Microbes in Human Welfare — Class 12 Biology Notes & Practice

Let us start somewhere that has nothing to do with a textbook. It is an ordinary school morning. You wake up, and there is a bowl of curd on the table. The bread in your tiffin is soft and full of little holes. In the medicine cabinet there is a half–finished strip of antibiotic tablets from last month’s throat infection. Outside your gate, a drain carries yesterday’s waste water away. In a village kitchen a few hundred kilometres away, a blue flame is burning on a stove that runs on cattle dung. And in a field somewhere, a paddy crop is quietly being fed nitrogen it never paid for.

Every single one of those things happened because of a microbe. Not one of them is visible. That is the whole chapter, and that is also the promise I want to make to you at the start: Microbes in Human Welfare is not a chapter you have to memorise from nowhere. It is a chapter you can rebuild from your own morning, if somebody shows you where to look.

I will be honest with you about why students find this chapter hard, because pretending otherwise helps nobody. It is almost entirely a naming chapter. There is very little to derive and almost nothing to calculate. What there is instead is a long list of organisms paired with a long list of products, and in the exam you either produce the right genus or you do not. A student who writes “Lactobacillus” for bread instead of curd loses the whole mark, no matter how beautifully the rest of the answer is written. So the danger here is not difficulty. The danger is blur — names sliding into each other the night before the paper.

So we are going to do something different. Instead of learning a list, we are going to walk through your day and hang each microbe on the place it actually shows up. Curd at breakfast. Bread in the tiffin. Tablets in the cabinet. The drain outside. The gas stove. The crop in the field. Every section below comes back to that walk, on purpose, so that in the exam hall you are not fishing in an empty list — you are simply remembering where in your day the answer lives. Along the way we will build one master microbes and their products table class 12 students can revise from in five minutes, a mnemonic that chains microbe to product, a separate mnemonic for the sewage sequence, and a full set of microbes in human welfare class 12 important questions with model answers at the end.

Take a breath. This is one of the friendliest scoring chapters in Unit VIII if you organise it once, properly. Let us organise it.

What You’ll Learn

Here is the whole chapter on one screen. Every link below jumps straight to that section, so if you are revising at 11 p.m. and only need the sewage treatment sequence, go straight there and come back later.

Your Game Plan

If you follow this order, the chapter takes about three focused sittings. Do not try to swallow it in one.

1. Read the day–walk once, slowly. Do not memorise anything yet. Just notice where each microbe sits in an ordinary day.
2. Do the household and industrial sections together. They are pure naming. Say each name out loud with its product — out loud matters more than you think.
3. Take sewage and biogas as one block. They share the same anaerobic digester, so learning them together saves you half the work.
4. Take biocontrol and biofertilisers as one block. Both are “microbes helping the farmer”; one kills pests, one feeds plants.
5. Write the master table from memory. Blank sheet, no peeking. Whatever you miss is exactly what you should revise tomorrow.
6. Then do the worksheet at the end and mark yourself honestly against the model answers.

Study Notes

Where Microbes Live — And Why They Matter To Us

Before we name a single organism, let us fix what we are even talking about. Microbes are organisms too small to be seen with the naked eye. That is a size definition, not a family definition — which is why the group is such a mixed crowd. It includes bacteria, fungi (moulds and yeasts), protozoa, microscopic algae, and also viruses, viroids and prions, which are not fully “alive” in the ordinary sense but are studied alongside microbes because of what they do to us.

Now, where do they live? The honest answer is: almost everywhere you can imagine, and several places you cannot. They are in soil and in water. They float in the air. They live several metres deep inside soil, and under thick layers of snow. They survive in hot springs at temperatures that would cook you, and in water so acidic that it would strip metal. They live on your skin, in your mouth, and in enormous numbers inside your gut — and inside the guts of cattle, which is a detail that will come back and become worth five marks when we reach biogas.

