# 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:

 Label BOD value Sample A. 20 mg/L Secondary effluent discharge from a sewage treatment plant B. 8 mg/L River water C. 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.

 Drug Function Microbe 1. Cyclosporine −A Immuno suppressive drug Trichoderma polysporum 2. Statin Blood cholesterol lowering agent Monascus purpureus

Q.13:

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.

(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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