Microbial World Class 7 Science Notes and Questions

Please refer to Microbial World Class 7 Science notes and questions with solutions below. These revision notes and important examination questions have been prepared based on the latest Science books for Class 7. You can go through the questions and solutions below which will help you to get better marks in your examinations.

Class 7 Science Microbial World Notes and Questions

Micro- organisms are microscopically small organisms. These include Bacteria, Viruses, Fungi, Protozoa and some Algae. Micro-organisms can flourish in all kinds of environment. Some even live inside human intestine as endoparasites. Few can reproduce independently, while some multiply only by attaching themselves to specific hosts. These are very hard and can survive under texteme unfavourable conditions by forming cysts around them and remain inactive. Under favorable conditions, they come out from cysts and start their regular activity.
Micro- organisms are useful as well as harmful and cause various diseases.
Viruses are in between living and non- living characters. In free state, viruses cannot grow.
They grow on other living cells, viruses are obligate parasites.

Bacteria:-
They are unicellular micro- organisms and their sizes vary between 0.3 and 2  m in diameter. They differ from plant and animal cell in that their nucleus is not a distinct organelle surrounded by a membrane. Bacteria are like plant cells in that they are surrounded by a rigid cell wall. Majority of them posses cilia and flagella for movement. Some are even photosynthetic.

Economic Importance of Bacteria
• Some bacteria increase soil fertility by fixing free atmospheric nitrogen into
nitrogenous compounds, e.g., Clostridium, Azotobacter and Rhizobium.
• Some decompose dead animal and plant materials to form nutrients in the soil.
• Acetobacter aceti bacteria produce vinegar from sugar solution.
• Certain bacteria are present in the intestines of cattles and are helpful in the digestion of cellulose.
• Some bacteria help in tanning of leather.
• Some medicines, e.g., streptomycin is produced by Streptomyces grisieus bacteria.
• Some human diseases are caused by bacteria, e.g., Tuberculosis, Typhoid, Diptheria etc.
• Some bacteria help in the disposal of sewage by converting organic faecal matter into
manure and humus.

Fungi:
Fungi are simple plants that reproduce mostly by means of spores and which lack chlorophyll,
hence are either saprophytes or parasites. Fungi include unicellular as well as multicellular
organisms and reproduce asexually and sexually. Moulds and yeast are major representative of fungi.

Economic Importance of Fungi :
• Because of high content of vitamin B 1, B ₁₂ and C, yeast are used for preparation of
digestive tablets.
• Bread and cakes are made by the action of yeast.
• Morchella, Agaricus and Lycoperdon are most commnon edible fungi.
• Wines and beers are produced by the action of fungi.
• Few very important medicines, such as penicillin is made by the fungi- Penicillium
notatum and penicillium chrysogenum.
• Some fungi cause disease in plant, while others cause athletes foot and ring worm in humans.
• Few fungi also cause diseases in animals.
• Amanitia is very poisonous fungus to man.LSD drug is extracted from the fungi- Claviceps.

Algae :
Algae is a large and extremely diverse group of plants including some prokaryotic forms,
although the majority are eukaryotic. The plant body may be unicellular or multicellular.
They are mostly aquatic. Most of them contain chlorophyll and perform photosynthesis.
Their sizes range from 1.0 micron (microscopic unicellular forms) to several meters long
sea weeds.

Economic Importance of Algae
• Some algae are utilized as food in Japan and China, e.g., Porphyra.
• Chlorella alga contains proteins and vitamins A and D in abundant quantity.
• Gel i d iu m an d Gr acil ar ia algae p r odu ce Agaragar, often used in the manufacture of ice creams.
• Nostoc and Anabaena algae convert atomospheric nitrogen into nitrogen compounds which are absorbed by higher plants.
• Kelp is a brown algae and is rich source of iodine and potassium.
• Some algae cause water blooms in water reservoirs and emit bad smell.
• Carrageenin extract, which is another product of algae origin, acts as a blood coagulant.
• An effective vermifuge is obtained from the extracts of Codium, Alsidium, Digenea and Durvillen.

Protozoa :
Protozoans are unicellular (single cell) animals and are microscopic. These animals do not
posses a cell wall. They have variable structure, such as amoeba does not posses a definite
structure and its pseudopodia can change its shape. Paramecium has a slipper like shape and
the body is covered by numerous cilia.

