NCERT Class 12 Biology Ch 8 Microbes in Human Welfare Solutions

Concept used. Microbes are micro-organisms (bacteria, fungi, protozoa, viruses) too small to be seen with the unaided eye. They are visible only when present in very large numbers (forming colonies, films, or clumps) or when viewed under a microscope. To demonstrate their presence in a classroom, we pick a household sample where the microbial population is naturally very high and easy to make a smear or wet-mount from.

1. Best sample to carry: a small spoonful of fresh curd (dahi) in a clean, capped vial. A small drop of curd added to fresh milk acts as inoculum (or ``starter'') because that drop already contains millions of lactic acid bacteria (LAB), chiefly Lactobacillus. So a teaspoon of home-made curd packs an extremely high microbial load in a tiny volume, which is perfect for microscopy.
2. Why curd works so well. The LAB in curd:
   • are abundant (densities of 10⁸ to 10⁹ cells per mL),
   • are easy to stain (a drop of curd + a drop of methylene blue on a slide is enough),
   • show a clear rod shape (Lactobacillus) or chain-of-cocci shape (Streptococcus) under 400× to 1000× magnification,
   • are non-pathogenic (safe to handle in a school lab).
3. How to prepare the slide in the lab.
   • Take a tiny drop of curd on a clean glass slide.
   • Add 1–2 drops of distilled water and mix with a needle to thin the smear.
   • Add a drop of methylene blue stain, wait 1–2 minutes, blot the excess.
   • Place a coverslip and view under the high-power objective of a compound microscope.
   • You will see clusters of rod-shaped Lactobacillus bacteria - direct visual proof that microbes were present in the curd you carried from home.
4. Other equally good samples: yeast paste from bread dough or idli/dosa batter (shows budding Saccharomyces cerevisiae cells), a slimy bit of pond water, or scrapings from a slice of bread on which mould has just begun to appear.

Carry a small spoonful of curd (or a pinch of idli/dosa batter or bread dough). Curd contains millions of Lactobacillus cells per drop, which can be easily stained with methylene blue and seen under a compound microscope as rod-shaped bacteria.

Concept used. Many microbes break down (ferment) sugars and other organic substrates anaerobically (or facultatively). The metabolic by-products almost always include one or more gases. The gas evolved depends on the microbe and the substrate. We can recognise gas release by visible bubbling, by ``puffing-up'' of dough, by holes in cheese, or by flammable biogas in a closed digester.

inlinerecall[Two common fermentation pathways] • Alcoholic fermentation (yeast on sugars):1pt] $C6H12O6 ->[yeast] 2 C2H5OH + 2 CO2$
[3pt] $•$ Methanogenesis (anaerobic bacteria on organic waste, especially cellulose-rich dung):
[1pt] Mixed organic matter $→$ $CH4 + CO2 + H2 + H2S$ (biogas). inlinerecall

steps Example 1 - Puffing of bread dough and idli/dosa batter ($CO2$ release). Bread dough is fermented by baker's yeast, Saccharomyces cerevisiae. Yeast respires the sugars in flour anaerobically, producing ethanol and $CO2$. The $CO2$ bubbles get trapped in the gluten/protein network, making the dough rise. [ C6H12O6 S. cerevisiae 2 C2H5OH + 2 CO2 (↑) The same CO2 release happens in idli and dosa batter, fermented by a mixed flora of bacteria and yeasts. Example 2 - Holes (``eyes'') in Swiss cheese (CO2 release). Propionibacterium shermanii (called P. sharmanii in NCERT) ferments lactate in cheese curd to propionic acid, acetic acid and large volumes of CO2. Trapped CO2 bubbles form the characteristic large round ``eyes'' that Swiss cheese is famous for. Example 3 - Production of biogas from cattle dung (CH4 + CO2 + H2 release). Methanogenic bacteria like Methanobacterium thrive anaerobically on the cellulose in cattle dung. They produce a gas mixture rich in methane (CH4), with CO2, H2 and traces of H2S. This flammable mixture is biogas (``gobar gas''), used for cooking and lighting in rural India. Example 4 - Brewing of beer, wine, whisky (ethanol + CO2). Saccharomyces cerevisiae (brewer's yeast) ferments malted cereals or fruit juices. The visible bubbling of fermenting must in a brewery is CO2 escaping. The same reaction as in step 1. Example 5 - Anaerobic sludge digester of a sewage plant. The remaining sludge from secondary treatment is fed to anaerobic digesters, where bacteria release CH4 + CO2 + H2S. This is the same biogas, harvested as fuel right at the sewage plant. steps

Curd + idli/dosa/bread dough rising, holes in Swiss cheese, fermenting beer/wine bubbling, biogas from dung, and gas from sewage digesters - all demonstrate that microbes release gases (CO2, CH4, H2, H2S) as metabolic by-products.