Here is the mental shift I want you to make. Most students meet microbes first as villains — the things that cause typhoid, malaria, dengue. That is the previous chapter’s job. This chapter is the other half of the story, and it is the bigger half. Microbes are also the reason your curd sets, your bread rises, your infections are curable, your city’s sewage does not poison its river, and your farmer’s field stays fertile. If you have already studied how decomposers keep nutrients cycling through an ecosystem in Class 10, you have already met this idea once; here we are simply watching humans put those same decomposers deliberately to work.

That four–box structure is worth seeing as a picture before you meet the names one by one. Study the tree below for a minute now, and then come back to it after you finish the chapter and try to rebuild it on a blank page.

Why this works. Two–mark questions in this chapter are almost always “give me two clean facts”. The examiner is not looking for a paragraph. Give the two facts, label them (i) and (ii), and stop. Over–writing on a 2–marker is one of the commonest ways students run out of time on a 70–mark paper.

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Microbes In Household Products

Back to your breakfast table. This is the section where the chapter stops feeling abstract, because you have eaten every example in it.

The curd. Last night somebody added a spoon of old curd to warm milk and left it covered. That spoon was not magic; it was an inoculum, a starting population of bacteria called lactic acid bacteria (LAB), of which the one you must name is Lactobacillus. These bacteria multiply in the milk and convert milk sugar into lactic acid. The acid does two things at once: it curdles the milk proteins so the milk sets, and it gives curd its slight sourness. There is a bonus that CBSE loves to ask about — LAB also improve the nutritional quality of the milk by increasing vitamin B₁₂. And in your own stomach, LAB play a useful role in checking disease–causing microbes.

Why this works. Notice that the question gave you a scene, not a term. Half the marks in this chapter are hidden inside little scenes like that: “dough left overnight puffs up”, “holes appear in a cheese”, “a mould killed the bacteria around it on a plate”. Train yourself to read the scene and immediately ask, “which of my four boxes is this, and which name lives there?”

The bread in your tiffin. Look at a slice of bread against the light. It is full of tiny holes, and it is soft precisely because of them. Those holes were bubbles of carbon dioxide. The dough for bread — and for dosa and idli batter too — is fermented by Saccharomyces cerevisiae, commonly called baker’s yeast. The yeast breaks down sugars in the dough without needing oxygen, and releases CO₂ gas. The gas cannot escape from the sticky dough, so it gets trapped as bubbles, and the dough puffs up — that is the word CBSE uses, and it is worth using it back. When the dough is baked, the yeast is killed and the bubbles are locked in as the holes you can see.

Toddy. A traditional drink of southern India, made by fermenting the sap of palms. It is on your syllabus purely as an example of a traditional fermented product, and one line is all it needs.

Cheese — and the two you must be able to tell apart. Cheese is one of the oldest foods humans have made with microbes, and different microbes give different cheeses their different characters. Two are named in your syllabus and they are asked constantly, usually together.

A word on spelling, because it matters here. Propionibacterium sharmanii is the form used in your NCERT text and it is the form to write in the exam; you will sometimes see it written elsewhere as P. shermanii. Either way, the genus Propionibacterium is the mark–bearing part. And notice that Penicillium roqueforti is not the same species as the Penicillium that gives us penicillin — same genus, completely different job. We will come to that in a moment.

Sketch practice: Draw Lactobacillus as rod-shaped bacterial cells beside budding Saccharomyces cerevisiae cells. Label the shapes clearly and write the food product associated with each microbe.

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Microbes In Industrial Products: Fermented Beverages

We now step out of your kitchen and into a factory. Before we start, one honest sentence, because you are old enough to be told the truth plainly rather than have it hidden: this topic is on your syllabus as biochemistry and microbiology, not as a recommendation. Alcohol carries real health risks, its consumption by minors is prohibited by law in India, and nothing here is a method to be tried. What the examiner wants from you is the microbe, the chemistry, and one clean distinction. That is exactly what we will do.

