Activities 2.8 and 2.9 — Microorganisms in Food — "The Invisible Living World: Beyond Our Naked Eye"

1. 14 marksCuriosity Grade 8, Chapter 2, Activity 2.8, pages 20-21

   Describe Activity 2.8. Why is bowl B set up at all, why is sugar added, why warm water, and why a damp cloth in a warm place?

   Hint. Every one of those four choices is there for a reason — three of them are about what yeast needs.

   Method. Take two bowls, A and B. Put 200 g of flour (atta or maida) and a pinch of sugar in each. Add a small amount of yeast powder to bowl A only and mix well. Leave bowl B without yeast. Knead both with warm water into a soft dough, cover with a damp cloth, keep in a warm place, and observe after 4-5 hours.

   Why bowl B exists. So that the two results can be compared. Bowl B is identical in every respect except that it has no yeast, so any difference between the bowls must be due to the yeast — nothing else was free to differ. Without it, a risen dough in bowl A could be explained by the warmth, the sugar or the kneading just as easily.

   Why each condition:

   Choice	Reason
   Sugar	Yeast needs food; it breaks down sugar to release energy
   Warm water	Yeast grows well in warm conditions
   Warm place	Keeps those conditions for the whole 4-5 hours
   Damp cloth	Stops the dough surface drying out, since microorganisms need moisture

   And if nothing has happened? The chapter says to leave the dough longer — a reminder that biological changes run on their own timetable, not the lesson's.

   ✦ Answer: bowl B is the comparison that isolates the effect of the yeast; sugar is the yeast's food, warm water and a warm place provide the warmth yeast needs to grow, and the damp cloth keeps the moisture it also requires.

2. 24 marksCuriosity Grade 8, Chapter 2, Activity 2.8, page 21

   What differences appear between the two bowls, and what is the role of yeast? Why does the dough smell different?

   Hint. Two products are formed, and each explains one of the observations.

   What you observe. The dough in bowl A, with yeast, has risen slightly, become fluffy, and has a different smell compared with the dough in bowl B.

   What yeast is. A microorganism belonging to the group of fungi. It grows well in warm conditions.

   What it does. Like other organisms, yeast respires and breaks down food to release energy for growth and life processes. Two things are produced:

   • Carbon dioxide, which forms bubbles — these are trapped in the dough and make it soft and fluffy, and the dough rises
   • A small amount of alcohol, which gives the dough its slightly different smell

   So each observation has its own cause. The rising and fluffiness come from the gas; the smell comes from the alcohol. They are two products of the same process.

   Where this is used. This special property of yeast is used in making breads, cakes and more.

   A related organism. Some bacteria, such as Lactobacillus, help ferment the batter for idli and dosa, and the dough for bhatura.

   ✦ Answer: bowl A rises, turns fluffy and smells different; yeast is a fungus that respires by breaking down sugar, releasing carbon dioxide, whose bubbles make the dough soft and fluffy, and a small amount of alcohol, which produces the smell.

3. 33 marksCuriosity Grade 8, Chapter 2, Activity 2.9, pages 21-22

   Describe Activity 2.9 and predict what happens in each bowl.

   Hint. The only difference between the bowls is temperature.

   Method. Take two small glass bowls, A and B. Pour lukewarm milk into A and cold milk into B. Add a small spoonful of curd to each and mix well. Cover both, keep A in a warm place and B in a cool place such as a refrigerator, for a few hours or overnight. Record predictions and observations in Table 2.4.

   Why add curd at all. The curd is the source of the bacteria. Curd contains several types of bacteria, one of which is Lactobacillus — adding a spoonful introduces them into the fresh milk.

   What the design isolates. Both bowls get the same milk and the same spoonful of curd; only the temperature differs. So any difference in the outcome can be attributed to temperature.

   Prediction. Bowl A, kept warm, should set into curd; bowl B, kept cold, should not.

   Observation. In bowl A the milk turns into curd after a few hours and becomes a little sour. In bowl B the milk has not curdled, though it might be a little sour.

   ✦ Answer: lukewarm milk plus curd kept warm (A) sets into curd and turns slightly sour; cold milk plus curd kept in the cold (B) does not curdle — the only variable between them being temperature.

4. 44 marksCuriosity Grade 8, Chapter 2, Activity 2.9, page 22

   Why does curd form in bowl A but not in bowl B? How does this differ from what yeast does to dough?

