Class 12 Biology Ch 8 Microbes in Human Welfare Formula Sheet

Ans. The sheet collates every numerical fact and microbe-product pair across NCERT Sections 8.1 to 8.6: lactic-acid fermentation, ethanol stoichiometry, citric-acid yield (0.9 g/g), penicillin potency (1500 to 1670 IU/mg), BOD ranges, the COD/BOD ratio, biogas composition (50 to 70% CH₄), Bt biopesticide mechanism, Rhizobium N-fixation (50 to 200 kg N/ha/yr) and the symbiotic-vs-free-living biofertiliser split. One printable revision page aligned to the 2026-27 NCERT.

Ans. Yes. Every entry matches the current 2026-27 print of NCERT Biology. Sections 8.1 through 8.6 are retained in full, and the sheet flags both classical numerical facts (BOD, biogas %, ethanol yield) and the NCERT-extension topics (PHA bioplastic, leghaemoglobin chemistry) exactly as they appear in the latest edition.

Ans. NEET pulls 2 to 4 questions a year from this chapter, almost all recall-based numericals or microbe-to-product matchings: percentage of methane in biogas, the BOD discharge limit, the symbiotic-vs-free-living N-fixation rate, the citric-acid yield and the Penicillium G potency. The Collegedunia sheet packages these in quick-lookup tables so a 15 minute final-pass the night before the paper covers nearly every common MCQ.

Ans. Biogas is a mixture of methane (CH₄) 50 to 70%, carbon dioxide (CO₂) 25 to 30%, hydrogen (H₂) 5 to 10% by volume, with traces of H₂S and N₂. Methane is the combustible fraction. Calorific value is 20 to 26 MJ/m³, about 60% of natural gas. Biogas plants in India are popularised by IARI and KVIC.

Ans. BOD (Biochemical Oxygen Demand) is the oxygen consumed by aerobic microbes to oxidise all the organic matter in 1 L of water over 5 days at 20 °C, measured in mg O₂/L. Clean river water has BOD < 5; raw sewage 200 to 600; the discharge standard for safe effluent is BOD ≤ 20 mg O₂/L. COD/BOD for domestic sewage is 1.5 to 2.5; a ratio much greater than 3 implies a large non-biodegradable industrial fraction.

Ans. Symbiotic fixers live inside legume root nodules (Rhizobium) or in Azolla (Anabaena) and fix 50 to 200 kg N/ha/yr. Free-living fixers (Azospirillum, Azotobacter, Nostoc, Oscillatoria) fix only 20 to 30 kg N/ha/yr. In Rhizobium the nitrogenase enzyme is O₂-sensitive and is protected by leghaemoglobin, which gives an active nodule its pink colour.

Ans. Antibiotics target peptidoglycan, 70S ribosomes or specific bacterial enzymes. Viruses lack peptidoglycan, lack ribosomes (they use the host's machinery) and lack the enzymes antibiotics inhibit, so antibiotics have no target on a virus. The common cold and COVID-19 therefore need antivirals (acyclovir, interferons) and not antibiotics. NEET tests this assertion-reason almost every year.

Ans. Bacillus thuringiensis produces spores containing Cry protein crystals. When a lepidopteran larva (e.g. cotton bollworm) eats the spray, the alkaline gut pH (≥ 9.5) solubilises the crystal, releasing the toxin. The toxin perforates the midgut and the larva dies in about 48 hours. Mammals, birds and fish have acidic stomachs and are not harmed. Cry genes (cryIAc, cryIIAb) have been cloned into Bt cotton and Bt maize.

Ans. Chapter 8 contributes around 5 to 7 marks in the CBSE Class 12 Biology board paper, typically split across a 2 mark short-answer on STP / biogas plant design and a 3 to 5 mark long-answer on industrial fermentation (penicillin, citric acid) or biofertilisers. NEET tests 2 to 4 questions a year, almost always 1 mark numericals or microbe-product MCQs.