Concept used. Lactic acid bacteria (LAB) are a group of Gram-positive, rod- or sphere-shaped bacteria that anaerobically ferment lactose (milk sugar) and other carbohydrates to lactic acid. The common genera are Lactobacillus, Streptococcus, Lactococcus and Leuconostoc. LAB are non-pathogenic and are widely used in food preservation, nutrition and human health (as probiotics).

1. Foods that contain LAB.
   • Curd / yoghurt / dahi - the classic example. Lactobacillus converts milk to curd.
   • Cheese - many varieties (cheddar, mozzarella, Roquefort) use LAB during ripening.
   • Idli and dosa batter - fermented by a mixed culture of LAB and wild yeasts.
   • Dhokla, kanji, sauerkraut, kimchi, pickles, yakult - all rely on LAB fermentation.
   • Fermented butter milk and lassi - products of LAB.
2. Useful applications of LAB.
   • Conversion of milk to curd. LAB produce acids that coagulate and partially digest the milk proteins (mainly casein). This curdles the milk and turns it into a thick, tangy semi-solid.
   • Improving nutritional quality. During curd formation LAB increase the content of vitamin B₁₂ (cyano-cobalamin). Curd is therefore a better B₁₂ source than plain milk - especially valuable for vegetarians.
   • Checking disease-causing microbes in the human gut. LAB are part of our natural intestinal flora. The lactic acid they secrete lowers gut pH, suppressing pathogenic bacteria. This is the principle behind probiotic drinks like Yakult.
   • Food preservation. Lactic acid acts as a natural preservative - it inhibits the growth of spoilage microbes. This is why curd, pickles, sauerkraut and kimchi keep for weeks without refrigeration.
   • Texture, flavour and aroma of cheese. LAB metabolism produces small molecules (diacetyl, acetate, propionate) that give cheese its characteristic sharp/tangy flavour.
   • Production of dough for idli, dosa, dhokla. LAB sourness + CO2 release together give the puffed, slightly sour traditional Indian breakfast foods.

LAB are found in curd, cheese, idli/dosa batter, dhokla, kanji, lassi, sauerkraut, kimchi and pickles. They convert milk to curd, increase vitamin B₁₂ content, suppress harmful gut microbes, preserve food, flavour cheese, and leaven traditional Indian breads.

Concept used. Indian traditional cuisine uses microbial fermentation extensively. Different microbes (mostly LAB and wild yeasts) act on the carbohydrates and proteins of wheat, rice or Bengal gram (chana, besan) flours to produce: a sour-tangy taste from lactic acid, a puffed/spongy texture from CO2, plus improved digestibility and nutritional value (more B-complex vitamins, lower phytic acid).

1. Foods from wheat (or wheat flour, maida, suji).
   • Bread / pav / kulcha - leavened by baker's yeast Saccharomyces cerevisiae; CO2 release makes the dough rise.
   • Bhatura / naan - yoghurt + flour fermented overnight by LAB and yeast.
   • Jalebi batter - fermented overnight, microbes contribute to the sourness and bubbles.
2. Foods from rice (or rice flour) usually mixed with urad/chana dal.
   • Idli - rice + urad dal batter, fermented overnight by LAB (∼Leuconostoc mesenteroides) and wild yeasts; steam-cooked.
   • Dosa - same batter, pan-cooked thin.
   • Uttapam, appam - fermented rice batters.
   • Khaman / dhokla - uses chickpea/Bengal-gram flour with a rice component; LAB fermentation gives the spongy, mildly sour cake.
3. Foods from Bengal gram (chana, besan).
   • Dhokla - besan (Bengal-gram flour) + semolina batter fermented by LAB; steamed.
   • Khandvi - besan-based rolled snack; fermentation by LAB during batter preparation gives the slight tang.
4. Microbes involved (summary).
   • Saccharomyces cerevisiae - baker's yeast, leavens bread/naan/bhatura/jalebi.
   • Lactobacillus, Streptococcus, Leuconostoc mesenteroides - LAB that sour and aerate idli/dosa/dhokla batter.
   • Wild yeasts and lactobacilli together provide both gas (CO2) and sourness (lactic acid).

