Chapter 8: Microbes in Human Welfare (Biology and Human Welfare)
Exam-ready answers to all 15 exercises of Chapter 8, Microbes in Human Welfare (NCERT Class 12 Biology, 2026-27): curd as a microbe sample, gases released by microbes, lactic acid bacteria, fermented Indian foods, antibiotics and the fungi that make them, sewage and its treatment, BOD problem, biogas, biocontrol and biofertilisers, cyclosporin A, statins, SCP and soil microbes. All 15 questions are answered, with the key answer highlighted.
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 with the help of a microscope, which sample would you carry and why?
Solution
Curd. It contains a large number of lactic acid bacteria (Lactobacillus), which are easy to see under a microscope after a simple stain. (Idli or dosa batter, which contains bacteria and yeast, would also work.)
A drop of curd: it is full of Lactobacillus bacteria.
In which way have microbes played a major role in controlling diseases caused by harmful bacteria?
Solution
Microbes produce antibiotics, chemical substances that kill or stop the growth of disease-causing bacteria. Penicillin, the first antibiotic, was obtained from the mould Penicillium notatum. Antibiotics have greatly reduced deaths from diseases like plague, whooping cough, diphtheria, leprosy and pneumonia.
Microbes produce antibiotics (e.g. penicillin from Penicillium) that kill disease-causing bacteria.
What is sewage? In which way can sewage be harmful to us?
Solution
Sewage is the municipal waste water from homes and cities, containing human excreta and large amounts of organic matter and microbes.
It is harmful because:
it contains pathogenic microbes that cause diseases such as cholera, typhoid and dysentery;
if discharged untreated into rivers, its organic matter is decomposed by microbes that use up the dissolved oxygen, killing fish and other aquatic life;
it pollutes drinking water sources and soil.
Municipal waste water with excreta and organic matter; it spreads disease and pollutes water bodies (depletes oxygen).
What is the key difference between primary and secondary sewage treatment?
Solution
Primary treatment is physical: removal of floating debris by sequential filtration and of grit (soil, small pebbles) by sedimentation. Solids settle as primary sludge.
Secondary (biological) treatment uses microbes: the primary effluent is aerated in large tanks, where aerobic microbes form flocs and consume most of the organic matter, greatly reducing the BOD. The sludge is then digested anaerobically.
Primary treatment is physical (filtering and settling); secondary treatment is biological (microbes digest organic matter).
Do you think microbes can also be used as source of energy? If yes, how?
Solution
Yes. Biogas, mainly methane, is produced by methanogenic bacteria (e.g. Methanobacterium) acting anaerobically on cattle dung (gobar), sewage sludge or other organic waste. Biogas is used as a fuel for cooking and lighting, and the leftover slurry is used as manure.
Yes: methanogens produce biogas (methane) from cattle dung and organic waste.
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 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?
Solution
BOD (biochemical oxygen demand) is higher when there is more organic matter, so a higher BOD means more pollution.
C (400 mg/L) is the most polluted: untreated sewage.
A (20 mg/L): secondary effluent from the treatment plant.
B (8 mg/L): river water (relatively clean).
C is most polluted. A = secondary effluent, B = river water, C = untreated sewage.
Find out the role of microbes in the following and discuss it with your teacher. (a) Single cell protein (SCP) (b) Soil.
Solution
(a) SCP: microbes such as Spirulina (an alga), yeast and some bacteria are grown on cheap materials (waste water from potato factories, straw, molasses, sewage) to produce protein-rich biomass for human and animal food. For example, 250 g of Methylophilus methylotrophus can produce about 25 tonnes of protein in a day.
(b) Soil: microbes decompose dead organic matter and recycle nutrients; form humus; fix nitrogen (Rhizobium, Azotobacter, cyanobacteria); help plants take up phosphorus (mycorrhiza); and some act as biocontrol agents against plant pathogens.
(a) Protein-rich microbial biomass as food. (b) Decomposition, nutrient recycling, humus formation, nitrogen fixation and phosphorus uptake.
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.
