Microbes in Human Welfare — NCERT Solutions

Yes, microbes can definitely be used as a source of energy.

How microbes are used as a source of energy:

1. Biogas Production:

• Methanogens (e.g., Methanobacterium) are anaerobic bacteria that decompose organic matter (animal dung, plant waste, sewage sludge) and produce biogas.
• Biogas is a mixture of gases, mainly methane ($CH_4$, ~75%), $CO_2$, $H_2S$, and $H_2$.
• Methane is a clean fuel with high calorific value.
• Biogas plants (gobar gas plants): Cattle dung (gobar) mixed with water is fed into an anaerobic digester tank. Methanogens act on it and produce biogas, which is collected and used for cooking and lighting in rural areas.

$$\text{Organic matter} \xrightarrow{\text{Methanogens (anaerobic)}} CH_4 + CO_2 + H_2S$$

2. Bioethanol:

• Yeast (Saccharomyces cerevisiae) ferments sugarcane juice, corn starch, or cellulose to produce ethanol (bioethanol).
• Bioethanol is used as a biofuel, blended with petrol to run vehicles.

$$\text{Glucose} \xrightarrow{\text{Yeast}} 2C_2H_5OH + 2CO_2$$

3. Microbial Fuel Cells (MFCs):

• Certain bacteria can transfer electrons from organic matter oxidation to an electrode, generating electricity directly.

4. Biohydrogen:

• Some microbes (e.g., Clostridium, cyanobacteria) can produce hydrogen gas through fermentation or photolysis of water, which can be used as a clean fuel.

Conclusion: Microbes, especially methanogens and yeast, are important sources of renewable energy in the form of biogas and bioethanol, and their use reduces dependence on fossil fuels.

Microbes as Alternatives to Chemical Fertilisers — Biofertilisers:

1. Nitrogen-fixing bacteria:

• Rhizobium forms symbiotic nodules on roots of leguminous plants and fixes atmospheric $N_2$ into organic nitrogen (ammonia), enriching soil nitrogen.
• Azotobacter and Azospirillum are free-living nitrogen-fixing bacteria that enrich soil nitrogen without forming symbiosis.
• ***Anabaena*, Nostoc, *Oscillatoria*** (cyanobacteria) fix atmospheric nitrogen in paddy fields and also add organic matter to soil.

2. Phosphate solubilising microbes:

• Fungi of genus Glomus form mycorrhizal associations with plant roots. The fungal symbiont absorbs phosphorus from soil and passes it to the plant, reducing the need for phosphatic fertilisers.

3. Cyanobacteria as biofertilisers:

• Blue-green algae (cyanobacteria) fix nitrogen and add organic matter to soil, increasing soil fertility in paddy fields.

Microbes as Alternatives to Chemical Pesticides — Biocontrol Agents:

**1. Bacillus thuringiensis (Bt):**

• This bacterium produces Bt toxin (crystal proteins / Cry proteins) that kills insect larvae (caterpillars, beetles, flies) by disrupting their gut.
• Bt spores are sprayed on crops as a biopesticide.
• Bt toxin genes have also been incorporated into crop plants (Bt cotton, Bt brinjal) to make them pest-resistant.

**2. Trichoderma (fungus):**

• Free-living fungi in the soil that act as biocontrol agents against several plant pathogens, protecting plant roots.

**3. Baculoviruses (Nuclear Polyhedrosis Viruses — NPV):**

• These are species-specific viruses that attack and kill insect pests without harming other organisms, plants, or the environment.
• They are excellent biocontrol agents for integrated pest management (IPM).

4. Predatory microbes:

• The bacterium Bacillus sphaericus is used to control mosquito larvae.

Conclusion: By using biofertilisers (Rhizobium, Azotobacter, mycorrhiza, cyanobacteria) and biocontrol agents (Bt, Trichoderma, baculoviruses), the use of chemical fertilisers and pesticides can be significantly reduced, leading to sustainable and eco-friendly agriculture.

Concept: BOD (Biochemical Oxygen Demand) is the amount of oxygen (in mg/L) consumed by microbes to decompose the organic matter present in one litre of water. Higher BOD = more organic matter = more pollution.