The microbe is the same yeast you already met in your tiffin: Saccharomyces cerevisiae, which in this industrial context is called brewer’s yeast. Remember that: one organism, two everyday names, depending on which job it is doing. In the bakery it is baker’s yeast; in the brewery it is brewer’s yeast. Same species.

The chemistry. The yeast is supplied with a sugar source — malted cereals (grain in which the starch has been converted to sugar) or fruit juices. In the absence of oxygen it carries out fermentation, breaking down the sugar into ethanol and carbon dioxide and releasing a small amount of energy. This is the same anaerobic breakdown pathway you first met years ago; if the logic of respiration without oxygen feels rusty, it is worth spending ten minutes with the anaerobic respiration section of the Class 10 Life Processes notes before going further, because the whole of the biogas section later rests on the same idea.

Why this works. Comparison questions are scored on the axis of comparison, not on the volume of writing. Here the axis is a single word — distillation. Once you have identified the axis, both halves of the answer write themselves. Look for the axis first in every “differentiate between” question in the paper.

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Microbes In Industrial Products: Antibiotics

Now open the medicine cabinet in your house. Somewhere in it there is a strip of tablets prescribed the last time somebody at home had a bad throat or a wound that would not settle. That strip exists because of an accident on a laboratory bench in London in 1928, and it is one of the genuinely great stories in biology. It deserves better than being reduced to three names you cram at midnight, so let us tell it properly — you will remember it far better that way.

The word itself is a good starting point. “Antibiotic” comes from “anti” (against) and “bios” (life) — literally, against life. An antibiotic is a chemical substance produced by one microbe that in low concentrations kills or retards the growth of other microbes. That definition, exactly as written, is a one–mark answer that comes up regularly.

The story. Alexander Fleming was working with Staphylococci bacteria. He had left a culture plate unwashed, and when he came back to it he found a mould growing on it — and, around that mould, a clear ring where no bacteria were growing. Something the mould was releasing was killing them. The mould was Penicillium notatum, and Fleming named the chemical after it: penicillin. This was the first antibiotic to be discovered.

But Fleming did not know how powerful he was holding. It was Ernest Chain and Howard Florey who did the work that established penicillin’s full potential as an effective antibiotic and made it usable as a real medicine. Its first great test was the Second World War, where it was used to treat wounded soldiers. All three men — Fleming, Chain and Florey — shared the Nobel Prize in 1945.

One more name, and it is the one students forget. The species used for the industrial, large–scale production of penicillin is Penicillium chrysogenum. So: P. notatum is the discovery story, P. chrysogenum is the factory.

What changed because of it. This is not decoration; it is a favourite two–mark question. Antibiotics dramatically improved our capacity to treat deadly diseases such as plague, whooping cough (kali khansi), diphtheria (gal ghotu) and leprosy, which had killed millions of people. Before antibiotics, a scratch that turned septic could genuinely kill you. That is the scale of the change.

Sketch practice: Draw Fleming’s culture plate with a Penicillium colony and a clear zone around it in the surrounding Staphylococcus lawn. Label the inhibition zone and connect it to antibiotic action.

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Microbes In Industrial Products: Chemicals, Enzymes And Bioactive Molecules

Stay in the medicine cabinet for a moment, then look around the rest of the house. The lemony tang in the packet of soft drink powder. The vinegar bottle in the kitchen. The detergent that lifts an oil stain your grandmother’s soap never could. The clear, un–cloudy bottled juice. Behind every one of them is a microbe in a fermenter somewhere, and this section is where the largest cluster of names in the chapter lives. Take it slowly.

Part one: the organic acids. Four acids, four microbes. There is a beautiful shortcut hiding here that almost nobody points out to students, and once you see it you will never lose these marks again.

Part two: the enzymes. Microbes are also grown industrially simply to harvest the enzymes they make. Three groups are named on your syllabus, and each one is tied to a very concrete use — learn the use and the name follows.