Economic Importance of Protozoa
• Some protozoans feed on bacteria and fungi, which are decomposers of organic matter,
thus finally degrade the wastes.
• Some are found in intenstine of termites and help in the digestion of cellulose.
• Serveral protozoans cause human diseases.
• Some animals feed on diatoms and other protozoans

Viruses :
Viruses are submicroscopic parasitic micro organisms often called microbes. They comprise
of a protein or protein / lipid sheath containing nucleic acid (DNA / RNA). Viruses are inert
outside the living host cell and can multiply only in host cells. They are in between non- living
and living as they possess both the characters. Tobacco plant and foot and mouth disease
(FMDV) in cattles. Viruses parasitize bacteria are called Bacteriophage.

Micro- organism and Diseases :
Disease causing micro- organisms are called Pathogenic micro- organisms. Microbial
diseases which can spread through air, water, food or physical contact are known as
Communicable Diseases. For example, common cold, cholera, and chicken pox etc.
Sometimes insects act as carriers of pathogenic micro-organisms, e.g., housefly spreads
cholera. Anopheles mosquito spreads malarial fever. Carriers of specific micro-organisms
are called Vectors. Thus, Anopheles and Aedes are mosquito vectors causing malaria and
yellow fever respectively.

List of Bacterial diseases
1. Cholera 2. Typhoid 3. Tuberculosis 4. Diphtheria
5. Pneumonia 6 .Tetanus 7. Leprosy

List of Viral (Virus) diseases
1. Small pox 2. Common cold 3. Chicken pox 4. Influenza
5. Measles 6. Mumps 7. Poliomyelitis 8. Rabies 9. AIDS

List of diseases caused by Protozoans
1. Malaria
2. Amoebic dysentery
3. Sleeping sickness
4. Kala-azar

List of diseases caused by Fungi
1. Ringworm (tinea)
2. Athlete’s food

Uses of Micro-organisms :

1. Medicinal uses of micro-organisms
It has been scientifically found that the lethal effect of one micro- organism can check the
growth of another micro- organism. Thus substances produced by micro- organisms that
kill or prevent growth of other micro- organisms are called Antibiotics. This property of microorganism
is made use of in the treatment of bacterial and fungal infections. Alexander Fleming
first noted this property of micro- organisms in 1928. The production of first antibiotic drug
for medicinal use was Penicillin by Chain and Florey. Other well known antibiotics are
Streptomycin, Tetracycline, Gentamycin etc. Some of them are made from bacteria and
others from fungi.

2. Vaccination:-
Vaccination is the method of inducing immunity to infectious disease caused by bacteria
and viruses. Starting from the information that farm-workers, who had accidently acquired
cowpox by milking infected cows, were resistant to smallpox. Jenner inoculated cowpox
material into non- immune persons who then showed resistance to smallpox. Later
Pasteur extended this work to cholera, human Anthrax and Rabies. Pasteur established
that smallpox and few other diseases were caused by viruses. Thus, Pasteur started
the practice of injecting a small amount of
pathogens into the body to develop resistance against a specific disease, and applied the
same to other diseases, such as cholera and tuberculosis and called this practice
Vaccination. The term vaccination became general for all methods of inducing immunity by
inoculation of products of the infectious organism. Antitoxins were soon developed in
which specific immunity to disease toxins was induced. One of the body’s defence
mechanisms is called antibodies (proteins) which are made by specialized white cells to
counter foreign proteins known as antigens (pathogens). Vaccination leads to the
formation of antibodies and the ability to produce large quantities rapidly at the time of
disease infection. A specific antibody is made for each antigen. After vaccination, the
injected viral cells (pathogens) are encountered by the specialized white blood corpuscles
and kill the pathogens by producing antibodies. These antibodies once formed remain for
some time in the body and fight any infection by that pathogen in future. This type of
resistance created in the body is called immunity and the process of vaccination against
diseases is called immunization.

