NCERT Solutions for Class 12 Biology Chapter 8: Microbes in Human Welfare

The easiest and most reliable sample to carry from home is a small amount of fresh curd (dahi).

Curd is formed when Lactobacillus and other lactic acid bacteria (LAB) grow in milk in enormous numbers, producing acids that coagulate and partially digest the milk proteins and convert the milk into curd. Because a small amount of curd used as a starter already “contains millions of LAB”, a tiny smear of curd placed on a glass slide, stained appropriately and viewed under a compound microscope, will readily reveal large numbers of rod-shaped/chain-forming bacteria — essentially the same kind of view as the magnified bacterial photographs (rod-shaped and spherical bacteria) shown in the chapter’s own figures.

Curd is a particularly good choice because it is (i) available in every household, (ii) safe and easy to handle in a school laboratory, and (iii) guaranteed to contain a very high, visible density of live bacteria, unlike, say, a sample of tap water, which may contain far fewer and more scattered micro-organisms.

Conclusion: curd is the ideal home sample because the lactic acid bacteria that convert milk into curd are present in such enormous numbers that they are easily visible under a school laboratory microscope, directly demonstrating that microbes, though invisible to the naked eye, are abundant in everyday food.

Several processes described in this chapter show microbes releasing gaseous end-products while metabolising their food source — the type of gas produced depends on the microbe and the organic substrate it uses.

1. Dough fermentation (idli/dosa): bacteria that ferment the batter produce CO₂ gas, which is what makes the dough ‘puff up’ before cooking.
2. Swiss cheese: the large holes characteristic of Swiss cheese are formed because the bacterium Propionibacterium sharmanii produces a large amount of CO₂ while ripening the cheese.
3. Anaerobic sludge digesters (sewage treatment): anaerobic bacteria digesting activated sludge produce a mixture of methane, hydrogen sulphide and carbon dioxide, together called biogas.
4. Biogas/gobar gas production: methanogens such as Methanobacterium, growing anaerobically on cellulosic material (e.g., cattle dung), produce a large amount of methane along with CO₂ and H₂.

Conclusion: from the everyday puffing of idli/dosa batter and the holes in Swiss cheese to the methane-rich biogas generated in sludge digesters and gobar-gas plants, microbial metabolism visibly and usefully releases gases such as CO₂, methane and hydrogen sulphide as natural by-products of their growth.

Lactic acid bacteria (LAB), such as Lactobacillus, are found in curd (dahi), made by inoculating milk with a starter culture of LAB.

Useful applications of LAB:

1. Converting milk to curd: LAB grow in milk and produce acids that coagulate and partially digest the milk proteins, converting it into curd.
2. Improving nutritional quality: the process of curd formation increases the amount of vitamin B12 in the milk.
3. Protecting digestive health: LAB present in our stomach play a beneficial role in checking disease-causing microbes, helping maintain a healthy gut environment.
4. Acting as starter culture: a small amount of curd added to fresh milk as an inoculum, containing millions of LAB, is enough to multiply at suitable temperatures and convert an entire batch of milk into curd.

Conclusion: lactic acid bacteria are found abundantly in curd, where they not only convert milk into a nutritious, vitamin-B12-enriched food but also, once consumed, help protect the digestive system by checking the growth of disease-causing microbes in the stomach.

Microbial fermentation is central to many everyday Indian foods made from wheat, rice and Bengal gram (chickpea/besan):

In each case, the same underlying principle from this chapter applies: micro-organisms ferment a cereal/pulse-based batter or dough, releasing CO₂ gas that gives the food its characteristic light, puffed-up or spongy texture, exactly as described for idli-dosa batter and bread dough.

Conclusion: wheat gives us microbially-leavened bread, rice (with black gram) gives us fermented idli and dosa batter, and Bengal gram gives us fermented, steamed snacks like dhokla — in every case, microbial fermentation and the CO₂ it releases are what make these traditional foods light and palatable.

Microbes have controlled bacterial diseases chiefly through the discovery and production of antibiotics — chemical substances produced by some microbes that can kill or retard the growth of other, disease-causing microbes.

1. The discovery of Penicillin. While working on Staphylococci bacteria, Alexander Fleming noticed, quite by chance, that a mould was growing on one of his unwashed culture plates, and that Staphylococci could not grow in the area around this mould. He found this was due to a chemical produced by the mould, which he named Penicillin, after the mould Penicillium notatum.
2. Establishing its therapeutic potential. Penicillin’s full potential as an effective antibiotic was established later by Ernest Chain and Howard Florey. The antibiotic was then used extensively to treat American soldiers wounded in World War II.
3. Recognition. Fleming, Chain and Florey were jointly awarded the Nobel Prize in 1945 for this discovery.
4. Impact on disease control. Following Penicillin, many other antibiotics were purified from other microbes. These antibiotics have greatly improved our capacity to treat once-deadly bacterial diseases such as plague, whooping cough (kali khansi), diphtheria (gal ghotu) and leprosy (kusht rog), which used to kill millions of people worldwide.