• Lipases — used in detergent formulations, where they help in removing oily stains from laundry. (Lipid → lipase → oil. The name tells you the job.)
• Pectinases and proteases — used to clarify bottled fruit juices. Freshly squeezed juice is cloudy because of suspended pectin and protein; these enzymes break them down and the juice turns clear.
• Streptokinase — produced by the bacterium Streptococcus and modified by genetic engineering, it is used as a “clot buster” for removing clots from the blood vessels of patients who have had a myocardial infarction (heart attack).

Part three: the bioactive molecules. These three are pure medicine, and they are the most valuable names in the section because a full mark hangs on each. The trick is to hold each one by the patient it helps rather than by its chemistry.

Why this works. Notice what we did across this whole section: we never learned a list. We learned a lemon (citric acid), a vinegar bottle (acetic acid), a laundry stain (lipase), a cloudy juice (pectinase) and three patients in a corridor. Objects and people stick in memory; lists do not. When you revise, revise the objects.

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Microbes In Sewage Treatment — Primary And Secondary Treatment

Step outside your gate now. There is a drain there, and everything that leaves your bathroom and kitchen goes into it. Multiply your house by a few million and you have the problem every city faces: an enormous volume of municipal waste water, called sewage, which contains large amounts of organic matter and huge numbers of microbes, many of which are pathogenic.

You cannot simply pour that into a river. So it is taken to sewage treatment plants (STPs), and here is the part that should genuinely impress you: the plant does not use chemicals to clean the water. It uses microbes to clean up the mess made by microbes. It is the same principle you saw with decomposers in an ecosystem, only engineered, at city scale.

This is the most heavily weighted single topic in the chapter and it is where five–mark questions live. It is also the only part with a genuine sequence, which is why students scramble it. So before we walk through it in words, look at the whole thing as one picture — and note especially the bars along the bottom, which are the story of the BOD.

Primary treatment: physical, not biological. This first stage involves the physical removal of particles — large and small — from the sewage. It is done in two steps. First, floating debris is removed by sequential filtration. Then grit — soil and small pebbles — is removed by sedimentation, that is, by letting it settle out under gravity.

Two new words come out of this stage and you must be able to use them precisely. All the solids that settle at the bottom form the primary sludge. The liquid that is left floating above it is the supernatant, and this is called the primary effluent. It is this primary effluent, and not the sludge, that is taken forward for secondary treatment.

Secondary (biological) treatment: where the microbes go to work. The primary effluent is passed into large aeration tanks. There, two things happen at once: the liquid is constantly agitated mechanically, and air is pumped into it. Why? Because we are about to grow aerobic microbes on purpose, and aerobic microbes need oxygen. Pumping air is not an incidental detail — it is the whole design.

Given all that oxygen and all that organic matter to eat, useful aerobic microbes grow vigorously and form flocs. A floc is a lovely thing to picture: masses of bacteria associated with fungal filaments (mycelia), forming mesh–like structures. Think of a floating net of fungal threads with bacteria packed all over it. And while these flocs grow, they consume the major part of the organic matter in the effluent.

That consumption is the entire point, because it significantly reduces the BOD of the effluent. Which brings us to the term that decides more marks in this chapter than any other.

Let me make sure that definition is not just words for you. Imagine you drop a spoon of sugar into a bottle of river water teeming with bacteria and seal it. The bacteria feast on the sugar, and to do that they consume dissolved oxygen. Measure how much oxygen disappeared and you have measured, indirectly, how much food was in there. Now imagine that instead of sugar it is a city’s untreated sewage flowing into a river. The bacteria in the river consume its organic matter, stripping the oxygen out of the water as they go — and the fish, which need that same dissolved oxygen, suffocate. High BOD is dangerous not because the number is large but because the organic matter behind it steals oxygen from everything else living in the water.

Why this works. Notice that you did not need to remember a single figure to answer that. You needed one principle — BOD falls as treatment proceeds — and the arithmetic followed. Whenever numbers appear in this chapter, ask what principle orders them, and let the principle do the work.