Wheat: bread, bhatura, naan, kulcha (yeast + LAB). Rice: idli, dosa, uttapam, appam (LAB + wild yeast on rice-dal batter). Bengal gram: dhokla and khandvi (LAB on besan/rice batter).

Concept used. Antibiotics are chemical substances produced by some microbes (mostly fungi, actinomycetes and bacteria) that can kill or retard the growth of other microbes - especially the disease-causing ones. The word anti-biotic literally means ``against life'' (with respect to harmful microbes) and pro-life (with respect to humans).

1. Microbes are the natural source of nearly every antibiotic in clinical use. Examples:
   • Penicillin - from the fungus Penicillium notatum (and later P. chrysogenum).
   • Streptomycin - from the bacterium Streptomyces griseus.
   • Tetracycline - from Streptomyces aureofaciens.
   • Chloramphenicol - from Streptomyces venezuelae.
   • Erythromycin - from Streptomyces erythraeus.
2. How antibiotics kill or stop bacteria. They target structures or processes specific to bacteria (and absent in human cells), e.g.:
   • Penicillin breaks down the peptidoglycan cell wall of bacteria.
   • Streptomycin and tetracycline jam the 70S bacterial ribosome, stopping protein synthesis.
   • Result: bacteria die or stop dividing; human cells are unaffected.
3. Diseases brought under control by microbial antibiotics.
   • Plague (caused by Yersinia pestis).
   • Whooping cough or kali khansi (Bordetella pertussis).
   • Diphtheria or gal ghotu (Corynebacterium diphtheriae).
   • Leprosy or kusht rog (Mycobacterium leprae).
   • Pneumonia, tuberculosis, typhoid, cholera and many more.
4. World War II turning point. Penicillin was used on a large scale to treat wounded American soldiers in World War II. It saved millions of lives that would otherwise have been lost to bacterial wound infections.
5. Net impact. Before antibiotics, infectious diseases were a leading cause of death worldwide. Today, with microbial antibiotics, most bacterial infections are easily treatable. We literally cannot imagine modern medicine without antibiotics - and we owe each of them to a microbe.

Microbes produce antibiotics - natural chemicals like penicillin, streptomycin, tetracycline, chloramphenicol and erythromycin. These kill or stop disease-causing bacteria, bringing under control deadly illnesses such as plague, diphtheria, whooping cough, leprosy, pneumonia and tuberculosis.

Concept used. An antibiotic is a microbial product that kills or inhibits other microbes. Fungi were the very first organisms found to make antibiotics, and they remain a major source of broad-spectrum antibiotics today. The most famous antibiotic-producing fungi belong to the genus Penicillium.

1. Species 1: Penicillium notatum. This is the original mould that Alexander Fleming observed on his contaminated Staphylococcus plate in 1928. It produces penicillin, the first commercially-used antibiotic and the prototype of all β-lactam antibiotics. Penicillin destroys the peptidoglycan cell wall of Gram-positive bacteria.
2. Species 2: Penicillium chrysogenum. A higher-yielding cousin of P. notatum; it produces penicillin in much larger amounts and is the strain most modern commercial penicillin fermentations actually use. Industrial strain improvement (mutagenesis + selection) on P. chrysogenum has raised penicillin titres from a few mg/L to several g/L.
3. Other antibiotic-producing fungi (for extra marks).
   • Cephalosporium acremonium - produces cephalosporin (closely related to penicillin).
   • Trichoderma polysporum - produces cyclosporin A (immunosuppressant, not strictly an antibiotic but bioactive).
   • Aspergillus fumigatus - produces fumagillin (antiamoebic).

Two fungi that produce antibiotics: Penicillium notatum and Penicillium chrysogenum - both sources of penicillin. (Cephalosporium acremonium, producing cephalosporin, is another good example.)

Concept used. Sewage is the term for the municipal waste water generated by households and other establishments in towns and cities. It consists mainly of human and animal excreta, used kitchen water, soap, detergents, paper, rags and food remains. Crucially, it is rich in two things: organic matter and microbes - many of which are pathogenic (disease-causing).