3. Commercial uses:-
Certain substances are made by the help of micro- organisms which are very useful to us such
as curd, bread, penicillin, alcohol and organic acids. Yeasts are used for the commercial
production of wines and alcohols from carbohydrates. Yeasts (fungi) break down the carbohydrate
molecules by their enzymes and produce alcohol by
chemical reaction. Certain bacteria break down carbohydrates (potato and corn) to produce
organic acids, e.g. acetic acid and lactic acid. Certain leguminous plants (pulse) have
nitrogen fixing bacteria in their root nodules which fix atmospheric nitrogen into soluble
nitro-genous compounds (nitrates) . Plants fulfil their nitrogenous need in the form of nitrates
and for proteins. These nitrogen fixing nodule bacteria are mostly Rhizobium. All pulses are
mostly rich in proteins.

Storage and Preservation:
Many micro- organisms grow and spoil useful materials, such as wood, leather, textile,paper
and grains. Thus these substances are well kept and treated to avoid the attack of microorganisms.
Some methods of preservation are described below,

Chemical method:-
Useful substances are often treated with chemicals to inhibit the growth of harmful microorganisms.
Insecticides and fungicides are often used to protect the substances from the
attack of these microbes. It is observed that food and pickles are often spoiled by the growth
of micro- organisms. To check the spoilage of edible substances, certain preservatives are
used. Acid preservatives are mostly used to check the activities of micro-organisms. These
acids check the enzymes action of microbes. Salt is also used as preservatives. Salt draw
water from substances and thus check the growth of microbes. Salts and acetic acid are
used as preservatives in pickles, while sodium benzoate or sodium metasulphate is used in
jams and squashes for preservation.

Heat and cold treatments :
Water is often boiled and then cooled before drinking. Thus the harmful microbes present are eliminated. Similarly
milk is also boiled before keeping it for some duration. Boiling is a way of
killing microbes although the spores of micro- organisms are not destroyed by boiling. Edible
substances are kept in refrigerators. Both cold and hot treatments are used to keep things for longer duration. By both ways the enzymes present in micro- organisms are inactivated, thus microbes are unable to spoil the food. Pasteurization of milk is a process of sterilizing milk for improving storage qualities. This is done by keeping the mik at about 65°C for 30 minutes and then suddenlly chilling it. Disease producing bacteria, particularly those causing tuberculosis, are thus destroyed with a minimum effect on the flavour.

Isolation :
Generally materials are stored in closed, sometimes airtight containers to check the growth of harmful microbes. After harvestation, grains are kept in large containers called ‘silos’ he chances
of microbial growth are reduced in this way but cannot be checked entirely. Best way of storing
materials, the containers must be sterilized by different methods. Microbes often spoil edible
things by breaking their molecules and producing amines; which have foul smell. Some microbes
completely spoil the food by producing poisonous toxins. These toxins produce illness and even
sometimes prove fatal. Therefore, it is quite neccessary to store food properly.

Points to Remember:-
• Blue green algae or cyanobacteia are prokaryotic and autotrophic organisms.
Ex. Anabaena, Nostoc, Oscillatoria, Spirulina
• Blue green algae fix atmospheric nitrogen and increase soil fertility. They have
specialized cells to fix nitrogen, such cell named heterocysts.
• Diatoms are unicellular eukaryotic organisms. Their cell wall contain cellulose but is
largely made up of silica. This silicated cell wall is called shell or frustule.
• When diatoms die, their covers (cell wall) settle at the bottom of the oceans or lake.
Over the year these form diatomaceous earth or diatomite or keiselgurh.
• Diatoms are also known as golden aglae or jewels of sea.
• The wine (brewery) and bread (bakery) industries are based on fermentation activity of yeast.
• Yeast is used in the production of riboflavin (vitamin-B 1)
• Mycoplasmas are also known as cell- wall less bacteria.
• The first polio vaccine was prepared by Jonas Salk (1953) by killing polio virus with formaldehyde.
• Sabin prepared an oral polio vaccine (OPV). It consists of living but attenuated polio virus.
• Mycoderma aceti and Acetobacter aceti used for large scale production of acetic acid (vinegar).
• Mycorrhiza are the symbiotic associations of fungal hyphae and roots of higher plants.
• Lichens are the symbiotic associations of fungi and algae.
• Bacillus, Pseudomonas and Proteus are used for the treatmemt of sewage.
• Christian Gram developed a technique to stain bacteria, called Gram staining.
• Gram positive bacteria retain the stain while the Gram negative ones do not. The two
types of bacteria primarily differ in the structure and composition of cell walls.
• Gram positive bacteria e.g. Salmonella, Spirillum, Escherichia coli.
• Gram negative bacteria e.g. Staphylococcus, Streptococcus.
• Kelps (Sea weeds algae) are the sources of iodine and potassium.
• An antibiotic chlorellin is obtained from the algae chlorella.
• Claviceps purpurae: A narcotic drug- LSD (Lysergic acid Diethylamide) is obtained from it.
• Neurospora- red or pink mold known as Drosophila of Plant Kingdom.
• Bioremediation- The use of microorganisms to clean up the environment or to remove
pollutants from the environment.
• Plant diseases caused by bacteria
Citrus canker, fire blight of pear, Leaf blight of rice, Crown gall, Black root of cabbage, Red
stripe of sugarcane, Wilt of potato.
• Plant disease caused by viruses. Yellow vein mosaic of ladyfinger, Tobbaco mosaic,
Bunchy top of banana, Leaf roll of potato.
• Plant diseases caused by fungi, Late blight of potato caused by Phytophthora infestans.
This disease is known as Famine of Ireland (1845).

