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NCERT Solutions Curiosity Chapter 2 Activity 2.8: Let us perform — Microorganisms and food

Book page 20 Updated on2026-09-05

Q1.
Did you find any change in the volume, smell, or texture of the dough?
Answer

Yes — but only in bowl A, the one with yeast.

Bowl A (flour + sugar + yeast + warm water)Bowl B (flour + sugar + warm water, no yeast)
VolumeHas risen — the dough is clearly bigger than when it was kneadedAlmost unchanged
TextureSoft, spongy and fluffy; small holes are seen when it is torn openFirm and even, no holes
SmellA different, slightly sharp smellThe plain smell of dough
Tip: if you see no change after 4–5 hours, leave the dough for some more time. The book says so — the yeast needs time to multiply before the change becomes visible. Bowl B is not wasted effort: it is the comparison, and without it you could not be sure the yeast caused the change.
Q2.
Why does this happen? What is the role of yeast? Why did we add sugar and warm water to the flour?
Answer

Yeast is a living microorganism — a fungus — and the dough rises because the yeast is respiring inside it.

Follow what happens in bowl A:

  1. Yeast belongs to the group of microorganisms called fungi. Like other organisms, it respires and breaks down food to release energy for its growth and to carry out life processes (you met this in ‘Life Processes in Animals’, Grade 7).
  2. During this process carbon dioxide is released.
  3. The kneaded dough is soft and stretchy, so the gas cannot escape freely. It collects as thousands of tiny bubbles trapped inside.
  4. Each bubble pushes the dough outwards, so the dough rises and becomes soft and fluffy. The holes you see on tearing it open are those bubbles.
  5. Yeast also produces a small amount of alcohol during this process, which is what gives the dough its slightly different smell.
Sugar in thedough Yeast respires (warm place, moist dough) energy for its growth Carbon dioxidebubbles → dough rises A little alcoholthe changed smell
Why the dough in bowl A rises: yeast breaks down sugar and releases carbon dioxide and a little alcohol.

Why sugar was added: the sugar is the yeast's food. Breaking it down is what releases the energy the yeast needs — and the carbon dioxide is the by-product of that breaking down. Without an easily available food the yeast would work far more slowly.

Why warm water was used: yeast grows well in warm conditions. Warm water wakes the dry yeast powder up, spreads it through the flour and keeps the dough at the temperature at which the yeast multiplies fastest. Cold water would leave it barely active — which is exactly what you will see in the curd experiment in Activity 2.9.

Did you know? This special property of yeast is used in making breads, cakes and more. Some bacteria, such as Lactobacillus, do the same job in the batter for idli and dosa and in the dough for bhatura.
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