1. What is sewage? (the textbook definition).
   • Sewage = municipal waste water from cities and towns.
   • A major component is human excreta (urine + faeces).
   • Other components: detergents, soaps, food scraps, kitchen oil, hair, paper, used water from baths and toilets, and (often) industrial effluents and storm-water runoff that join the sewer line.
   • Two main constituents matter biologically: large amounts of organic matter and large numbers of microbes.
2. Why sewage is dangerous if released untreated.
   • Many of the microbes in sewage are pathogens. They cause water-borne diseases like cholera, typhoid, dysentery, hepatitis A and E, polio, diarrhoea, gastroenteritis and parasitic infections like amoebiasis, ascariasis and giardiasis.
   • Drinking water contaminated with sewage is the main route by which these diseases spread, often in explosive outbreaks (especially after floods).
   • Sewage also contains parasitic worm eggs (round-worm, hook-worm), which can survive long periods in water and soil.
3. Environmental harm.
   • Untreated sewage discharged into rivers and lakes raises the BOD of the water (see Q11). Aerobic bacteria consume the dissolved oxygen, leading to fish kills.
   • Excess nitrogen and phosphate in sewage cause eutrophication - explosive algal blooms followed by anoxic conditions that destroy aquatic life.
   • Toxic chemicals (heavy metals, pesticides, drug residues) in untreated sewage accumulate up the food chain.
4. Public-health corollary. This is precisely why every city must run sewage treatment plants (STPs). The Ganga Action Plan and Yamuna Action Plan of the Indian Ministry of Environment and Forests were launched to add STP capacity for these heavily-polluted rivers.

Sewage is the municipal waste water of cities (mostly human excreta + organic matter + microbes). It is harmful because the pathogenic microbes in it cause water-borne diseases like cholera, typhoid, dysentery, hepatitis and polio, while its organic and nutrient load pollutes rivers, depletes dissolved oxygen, and kills aquatic life.

Concept used. Sewage treatment occurs in two main stages. The first stage is primary treatment - purely a physical process (filtration, sedimentation) that removes solid particles. The second stage is secondary treatment - a biological process in which aerobic microbes consume the dissolved organic matter, sharply reducing the BOD of the effluent.

1. Primary treatment - physical removal of solids.
   • Floating debris (plastic, rags, leaves) is removed by sequential filtration through screens.
   • Grit (soil, small pebbles, sand) is removed by sedimentation in grit chambers.
   • In the primary settling tank, all heavier solids that settle by gravity form the primary sludge.
   • The supernatant liquid is called the primary effluent, which is then sent for secondary treatment.
   • No microbial action is involved here; it is purely physical.
2. Secondary (biological) treatment - microbial breakdown of dissolved organic matter.
   • Primary effluent is fed into large aeration tanks.
   • Tanks are constantly agitated mechanically; air is pumped in.
   • Aerobic microbes grow vigorously and form flocs - masses of bacteria associated with fungal filaments forming mesh-like structures.
   • These flocs consume most of the dissolved organic matter in the effluent.
   • This sharply reduces the BOD of the water.
   • The treated water is then passed to a settling tank; the bacterial flocs sediment as activated sludge.
   • Part of the activated sludge is recycled as inoculum to the aeration tank; the rest is sent to anaerobic sludge digesters where biogas is produced.
3. Tabulated comparison.

   1.25 tabular|p2.8cm|p5.0cm|p5.0cm|

   Feature & Primary treatment & Secondary treatment
   Nature of process & Physical (filtration, sedimentation) & Biological (microbial degradation)
   Role of microbes & None & Central: aerobic microbes form flocs and consume organic matter
   What is removed & Large/heavy solids (debris, grit, sludge) & Dissolved and fine suspended organic matter
   Effect on BOD & Small reduction & Large reduction in BOD
   Output & Primary effluent and primary sludge & Treated effluent (low BOD), activated sludge
   Typical equipment & Screens, grit chambers, settling tanks & Aeration tanks, settling tanks, digesters
   tabular

4. One-line key difference. Primary treatment is physical (removes solids, no microbes involved). Secondary treatment is biological (uses aerobic microbes in flocs to consume the organic matter and drastically reduce the BOD).

Primary treatment is a physical process (filtration + sedimentation) that removes large/small solids. Secondary treatment is a biological process where aerobic microbes form flocs in aeration tanks and consume the dissolved organic matter, sharply reducing the BOD of the effluent.

