NCERT Solutions: Chapter2 The Invisible Living World: Beyond Our Naked Eye (class 8 Science)

Yes, it would be fascinating to see the microorganisms like bacteria, fungi, protozoa, and viruses that are constantly interacting with our environment and even inside our bodies.[cite: 9] It would change how we perceive cleanliness, health, and the complexity of ecosystems around us.[cite: 9]

Observing this hidden world would reveal that even microscopic organisms exhibit complex behaviors such as movement, reproduction, response to stimuli, and forming colonies.[cite: 9] This would deepen our understanding of what qualifies as a living being and show that size does not limit the complexity of life.[cite: 9]

Yes, microorganisms constantly interact; some live symbiotically, while others compete for resources or prey on one another.[cite: 9] For example, bacteria can help in digestion or cause diseases, and fungi can decompose organic matter, enriching the soil.[cite: 9] These interactions form the foundation of many ecological processes.[cite: 9]

Here are key questions that spark scientific curiosity:[cite: 9]

• What do microorganisms eat to stay alive?[cite: 9]
• Can microorganisms see or feel each other, or do they just bump into things?[cite: 9]
• Why are some bacteria helpful (like in curd) and others harmful (like causing diseases)?[cite: 9]
• How do microorganisms survive in tough places, like hot springs or salty water?[cite: 9]

(i) Prediction and Explanation:
Correct Option: (c) Yeast produced a gas inside test tube B which inflated the balloon.[cite: 9]
Scientific Reason: The yeast cells undergo anaerobic respiration (fermentation) by breaking down the sugar solution, releasing Carbon Dioxide gas ($CO_2$).[cite: 9] The accumulated $CO_2$ gas builds pressure and inflates balloon B.[cite: 9]

(ii) Lime Water Test:
She wants to confirm that the gas produced during yeast fermentation is indeed Carbon Dioxide ($CO_2$).[cite: 9]
When the gas from balloon B is bubbled into lime water, the lime water turns milky due to the formation of an insoluble white precipitate of Calcium Carbonate ($CaCO_3$):[cite: 9]
$$Ca(OH)_2 + CO_2 \rightarrow CaCO_3 \downarrow + H_2O$$

Beans are leguminous crops that form a mutualistic symbiotic relationship with Rhizobium bacteria residing in their root nodules.[cite: 9]

• Rhizobium bacteria have the unique capacity to trap atmospheric nitrogen ($N_2$) and convert it into soluble nitrogenous compounds (nitrates/nitrites) that plants can readily absorb.[cite: 9]
• In return, the bean plant supplies food and shelter to the bacteria.[cite: 9]
• Because beans naturally enrich the soil with nitrogen, the second farmer does not need to invest in synthetic nitrogen-rich fertilizers.[cite: 9]

Snehal is testing whether mixing fruit and vegetable peels with dry leaves facilitates faster and better microbial decomposition into organic manure.[cite: 9]

• Pit A combines nitrogen-rich wet kitchen waste (peels) with carbon-rich brown organic matter (dried leaves), providing balanced aeration and moisture for aerobic composting microbes.[cite: 9]
• Therefore, Pit A will decompose much more efficiently into nutrient-rich compost/manure.[cite: 9]

1. (i) Bacteria: Ubiquitous single-celled organisms thriving in water, soil, air, extreme habitats, and inside human intestines (gut microbiota) assisting digestion.[cite: 9]
2. (ii) Yeast (Fungus): Unicellular fungus that ferments dough sugars, liberating $CO_2$ gas bubbles that make baked goods soft and fluffy.[cite: 9]
3. (iii) Rhizobium: Nitrogen-fixing bacterium inhabiting the root nodules of leguminous pulse crops.[cite: 9]

Experimental Procedure: Take four identical slices of bread and place them under four different environmental conditions:[cite: 9]

Conclusion: Microorganisms require an optimal combination of warmth, moisture, and air to flourish.[cite: 9]

• Observations: The bread slice near the sink shows visible fungal growth (mold patches), whereas the slice in the refrigerator remains clear with no or very slow microbial development.[cite: 9]
• Reason: The area near the sink provides warm temperature and high humidity/moisture that stimulate fungal spore germination.[cite: 9] In contrast, low temperatures inside the refrigerator deactivate enzymes and inhibit microbial multiplication.[cite: 9]