Conclusion: microbes have controlled bacterial disease primarily by giving us antibiotics — beginning with Fleming’s chance discovery of Penicillin from the mould Penicillium notatum, developed into a usable drug by Chain and Florey — which today allow us to treat diseases like plague, diphtheria, whooping cough and leprosy that were once massive killers.

Antibiotics are chemical substances produced by certain microbes, including several species of fungus, that kill or retard the growth of other, disease-causing microbes. Two well-known antibiotic-producing fungal species are:

1. Penicillium notatum — the mould from which Penicillin, the first antibiotic ever discovered, was obtained. This is the very mould Alexander Fleming observed inhibiting Staphylococci growth on his culture plate (industrially, the related, higher-yielding species Penicillium chrysogenum is now used for large-scale penicillin production).
2. Cephalosporium acremonium — a fungus from which cephalosporin antibiotics, a different and widely used class of antibiotics, are obtained.

Conclusion: Penicillium notatum (source of penicillin) and Cephalosporium acremonium (source of cephalosporin) are two fungal species whose antibiotic-producing ability has been put to large-scale medical use.

Sewage is the municipal waste water generated daily in large quantities in cities and towns. A major component of sewage is human excreta, and it contains large amounts of organic matter and microbes, many of which are pathogenic.

Sewage is harmful in the following ways:

1. Spread of water-borne disease. Because sewage carries pathogenic micro-organisms from human excreta, discharging it untreated into rivers or streams can spread diseases such as typhoid, cholera and amoebiasis to anyone using that water.
2. High pollution load. Sewage has a very high BOD (biochemical oxygen demand) — a large amount of oxygen would be consumed if all its organic matter were oxidised by bacteria — so discharging it directly severely depletes the dissolved oxygen of the receiving water body, harming or killing fish and other aquatic life.
3. Contamination of water bodies. This is exactly why NCERT states that sewage “cannot be discharged into natural water bodies like rivers and streams directly” and must first be treated in Sewage Treatment Plants (STPs).
4. Real-world consequence of inadequate treatment. Because urbanisation has increased sewage generation faster than the number of STPs, much of it is discharged untreated into rivers, increasing their pollution and the incidence of water-borne diseases — the specific problem that India’s Ganga Action Plan and Yamuna Action Plan were launched to address.

Conclusion: sewage is organic-matter- and pathogen-rich municipal waste water that, if discharged untreated, spreads water-borne disease and severely depletes the oxygen of natural water bodies, which is why it must always be processed through a sewage treatment plant before release.

The key difference is that primary treatment is purely physical, while secondary treatment is biological, carried out by living micro-organisms.

Primary treatment involves the physical removal of particles — large and small — from the sewage through filtration (removing floating debris) and sedimentation (removing grit: soil and small pebbles). The solids that settle form the primary sludge, and the supernatant liquid, the effluent, is passed on for secondary treatment. No micro-organisms are deliberately used at this stage.

Secondary (biological) treatment passes the primary effluent into large aeration tanks, where it is mechanically agitated and air is pumped in. This allows vigorous growth of aerobic micro-organisms into flocs (masses of bacteria associated with fungal filaments, forming mesh-like structures), which consume most of the organic matter in the effluent and thereby significantly reduce its BOD. The flocs are then allowed to settle in a settling tank as activated sludge.

Conclusion: primary treatment physically removes solid debris and grit from sewage, while secondary treatment relies on aerobic micro-organisms in aeration tanks to biologically break down the dissolved and suspended organic matter, which is the key difference between the two stages.

Yes. Microbes are used as a source of energy through the production of biogas — a mixture of gases, predominantly methane, produced by microbial activity, which is inflammable and can be used as fuel.

1. Certain bacteria, called methanogens (e.g., Methanobacterium), grow anaerobically on cellulosic material and produce a large amount of methane along with CO₂ and H₂. These bacteria are commonly found in the anaerobic sludge formed during sewage treatment, and also in the rumen (part of the stomach) of cattle, where they help digest the cellulose present in cattle feed.
2. Because cattle dung (gobar) is rich in these methanogenic bacteria, it can be used to generate biogas, popularly called gobar gas.
3. In a biogas plant, a slurry of dung is fed into a deep (10–15 feet) concrete tank. A floating cover placed over the slurry rises as gas accumulates due to microbial activity. The plant has an outlet connected to a pipe that supplies biogas to nearby houses for cooking and lighting, while the spent slurry is removed through another outlet and can be used as fertiliser.
4. This technology was developed in India mainly through the efforts of the Indian Agricultural Research Institute (IARI) and the Khadi and Village Industries Commission (KVIC), and biogas plants are especially common in rural areas where cattle dung is abundantly available.