Given BOD values:

• Sample A = $20 \text{ mg/L}$
• Sample B = $8 \text{ mg/L}$
• Sample C = $400 \text{ mg/L}$

Most Polluted Sample:
$$\text{Sample C with BOD} = 400 \text{ mg/L is the most polluted.}$$

Assigning correct labels:

Conclusion:

• Sample C ($400 \text{ mg/L}$) → Untreated sewage water (most polluted)
• Sample A ($20 \text{ mg/L}$) → Secondary effluent from sewage treatment plant
• Sample B ($8 \text{ mg/L}$) → River water (relatively clean, least polluted)

Note: The BOD of clean river water is typically less than $10 \text{ mg/L}$, secondary effluent is around $10$–$30 \text{ mg/L}$, and untreated sewage can be several hundred mg/L.

1. Cyclosporin A (Immunosuppressive drug):

• Source microbe: Trichoderma polysporum (a fungus)
• Use: Cyclosporin A is used as an immunosuppressive agent in organ transplant patients to prevent rejection of the transplanted organ. It suppresses the immune response of the recipient's body.

2. Statins (Blood cholesterol-lowering agents):

• Source microbe: Monascus purpureus (a yeast / fungus)
• Example: Lovastatin (first statin discovered)
• Use: Statins competitively inhibit the enzyme HMG-CoA reductase, which is involved in cholesterol biosynthesis in the liver, thereby lowering blood cholesterol levels.
• Statins are used to treat hypercholesterolaemia and reduce the risk of cardiovascular diseases.

Summary Table:

Conclusion: Both Cyclosporin A and Statins are important bioactive molecules produced by fungi that have revolutionised medicine in the fields of organ transplantation and cardiovascular disease management.

(a) Single Cell Protein (SCP):

Definition: Single Cell Protein refers to the dried cells of microorganisms (bacteria, yeast, algae, or fungi) that are used as a protein supplement in human food or animal feed.

Role of Microbes in SCP:

• Microbes have a very high protein content (40–80% of dry weight) and can be grown rapidly on cheap substrates (industrial waste, agricultural waste, molasses, whey, etc.).
• Microbes used for SCP production:
• Spirulina (cyanobacterium) — grown on waste water; rich in protein, fats, carbohydrates, vitamins, and minerals.
• Methylophilus methylotrophus (bacterium) — grown on methanol; used as animal feed ("Pruteen").
• Saccharomyces cerevisiae (yeast) — used as food supplement.
• Fusarium (fungus) — produces "Quorn" (mycoprotein) used as meat substitute.
• Advantages:
• Reduces pressure on conventional agricultural protein sources.
• Can be produced on industrial scale using waste materials.
• Provides protein to malnourished populations.
• Reduces environmental pollution by using industrial effluents as substrate.

(b) Role of Microbes in Soil:

Microbes play extremely important roles in maintaining soil health and fertility:

1. Decomposition / Nutrient Cycling:

• Bacteria and fungi decompose dead organic matter (leaf litter, animal remains) into simpler inorganic compounds, releasing nutrients back into the soil (carbon, nitrogen, phosphorus cycles).

2. Nitrogen Fixation:

• Free-living bacteria (Azotobacter, Azospirillum, cyanobacteria) and symbiotic bacteria (Rhizobium in legume root nodules) fix atmospheric $N_2$ into ammonia, enriching soil nitrogen.

3. Nitrification:

• Nitrosomonas converts $NH_3$ → $NO_2^-$; Nitrobacter converts $NO_2^-$ → $NO_3^-$ (nitrates), which are absorbed by plants.

4. Denitrification:

• Pseudomonas and other bacteria convert nitrates back to $N_2$, completing the nitrogen cycle.

5. Mycorrhizal associations:

• Fungi (Glomus) form mycorrhiza with plant roots, helping in phosphorus absorption and improving plant growth.

6. Soil Structure:

• Microbial activity improves soil texture, aeration, and water-holding capacity.

7. Biocontrol:

• Trichoderma and other soil microbes suppress soil-borne plant pathogens.