Settling, and the fate of the sludge. Once the BOD has been reduced significantly, the effluent is passed into a settling tank, where the bacterial flocs are allowed to sediment. This sediment has a name of its own: it is the activated sludge.

Now the activated sludge is split, and this split is a classic exam point. A small part of it is pumped back into the aeration tank to serve as the inoculum — a starter culture, exactly like the spoon of old curd in your milk. It is the same idea, at a scale of millions of litres. The remaining major part is pumped into large anaerobic sludge digesters.

Inside those digesters there is no oxygen, so a different crowd takes over: other kinds of bacteria, which grow anaerobically, digest the bacteria and the fungi in the sludge. During this digestion they produce a mixture of gases — methane, hydrogen sulphide and carbon dioxide. That mixture is biogas, and because it is inflammable it can be used as a source of energy. So the plant does not just clean water; it hands you back fuel. Hold that thought — the next section is entirely about it.

Finally, the effluent from the secondary treatment plant is generally released into natural water bodies such as rivers and streams. In India, this is not a theoretical concern: the Ministry of Environment and Forests initiated the Ganga Action Plan and the Yamuna Action Plan to save our major rivers by building a large number of sewage treatment plants.

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Microbes In Production Of Biogas

Now travel from your street to a village kitchen. There is a blue flame burning on the stove, and nobody bought a cylinder for it. The fuel came from the cattle standing in the yard. This is the section where the chapter stops being about cleanliness and starts being about energy — and it connects straight back to the digester you just left.

Biogas is a mixture of gases, predominantly methane, produced by microbial activity, and it is used as a fuel. Learn that sentence; it is a clean one–mark definition.

Who makes it. Certain bacteria grow anaerobically on cellulosic material and, in doing so, produce large amounts of methane along with carbon dioxide and hydrogen. Because methane is the headline product, these bacteria are collectively called methanogens, and the one you must be able to name is Methanobacterium.

Where they already live — and this is the elegant bit. You have already met methanogens once, in the anaerobic sludge digesters of the sewage plant. But they are also found somewhere much closer to a farmer: in the rumen of cattle. A cow eats grass and straw, which is largely cellulose, and a cow cannot digest cellulose on its own. The methanogens in its rumen help in the breakdown of cellulose and play an important role in the nutrition of cattle.

Follow that through to its conclusion and the whole technology falls out of it. If the rumen is full of methanogens, then the excreta of cattle — the dung, commonly called gobar — is rich in these bacteria. So you do not need to buy or culture a microbe at all. The dung arrives with its own workforce already inside it. That is precisely why dung can be used for the generation of biogas, commonly called gobar gas.

How the plant is built. A biogas plant consists of a concrete tank, about 10–15 feet deep, into which bio–wastes and a slurry of dung are fed. Over the slurry sits a floating cover, which keeps on rising as the gas is produced in the tank by microbial activity — that rising dome is how you can tell from outside that the plant is working. The plant has an outlet connected to a pipe, which supplies the biogas to nearby houses. The spent slurry is removed through another outlet and can be used as fertiliser. Nothing is wasted: gas out of the top, manure out of the bottom.

Who developed it in India. The technology of biogas production was developed in India mainly due to the efforts of the Indian Agricultural Research Institute (IARI) and the Khadi and Village Industries Commission (KVIC). Those two names are worth a mark on their own and students routinely leave them out.

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Microbes As Biocontrol Agents

Last stop on our walk: the field where your food is grown. And here we have to start with a question of attitude rather than of fact, because CBSE genuinely asks about it.

Biological control is the use of biological methods for controlling plant diseases and pests. It stands opposed to the habit of drenching a field in chemical insecticides and weedicides, which are toxic, harm the people who apply them and the people who eat the crop, and kill a great deal more than the pest. The organic farmer works from a different starting assumption: not that every insect must be eradicated, but that a field is a web of interlinked organisms in which pests and predators keep each other in check. The aim is control, not eradication — keeping the pest population at a manageable level rather than sterilising the land. That single sentence is often the whole answer to “what is the approach of an organic farmer to pest control?”

