby hospitals.
How do microbes clean up the environment?
Microbes o r microorganisms decompose organic waste and dead remains o plants and animals and convert them into s i m p l e r
substances (which can again be used by other plants and animals) by the process o biodegradation. T h u s , they help us in getting
r i d o h a r m u l and s m e l l y substances and clean up the environment.
VACCINE
S

14. HARMFUL
MICROORGANISMS
A house l y m a y sit on the garbage and animal excreta and
the pathogens stick to their body. When they sit on uncovered
ood, these pathogens get trans e r r e d to the ood. When
someone eats this contaminated ood, he o r she m a y a l l
sick.
T o avoid this, w e m u s t keep the ood covered.

15. COMMON DISEASES AND
THEIR MODES OF
TRANSMISSION

16. HOW TO PREVENT DISEASES THAT SPREAD THROUGH A I R O R
CONTACT?
To prevent diseases that spread through the air, keep the patient in complete isolation and keep his or her
personal belongings a w a y r o m others. Vaccination at the suitable age can prevent the onset o
tuberculosis, chicken pox, polio, and measles.
How to prevent diseases that spread through w a t e r or ood?
To prevent the spread o polio, cholera, typhoid and hepatitis A, vaccination is e ective. One should also
maintain personal hygiene and good sanitary habits and dr ink boiled drinking w a t e r.
One should also eat p r o p e r l y cooked ood to avoid diseases l i k e cholera.
How to prevent diseases that spread through mosquitoes?
We can stop the spreading o diseases caused by mosquitoes (such as m a l a r i a and dengue) by not a l l o w i n g
the mosquitoes to breed. We should keep our surroundings clean and dr y, and not l e t w a t e r collect
anywhere - in coolers, tyres, and l o w e r pots etc.
We should also s pr ay insecticides and use mosquito repellents
and mosquito nets to protect ourselves r o m mosquito bites.

17. COMMON DISEASES ANDTHEMICROORGANISMS
THAT CAUSETHEM

18. Diseases Caused by Microorganisms in
Anim als
BacillusAnthrax: A dangerous disease that
a ects human and c attle is caused by a
bacterium c a l l e d B a c i l l u s Anthracis.
Foot and mouth disease in Cattle: I t is
caused by a v i r u s c a l l e d Foot-and-mouth-
disease Virus (FMDV) or Picornavirus.
Diseases Caused by Microorganisms in
Plants
Microorganisms can cause diseases in
pl ant s and r edu ce cr op yi el d. Som e o t he
plants in which they cause diseases are:

19. T h e plants can be protected by using chemicals that k i l l
these microbes.
Common Diseases in Plants caused by Microbes
C i t r u s Canker is caused by Bacteria and spreads through
A i r.
Rust o Wheat is caused by Fungi and spreads through A i r or
Seeds.
Yellow Vein Mosaic o Okra (Bhindi) is caused by Virus and
spreads through I n s e c t s .

20. Food Preservation
Why do w e need to preserve ood?
We need to preserve ood because
microorganisms that g r o w on ood can
sometimes produce toxic substances
which are poisonous to us. I w e
consume this spoilt ood, w e can become
seriously i l l or die. Hence, w e need to
preserve ood r o m being spoilt.

22. Nitrogen Fixation
Nitrogen constitutes 78% o o u r atmosphere.
I n living organisms, i t is ound in:
Proteins,
Nucleic Acids,
Chlorophyll, and
Vitamins.
Atmospheric nitrogen cannot be used d i r e c t l y by the plants and
animals. I t gets ixed by either lightning or n a t u r a l nitrogen
ixers.

24. NITROGEN
CYCLE

25. A step-by-step explanation o Nitrogen Cycle
Nitrogen Fixation: Atmospheric nitrogen is converted by lightning o r certain bacteria
l i k e Rhizobium, Azotobacter and blue-green algae (present in soil) into compounds
usable by plants.
N i t r i ication: Ammonia conversion into nitrites by Nitrosomonas and u r t h e r
conversion o nitrites into nitrates by Nitrobacter. Plants take up nitrogen in o r m o
ammonia o r nitrates.
Assimilation: Roots o plants absorb these nitrogenous compounds r o m soils and
plants use them to synthesize proteins and other compounds.
A n i m a l s eeding on plants get these proteins and nitrogen compounds.
Ammoni ication: When plants and animals die, bacteria and ungi present in the soil
convert the nitrogenous wastes into compounds that can be used by plants again.
Denitri ication:Nitrates can be converted into nitrogen gas which is released back in
the atmosphere by certain bacteria. Eg. Pseudomonas

