Biology Class 12 NCERT Solutions: Chapter 10 Microbes in Human Welfare Part 4

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Q.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?


  • Biological oxygen demand (BOD) is the method of determining the amount of oxygen required by micro-organisms 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 effluent discharge from a sewage treatment plant is most polluted.

  • Thus, sample A is secondary effluent 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:

Table of BOD Value and Sample
Table of BOD Value and Sample


BOD value



20 mg/L

Secondary effluent discharge from a sewage treatment plant


8 mg/L

River water


400 mg/L

Untreated sewage water

Q.12 Find out the name of the microbes from which Cyclosporine A (an immunosuppressive drug) and Statins (blood cholesterol lowering agents) are obtained.


Table of Drug and this Function
Table of Drug and this Function





Cyclosporine −A

Immuno suppressive drug

Trichoderma polysporum



Blood cholesterol lowering agent

Monascus purpureus


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)

  • 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.

Image of Single Cell Protein

Image of Single Cell Protein

(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 fix atmospheric nitrogen into usable form.

  • Rhizobium is a symbiotic bacteria found in the root nodules of leguminous plants.

  • Azospirillium and Azotobocter are free living nitrogen-fixing bacteria, whereas Anabaena, Nostoc, and Oscillitoria are examples of nitrogen-fixing cyanobacteria

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