The animals first, because they are the easiest marks in the chapter. Two are named and they are asked as a pair:

• Ladybird — beetles with red and black markings — are useful for getting rid of aphids.
• Dragonflies are useful for getting rid of mosquitoes.

Now the microbes. There are exactly three, and here is the structure that makes them impossible to forget: one is a bacterium, one is a fungus, one is a virus. One from each level. If you can remember “bacterium, fungus, virus”, you will never be stuck for the third.

The bacterium: Bacillus thuringiensis, universally shortened to Bt. This is the most detailed mechanism in the section, so read it slowly. Dried spores of Bacillus thuringiensis are available in sachets. The spores are mixed with water and sprayed onto vulnerable plants such as brassicas and fruit trees, where they are eaten by the insect larvae (caterpillars). In the gut of the larvae the toxin is released, and the larvae are killed.

But why does it kill the caterpillar and not the plant, or you? Because of a beautiful safety catch built into the chemistry. The toxic insecticidal protein exists in the bacterium as inactive protoxin crystals. It only becomes dangerous once ingested by the insect, because the alkaline pH of the insect gut solubilises the crystals and converts the protoxin into the active form of the toxin. Your own stomach is strongly acidic, not alkaline, so the crystals never activate. The insect’s own gut is what arms the weapon. That is why Bt is a biocontrol agent and not a poison sprayed over everything.

You will meet Bacillus thuringiensis again very soon, because the Bt toxin gene has been isolated and introduced into plants such as Bt cotton, making the plant itself resistant to insect attack. Understanding how that gene is cut, carried and expressed is the job of the biotechnology unit, and it builds directly on how a gene is actually read and copied inside a cell. Once the gene is in the plant, the resistance it confers is inherited by the next generation of plants in exactly the way a trait passes from one generation to the next.

The fungus: Trichoderma. Species of Trichoderma are free–living fungi that are very common in the root ecosystems, and they are effective biocontrol agents of several plant pathogens. One line, and remember it is a fungus and it works in the soil around roots.

The virus: Baculoviruses. Baculoviruses are pathogens that attack insects and other arthropods. The majority of those used as biological control agents belong to the genus Nucleopolyhedrovirus. What makes them special is that they are species–specific, with narrow–spectrum insecticidal applications. They have no negative impact on plants, mammals, birds, fish or even on non–target insects. That combination makes them especially desirable in Integrated Pest Management (IPM) programmes, and particularly useful when beneficial insects are being conserved to help maintain a healthy ecosystem.

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Microbes As Biofertilisers

We are still in the field, but the question has changed. Biocontrol was about protecting the crop. Biofertilisers are about feeding it — and feeding it without the chemical fertiliser bags that cost the farmer money and, over years, damage the soil and run off into the water table.

Biofertilisers are organisms that enrich the nutrient quality of the soil. The main sources are bacteria, fungi and cyanobacteria — three sources, and each has its own named examples. Get the three sources straight first, and the names slot in underneath.

Source one: bacteria that fix nitrogen. The air above every field is about 78% nitrogen, and not one plant can use a molecule of it directly. Nitrogen fixation is the conversion of that free atmospheric nitrogen into forms a plant can absorb, and certain bacteria do it for free. They split into two groups by where they live.

• Rhizobium — symbiotic. It forms a symbiotic association with the root nodules of leguminous plants (peas, gram, beans, groundnut) and fixes atmospheric nitrogen into organic forms that the plant uses as nutrient. The plant gives it shelter and food; it gives the plant nitrogen. That is the bargain.
• Azospirillum and Azotobacter — free–living. These live free in the soil, need no host, and also fix atmospheric nitrogen, thereby enriching the nitrogen content of the soil.