Conclusion: yes — methanogenic bacteria like Methanobacterium, growing anaerobically on cellulosic waste such as cattle dung, produce methane-rich biogas that is piped directly into rural homes for cooking and lighting, making microbes a genuine, renewable source of energy.

This is accomplished in two separate ways — by using microbes as biofertilisers in place of chemical fertilisers, and as biocontrol agents in place of chemical pesticides.

(a) Reducing chemical fertilisers — biofertilisers. Biofertilisers are organisms that enrich the nutrient quality of soil, sourced mainly from bacteria, fungi and cyanobacteria: Rhizobium forms symbiotic root nodules on leguminous plants and fixes atmospheric nitrogen for the plant’s use; free-living bacteria such as Azospirillum and Azotobacter also fix atmospheric nitrogen in the soil; mycorrhiza (fungi of the genus Glomus) form symbiotic associations with plant roots, absorbing phosphorus from the soil and passing it to the plant while also improving resistance to root pathogens and tolerance to salinity/drought; and nitrogen-fixing cyanobacteria (Anabaena, Nostoc, Oscillatoria) serve as important biofertilisers in paddy fields, also adding organic matter to the soil.

(b) Reducing chemical pesticides — biocontrol agents. Biocontrol uses biological methods, rather than toxic chemical insecticides/pesticides, to manage pests: the bacterium Bacillus thuringiensis (Bt) is sprayed as dried spores onto crops, killing caterpillar larvae that ingest it while leaving other insects unharmed (its toxin genes have even been engineered directly into plants, as in Bt-cotton); the fungus Trichoderma is an effective biocontrol agent against several plant pathogens; and baculoviruses (mainly genus Nucleopolyhedrovirus) provide species-specific, narrow-spectrum control of insect pests with no harm to plants, mammals, birds, fish or non-target insects.

Conclusion: microbes reduce our dependence on toxic chemicals in agriculture in two complementary ways — nitrogen- and phosphorus-enriching biofertilisers such as Rhizobium and mycorrhizal fungi replace chemical fertilisers, while biocontrol agents such as Bacillus thuringiensis, Trichoderma and baculoviruses replace chemical pesticides, together making agriculture safer and more sustainable.

BOD (Biochemical Oxygen Demand) is the amount of oxygen that would be consumed if all the organic matter in one litre of water were oxidised by bacteria; it is therefore a direct, indirect measure of the organic matter/pollution load of a water sample — the greater the BOD, the greater the polluting potential.

Applying this principle to the three recorded values (A = 20 mg/L, B = 8 mg/L, C = 400 mg/L):

1. Sample C (400 mg/L) is the most polluted water sample — its BOD is dramatically higher than the other two, consistent only with untreated sewage water, which is loaded with organic matter and pathogenic microbes that have not yet been broken down by any treatment process.
2. Sample A (20 mg/L) has a BOD that is far lower than raw sewage but still clearly higher than the cleanest sample — this matches secondary effluent from a sewage treatment plant, whose organic load has already been very significantly reduced by aerobic microbial action in the aeration tank, but which is still not as clean as a relatively unpolluted natural water source.
3. Sample B (8 mg/L), the lowest of the three, matches river water, which the question tells us to assume is ‘relatively clean’.

Conclusion: sample C, with the highest BOD (400 mg/L), is the most polluted and must be untreated sewage; sample A (20 mg/L) is the secondary treated effluent, whose BOD has been greatly but not completely reduced; and sample B (8 mg/L), with the lowest BOD, is the relatively clean river water.

Both of these important bioactive molecules are obtained from fungi used in the pharmaceutical industry, as described in this chapter’s section on chemicals, enzymes and bioactive molecules:

Conclusion: Cyclosporin A, used to prevent organ-transplant rejection, is obtained from the fungus Trichoderma polysporum, while Statins, used to lower blood cholesterol by competitively inhibiting the cholesterol-synthesising enzyme, are obtained from the yeast Monascus purpureus.

(a) Single Cell Protein (SCP). SCP refers to protein-rich biomass obtained by growing micro-organisms — bacteria, yeast, fungi or algae — in large quantities on relatively inexpensive substrates (such as whey, molasses, or other agricultural/industrial waste). Because microbes have an extremely rapid rate of multiplication compared to plants or animals, they can be cultured to produce large amounts of protein-rich biomass in a very short time and a small land area, without depending on conventional crop or livestock farming. This harvested microbial biomass can then be processed and used as a protein supplement in human food or animal feed, helping address protein/nutritional deficiency in a growing population. A commonly cited example in this context is the cyanobacterium Spirulina, which is cultivated specifically as a single cell protein source because of its very high protein content.

