NCERT Solutions for Class 12 Biology Chapter 8 Microbes in Human Welfare

NCERT
SOLUTIONS
CLASS - 12th

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Class : 12th
Subject : Biology
Chapter : 10
Chapter Name : Microbes in Human Welfare

Q1 Bacteria cannot be seen with the naked eyes, but these can be seen with the help of a
microscope. If you have to carry a sample from your home to your biology laboratory to
demonstrate the presence of microbes under a microscope, which sample would you carry and why
?

Answer. Curd can be used as a sample for the study of microbes. Curd contains numerous lactic
acid bacteria (LAB) or Lactobacillus. These bacteria produce acids that coagulate and digest milk
proteins. A small drop of curd contains millions of bacteria, which can be easily observed under a
microscope.

Q2 Give examples to prove that microbes release gases during metabolism.

Answer. The examples of bacteria that release gases during metabolism are:
(a) Bacteria and fungi carry out the process of fermentation and during this process, they release
carbon dioxide. Fermentation is the process of converting a complex organic substance into a
simpler substance with the action of bacteria or yeast. Fermentation of sugar produces alcohol
with the release of carbon dioxide and very little energy.
(b) The dough used for making idli and dosa gives a puffed appearance. This is because of the
action of bacteria which releases carbon dioxide. This C02 released from the dough gets trapped in
the dough, thereby giving it a puffed appearance.

Q3 In which food would you nd lactic acid bacteria? Mention some of their useful applications.

Answer. Lactic acid bacteria can be found in curd. It is this bacterium that promotes the formation
of milk into curd. The bacterium multiplies and increases its number, which converts the milk into
curd. They also increase the content of vitamin B12 in curd. Lactic acid bacteria are also found in
our stomach where it keeps a check on the disease-causing microorganisms.

Q4 Name some traditional Indian foods made of wheat, rice and Bengal gram (or their products)
which involve use of microbes.

Answer. (a) Wheat: Product: Bread, cake, etc.
2. Rice: Product: Idli, dosa
2. Bengal gram: Product: Dhokla, Khandvi

Q5 In which way have microbes played a major role in controlling diseases caused by harmful
bacteria?

Answer. Several microorganisms are used for preparing medicines. Antibiotics are medicines
produced by certain microorganisms to kill other disease-causing microorganisms. These
medicines are commonly obtained from bacteria and fungi. They either kill or stop the growth of
disease-causing microorganisms. Streptomycin, tetracycline, and penicillin are common
antibiotics. Penicillium notatum produces chemical penicillin, which checks the growth of
staphylococci bacteria in the body. Antibiotics are designed to destroy bacteria by weakening their
cell walls. As a result of this weakening, certain immune cells such as the white blood cells enter
the bacterial cell and cause cell lysis. Cell lysis is the process of destroying cells such as blood cells
and bacteria.

Q6 Name any two species of fungus, which are used in the production of the antibiotics.

Answer. Antibiotics are medicines that are produced by certain microorganisms to kill other
disease-causing microorganisms. These medicines are commonly obtained from bacteria and
fungi. The species of fungus used in the production of antibiotics are:

Antibiotic || Fungus source
Penicillin || Penicillium notatum
Cephalosporin || Cephalosporium acremonium

Q7 What is sewage? In which way can sewage be harmful to us?

Answer. Sewage is the municipal waste matter that is carried away in sewers and drains. It
includes both liquid and solid wastes, rich in organic matter and microbes. Many of these microbes
are pathogenic and can cause several water- borne diseases. Sewage water is a major cause of
polluting drinking water. Hence, it is essential that sewage water is properly collected, treated,
and disposed.

Q8 What is the key difference between primary and secondary sewage treatment?

Answer.

Q9 Do you think microbes can also be used as source of energy? If yes, how?

Answer. Yes, microbes can be used as a source of energy. Bacteria such as Methane bacterium is
used for the generation of gobar gas or biogas. The generation of biogas is an anaerobic process in
a biogas plant, which consists of a concrete tank (10—15 feet deep) with suf cient outlets and
inlets. The dung is mixed with water to form the slurry and thrown into the tank. The digester of
the tank is lled with numerous anaerobic methane-producing bacteria, which produce biogas
from the slurry.
Biogas can be removed through the pipe which is then used as a source of energy, while the spent
slurry is removed from the outlet and is used as a fertilizer.

Q10 Microbes can be used to decrease the use of chemical fertilisers and pesticides. Explain how
this can be accomplished.

