NCERT Solutions for Class 12 Biology Chapter 10 Microbes in Human Welfare


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




Importance in society



The first antibiotic and is used for the treatment of several life-threatening bacterial infections.



A clean, environmentally friendly, sustainable source of energy made of biological waste product. Biogas production can be carried out by unskilled people and is not expensive.  It provides energy for cooking and generating electricity in the areas with limited resources.


Citric Acid

The dominant use of citric acid is as a flavouring agent and preservative in food and beverages. Industrial-scale production of citric acid began during world war I.



Curd has many health benefits for humans. It is a natural probiotic which keeps the microflora of the gut healthy and balanced. Curd is also a source of nutrients like calcium, vitamin-B12, etc.



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


Cyclosporin A

An immunosuppressive drug

Tolypocladium inflatum (fungus)





Blood cholesterol lowering agents


Monascus purpureus (fungus)

Aspergillus terreus (fungus)


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


Municipal wastewater that contains human excreta and effluent from the kitchen and bathrooms is collectively called sewage. Sewage contains large amounts of organic matter and also microbes some of which can be harmful to humans and other living beings. Sewage water is a major source of water pollution. Therefore, it is important and mandatory to collect, treat and dispose of sewage responsibly.


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




Penicillium notatum


Streptomyces griseus



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


Lactic Acid Bacteria (LAB) can be found in curd. LAB converts milk into curd. Lactobacillus (LAB) is also used for commercial and industrial production of lactic acid. Casein, an important milk protein, is soluble at a neutral pH, but insoluble in acid. When milk turns sour, casein precipitates leading to the thickening of the product.

Lactose (milk sugar) + Lactobacillus  → Lactic acid

This lactic acid causes casein to curdle and form the curd.


How do biofertilisers enrich the fertility of the soil?


Biofertilisers are organisms that enrich the nutrient quality of the soil. The main sources of biofertilisers are bacteria, fungi, and cyanobacteria. Biofertilisers can enrich the fertility of the soil in the following ways:

  1. Nitrogen enrichment – Bacteria like Azospirillum and Azotobacter are free-living in the soil and can fix atmospheric nitrogen thus increasing the nitrogen content of the soil. Rhizobium another nitrogen-fixing bacteria forms nodules on the plant roots and lives symbiotically. Cyanobacteria are widely distributed in aquatic and terrestrial environments many of which can fix atmospheric nitrogen, e.g. Anabaena, Nostoc, Oscillatoria, etc. In paddy fields, cyanobacteria serve as an important biofertiliser.
  2. Phosphorous enrichment - Fungus like Glomus form Mycorrhiza live symbiotically with a plant and help in absorbing phosphorus for the plant.


Three water samples namely river water, untreated sewage water and secondary effluent discharged from a sewage treatment plant were subjected to BOD test. The samples were labeled 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 20 mg/L, 8 mg/L and 400 mg/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?


BOD refers to the amount of the oxygen that would be consumed if all the organic matter in one litre of water is oxidised by bacteria. The BOD test measures the rate of uptake of oxygen by micro-organisms in a sample of water and thus, indirectly, BOD is the measure of the organic matter present in the water. The greater the BOD of waste water more is its polluting potential.



BOD values

Water sample types


20 mg/L

Secondary effluent

This is the effluent that has been treated before disposing into the natural water bodies.


8 mg/L

River water

This sample has the least amount of organic matter. Therefore, it can be considered the river water. 


400 mg/L

Untreated sewage water

This is the municipal sewage that is collected for treatment in the STP.


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


Yes, microbes can be used as a source of energy. One of the most common examples of microorganisms being used for obtaining energy is biogas. 

Biogas: Methanogens are anaerobic bacteria that breakdown cellulosic material producing large amounts of methane along with carbon dioxide and hydrogen. These bacteria reside in the stomach of cattle and help the animal to digest the cellulose. Cattle dung is rich in these bacteria and is used in the production of biogas. The biogas plant consists of a concrete tank (10-15 feet deep) into which biowastes are collected and the slurry of dung is fed. A floating cover is placed over the slurry, which keeps on rising as the gas is produced in the tank due to the microbial activity. The biogas plant has an outlet, which is connected to a pipe to supply biogas to nearby houses.