Source two: fungi — the mycorrhiza. Many fungi form a symbiotic association with the roots of higher plants, and such associations are called mycorrhiza. The genus you must name is Glomus, many members of which form mycorrhiza.

Notice the difference in what is being delivered, because this is a favourite one–mark trap. Rhizobium brings nitrogen. In a mycorrhiza, the fungal symbiont absorbs phosphorus from the soil and passes it on to the plant. Different nutrient, different partner. And the benefits do not stop at phosphorus — plants having such associations also show resistance to root–borne pathogens, tolerance to salinity and drought, and an overall increase in plant growth and development. Those four extra benefits are frequently worth two marks on their own.

Source three: cyanobacteria. These are autotrophic microbes widely distributed in aquatic and terrestrial environments, and many of them can fix atmospheric nitrogen. The three names on your syllabus are Anabaena, Nostoc and Oscillatoria.

Their star turn is in the paddy field. A rice field is flooded, which makes it a shallow aquatic habitat — ideal for cyanobacteria. In paddy fields, cyanobacteria serve as an important biofertiliser. Beyond fixing nitrogen, blue–green algae also add organic matter to the soil and increase its fertility.

And there is one partnership so useful that Indian farmers have used it for generations: Azolla, a small floating water fern, harbours the cyanobacterium Anabaena azollae in the cavities of its leaves. Grow Azolla on the surface of a flooded paddy field and the Anabaena inside it fixes nitrogen continuously; when the fern dies and decomposes, that nitrogen is released into the soil for the rice. A free fertiliser factory floating on the water. Whether the crop then goes on to set good grain depends on the flowering and fertilisation machinery you study in sexual reproduction in flowering plants — nutrition and reproduction are the two halves of a good harvest.

Sketch practice: Draw a longitudinal section of a leguminous root nodule and label the region containing Rhizobium bacteroids. Add one note linking the association with biological nitrogen fixation.

Sketch practice: Draw a plant root associated with Glomus and extend fungal hyphae into the surrounding soil. Label the increased absorption area and note the phosphorus benefit.

Sketch practice: Draw floating Azolla and show Anabaena azollae inside a leaf cavity. Label both partners and connect the association with nitrogen enrichment in paddy fields.

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Microbes And Their Products Table Class 12 — The One Table That Wins Marks

Everything above now collapses into this. If you learn nothing else from this page, learn this table — and learn it by writing it, not by reading it. The last column is the day–walk we started with, so that in the exam you can find the row by remembering the place.

Sewage Treatment Notes Class 12 Biology — And A Two–Minute Rapid Revision

This is the page to read on the morning of the exam. Sewage first, because it carries the most marks and has the most order to lose.

• Sewage sequence: sewage → primary (physical): sequential filtration removes floating debris, sedimentation removes grit → primary sludge settles, primary effluent (supernatant) goes on → secondary (biological): aeration tank, agitation, air pumped in, aerobic microbes grow as flocs (bacteria + fungal mycelia) → they eat the organic matter, so BOD falls → settling tank → activated sludge → small part back as inoculum, major part to the anaerobic sludge digester → biogas → treated effluent to the river. Mnemonic: Filter Sand, Add Foam, Sink Deep.
• BOD: the oxygen that would be consumed if all the organic matter in one litre of water were oxidised by bacteria. Greater the BOD, greater the polluting potential.
• Household: Lactobacillus → curd; Saccharomyces cerevisiae → bread and beverages; Propionibacterium sharmanii → Swiss cheese holes; Penicillium roqueforti → Roquefort flavour; toddy → fermented palm sap. Mnemonic: Lassi, Slice, Punch, Roquefort.
• Antibiotics: Fleming → Penicillium notatum → penicillin; Chain and Florey established its potential; Nobel Prize 1945; P. chrysogenum for industrial production; finish the prescribed course.
• Acids: citric → Aspergillus niger (the exception); acetic → Acetobacter aceti; butyric → Clostridium butylicum; lactic → Lactobacillus.
• Enzymes and medicines: lipase → oily stains in detergents; pectinase and protease → clarify juices; streptokinase (Streptococcus) → clot buster; cyclosporin A (Trichoderma polysporum) → immunosuppressant; statins (Monascus purpureus) → lower cholesterol.
• Biogas: methanogens, chiefly Methanobacterium, anaerobic, on cellulosic material; present in the cattle rumen, hence dung is rich in them; plant = concrete tank 10–15 ft deep + floating cover + gas outlet + spent slurry outlet; developed in India by IARI and KVIC.
• Biocontrol: bacterium Bacillus thuringiensis (protoxin activated by alkaline larval gut); fungus Trichoderma; virus Nucleopolyhedrovirus (species–specific, narrow spectrum); ladybird → aphids; dragonfly → mosquitoes.
• Biofertilisers: Rhizobium (symbiotic, nodules); Azotobacter and Azospirillum (free–living); Glomus (mycorrhiza, phosphorus); Anabaena, Nostoc, Oscillatoria (cyanobacteria, paddy fields); Azolla harbours Anabaena azollae. Mnemonic: Rich Aunts Always Give Nice Advice.