Concept used. Yes. Certain anaerobic microbes called methanogens grow on cellulose-rich organic matter and release a mixture of gases - chiefly methane (CH4), with CO2, H2 and traces of H2S. This combustible mixture is called biogas. It is harvested in a biogas plant, also called a gobar-gas plant, and used as a clean fuel for cooking, lighting, and even running small engines.

1. Yes - microbes are a major source of renewable energy. The most important example is biogas, produced by methanogens from organic waste.
2. Microbe responsible. Methanogens are obligate anaerobic bacteria (e.g. Methanobacterium, Methanococcus, Methanosarcina). They live in three main environments:
   • the rumen (stomach) of cattle, where they help digest cellulose;
   • anaerobic sludge of sewage treatment plants;
   • anaerobic chambers of biogas plants (the topic of this question).
   Because cattle dung is naturally rich in these bacteria, the dung is the perfect starter for a biogas plant - hence the name ``gobar gas''.
3. Composition of biogas. Biogas is roughly:
   • 50–70% methane (CH4), the main fuel,
   • 25–45% carbon dioxide (CO2),
   • small amounts of H2, H2S, N2.
4. Construction of a biogas plant (see Fig 8.8 above).
   • A concrete tank, 10–15 feet deep, called the digester, is built underground.
   • A slurry of cattle dung + water is fed in through the dung inlet.
   • A floating metal cover (the gas-holder) sits over the slurry. As gas is produced, the dome rises.
   • An outlet pipe carries the biogas to nearby houses for cooking and lighting.
   • The spent slurry (rich in nitrogen and phosphorus) is pumped out through another outlet and used as fertiliser on fields.
5. The reactions. Under strictly anaerobic conditions:
   • Cellulose and other organic matter → organic acids + alcohols (by fermentative bacteria).
   • Organic acids/alcohols → acetate + H2 + CO2 (by acetogenic bacteria).
   • Acetate + H2 + CO2 → CH4 + CO2 (by methanogens). For example: CO2 + 4 H2 -> CH4 + 2 H2O CH3COOH -> CH4 + CO2
6. Other microbial energy routes.
   • Bioethanol - Saccharomyces cerevisiae ferments sugar/cellulose to ethanol used as transport fuel.
   • Hydrogen production - certain bacteria (e.g. Clostridium butyricum) and cyanobacteria evolve H2 during photosynthesis or anaerobic fermentation.
   • Microbial fuel cells - bacteria oxidise organic waste at an electrode, generating an electric current directly.
7. Indian context. Biogas technology was developed largely in India by Indian Agricultural Research Institute (IARI) and Khadi and Village Industries Commission (KVIC). Biogas plants are most useful in rural India, where cattle dung is plentiful, and they double up as fertiliser sources.

Yes. Methanogenic bacteria like Methanobacterium digest cellulose-rich organic matter (cattle dung, plant waste, sewage sludge) anaerobically and release biogas - a mixture of CH4, CO2, H2 - that is used as a clean fuel for cooking, lighting and small engines. Bioethanol, biohydrogen and microbial fuel cells are other microbial energy routes.

Concept used. Microbes can replace chemical inputs in two distinct ways:

• as biofertilisers - organisms that enrich the soil with nitrogen, phosphorus or organic matter (substitute for chemical fertilisers like urea, DAP);
• as biocontrol agents - organisms that kill or suppress pests, weeds or plant pathogens (substitute for chemical pesticides and weedicides).

