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

Here we provide NCERT Solutions for Class 12 Biology Chapter 10 Microbes in Human Welfare for English medium students, Which will very helpful for every student in their exams. Students can download the latest NCERT Solutions for Class 12 Biology Chapter 10 Microbes in Human Welfare pdf, free NCERT solutions for Class 12 Biology Chapter 10 Microbes in Human Welfare book pdf download. Now you will get step by step solution to each question.

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

Question 1.
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?
Microbes or microorganisms like bacteria, fungi and protozoans are omnipresent (i.e., found everywhere) but are not visible to naked eye because they have a size of 0.1 mm or less. The common household product that shows the presence of bacteria is curd. Milk is converted to curd by Lactobacillus bulgaricus and Streptococcus thermophillus.

Question 2.
Give examples to prove that microbes release gases during metabolism.
(i) Making of dough for bread, dosa, and idli with the help of fermenting microbes. Heat expels the gases and makes the food spongy.
(ii) Production of biogas

Question 3.
In which food would you find lactic acid bacteria ? Mention some of their useful applications.
Microorganisms such as Lactobacillus and others commonly called lactic acid bacteria (LAB) grow in milk. They convert lactose sugar of milk into lactic acid. Lactic acid causes coagulation of milk protein casein. Milk is changed into curd, yogurt and cheese etc.

  • Curd – Indian curd is prepared by inoculating cream and skimmed milk with Lactobacillus acidophilus at a temperature of about 40°C or less.
  • Yogurt – It is produced by curdling milk with the help of Streptococcus thermophilus and Lactobacillus bulgaricus. It has a flavor of lactic acid and acetaldehyde. It is often sweetened and flavored with fruit.
  • Cheese – It consists of milk curd separated from liquid part. In preparation of raw cheese milk is curdled with the help of lactic acid bacteria.
    In our stomach too, the LAB play very beneficial role in checking disease causing microbes.

Question 4.
Name some traditional Indian foods made of wheat, rice and Bengal gram (or their products) which involve use of microbes.
Idli, Dhokla, Dosa. Several food items such as dosa, idli, jalebi and bread are prepared by fermentation process in which one or more kinds of microbes are used.

Question 5.
In which way have microbes played a major role in controlling diseases caused by harmful bacteria?
Microbes are very useful to combat disease causing harmful bacteria. A number of antibiotics have been isolated from microorganisms. An antibiotic is a substance which in low concentration inhibits the growth and metabolic activity of pathogenic organisms without harming the host. Penicillin was the first antibiotic to be discovered by Alexander Fleming from fungus Penicillium notation. Antibiotics are obtained from lichens, fungi, eubacteria and actinomycetes. Some common antibiotics and their sources are as follows :

  1. Polymyxin – Bacillus polymyxa
  2. Chloramphenicol – Streptomyces venezuelae
  3. Neomycin – Streptomyces fradiae
  4. Tetracycline (Terramycin) – Streptomyces rimosus
  5. Cephalosporin – Cephalosporium acremonium

Question 6.
Name any two species of fungus, which are used in the production of the antibiotics
Penicillium notatum provides antibiotic penicillin and antibiotic fumagillin is obtained from Aspergillus fumigatus.

Question 7.
What is sewage ? In which way can sewage be harmful to us?
Sewage is a collective term used to represent municipal waste water (both liquid and solid wastes) generated in cities and towns which is carried off in sewers. Chemically, the sewage consists of approximately 99% water and 1% solid waste including inorganic and organic matter. The microorganisms present in sewage include bacteria (coli-forms, streptococci, clostridia, lactobacilli), microfungi, protozoa and microalgae. Proper sewage disposal is of prime importance, because disposal of untreated sewage in river and other water bodies may be harmful in the following ways :

  • It results in dissemination of water borne diseases caused by microorganisms.
  • It may cause depletion of dissolved oxygen (DO) in water. Reduction in oxygen availability may kill aerobic aquatic microorganisms.
  • Untreated sewage produces offensive odor.

Question 8.

What is the key difference between primary and secondary sewage treatment?
Primary (1°) treatment is a physical process that involves for removal of particulate as settelable particle.Secondary (2°) treatment is purely a biological treatment involving microbial oxidation.

