But how does this process (digestion) happen?
Food is pushed through a long tube called the alimentary canal. At different spots along that tube special liquids (digestive juices) break big food pieces into tiny ones the body can use.
What do you think is the role of saliva in your mouth?
Saliva makes food wet and soft so we can swallow it, and it starts digestion by changing starch (found in rice, chapati, etc.) into a little sugar.
Activity 9.1 (Chewed rice vs. plain rice & iodine)
a. What causes the blue-black colour in test-tube A?
Iodine turns blue-black when starch is present, so the colour shows starch is still there.
b. Why is there little or no blue-black colour in test-tube B?
Chewing mixed the rice with saliva. Saliva began breaking the starch into sugar, so almost no starch was left to change colour.
c. Would chewing longer change the colour even more?
Yes. More chewing means more saliva time, so still more starch would change into sugar and the blue-black colour would fade even further.
Why do cows (and some other animals) keep chewing even when they are not eating fresh grass?
Cows first swallow grass partly-chewed. Later they bring that half-digested food back up to the mouth and chew it again. This is called rumination, and cows are ruminants.
How do we breathe?
Air enters through the nostrils, travels down the windpipe into two lungs. When we inhale, ribs move out and a big muscle called the diaphragm moves down, pulling air in. When we exhale, ribs move in and diaphragm moves up, pushing air out.
In the model (Fig. 9.9) what do the balloons and the rubber sheet represent?
The balloons act like our lungs. The rubber sheet acts like the diaphragm.
What do we breathe out?
Exhaled air has less oxygen and much more carbon-dioxide than the air we inhaled.
Are breathing and respiration the same?
No. Breathing is just moving air in and out of the lungs.
Respiration is a chemical step inside cells: oxygen breaks down food like glucose to give energy, producing carbon-dioxide and water.
Do other animals breathe the same way humans do?
No. Land animals such as birds, lions, goats, and lizards use lungs (though lung shape differs).
Fish use gills to take oxygen dissolved in water.
Adult frogs use lungs on land and breathe through moist skin in water.
Earthworms breathe only through their skin.
Let Us Enhance Our Learning – Answers
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Journey of food:
Mouth → Oesophagus (food-pipe) → Stomach → Small intestine → Large intestine → Anus -
Rice-flour test-tube experiment
| Test-tube | What happens with iodine | Why |
|---|---|---|
| A – plain rice-flour | Turns blue-black | Starch is present |
| B – chewed rice-flour | Little / no blue-black | Saliva has changed much starch to sugar |
| C – boiled mashed potato | Also blue-black (potato contains starch) | Starch still present |
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Role of the diaphragm:
It moves downward when we inhale and upward when we exhale, helping air go in and out of the lungs. -
Matching
| Part | Function |
|---|---|
| (i) Nostrils | (a) Fresh air enters |
| (ii) Nasal passages | (d) Tiny hair and mucus trap dust |
| (iii) Windpipe | (e) Air reaches the lungs |
| (iv) Alveoli | (b) Exchange of gases occurs |
| (v) Rib cage | (c) Protects lungs |
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Question to Anil who says breathing = respiration:
“If breathing only moves air, where and how does your body change food into energy? Isn’t that extra chemical step called respiration?” -
Which statement is correct?
Tanu: “We inhale air rich in oxygen.”
Why: Air is a mixture; about one-fi fth of it is oxygen. -
Why do we often sneeze after inhaling dusty air?
Dust irritates the lining inside the nose. Sneezing is the body’s quick way to blow out those dust particles. -
Why was Anusha breathing faster than Paridhi after the run?
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She may have run faster or longer, so her muscles needed more oxygen.
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Her fitness level might be lower, so her body must breathe faster to get enough oxygen.
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What was Yadu trying to test with rice-flour, water and saliva?
He wanted to show that saliva breaks down starch (in rice-flour) so iodine colour disappears or lessens. -
What is Rakshita investigating with lime-water and two test-tubes?
She is comparing carbon-dioxide in inhaled vs. exhaled air.
To confirm she checks which tube of lime-water turns milky faster; the faster one got exhaled air, proving more CO₂ is present.