In-text — Microbes and the Environment — "The Invisible Living World: Beyond Our Naked Eye"

1. 13 marksCuriosity Grade 8, Chapter 2, Section 2.4, page 18

   Why does food such as lemon, tomato or orange rot when left outside, and why do pickles and murabbas not spoil in the same way?

   Hint. The chapter answers the second half in a speech bubble on the same page.

   Why food rots. The powdery or cotton-like growth on a rotting fruit is a microbial infection. Microorganisms are found everywhere — in water, soil, air, and on food itself — so they do not need to be introduced; they are already present, or arrive from the air and the surfaces the food touches. Given a moist, nutrient-rich surface at a comfortable temperature, they grow and multiply, breaking the food down.

   Why pickles and murabbas resist. Because of what is added to them: many spices along with salt or sugar, which act as preservatives. A high concentration of salt or sugar does not allow these organisms to grow.

   The general principle. Preserving food is not about keeping microbes out — that is nearly impossible — but about making conditions unsuitable for their growth. Heavy salt or sugar does it chemically; the refrigerator does it by removing warmth, which is exactly what exercise question 7 tests with two slices of bread.

   ✦ Answer: food rots because microorganisms are present everywhere and multiply on moist, nutrient-rich food; pickles and murabbas keep because their high concentration of salt or sugar acts as a preservative and does not allow microbes to grow.

2. 24 marksCuriosity Grade 8, Chapter 2, Activity 2.7, pages 18-19

   Describe Activity 2.7 and its result. What has happened to the fruit and vegetable peels, and how?

   Hint. It takes two to three weeks, and the answer is a change of colour and texture.

   Method. Half-fill an empty container with garden soil. Add fruit and vegetable peels. Put a layer of soil on top and leave it aside. After 2-3 weeks, look at the contents again.

   Result. The peels have turned into a dark-coloured material. This is manure — rich in nutrients, and it increases the fertility of the soil.

   How it happened. The soil was not inert. As Activity 2.6 showed, soil contains many microorganisms; some of these, particularly fungi and bacteria, act on the plant waste and slowly break it down into simpler, nutrient-rich manure.

   Why the peels were buried under soil. Covering them puts them in contact with the soil microorganisms that do the work, and helps hold the moisture they need.

   What conditions it needs. The chapter states that manure formation occurs at optimal temperature and appropriate moisture level — which is why the same process is slow in a refrigerator and in bone-dry soil.

   Where you have seen it. Gardeners collecting dry leaves and plant waste into pits are doing exactly this, to make natural manure.

   ✦ Answer: buried fruit and vegetable peels turn into a dark, nutrient-rich manure after 2-3 weeks, because fungi and bacteria in the soil break the plant waste down into simpler substances — a process needing suitable temperature and moisture.

3. 34 marksCuriosity Grade 8, Chapter 2, Section 2.4.1, page 19

   What is decomposition, and what would happen if microorganisms did not exist on Earth?

   Hint. Follow the nutrients — where do they come from, and where do they go?

   What decomposition is. Microorganisms break down the complex substances of fallen leaves into simpler substances rich in nutrients. Those nutrients go back into the soil and help plants grow better. Microorganisms decompose the bodies of dead animals too, and helpful bacteria can decompose animal wastes like dung.

   Why it is called recycling. The nutrients in a leaf came from the soil in the first place. Decomposition returns them, so the same material is used again by the next generation of plants. Nothing new is created; it circulates.

   If microorganisms did not exist:

   • Dead leaves, plant waste, dead animals and dung would pile up without breaking down — the chapter's own observation is that such material disappears from a garden after some time, and it would simply stop disappearing
   • The nutrients locked inside that dead matter would never return to the soil, so soil fertility would fall and plants would grow poorly
   • There would be no natural manure, and no biogas
   • Legumes would lose Rhizobium, so nitrogen from the air could not be trapped
   • Curd, bread, idli, dosa and bhatura could not be made
   • Our own digestion, which depends on gut bacteria, would be affected

   ✦ Answer: decomposition is the breaking down of complex dead matter into simple nutrient-rich substances that return to the soil; without microorganisms waste would accumulate undecomposed, nutrients would never be recycled, soil fertility would collapse, and much of our food and digestion would fail.

4. 43 marksCuriosity Grade 8, Chapter 2, A step further, page 20

   What is biogas, how is it produced, and what is it used for?

   Hint. The bacteria responsible do their work without oxygen.

   Who makes it. Many microorganisms, such as bacteria and fungi, live in an oxygen-free environment. Some of these bacteria can decompose plant and animal waste from the environment, or household wastewater.

   What is released. During that process they release a mixture of gases called biogas, composed primarily of carbon dioxide together with a high proportion of methane.

   What it is used for. As a fuel — for cooking, heating, generating electricity, and even running vehicles.

   Why this is a good arrangement. The same process solves two problems at once: it disposes of waste that would otherwise accumulate, and it yields a usable fuel. This is decomposition again, but run without oxygen so that the products are combustible gases rather than only manure.

   An Indian connection. The chapter's project section notes that India has a long history of biogas production, with one of our oldest biogas plants set up in the late 1850s.

   ✦ Answer: biogas is a mixture of gases — mainly carbon dioxide with a high proportion of methane — released by bacteria decomposing plant, animal or household waste in an oxygen-free environment, and it is used as fuel for cooking, heating, electricity generation and vehicles.

5. 54 marksCuriosity Grade 8, Chapter 2, Be a scientist, page 20

   Who was Ananda Mohan Chakrabarty and what did he do? What is a patent? What other problems do you think microorganisms could help solve?

