Ch 2 introduces the world of microorganisms — bacteria, viruses, fungi, protozoa, and algae. Students see how they are both "friends" (fermentation, medicine, soil fertility) and "foes" (diseases, food spoilage).
Bacteria: Lactobacillus makes curd from milk. Yeast: used in bread (CO₂ makes dough rise) and alcohol (fermentation). Antibiotics: penicillin from Penicillium fungus kills bacteria. Vaccines: weakened/dead microbes that train our immune system (discovered by Edward Jenner). Decomposers: bacteria/fungi break dead matter into compost.
Diseases: bacteria (tuberculosis, cholera), viruses (common cold, COVID-19, polio), protozoa (malaria, dysentery), fungi (ringworm). Food spoilage: microbes grow on food, making it unfit. Preservation: salt, sugar, vinegar, oil, pasteurisation (heating), refrigeration, drying.
Download: https://ncert.nic.in/textbook/pdf/hesc102.pdf | Complete book: https://ncert.nic.in/textbook/pdf/hesc1ps.zip
Pasteurisation is heating milk to about 70°C for 15-30 seconds and then quickly cooling it. Named after Louis Pasteur, it kills harmful microbes without significantly changing the milk's taste. This extends the shelf life of milk.
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