MCQs for NCERT Class 7 Science Microbial World

Question: Micro- organisms can be seen by the aid of –
(A)Microscope
(B) Hand lens
(C) Naked eye
(D) None

Question: Viruses are –
(A) Living organisms
(B) Non- living organisms
(C) In between living and non-living organisms
(D) None of the above

Question: Bacteria is regarded as plant cell, because-
(A) It has clear nucleus
(B) It has rigid cell wall
(C) It reproduces by binary fission
(D) It is parasitic

Question: Bacteria are-
(A) Aerobic only
(B) Non- aerobic only
(C) Aerobic and non- aerobic both
(D) None of the above

Question: Which of the following beneficial bacteria is found in the colon of man?
(A) Lactobacillus
(B) Pseudomonas
(C) Clostridium
(D) Escherischia coli

Question: The bacterial action in milk is –
(A) Alkaline
(B) Acidic
(C) Neutral
(D) Saline

Question: Which bacteria produces vinegar in sugar solutions?
(A) Escherischia (
B) Azotobacter
(C) Acetobacter aceti
(D) Rhizobium

Question: Streptomycin is produced by –
(A) Fungi
(B) Yeast
(C) Bacteria
(D) Virus

Question: Which of the following is present in the intestines of termites to digest cellulose?
(A)Yeasts
(B) Bacteria
(C) Protozoans
(D) Algae

Question: Typhoid is caused by –
(A) Fungi
(B) Virus
(C) Bacteria
(D) Protozoa

Question: Which of the following grows on wet bread?
(A) Yeast
(B) Mould
(C) Both A and B
(D) None

Question: Yeasts cause dough in maida, due to the formation of –
(A)Oxygen
(B) Nitrogen
(C) Carbon dioxide
(D)Carbon monoxide

Question: Which of the following is used in the preparation of digestive tablets?
(A) Yeast
(B) Mould
(C) Algae
(D) None of these

Question: Edible fungi is –
(A)Morchella
(B) Agaricus
(C) Lycoperdon
(D) All the above

Question: Antibiotic penicillin is obtained from –
(A) Bacteria
(B) Moulds (
C) Yeasts
(D) Algae

Question: Yeast helps in the production of –
(A)Oxygen
(B) Glucose
(C) Alcohol
(D) Salts

Question: Bacteria generally multiply by –
(A) Binary fission
(B) Multiple fission
(C) Longitudinal fission
(D) All of the above

Question: Which of the following is used as culture medium of bacterial growth ?
(A) Iodine
(B) Agar- agar
(C) Sugar
(D) Alcohol

Question: Mushroom is a type of –
(A) Yeast
(B) Algae
(C) Mould
(D) Protozoa

Question: LSD drug is extracted from a –
(A) Algae
(B) Fungi
(C) Virus
(D) Bacteria

Question: Most algae can make their own food because they conatin-
(A)Clear nucleus
(B) Pigments
(C) Chlorophyll
(D) None of the above