26. NCERT
EXERCISE 1.F i l l in the blanks.
( a) M i cr oor ganism s can be seen w i t h t he hel p o a
.
( b ) B l u e green algae ix d i r e c t l y r o m a i r
and enhance f e r t i l i t y o soil.
(c) Alcohol is produced w i t h the help o .
(d) Cholera is caused by .
Soln:
a) Microscope
b) Nitrogen
c) Yeast
d) Bacteria- Vibrio cholera
2. Tick the correct answer.
(a) Yeast is used in the production of
(i) sugar (ii) alcohol (iii) hydrochloric acid (iv) oxygen
(b) The following is an antibiotic
(i) Sodium bicarbonate (ii) Streptomycin (iii) Alcohol (iv) Yeast
(c) Carrier of malaria-causing protozoan is
(i)female Anopheles mosquito (ii) cockroach (iii) housefly (iv)
butterfly
(d) The most common carrier of communicable diseases is
(i)ant (ii) housefly (iii) dragonfly (iv) spider
(e) The bread or idli dough rises because of
(i) heat (ii) grinding (iii) growth of yeast cells (iv) kneading
(f) The process of conversion of sugar into alcohol is called
(i) nitrogen fixation (ii) moulding (iii) fermentation (iv)
infection Soln:
a) (ii) alcohol
b) (ii) Streptomycin
c) (i) female Anopheles mosquito
d) (ii) housefly
e) (iii) growth of yeast cells
f) (iii) fermentation

27. A B
( i ateria (e) Causing cholera
( i i ) Rhizobium ( ) Fixing nitrogen
( i i i ) Lactobacillus
() Yeast ( c ) Nonbread
(v) A protozoan ( d ) Csing m a a r i a
(vi ) A vi rs
4. Can microorganisms be seen w i t h the
naked eye? I not, how can they be seen?
Soln:
Microorganisms cannot be seen by the
naked eye because they a r e very s m a l l .
T h e y can be seen by using a microscope.
5. What a r e the major groups o microorganisms?
Soln:
M ajor gr ou p o m i cr oor gani sm s ar e as ol l ow s
B a c t e ri a
Fu ngi
Protozoans
A l g a e
Vi r u ses
(i) Bacteria (e) Causing cholera
(ii) Rhizobium (a) Fixing nitrogen
(iii) Lactobacillus (b) Setting of curd
(iv) Yeast (c) Baking of bread
(v) A protozoan (d) Causing malaria
(vi) A virus (f) Causing AIDS
(i) Bacteria (a) Fixing nitrogen
(ii) Rhizobium (b) Setting of curd
(iii) Lactobacillus (c) Baking of bread
(iv) Yeast (d) Causing malaria
(v) A protozoan (e) Causing cholera
(vi) A virus (f) Causing AIDS
(g) Producing
antibodies

28. 6. Name the microorganisms
which can ix atmospheric
nitrogen in the soil.
Soln:
Rhi zobi u m
7. Write 10 lines on the usefulness of microorganisms in
our lives.Soln:Uses of microorganisms are listed
belowThey are helpful in cleaning up of the environment.
For example, the organic wastes (vegetable peels, remains
of animals, faeces, etc.) are broken down into harmless
and usable substances by bacteria.Curd contains several
microorganisms. Of these, the bacterium, Lactobacillus
promotes the formation of curd.
It multiplies in milk and converts it into curd.Bacteria are
also involved in the making of cheese, pickles and many
other food items.Bacteria and yeast are also helpful for
fermentation of rice idlis and dosa batter.Yeast reproduces
rapidly and produces carbon dioxide during
respiration. Bubbles of the gas fill the dough and
increase its volume. This is the basis of the use of yeast
in the baking industry for making breads, pastries and
cakes.Microorganisms are used for the large scale
production of alcohol, wine and acetic acid
(vinegar).Antibiotics are being produced from bacteria and
fungi.Microorganisms are helpful in producing vaccines
against disease-causing agents.Some bacteria (Fig. 2.7) are
able to fix nitrogen from the atmosphere to enrich the soil
with nitrogen and increase its fertility.Microorganisms
decompose dead organic waste of plants and animals to
convert them into simple substances. These substances
are again used by other plants and animals. Thus,
microorganisms can be usedto degrade the harmful and
smelly substances and thereby clean up the environment.

29. 8. Write a short paragraph on the
h a r m u l e ects o
9. WHAT ARE ANTIBIOTICS?
WHAT
microorganisms.
Soln:
Microorganisms cause diseases
in plants and animals Ex; Cholera
in humans is caused by bacteria
c a l l e d Vibrio cholera. I n animals,
the v i r u s causes oot and mouth
diseases. Microorganisms spoil
the oods by decomposing the ood
m a te r ia l. Some microorganisms
spoil leather and clothes.
Microorganisms are responsible
or ood poisoning.
precautions m u s t be taken
w h i l e taking antibiotics?
Soln:
Chemical agents that k i l l or
stop the g r o wth o disease-
causing microorganisms are
c a l l e d antibiotics. Antibiotics
should be taken only on the
advice o a quali ied doctor.
Also one m u s t complete the
course prescribed by the
doctor.

30. Y OU