1. Biofertilisers - substitutes for chemical fertilisers.
   • Symbiotic nitrogen-fixers. Rhizobium bacteria live in root nodules of leguminous plants (peas, beans, gram). They fix atmospheric N2 into ammonia/amino-form nitrogen, which the plant uses as nutrient. The plant in return supplies sugars to the bacteria - a mutualism.
   • Free-living nitrogen-fixers in soil. Azotobacter and Azospirillum live freely in soil and fix atmospheric N2, enriching soil nitrogen content.
   • Cyanobacteria (blue-green algae). Anabaena, Nostoc, Oscillatoria fix atmospheric nitrogen and also add organic matter. Especially important in paddy fields, where they serve as natural biofertilisers.
   • Mycorrhiza - fungal symbionts. Many fungi of the genus Glomus form symbiotic associations with plant roots (called mycorrhiza). The fungus absorbs phosphorus from soil and passes it to the plant. The plant gains: itemize
   • better phosphorus uptake,
   • resistance to root-borne pathogens,
   • tolerance to salinity and drought,
   • overall increase in growth.
   itemize
2. Biocontrol agents - substitutes for chemical pesticides.
   • Bacillus thuringiensis (Bt) - produces a toxin that kills butterfly caterpillars but is harmless to other insects, mammals and birds. Sold as dried spores in sachets; sprayed on brassicas and fruit trees.
   • Trichoderma (fungus) - free-living root-zone fungus that is an effective biocontrol against several soil-borne plant pathogens.
   • Baculoviruses (genus Nucleopolyhedrovirus) - viruses that infect only specific insect pests; no harm to plants, mammals, birds, fish or beneficial insects. Ideal for species-specific pest control.
   • Ladybird beetles and dragonflies - though not microbes, are classic biocontrol agents against aphids and mosquitoes respectively.
3. Combined outcome.
   • Less use of urea, superphosphate, DAP ⇒ less pollution of soil and groundwater.
   • Less use of toxic pesticides (DDT, organophosphates) ⇒ less harm to non-target organisms and to humans.
   • Soil structure and biodiversity are preserved (a key tenet of organic farming).

Microbes act as biofertilisers - Rhizobium, Azotobacter, Azospirillum, cyanobacteria (Anabaena, Nostoc) and mycorrhizal fungi (Glomus) enrich soil with nitrogen, phosphorus and organic matter - and as biocontrol agents - Bacillus thuringiensis, Trichoderma and baculoviruses kill pests and plant pathogens - thereby cutting our dependence on chemical fertilisers and pesticides.

Concept used. BOD (Biochemical Oxygen Demand) is the amount of oxygen (in mg per litre of water) that would be consumed if all the organic matter in one litre of water were oxidised by bacteria. BOD is therefore a measure of the organic-matter load in the water: higher BOD ⇒ more organic pollution ⇒ more polluted water.

1. Match each BOD value to the corresponding water type.
   • Sample B: 8 mg/L ⇒ very low BOD ⇒ clean water ⇒ river water.
   • Sample A: 20 mg/L ⇒ moderate BOD ⇒ partly cleaned ⇒ secondary effluent from the STP.
   • Sample C: 400 mg/L ⇒ very high BOD ⇒ heavy organic load ⇒ untreated sewage.
2. Which sample is most polluted? Sample C (BOD = 400 mg/L) is the most polluted. Such a heavy organic load would, if discharged into a water body, allow bacteria to consume nearly all the dissolved O2 - killing fish and other aerobic aquatic life.
3. Reasoning in one line. BOD ordering: C (400) ≫ A (20) > B (8). So the pollution ordering is: untreated sewage ≫ secondary effluent > river water - which is exactly what we expect physically.
4. Tabulated answer.
   1.2 tabular|c|c|l|

   Sample & BOD (mg/L) & Identity
   A & 20 & Secondary effluent (after STP)
   B & 8 & River water (clean)
   C & 400 & Untreated sewage (most polluted)
   tabular

A = secondary effluent (20 mg/L); B = river water (8 mg/L, the cleanest); C = untreated sewage (400 mg/L, the most polluted).

Concept used. Microbes synthesise many bioactive molecules that we use as drugs. Two important examples from this chapter are cyclosporin A (an immunosuppressant used during organ transplants) and statins (cholesterol-lowering agents). Both are produced by specific fungi.

1. Cyclosporin A is obtained from Trichoderma polysporum, a fungus.
   • Use: it is used as an immunosuppressive agent in organ-transplant patients. By suppressing the recipient's immune response, it prevents the body from rejecting the transplanted organ.
   • Mechanism: cyclosporin A binds the protein cyclophilin and inhibits calcineurin, blocking T-lymphocyte activation.
   • Discovery: extracted in 1971; revolutionised organ transplantation (kidney, liver, heart).
2. Statins are obtained from Monascus purpureus, a yeast (a fungus too, but reproduces by budding like a yeast).
   • Use: statins are used as blood-cholesterol lowering agents. They reduce LDL (∼``bad'' cholesterol) and lower the risk of heart attacks and strokes.
   • Mechanism: statins competitively inhibit the enzyme HMG-CoA reductase, which is the rate-limiting step in cholesterol biosynthesis. Less enzyme activity ⇒ less cholesterol made by the liver ⇒ lower blood cholesterol.
   • Modern commercial statins (atorvastatin, simvastatin) are chemically modified versions of the natural Monascus product.
3. One-line summary.
   • Cyclosporin A ← Trichoderma polysporum (fungus).
   • Statins ← Monascus purpureus (yeast).