   Hint. The two microorganisms produce different substances from the same kind of raw material.

   The organism. Curd contains several types of bacteria; one of them is Lactobacillus.

   What it does. It feeds on the sugar in the milk (lactose), multiplies, and ferments the milk to form curd.

   Why it turns sour. Instead of producing alcohol as yeast does, these bacteria produce lactic acid, and that acid is what makes curd sour.

   Why A and not B. These bacteria grow well in warm conditions. Bowl A was kept warm, so they multiplied and fermented the milk; bowl B was kept cold, which slows their growth so much that the milk does not set.

   Comparing the two activities:

   	Yeast (Activity 2.8)	Lactobacillus (Activity 2.9)
   Group	Fungus	Bacterium
   Food	Sugar in the flour	Lactose, the sugar in milk
   Main product	Carbon dioxide + a little alcohol	Lactic acid
   Effect	Dough rises, becomes fluffy, smells different	Milk sets into curd and turns sour
   Needs warmth?	Yes	Yes

   The shared principle. Both are fermentation: a microorganism breaking down sugar for energy, with the by-product changing the food. Which by-product you get depends on which organism is doing it.

   ✦ Answer: Lactobacillus feeds on the milk sugar lactose, multiplies and ferments the milk, producing lactic acid which sets it and makes it sour; it grows well in warmth, so bowl A sets and cold bowl B does not — unlike yeast, which produces carbon dioxide and alcohol rather than lactic acid.

5. 54 marksCuriosity Grade 8, Chapter 2, Section 2.4.2, page 22

   What is Rhizobium, where does it live, and why do farmers grow legumes in rotation with other crops?

   Hint. The swollen structures on the roots are not a disease.

   What and where. Rhizobium is a bacterium. It forms swollen regions called root nodules on the roots of certain legumes — beans, peas and lentils — and lives inside them.

   What it does. These bacteria trap nitrogen from the air and make it useful for the plants. This helps the plants grow well without chemical fertilisers.

   Why crop rotation follows. Because the nitrogen the bacteria fix does not all leave with the harvest — growing legumes naturally increases the nitrogen in the soil and keeps it healthy for the next crop. So a farmer alternates legumes with other crops, and the legume year restores what the other crop took out.

   Why it is a fair exchange. The plant supplies the bacteria with a place to live and food; the bacteria supply the plant with usable nitrogen. Neither could do the other's job.

   Why the air alone is not enough. Nitrogen is abundant in the air, but plants cannot use it in that form. The bacteria's contribution is not fetching nitrogen but converting it into a form the plant can take up.

   ✦ Answer: Rhizobium is a bacterium living in the root nodules of legumes such as beans, peas and lentils, which traps nitrogen from the air and makes it available to the plant; growing legumes in rotation therefore enriches the soil with nitrogen for the next crop and reduces the need for chemical fertiliser.

6. 64 marksCuriosity Grade 8, Chapter 2, Section 2.4.3 and Ever heard of…, pages 22-23

   What are microalgae, why do they matter, and what is Spirulina?

   Hint. One statistic in this section is larger than students expect.

   What they are. Microscopic plant-like organisms living in water, soil, air and even on trees. They make their own food using sunlight.

   Why they matter.

   • While making food they release oxygen — and produce more than half of the Earth's oxygen supply
   • They are rich in nutrients and serve as food for many aquatic animals
   • Some are used by humans as health supplements and medicines
   • They help clean water, and are used to make biofuel

   The threat. Pollution, climate change and habitat destruction are threatening their diversity and abundance, so conserving them matters for the environment and for maintaining the oxygen balance on Earth.

   Spirulina. A microalga called a superfood. It is a good source of vitamin B12, and is more than 60 per cent protein by body weight, with only a small amount of fat and sugar. Farming it is becoming a feasible livelihood: set a clear glass tank in a bright place away from direct sunlight, cover with a shade net or keep it at moderate temperature, fill with pond water, add living Spirulina collected from a pond, stir twice a week, and after 3-6 weeks harvest by filtering through a fine cloth.

   Named examples. Spirulina, Chlorella and diatoms.

   ✦ Answer: microalgae are microscopic plant-like organisms that make their own food using sunlight, produce more than half the Earth's oxygen, feed aquatic animals, clean water and yield biofuel; Spirulina is one, valued as a superfood at over 60% protein and a source of vitamin B12, and can be farmed in a tank of pond water and harvested after 3-6 weeks.

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.