Question 9.
Do you think microbes can also be used as source of energy ? If yes, how?
Microbes can be used as source of energy. Biogas is a mixture of gases produced from degradable organic matter by the activity of various anaerobic microorganisms and it may be used as fuel.

The microorganisms involved in biogas production are mainly facultative and strictly anaerobic bacteria. The most important among them are methanogenic archaebacteria, represented by Methanobacterium. The major component of biogas is methane (about 50-68%) which is highly inflammable. The other gases are CO2 (25-35%), hydrogen (1-5%), nitrogen (2-7%), oxygen (0-0.1%) and H2S (traces).

Biogas is commercially produced inside the biogas plant. The plant is fed with a mixture of dung and water (1 : 1 ratio). Dung is excreta of cattle, commonly called ‘gobar’, it is because of this, the plant is commonly called ‘gobar gas plant’. Cattle dung is a rich source of cellulosic material from plants. Biogas is used as fuel for heating, cooking, lighting, power for irrigation and other purposes as an alternative of fire wood, kerosene, dung cakes or even electricity and LPG. It is considered as ecofriendly and pollution free source of energy.

Question 10.
Microbes can be used to decrease the use of chemical fertilizers and pesticides. Explain how this can be accomplished.
Due to its hazardous nature and anti environment effect the use of chemical fertilisers and chemical insecticides are very illegitimate. Development of biofertilisers and bioinsecticides have enabled us to reduce the use of chemical fertilisers and chemical insecticides. Microbes are very important biological agents as biofertilisers and biopesticides.
Microbes as biofertilisers

  1. Free living nitrogen fixing bacteria – Azotobactcr, Clostridium.
  2. Free living nitrogen fixing cyanobacteria – Anabaena, Nostoc.
  3. Symbiotic nitrogen fixing bacteria – Rhizobium
  4. Symbiotic nitrogen fixing cyanobacteria – Azolla – Anabaena
  5. Mycorrhiza – symbiotic association between fungi and roots of a higher plant.

Microbes as bio-pesticides :-
Biopesticides are those biological agent that are used to control weeds, insects and pathogens. The microorganisms used as biopesticides are viruses, bacteria, protozoa, fungi and mites. Some of the biopesticides are being used at a commercial scale. Most important example is soil bacterium Bacillus thuringiensis (Bt). Spores of this bacterium produce the insecticidal Cry protein. This bacterium was the first biopesticide to be used on a commercial scale in the world. Through the use of genetic engineering the scientists have introduced B. thuringiensis toxin gene into plants. Such plants are resistant to attack by insect pests.

Question 11.
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 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 ?
BOD (Biological Oxygen Demand) refers to the amount of the oxygen that would be consumed if all the organic matter in one litre of water were oxidised by bacteria. BOD is a measure of the organic matter present in the water. The greater the BOD of waste water, the more are the pollutants. BOD value of clean water is generally between 1 and 2 mg/L. As the amount of pollution increases, BOD is also increased and grossly polluted waters may have the BOD around 20 mg/L. In the given problem BOD values of the three samples A, B and C are 20 mg/L, 8 mg/L and 400 mg/L, respectively. Here sample C has greatest BOD value hence it is most polluted. If we correctly label the three samples, then sample A should be secondary effluent discharged from a sewage treatment plant (20 mg/L), sample B should be river water (8 mg/L) and sample C should be untreated sewage water (400 mg/L).

Question 12.
Find out the name of the microbes from which Cyclosporin A (an immunosuppressive drug) and Statins (blood cholesterol lowering agents) are obtained.
Cyclosporin A – Trichoderma polysporum. Statin – Monascus purpureus.