Cyclosporin A is obtained from the fungus Trichoderma polysporum (used as immunosuppressant during organ transplants). Statins are obtained from the yeast Monascus purpureus (used to lower blood cholesterol).

Concept used. Microbes are not only producers of food and medicine - they can also be food (Single Cell Protein) and they actively maintain the structure and fertility of soil. This question asks us to explain both roles.

1. (a) Role of microbes as Single Cell Protein (SCP).
   • Definition. Single Cell Protein refers to dried cells of microbes (algae, fungi, bacteria, yeasts) used as protein-rich human or animal food. The cells themselves are the food.
   • Why it matters. Global population growth (∼8 billion and rising) means conventional agriculture-and-livestock cannot meet protein demand sustainably. SCP grows very fast, on cheap substrates, and yields high-quality protein per unit area.
   • Common SCP microbes. itemize
   • Spirulina (a cyanobacterium) - grown on materials like sewage water and molasses; rich in protein, vitamins and minerals.
   • Methylophilus methylotrophus (a bacterium) - grown on methanol; produces tonnes of biomass per day.
   • Saccharomyces cerevisiae (yeast) - used as animal feed protein.
   • Fusarium graminearum (mycoprotein) - the source of ``Quorn'', a mycoprotein-based meat substitute.
2. Key advantages.
   • Microbes can double their mass in hours; cattle take years.
   • Grow on industrial waste (cellulose, methanol, molasses) - turns waste into protein.
   • Rich in essential amino acids, vitamins, minerals.
   • Smaller land and water footprint than meat or pulses.
3. NCERT example to remember. 250 kg of a cow produces ∼200 g of protein per day; 250 kg of Methylophilus methylotrophus bacteria can produce 25 tonnes of protein per day. The difference is staggering. itemize
4. (b) Role of microbes in soil.
   • Decomposition of organic matter. Saprophytic bacteria and fungi decompose dead plants, animals and animal excreta. The breakdown releases CO2 to the atmosphere and humus into the soil, improving soil structure and fertility.
   • Nitrogen fixation. itemize
   • Symbiotic: Rhizobium in root nodules of legumes.
   • Free-living: Azotobacter, Azospirillum.
   • Cyanobacterial: Anabaena, Nostoc, Oscillatoria.
5. Phosphorus solubilisation. Mycorrhizal fungi (Glomus species) form symbiotic associations with plant roots, mobilising soil phosphorus to the plant.
6. Sulphur and other mineral cycling. Microbes drive the nitrogen, sulphur and phosphorus cycles, recycling elements through ecosystems.
7. Disease suppression. Trichoderma and other antagonistic microbes outcompete root pathogens - improving plant health without chemicals.
8. Soil structure. Fungal hyphae and bacterial polymers bind soil particles into stable aggregates, improving aeration and water retention.
9. Biocontrol in the rhizosphere. Bacillus thuringiensis and similar microbes suppress harmful insects and worms. itemize
10. Putting both together. Microbes provide food directly (SCP) and indirectly (soil fertility supporting all crop growth). A handful of healthy soil contains more microbial cells than there are humans on earth.

(a) SCP = microbial biomass (Spirulina, Methylophilus methylotrophus, Saccharomyces, Fusarium graminearum) eaten as protein-rich food. (b) In soil, microbes decompose organic matter into humus, fix nitrogen (Rhizobium, Azotobacter, cyanobacteria), solubilise phosphorus (mycorrhiza/Glomus), suppress disease (Trichoderma), and recycle nutrients - keeping soil fertile.

Concept used. ``Most important to human welfare'' is judged by (i) the breadth of human need addressed (health, energy, food), (ii) the number of lives saved or quality-of-life improvements delivered, and (iii) how irreplaceable the product is. Health (life-and-death) is generally placed above food (daily quality of life), and food above industrial chemicals.

