Answers to all "Let Us Enhance Our Learning" questions of Chapter 9, Life Processes in Animals (NCERT Class 7 Science, Curiosity, 2026-27): the human digestive system, digestion of starch by saliva, breathing and respiration, the respiratory organs, and experiments with iodine and lime water. All 10 questions are answered, with the key answer highlighted.
Sahil placed some pieces of chapati in test tube A. Neha placed chewed chapati in test tube B, and Santushti took boiled and mashed potato in test tube C. All of them added a few drops of iodine solution. What would be their observations? Give reasons.
Solution
A (pieces of chapati): turns blue-black, because chapati contains starch, which gives a blue-black colour with iodine.
B (chewed chapati): shows little or no blue-black colour (it may stay brownish, or be much fainter). Saliva mixed during chewing breaks down the starch into sugar, which does not give a blue-black colour with iodine.
C (boiled mashed potato): turns blue-black, because potato is rich in starch and no saliva was added.
A: blue-black (starch); B: little or no blue-black, as saliva has digested the starch into sugar; C: blue-black (starch in potato).
What is the role of the diaphragm in breathing? (i) To filter the air (ii) To produce sound (iii) To help in inhalation and exhalation (iv) To absorb oxygen
Solution
(iii) To help in inhalation and exhalation. When the diaphragm moves down, the chest cavity becomes larger and air is drawn in; when it moves up, the cavity becomes smaller and air is pushed out.
Match the following: (i) nostrils (ii) nasal passages (iii) windpipe (iv) alveoli (v) ribcage — (a) fresh air from outside enters (b) exchange of gases occurs (c) protects lungs (d) tiny hair and mucus help to trap dust and dirt from the air we breathe (e) air reaches our lungs through this part
Anil claims to his friend Sanvi that respiration and breathing are the same process. What question(s) can Sanvi ask him to make him understand that he is not correct?
Solution
"Where does breathing happen, and where does respiration happen?" (Breathing happens in the lungs and airways; respiration happens in every cell of the body.)
"Does breathing release energy from food?" (No; breathing only takes in air and gives out air. Respiration is the process in which food (glucose) is broken down using oxygen to release energy.)
"Do plants and tiny organisms without lungs respire?" (Yes, they respire, though they do not breathe like us.)
"Can respiration happen without oxygen?" (In some cases, yes, for example in yeast or in our muscles during hard exercise.)
So breathing is only the intake of oxygen and removal of carbon dioxide, while respiration is the release of energy inside cells; breathing is just one part of the whole process.
E.g. "Where does each happen?" (lungs vs every cell) and "Which one releases energy from food?" (respiration); breathing only moves air in and out.
Which of the following statements is correct and why? Anu: We inhale air. Shanu: We inhale oxygen. Tanu: We inhale air rich in oxygen.
Solution
Anu is correct. We breathe in air, which is a mixture of gases: mostly nitrogen (about 78%), oxygen (about 21%), and small amounts of carbon dioxide, water vapour and other gases. We do not inhale pure oxygen (Shanu), and inhaled air is not "rich in oxygen" (Tanu); oxygen is only about one-fifth of it. Our body uses the oxygen from the air we inhale.
Anu: we inhale air, a mixture mostly of nitrogen with about 21% oxygen; it is neither pure oxygen nor oxygen-rich.
We often sneeze when we inhale a lot of dust-laden air. What can be the possible explanations for this?
Solution
The nose is lined with hair and sticky mucus that trap dust and dirt. When a lot of dust enters, it irritates the lining of the nasal passage. The body responds by sneezing, a sudden, forceful blast of air that throws out the dust particles and keeps them from reaching the lungs. Some people also sneeze because they are allergic to dust or pollen.
Dust irritates the lining of the nose, and sneezing forcefully expels the dust to protect the lungs (and some people are allergic to dust).
Paridhi and Anusha ran their morning workout and then counted their breaths per minute. Anusha was breathing faster than Paridhi. Provide at least two possible explanations.
Solution
Anusha may have run faster or for longer, so her muscles needed more energy and more oxygen; she breathes faster to get more oxygen and remove more carbon dioxide.
Paridhi may be fitter (used to regular exercise), so her lungs and heart work more efficiently and she recovers faster.
Anusha may have counted sooner after stopping, when breathing was still fast.
Differences in body size, age, health (e.g. a cold or asthma) or the weather can also change the breathing rate.
E.g. Anusha ran harder or longer and needed more oxygen; Paridhi may be fitter; or Anusha counted sooner after stopping.
Yadu took two test tubes A and B, added a pinch of rice flour to each, half-filled them with water and stirred. To B he added a few drops of saliva. After 35–45 minutes he added iodine solution to both. Test tube A turned blue-black and B did not (Fig. 9.15). What do you think he wants to test?
Solution
He wants to test whether saliva digests (breaks down) starch. In A (no saliva), the starch remains, so iodine gives a blue-black colour. In B, saliva has broken down the starch into sugar during 35–45 minutes, so the iodine does not turn blue-black. A is the control, showing that only the saliva made the difference.
Whether saliva breaks down starch: A (no saliva) stays blue-black, while in B saliva has digested the starch into sugar.
Rakshita took two test tubes A and B, filled them with lime water, and in A passed the surrounding (inhaled) air by sucking from a pipe, while in B the exhaled air was blown through a pipe (Fig. 9.16). What is she trying to investigate? How can she confirm her findings?
Solution
She is investigating whether the air we breathe out contains more carbon dioxide than the air we breathe in.
Confirmation: carbon dioxide turns lime water milky. The lime water in B (exhaled air) will turn milky quickly, while in A (inhaled air) it will take much longer or stay clear. She should pass air for the same length of time in both and repeat the experiment a few times to be sure. This shows that exhaled air has much more carbon dioxide.
Whether exhaled air has more carbon dioxide than inhaled air; lime water in B turns milky much faster than in A, which she can confirm by repeating with equal times.