Question 13.
Find out the role of microbes in the following:

  1. Single cell protein (SCP)
  2. Soil


  1. Single cell protein (SCP) – Microorganisms (e.g., bacteria, yeast, filamentous fungi, algae etc.) can be cultured on a commercial scale in a fermenter, treated in various ways, dried or used as food source or as animal feed are called single cell protein. The term SCP, however, is misleading as it sounds as if the protein is obtained from single cell. In fact, the biomass obtained from uni and multicellular organism is considered as SCP. Some common microorganisms, used in production of SCP are as follows:
    – Bacteria (e.g.,Methylophilus, Brevibacterium, etc.)
    – Cyanobacteria (e.g., Spirulina)
    – Yeasts (e.g., Saccharomyces cerevisiae, Candida utilis, etc.)
    – Filamentous fungi (e.g., Fusarium grami- nearum)
    – Algae (e.g., Chlorella)
    There are several advantages of SCP using as food. Some of them are listed below :
    – SCP is rich in high quality protein and poor in fat content.
    – The SCP can be produced in laboratories all the year round. Its production is not dependent on climatic factors.
    – The microorganisms, used in the production of SCP, are very fast growing and produce large amount of SCP from relatively very small area of land.
  2. Soil – Microbes are very useful to maintain and restore soil fertility. The fertility of soil depends not only on its chemical composition but also on the quantity and quality of useful microbes present in it. Moreover, if the composition of the soil is not upto the mark and poor in fertility, materials of biological origin are added into it to improve and maintain its fertility. These materials are grouped under the two broad categories manures and’ biofertilisers. Manures are of three types, farmyard manure, compost and green manure. Farmyard manure is the oldest manure known to mankind which is made up of dung of farm animals and plant remains etc, which are allowed to partial decay with the help of soil microorganisms. These microorganisms decompose complex organic debris into a dark amorphous substance (humus) and degradation products are easily assimilated by plants. The manure loosens the soil, increases its aeration and makes soil more fertile.
    Biofertilisers are the microorganisms which bring about soil nutrient enrichment, maximize the ecological benefits and minimize the environment hazards.

Question 14.
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
The correct order from view point of human welfare should be as follows: Penicillin > biogas > citric acid > curd Penicillin is an antibiotic, which is used to combat pathogenic microorganisms. Today, we cannot imagine a world without antibiotics because antibiotics have greatly improved our capacity 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 globe. It should be given first priority.

As we are going to face a great crisis of fossil fuels in near future, biogas can be legitimate and brilliant alternative of fossil fuels. It can be used as fuel for heating, cooking, lighting, power for irrigation and other purposes. It is considered as ecofriendly and pollution free energy source.
Citric acid is produced through the fermentation carried out by Aspergillus niger on many carbohydrates. Citric acid is used in medicines, dyeing, mirror silvering, manufacture of ink, flavouring and preservation of food and candies.

Curd is prepared by fermentation of milk. Fermentation agents are lactic acid bacteria. Curd is more nutritious than milk as it contains a number of vitamins and organic acids.

Question 15.
How do biofertilisers enrich the fertility of the soil?
Biofertilisers are the microorganisms which bring about soil nutrient enrichment, maximize the ecological benefits and minimize the environmental hazards. The main sources of biofertilisers are bacteria, fungi and cyanobacteria. Some of the biofertilisers and their importance in maintaining soil fertility are as follows :

  1. Free living nitrogen fixing bacteria fix atmospheric nitrogen in the soil and make it available for the higher plants. The best example is Azolobncter. Apart from this Clostridium, Bacillus polymyxa, Derxia etc., are also known to fix atmospheric nitrogen.
  2. Symbiotic nitrogen fixing bacteria, Rhizobium, form an efficient symbiotic relationship with leguminous plants and can fix upto 500 kg nitrogen per hectare of land. Rhizobium forms nodules on the roots of legume plants.
  3. Free living nitrogen fixing cyanobacteria ‘includes Anabaena, Nostoc, Aulosira, Stigonema etc. Aulosira fertilissimma is considered to be most active nitrogen fixer of rice fields in India.
  4. Nitrogen fixing cyanobacteria form symbiotic association with several plants, e.g., cycad roots, lichens, liverworts, Azolla (fern). Out of these, Azollci-Amibaena association is of great importance to agriculture. Anabaena azollae resides in the leaf cavities of fern. It fixes nitrogen.
  5. Fungi are also known to form symbiotic associations with plants (mycorrhiza). The fungal symbiont in these associations absorbs phosphorus from soil and passes it to the plant. Plants having such associations show other benefits also, such as resistance to root-borne pathogens, tolerance to salinity and drought, and an overall increase in plant growth and development.

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