Microbes in Human Welfare Class 12 Important Questions — Practice Worksheet

Ten questions, mixed marks, in the style CBSE actually asks. Do them on paper first, with the page scrolled so you cannot see the answers. Then open each one and mark yourself strictly — a name that is nearly right is wrong. Be tougher on yourself here than the examiner will be in March, and March will feel easy.

Q1. Name the microbe that converts milk into curd and state the acid it produces. (1 mark)

Q2. A student leaves idli batter overnight and finds it has risen and become spongy. Identify the microbe and the gas responsible, and explain why the batter rises. (2 marks)

Q3. Distinguish between the roles of Propionibacterium sharmanii and Penicillium roqueforti in cheese production. (2 marks)

Q4. (a) Who discovered penicillin and under what circumstances? (b) Name the two scientists who established its full potential, and the species used for its industrial production. (3 marks)

Q5. Match each product with its source microbe and state whether the microbe is a bacterium or a fungus: (i) citric acid (ii) acetic acid (iii) butyric acid (iv) statins. (2 marks)

Q6. Define BOD. Water sample X has a BOD of 400 mg/L and sample Y has a BOD of 12 mg/L. Which is more likely to be untreated sewage, and what would happen to fish in a river receiving sample X? (3 marks)

Q7. Describe secondary treatment of sewage. Why is it called biological treatment, and what are flocs? (5 marks)

Q8. A farmer in a village runs his kitchen stove on gas produced from cattle dung. (a) Name the group of bacteria and one specific genus responsible. (b) Why is cattle dung particularly suitable as the raw material? (c) Name the two Indian organisations that developed this technology. (3 marks)

Q9. Why are baculoviruses considered especially suitable for Integrated Pest Management? Name the genus most commonly used. (2 marks)

Q10. (a) Differentiate between a symbiotic and a free–living nitrogen–fixing biofertiliser, with one example each. (b) Which nutrient does a mycorrhizal fungus supply to the plant, and name one genus that forms mycorrhiza. (c) State two additional benefits shown by plants having mycorrhizal associations. (5 marks)

Now go back and look honestly at what you got wrong. In this chapter, almost every lost mark will be a name — not a concept. That is genuinely good news, because names are the easiest thing in the world to fix. Write the ones you missed on a slip of paper, stick it where you will see it tomorrow morning, and they will be gone by the weekend.

And that is the whole spirit of how we study here. You are not trying to become perfect tonight. You are trying to get one more answer right than you did yesterday — one more genus spelled correctly, one more sequence in the right order, one more mark. Do that every day between now and the exam and the arithmetic will look after itself. Small, steady, honest improvement beats one heroic all–nighter every single time.

Tomorrow morning, when you sit down to your curd, you will know exactly who made it. That is not a small thing. That is what an education actually feels like from the inside.