Answer. Microbes play an important role in organic farming, which is done without the use of
chemical fertilizers and pesticides. Bio-fertilizers are living organisms which help increase the
fertility of soil. It involves the selection of bene cial microorganisms that help in improving plant
growth through the supply of plant nutrients. Bio- fertilizers are introduced in seeds, roots, or soil
to mobilize the availability of nutrients. Thus, they are extremely bene cial in enriching the soil
with organic nutrients. Many species of bacteria and cyanobacteria have the ability to x free
atmospheric nitrogen. Rhizobium is a symbiotic bacteria found in the root nodules of leguminous
plants. Azospirillum and Azotobacter are free living nitrogen- xing bacteria, whereas Anabaena,
Nostoc and Oscillatoria are examples of nitrogen- xing cyanobacteria. Bio-fertilizers are cost
effective and eco-friendly. Microbes can also act as bio-pesticides to control insect pests in plants.
An example of bio-pesticides is Bacillus thuringiensis, which produces a toxin that kills the insect
pests. Dried bacterial spores are mixed in water and sprayed in agricultural elds. When larvae of

insects feed on crops, these bacterial spores enter the gut of the larvae and release toxins, thereby
it. Similarly, Trichoderma are free living fungi. They live in the roots of higher plants and protect
them from various pathogens. Baculoviruses is another bio-pesticide that is used as a biological
control agent against insects and other arthropods.

Q11 Three water samples namely river water, untreated sewage water and secondary ef uent
discharged from a sewage treatment plant were subjected to BOD test. The samples were labelled
A, B and C; but the laboratory attendant did not note which was which. The BOD values of the
three samples A, B and C were recorded as 20mg/L, 8mg/L and 400mg/L, respectively. Which
sample of the water is most polluted? Can you assign the correct label to each assuming the river
water is relatively clean?

Answer. Biological oxygen demand (BOD) is the method of determining the amount of oxygen
required by microorganisms to decompose the waste present in the water supply. If the quantity of
organic wastes in the water supply is high, then the number of decomposing bacteria present in
the water will also be high. As a result, the BOD value will increase. Therefore, it can be concluded
that if the water supply is more polluted, then it will have a higher BOD value. Out of the above
three samples, sample C is the most polluted since it has the maximum BOD value of 400 mg/L.
After untreated sewage water, secondary ef uent discharge from a sewage treatment plant is most
polluted. Thus, sample A is secondary ef uent discharge from a sewage treatment plant and has
the BOD value of 20 mg/L, while sample B is river water and has the BOD value of 8 mg/L.
Hence, the correct label for each sample is:

Q12 Find out the name of the microbes from which Cyclosporin A (immunosuppressive drug) and
Statins (blood cholesterol lowering agents) are obtained.

Answer.

Page : 190 , Block Name : Exercise

Q13 Find out the role of microbes in the following and discuss it with your teacher.
(a) Single cell protein (SCP)
(b) Soil

Answer. (a) Single cell protein (SCP)
A single cell protein is a protein obtained from certain microbes, which forms an alternate source
of proteins in animal feeds. The microbes involved in the preparation of single cell proteins are
algae, yeast, or bacteria. These microbes are grown on an industrial scale to obtain the desired
protein.
For example, Spirulina can be grown on waste materials obtained from molasses, sewage, and
animal manures. It serves as a rich supplement of dietary nutrients such as proteins,
carbohydrate, fats, minerals, and vitamins. Similarly, micro-organisms such as Methylophilus and
methylotrophus have a large rate of biomass production. Their growth can produce a large amount
of proteins.

(b) Soil Microbes play an important role in maintaining soil fertility. They help in the formation of
nutrient-rich humus by the process of decomposition. Many species of bacteria and cyanobacteria
have the ability to x atmospheric nitrogen into usable form. Rhizobium is a symbiotic bacteria
found in the root nodules of leguminous plants. Azospirillum and Azotobacter are free living
nitrogen- xing bacteria, whereas Anabaena, Nostoc, and Oscillatoria are examples of nitrogen-
xing cyanobacteria.

Page : 190 , Block Name : Exercise

Q14 Arrange the following in the decreasing order (most important rst) of their importance, for
the welfare of human society. Give reasons for your answer. Biogas, Citric acid, Penicillin and Curd

Answer. The order of arrangement of products according to their decreasing importance is:
Penicillin- Biogas - Citric acid — Curd
Penicillin is the most important product for the welfare of human society. It is an antibiotic, which
is used for controlling various bacterial diseases. The second most important product is biogas. It
is an eco-friendly source of energy. The next important product is citric acid, which is used as a
food preservative. The least important product is curd, a food item obtained by the action of
lactobacillus bacteria on milk Hence, the products in the decreasing order of their importance are

as follows:
Penicillin- Biogas - Citric acid — Curd

Page : 190 , Block Name : Exercise

Q15 How do biofertilizers enrich the fertility of the soil?

Answer. Bio-fertilizers are living organisms which help in increasing the fertility of soil. It involves
the selection of bene cial microorganisms that help in improving plant growth through the supply
of plant nutrients. These are introduced to seeds, roots,or soil to mobilize the availability of
nutrients by their biological activity. Thus, they are extremely bene cial in enriching the soil with
organic nutrients. Many species of bacteria and cyanobacteria have the ability to x free
atmospheric nitrogen. Rhizobium is a symbiotic bacteria found in the root nodules of leguminous
plants. Azospirillum and Azotobacter are free living nitrogen- xing bacteria, whereas Anabaena,
Nostoc, and Oscillatoria are examples of nitrogen- xing cyanobacteria. Bio- fertilizers are cost
effective and eco-friendly.

Page : 190 , Block Name : Exercise