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


Microbes such as fungi and bacteria produce antibiotics which are chemical substances that can kill or retard the growth of other (disease-causing) microbes.  Antibiotics can be used to treat potentially life threatening diseases like pneumonia to relatively mild conditions such as acne. Antibiotics have greatly improved our ability to treat deadly diseases such as plague, whooping cough (kali khansi), diphtheria (gal ghotu) and leprosy (kusht rog), which used to kill millions of people all over the world.


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


Wheat: Bread, kulchas and bhaturas

Rice: Dosa and idli

Bengal Gram: Dhokla and khandvi

Explanation: The microbes that ferment these batters are found naturally on urad dal which is used for making the batter hence, no inoculums are required to start the fermentation unlike in bread where yeast inoculum is used to initiate fermentation.


Give examples to prove that microbes release gases during metabolism.


Microbes produce different types of gaseous end-products during growth and metabolism. The type of gas produced depends upon the microbes and the organic substrates they utilise.

  1. Microbes found in the dough of idli, dosa or for making bread release CO2 during fermentation. The idli batter after being kept overnight becomes very fluffy with small bubbles. If the batter is mixed hard it flattens down because the entrapped air (carbon dioxide) escapes.
  2. Some microbes which grow anaerobically on cellulosic material produce a large amount of methane along with CO2 and H2. These bacteria are collectively called methanogens, and one such common bacterium is Methanobacterium. These bacteria reside in the stomach of cattle and help these animals to digest the cellulose. Cattle dung is rich in these bacteria and is used in the production of biogas, which can be used for cooking food and generating electricity.


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?


I would carry some fresh homemade yogurt/ curd because ‘Lactic acid bacteria’ (LAB) for example Lactobacillus grows in milk and converts it into curd. A small amount of curd contains millions of LAB and these can be easily observed under a microscope.

Method to observe LAB under the microscope:

  1. Make a thin smear of curd on a glass slide.
  2. Stain the smear according to the procedure for the Gram stain or with methylene blue.
  3. View the stained smear at 400x to determine the characteristic features of Lactobacillus like rod-shaped cells.
  4. Observe the morphology. LABs the rod- shaped bacteria will stain blue.


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


(a) Single Cell Protein (SCP)

SCP refers to the dried microbial cells or total protein extracted from pure microbial cell culture (algae, bacteria, filamentous fungi, yeasts), which can be used as food supplement for humans (food grade) or animals (feed grade). SCP, a non-conventional protein source can help solve the problem of malnutrition, food and feed shortage in the developing countries. The production of ‘Single Cell Protein’ can be done using waste materials as the substrate, specifically agricultural wastes such as wood shavings, sawdust, corn cobs, etc. and culturing yeast, bacteria, algae or fungi. Spirulina algae, is a good resource as it can be mass cultivated easily and is fast growing with high nutritional content.

(b) Soil

Microorganisms in soil are important because they affect the structure and fertility of different soils. Soil microorganisms can be classified as bacteria, actinomycetes, fungi, algae, and protozoa. Each of these groups has different functions in the soil they inhabit. Some examples of microbes, which are beneficial for soil quality are as follows:






Turns nitrite into nitrate, which results in the gain of oxygen and is also known as oxidation


Metabolises a wide range of chemicals and fertilisers

Anabaena, Nostoc, Oscillatoria

Nitrogen fixation

Azospirillum and Azotobacter

Fixes atmospheric nitrogen


Blue-green algae

Nitrogen fixation



Absorption of phosphorus


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


The chemical fertilisers and pesticides used in agricultural practices these days pose several health hazards to humans and also create an imbalance in the natural ecosystem.  To decrease the use of these chemicals, microbes have been employed through various methods like:

Biopesticides: Biological control of pests and diseases relies on using the natural pest-predator relationship to control diseases. Some microbes have proven to be effective biological control agents and have shown to have no impact on other useful plants or animals. For examples: 

  • Trichoderma species is a fungus that is commonly found in the root ecosystem. They are effective biocontrol agents of several plant pathogens.
  • Bacillus thuringiensis is the most widely used species of bacteria for biological control of insects.
  • Baculoviruses are pathogens that attack insects and other arthropods.