(b) Role of microbes in soil. Microbes are central to maintaining the natural fertility and health of soil: decomposer bacteria and fungi break down dead organic matter (plant litter, animal remains), recycling carbon, nitrogen, phosphorus and sulphur back into forms plants can use. Nitrogen-fixing microbes — the symbiotic bacterium Rhizobium in the root nodules of legumes, free-living bacteria like Azotobacter and Azospirillum, and nitrogen-fixing cyanobacteria such as Anabaena and Nostoc — convert atmospheric nitrogen into forms usable by plants, directly enriching soil fertility (exactly as described in the biofertiliser section of this chapter). Mycorrhizal fungi (e.g., Glomus) form symbiotic associations with plant roots, improving phosphorus uptake and conferring resistance to root pathogens and tolerance to drought/salinity. Together, these microbial processes are what keep soil fertile naturally, reducing the need for chemical fertilisers.

Conclusion: microbes serve as a rapid, land-efficient source of protein-rich biomass (SCP, e.g., from Spirulina), and in soil they act as nature’s own fertiliser factory — decomposing organic matter and fixing atmospheric nitrogen — both roles reducing our dependence on conventional agriculture and chemical fertilisers respectively.

This is an opinion-based question, and different, well-reasoned orderings can be justified; the key is to support the order with sound reasoning. One widely accepted ordering, based on breadth and life-criticality of impact, is:

Penicillin > Biogas > Curd > Citric acid

1. Penicillin (most important). As a life-saving antibiotic, Penicillin and the antibiotics that followed it directly save human lives by curing once-fatal bacterial diseases such as plague, diphtheria, whooping cough and leprosy. No other item in this list has as direct and irreplaceable an impact on human survival.
2. Biogas. Biogas is a renewable source of energy for cooking and lighting, used especially by rural households; it reduces dependence on firewood/fossil fuels, helps dispose of organic waste (cattle dung) usefully, and even yields spent slurry as fertiliser — a broad, everyday impact on quality of life and the environment for a very large population.
3. Curd. Curd is a nutritious, widely consumed daily food that improves digestion and vitamin B12 intake, contributing meaningfully to everyday nutrition and health, but it is not life-saving in the way Penicillin or energy security are.
4. Citric acid (least important of the four). Citric acid is a valuable industrial/food-flavouring chemical (used in soft drinks, food preservation, etc.), but it is the least critical to basic human survival or welfare compared to a life-saving antibiotic, a source of energy, or a staple nutritious food.

Conclusion: ranked by their life-saving potential and breadth of impact on human welfare, Penicillin (a life-saving antibiotic) is most important, followed by biogas (a renewable energy source), then curd (a daily nutritious food), with citric acid (a useful but non-essential industrial chemical) being the least critical of the four.

Biofertilisers are organisms — mainly bacteria, fungi and cyanobacteria — that enrich the nutrient quality of soil naturally, without the pollution associated with chemical fertilisers.

1. Symbiotic nitrogen fixation. Rhizobium forms nodules on the roots of leguminous plants in a symbiotic association, and fixes atmospheric nitrogen into organic forms that the plant can directly use as a nutrient.
2. Free-living nitrogen fixation. Bacteria such as Azospirillum and Azotobacter, living freely in the soil (without needing a plant partner), also fix atmospheric nitrogen, enriching the soil’s nitrogen content.
3. Mycorrhizal phosphorus transfer. Many fungi of the genus Glomus form a symbiotic association with plant roots called mycorrhiza; the fungal partner absorbs phosphorus from the soil and passes it on to the plant. Plants with mycorrhizal associations also show other benefits — resistance to root-borne pathogens, tolerance to salinity and drought, and overall better growth and development.
4. Cyanobacterial nitrogen fixation and organic enrichment. Autotrophic cyanobacteria such as Anabaena, Nostoc and Oscillatoria fix atmospheric nitrogen and serve as important biofertilisers, particularly in paddy fields; these blue-green algae also add organic matter to the soil, further increasing its fertility.

Conclusion: biofertilisers enrich soil fertility chiefly by fixing atmospheric nitrogen (via symbiotic Rhizobium, free-living Azotobacter/Azospirillum, and nitrogen-fixing cyanobacteria) and by improving phosphorus uptake through mycorrhizal fungi, offering a natural, pollution-free alternative to chemical fertilisers.