1. Continuous Bacterial Proliferation: Lactic acid bacteria (Lactobacillus) continue to consume lactose sugar and multiply, synthesizing larger quantities of lactic acid, which intensifies sourness.[cite: 9]
2. Temperature Acceleration: Warmer ambient room temperature significantly accelerates the metabolic and enzymatic rates of bacteria, increasing acid production speed.[cite: 9]

• (i) Action in Flask A: The yeast ferments the sugar solution anaerobically, generating carbon dioxide gas and a small amount of ethyl alcohol.[cite: 9]
• (ii) Observation in Test Tube B: The clear lime water turns milky.[cite: 9] This occurs because the $CO_2$ produced in flask A travels through the delivery tube into test tube B and reacts with calcium hydroxide to precipitate calcium carbonate.[cite: 9]
• (iii) If Yeast was Not Added: No fermentation occurs.[cite: 9] Without yeast enzymes, sugar does not break down, no $CO_2$ gas is produced, and the lime water in tube B remains clear.[cite: 9]

India pioneered biogas generation in the late 1850s.[cite: 9] Today, the Ministry of New and Renewable Energy (MNRE) implements the National Biogas and Manure Management Programme (NBMMP) to set up family-type and community biogas plants across rural India.[cite: 9]

• Core Objective: Provide clean, smoke-free cooking fuel to rural households, improve village sanitation, and prevent indoor air pollution.[cite: 9]
• Biological Process: Methanogenic anaerobic bacteria break down cattle dung, agricultural crop residues, and bio-waste in an oxygen-free digester, producing biogas (55–70% Methane + $CO_2$) and digested nutrient slurry (bio-manure).[cite: 9]
• Socio-Economic Benefits: Minimizes firewood gathering, decreases reliance on commercial LPG cylinders, prevents deforestation, and provides organic fertilizer.[cite: 9]
• Government Support: Capital subsidies, mason training, and technical assistance are provided to farmers.[cite: 9]

Fermentation utilizes microbes (bacteria/yeast) to break down complex sugars, enhancing digestibility, flavor, shelf-life, and nutrient bioavailability:[cite: 9]

Mushrooms are fruiting bodies of macro-fungi (not plants).[cite: 9] Their anatomical structure consists of:

• Pileus (Cap): The broad umbrella-shaped top structure protecting the spore-producing tissues.[cite: 9]
• Lamellae (Gills): Thin, radiating vertical plates beneath the cap where millions of microscopic spores develop.[cite: 9]
• Stipe (Stalk): The sturdy central stem that elevates the cap for air-dispersal of spores.[cite: 9]
• Annulus (Ring) & Volva (Base): Residual membrane tissue found in specific mushroom varieties.[cite: 9]
• Hyphae / Mycelium: Thread-like vegetative filaments spreading under the soil/substrate to absorb nutrients.[cite: 9]
• Microscopic View: Spores appear as tiny oval reproductive cells, and hyphae appear as transparent branched tubular networks.[cite: 9]

Mushroom farming requires vertical indoor space and utilizes agricultural waste effectively:[cite: 9]

1. Variety Selection: Choose Button mushroom (Agaricus bisporus) or Oyster mushroom (Pleurotus).[cite: 9]
2. Spawn Preparation: Procure disease-free, high-quality grain spawn (mushroom seeds).[cite: 9]
3. Substrate Preparation: Sterilize wheat/paddy straw or sawdust to eliminate competitor molds.[cite: 9]
4. Inoculation (Spawning): Thoroughly mix spawn with moist substrate and pack into perforated polybags.[cite: 9]
5. Spawn Running (Incubation): Maintain in a dark, warm room ($25\text{–}30^\circ\text{C}$) for 15–20 days until white mycelium covers the substrate.[cite: 9]
6. Cropping & Fruiting: Transfer bags to a well-ventilated, humid, and cool growing chamber.[cite: 9]
7. Harvesting: Gently twist and pluck mushrooms when the caps fully expand before gills release spores.[cite: 9]

Debate Outline ("Can mushroom farming be a viable business for rural youth?"):
• Affirmative Arguments: Low capital investment, high profit margin, minimal land requirements, quick 30-day harvest cycles, and high urban market demand.[cite: 9]
• Challenges to Address: Strict sterilization controls required to prevent contamination, temperature/humidity control in summer, and local cold-chain storage for quick distribution.[cite: 9]