1. Step 1 - Penicillin (most important).
   • It was the first antibiotic, isolated from the fungus Penicillium notatum.
   • Saved millions of lives during World War II and ever since by curing deadly bacterial infections (plague, pneumonia, diphtheria, leprosy, septicaemia).
   • Opened the entire era of antibiotic therapy - the single biggest jump in human life expectancy in the 20th century.
   • Directly saves lives - health trumps everything else.
2. Step 2 - Biogas (second).
   • Produced by anaerobic methanogens on dung/sewage; mostly CH4.
   • Provides clean cooking and lighting fuel to millions of rural households.
   • Solves an energy problem (fuel for cooking) AND a waste problem (cattle dung disposal) AND a fertiliser problem (spent slurry).
   • Indispensable in rural India, China and much of the developing world.
   • Health + environment angle: cleaner indoor cooking ⇒ fewer lung diseases; replaces firewood/kerosene.
3. Step 3 - Curd (third).
   • Made by lactic acid bacteria from milk.
   • A daily food for billions, especially in South Asia. Improves nutritional value of milk (more B₁₂). Acts as a probiotic, suppressing gut pathogens.
   • Important for daily nutrition but not directly life-saving the way antibiotics are.
4. Step 4 - Citric acid (least, in this list).
   • Produced by Aspergillus niger; used in foods, beverages, pharmaceuticals, detergents and as a preservative.
   • Useful and widespread, but it is mostly a flavouring/processing additive. People could live healthy lives without it.
   • No direct life-saving function; the most ``replaceable'' item in this list.
5. Final ordering. Penicillin > Biogas > Curd > Citric acid.

Penicillin > Biogas > Curd > Citric acid. Penicillin saves lives (antibiotic). Biogas powers and cleans up rural living. Curd is a daily nutrient and probiotic. Citric acid is a useful industrial additive but the most replaceable.

Concept used. Biofertilisers are living organisms (mostly bacteria, cyanobacteria and fungi) that enrich the nutrient quality of soil. They achieve this by (i) fixing atmospheric nitrogen, (ii) solubilising/mobilising phosphorus, and (iii) adding organic matter to soil. Together they reduce the need for chemical fertilisers and support sustainable agriculture.

1. Symbiotic nitrogen fixation - Rhizobium.
   • Rhizobium bacteria live inside root nodules of leguminous plants (peas, beans, gram, soybean).
   • Inside the nodule, they fix atmospheric N2 to organic nitrogen (NH3 → amino acids), which the plant uses as nutrient.
   • In return, the plant supplies sugars and a protected environment.
   • Net result: when legume roots decompose at the end of a season, the soil is enriched with nitrogen - which the next non-leguminous crop can use. This is why farmers rotate legumes with grain crops.
2. Free-living nitrogen fixers in soil.
   • Azotobacter (aerobic) and Azospirillum (microaerophilic) fix atmospheric N2 while living freely in soil.
   • They do not need a plant host, so they enrich soil nitrogen everywhere they grow.
3. Cyanobacteria - the paddy-field biofertilisers.
   • Autotrophic, photosynthetic microbes (often called blue-green algae).
   • Many fix atmospheric nitrogen, e.g. Anabaena, Nostoc, Oscillatoria.
   • In paddy fields they are major biofertilisers, supplying ∼25–30 kg N/hectare/season.
   • They also add organic matter to soil when they die and decompose - increasing humus and soil fertility.
4. Mycorrhiza - fungal biofertilisers.
   • Many fungi of the genus Glomus form symbiotic associations with plant roots, called mycorrhiza.
   • The fungus absorbs phosphorus from soil and passes it to the plant.
   • Other benefits to the plant: resistance to root-borne pathogens, tolerance to salinity and drought, overall increase in growth.
   • The fungus, in return, receives sugars from the plant.
5. Summary of how biofertilisers enrich soil.
   • Add nitrogen to soil (Rhizobium, Azotobacter, Azospirillum, Anabaena, Nostoc).
   • Supply phosphorus to plant roots (Glomus mycorrhiza).
   • Add organic matter / humus (cyanobacteria, decomposers).
   • Improve soil structure and water retention (microbial polymers, fungal hyphae).
   • Suppress soil-borne diseases (Trichoderma, antagonistic microbes).

Biofertilisers enrich soil by (i) fixing atmospheric N2 - Rhizobium (symbiotic in legume nodules), Azotobacter/Azospirillum (free-living), and cyanobacteria Anabaena/Nostoc (paddy fields), (ii) mobilising phosphorus to plants via mycorrhizal fungi (Glomus), and (iii) adding organic matter that improves soil structure, fertility and disease resistance.