   Hint. The last part is an invitation to think, not a fact to recall.

   Who he was. Ananda Mohan Chakrabarty (1938-2020), a scientist who studied bacteria.

   What he did. In 1971 he developed a special bacterium that could break down oil spills, helping to clean the environment. His discovery received a patent in 1980.

   What a patent is. The chapter defines it as a copyright given to a person so that no one else can copy, use or sell their invention without permission.

   Why he is remembered. His work showed that microorganisms could be used to solve environmental problems like pollution — not merely studied, but put to work.

   What else microbes might tackle — reasoning from what this chapter has already established, rather than guessing:

   • Plastic and other stubborn waste, extending the decomposition of leaves and peels to materials that do not break down naturally
   • Sewage and household wastewater, already partly handled by the biogas bacteria
   • Restoring soil fertility without chemical fertilisers, as Rhizobium does
   • Cleaning polluted water, which the chapter says microalgae help with
   • Producing fuel — biogas from waste, and biofuel from microalgae

   These are directions worth investigating, not settled achievements; the honest answer names the reasoning behind each rather than claiming it is already done.

   ✦ Answer: Ananda Mohan Chakrabarty (1938-2020) developed an oil-spill-degrading bacterium in 1971, patented in 1980 — a patent being a right preventing others from copying, using or selling an invention without permission — and his work showed microbes could solve pollution problems; plastic waste, sewage treatment, soil restoration, water cleaning and fuel production are plausible further directions.

6. 62 marksCuriosity Grade 8, Chapter 2, Our scientific heritage, page 19

   What do ancient Indian texts say about tiny living entities?

   Hint. One Sanskrit word, and a distinction drawn within it.

   The word. Ancient Indian texts, particularly the Vedas, contain references to the word 'Krimi', which means different tiny entities.

   The distinction drawn. Krimi covers both 'Drishya' (visible) and 'Adrishya' (invisible) entities. That is a striking distinction to find in a pre-microscope text: it acknowledges that some of these tiny beings cannot be seen at all, while still treating them as real.

   What the texts say about them. Various Vedic texts mention their beneficial and harmful effects — the same two-sided view this chapter takes, where microorganisms can be beneficial or harmful to us.

   Where it appears. The Atharvaveda also refers to Krimi.

   How to read this carefully. The texts record an observation — that unseen tiny entities affect health, both for good and ill — not a theory of microorganisms. What the microscope added, much later, was the ability to see them and find out what they are.

   ✦ Answer: the Vedas, including the Atharvaveda, refer to 'Krimi' — tiny entities both 'Drishya' (visible) and 'Adrishya' (invisible) — and describe their beneficial and harmful effects.

Solutions written by the tuition.in editorial team and checked against NCERT Curiosity - Textbook of Science for Grade 8 (hecu102.pdf), Chapter 2 'The Invisible Living World: Beyond Our Naked Eye', pages 8-27, Reprint 2026-27. HAND-WRITTEN throughout. Page count verified against the printed contents page (hecu1ps.pdf p.18): 20 pages, 8-27, matching exactly. Covers the 3 'Probe and ponder' prompts (p.8), all 9 activities (2.1 to 2.9), every in-text section (2.1 to 2.5), the 9 numbered 'Keep the curiosity alive' exercises (pp.25-26) and the 4 'Discover, design, and debate' projects (p.27). EXERCISE Q1 VENN DIAGRAM READ OFF A 500-DPI RENDER of p.25: three circles labelled 'Only in Animal Cell' (top), 'Only in Bacterial Cell' (lower left) and 'Only in Plant Cell' (lower right), with an arrow labelling the central three-way overlap 'Common to all three cells'. Each of those four labelled regions carries two ruled lines; the three PAIRWISE overlap regions carry none. Working from the book's own statements (p.13 'the cell wall in the plant cell provides rigidity'; p.24 'bacteria do not have a well-defined nucleus... instead they have a nucleoid'; and the Snapshots line 'Plant, fungal, and bacterial cells have an extra covering, called a cell wall'), the correct placement is: cytoplasm and cell membrane in the three-way overlap (exactly two items, matching the two ruled lines); chloroplast in plant-only; nucleoid in bacterial-only; NUCLEUS in the animal-plant pairwise overlap (plant cells have nuclei, bacteria do not); CELL WALL in the plant-bacterial pairwise overlap (not plant-only, since the book says bacterial and fungal cells have walls too); and NO listed part is unique to the animal cell. The solution states both traps explicitly. FACTUAL FIGURES TAKEN VERBATIM FROM THE BOOK: Hooke's Micrographia 1665, microscope 200-300x, cork, honeycomb, first use of 'cell'; Leeuwenhoek 1660s, Father of Microbiology; ordinary microscope 100-400x; electron microscope about 10,00,000x; ostrich egg yolk 130-170 mm as the largest known cell; Ananda Mohan Chakrabarty 1938-2020, oil-degrading bacterium 1971, patent 1980; biogas mainly carbon dioxide with a high proportion of methane; one of India's oldest biogas plants late 1850s; microalgae produce more than half of Earth's oxygen; Spirulina over 60 per cent protein by body weight and a source of vitamin B12, harvested after 3-6 weeks. The MNRE biogas-programme project answer deliberately does NOT assert current programme details - it names the programmes to search for and tells the student to verify against the current official MNRE page, since names and subsidy rules change.. Questions are referenced from the NCERT textbook for identification.