Question: Most fungi cannot make their own food due to lack of –
(A)Chlorophyll
(B) Pigments
(C) Clear nucleus
(D) All the above

Question: The red colour of red – sea is due to –
(A) Brown algae
(B) Green algae
(C) Red algae
(D) All the above

Question: China grass is –
(A) Fungi
(B) Algae
(C) Both A and B
(D) none of these

Question: Which of the following algae produce Agar-agar?
(A)Gelidium (
B) Gracilaria
(C) Both A and B (
D) None of these

Question: Kelp is –
(A) Red algae
(B) Blue-green algae
(C) Brown algae
(D) All the above

Question: Rich source of iodine is –
(A) Red algae
(B) Green algae
(C) Blue- Green algae
(D) Brown algae

Question: Which of the following protozoan possesses definite shape?
(A) Amoeba
(B) Paramecium
(C) Both A and B
(D) None of these

Question: Pasteurization of milk destroys its-
(A) Vitamins
(B) Proteins contents
(C) Fat contents
(D) Pathogenic bacteria

Question: Viruses are inert-
(A)Outside host cell
(B) Inside host cell
(C) Both A and B
(D) None of the above

Question: Bacteriophage virus-
(A) Parasitize man
(B) Parasitize bacteria
(C) Parasitize cattles
(D) Parasitize dogs

Question: The host for malaria causing protozoan is –
(A) Anopheles mosquito
(B) Dog
(C) Earthworm
(D) Housefly

Question: The most common carrier of communicable diseases is –
(A)Mosquito
(B) Cow
(C) Dog
(D) Housefly

Question: Leprosy is caused by –
(A) Bacteria
(B) Fungi
(C) Protozoan
(D) Virus

Question: AIDS is a –
(A) Baterial disease
(B) Viral disease
(C) Protozoan disease
(D) Fungal disease

Question: Malaria is caused by a –
(A) Virus
(B) Bacteria
(C) Protozoa
(D) Fungus

Question: Athlets’ foot and ringworm are caused by –
(A) Bacteria
(B) Fungi
(C) Protozoa
(D) None

Question: Which of the following is an antibiotic?
(A) Insulin
(B) Streptomycin
(C) Dextrose
(D) Alcohol

Question: Salting the food helps to preseve it by-
(A) Extracting the water from it
(B) Increasing alkalinity
(C) Lowering the temperature
(D) None of the above

Question: Disease causing micro-organisms are known as –
(A)Microbes (B) Pathogens (C) Antibody (D) All the above

Question: The process of immunization is known as-
(A) Pathogenesis
(B) Sterilization
(C) Immunity
(D) All the above

Question: Atmospheric nitrogen is fixed in leguminous roots by-
(A)Chorophyll
(B) Carbon dioxide
(C) Bacteria
(D) All the above

Question: Plants take nitrogen in the form of –
(A)Nitrates
(B) Nitrites
(C) Ammonia
(D) All the above

Question: Pickles are mostly preserved by adding-
(A) Sodium benozoate
(B) Acetic acid
(C) Hydrochloric acid
(D) Sulphuric acid

Question: Jams and squashes are preserved by adding-
(A) Acetic acid
(B) Sodium benzoate
(C) Hydrochloric acid
(D) Sulphuric acid

Question: In Blue green algae, the structure specialized for nitrogen fixation is
(A) Thylakoid
(B) Harmogonia
(C) Heterocyst
(D) Endospore

Question: The most ancient group of organisms on the earth is
(A) Eubacteria
(B) Cyanobacteria
(C) Archaebacteria
(D) PPLO

Question: The similaity between bacterium and cyanobacterium is in the presence of
(A) Flagella
(B) Nucleoid
(C) Size
(D) 80S ribosomes

Which one of the following statements is correct?
(A) Legumes fix nitrogen through specialised bacteria that lives on their leaves.
(B) Legumes are incapable of fixing nitrogen
(C) Legumes fix nitrogen only through specialised bacteria that lives in their roots
(D) Legumes fix nitrogen independently of the specialised bacteria that live in their roots

Question: Rocky mountain spotted fever is caused by
(A)Rickettsia
(B) Fungi
(C) Bacteria
(D) PPLO

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