Biofertilisers: They add nutrients through the natural processes of nitrogen fixation, solubilising phosphorus, and stimulating plant growth through the synthesis of growth-promoting substances. They can be applied to seed, plant surfaces, or soil. They colonise the interior of the plant and promote growth by increasing the supply or availability of primary nutrients to the host plant. For examples:

  • Azotobacter can be used with crops like wheat, maize, mustard, cotton, potato, and other vegetable crops.
  • Azospirillum inoculations are recommended mainly for sorghum, millets, maize, sugarcane, and wheat.
  • Blue-green algae fix atmospheric nitrogen and are used as inoculations for paddy crop.


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


Primary Sewage Treatment

Secondary Sewage Treatment

Mechanical, inexpensive and simple process

Biological, expensive and complicated process

Involves simple filtration and sedimentation of sewage for the removal of debris

Involves the use of microbes to breakdown the organic matter in the sewage

Explanation: Before being drained into water bodies like rivers and lakes, sewage needs to be treated to remove the organic matter. This is carried out in two stages:

Primary Sewage Treatment: The initial phase of sewage treatment involves physical removal of particles. These are removed in two stages;

  1. Sequential filtration for removing floating debris
  2. Sedimentation for removing grit (soil and small pebbles)

All the solids that settle form the primary sludge and the supernatant forms the effluent. The effluent from the primary settling tank is taken for secondary treatment.

Secondary sewage treatment or Biological treatment: This is also carried out in stages before the water can be returned to natural water sources.

  1. Aeration tank: The primary effluent is passed into large aeration tanks where it is constantly agitated mechanically and air is pumped into it. This allows vigorous growth of useful aerobic microbes into flocs (masses of bacteria associated with fungal filaments to form mesh-like structures). While growing, these microbes consume the major part of the organic matter in the effluent. This significantly reduces the BOD (biochemical oxygen demand) of the effluent. BOD refers to the amount of the oxygen that would be consumed if all the organic matter in one litre of water is oxidised by bacteria. The sewage water is treated till the BOD is sufficiently reduced after which the effluent is then passed into a settling tank.
  2. Settling tank: Here the bacterial ‘flocs’ are allowed to sediment. This sediment is called activated sludge. A small part of the activated sludge is pumped back into the aeration tank to serve as the inoculum. The remaining major part of the sludge is pumped into large tanks called anaerobic sludge digesters.

Anaerobic sludge digesters: Here anaerobic bacteria digests the bacteria and fungi in the sludge. The effluent from the secondary treatment plant is generally released into natural water bodies like rivers and streams.

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Tips & Strategies for Class 12 Exam Preparation

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2. Class 12 is important for your career, therefore follow your time table religiously.

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5. Most importantly, be confident.

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Frequently Asked Questions

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Math is a subject that analyses the critical and analytical thinking of a student and tests numerical questions. So, the best way to prepare for Math is by studying the NCERT solutions. Make a timetable, jot down the important formulas, and theorems, make time for revision and give as much time as you can give to practicing questions. Solve a maximum number of questions and time your efforts. Extramarks - The Learning App has several sample papers along with NCERT 12 solutions that can be used for practicing for class 12 Math exam.

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2. Follow your timetable religiously. Save time for relaxing activities like meditation, swimming or sleeping.

3. Make brief notes containing important answers, character sketches, theorems, formulae, etc. Make clear notes so you can study them before the exam.

4. Learn from class 12 NCERT solutions given on Extramarks website.

5. Be confident that you can crack these exams and take time off to relax.

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