Conclusion: Microbes are indispensable for soil fertility and ecosystem functioning.

Decreasing order of importance for human welfare:

$$\boxed{\text{Penicillin} > \text{Biogas} > \text{Curd} > \text{Citric acid}}$$

Reasons:

1. Penicillin (Most Important):

• Penicillin is an antibiotic produced by Penicillium notatum.
• It revolutionised medicine by providing an effective treatment against deadly bacterial infections like pneumonia, diphtheria, whooping cough, syphilis, and many others.
• Before antibiotics, these diseases killed millions of people. Penicillin has saved hundreds of millions of lives worldwide.
• Its discovery is considered one of the greatest achievements in medical history.
• Impact: Life-saving; affects the most fundamental aspect of human welfare — health and survival.

2. Biogas (Second):

• Biogas (mainly methane) is produced by methanogens from organic waste.
• It provides a clean, renewable source of energy to millions of rural households for cooking and lighting.
• It reduces dependence on fossil fuels, reduces deforestation (less firewood needed), and reduces greenhouse gas emissions.
• The spent slurry is an excellent organic manure.
• Impact: Addresses energy needs and environmental sustainability for a large population.

3. Curd (Third):

• Curd is produced by Lactic Acid Bacteria (LAB) fermenting milk.
• It is a nutritious food consumed daily by millions; it is more digestible than milk, contains probiotics, and has higher vitamin $B_{12}$ content.
• It also acts as a food preservative and is the basis of many dairy products.
• Impact: Important for nutrition and food security, but less critical than medicines or energy.

4. Citric Acid (Least Important among these):

• Citric acid is produced by Aspergillus niger from molasses.
• It is used as a food additive (flavouring, preservative), in pharmaceuticals, and in the cosmetic industry.
• While useful, it is not as critical to human survival and welfare as antibiotics, energy, or staple food.
• Impact: Industrial/commercial importance, but not essential for survival.

Conclusion:
$$\text{Penicillin} > \text{Biogas} > \text{Curd} > \text{Citric acid}$$

Definition: Biofertilisers are organisms (mainly bacteria, fungi, and cyanobacteria) that enrich the nutrient quality of the soil through natural biological processes.

How biofertilisers enrich soil fertility:

1. Nitrogen Fixation by Bacteria:

(a) Symbiotic nitrogen fixation:

• Rhizobium bacteria form symbiotic associations with roots of leguminous plants (pea, bean, soybean, groundnut), forming root nodules.
• They fix atmospheric $N_2$ into ammonia ($NH_3$), which is converted to organic nitrogen used by the plant.
• When the plant dies, this fixed nitrogen is released into the soil, enriching it.

$$N_2 + 8H^+ + 8e^- \xrightarrow{\text{Nitrogenase}} 2NH_3 + H_2$$

(b) Free-living nitrogen fixation:

• Azotobacter and Azospirillum are free-living bacteria in soil that fix atmospheric nitrogen independently, directly enriching soil nitrogen content.

2. Nitrogen Fixation by Cyanobacteria:

• ***Anabaena*, Nostoc, *Oscillatoria*** are cyanobacteria (blue-green algae) that fix atmospheric nitrogen.
• They are especially important as biofertilisers in paddy fields.
• They also add organic matter to the soil, improving its structure and fertility.

3. Phosphorus Solubilisation by Mycorrhizal Fungi:

• Fungi of genus Glomus form mycorrhizal associations with plant roots.
• The fungal hyphae have a large surface area and absorb phosphorus from soil (even from insoluble forms) and transfer it to the plant.
• Plants with mycorrhizal associations also show:
• Resistance to root-borne pathogens
• Tolerance to salinity and drought
• Overall increase in plant growth and development

4. Addition of Organic Matter:

• Decomposition of microbial biomass adds humus and organic matter to soil, improving soil texture, water retention, and aeration.

Summary Table:

Conclusion: Biofertilisers enrich soil fertility by fixing atmospheric nitrogen, solubilising phosphorus, and adding organic matter to the soil through natural biological processes, thereby reducing the need for chemical fertilisers and promoting sustainable agriculture.